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Atmos. Chem. Phys., 25, 1333–1351, 2025
https://doi.org/10.5194/acp-25-1333-2025
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Research article
Impact of mineral dust on the global nitrate aerosol
direct and indirect radiative effect
Alexandros Milousis1, Klaus Klingmüller2, Alexandra P. Tsimpidi1, Jasper F. Kok3,
Maria Kanakidou4,5,6, Athanasios Nenes5,7 , and Vlassis A. Karydis1
1Institute of Climate and Energy Systems: Troposphere (ICE-3), Forschungszentrum
Jülich GmbH, Jülich, Germany
2Max Planck Institute for Chemistry, Mainz, Germany
3Department of Atmospheric and Oceanic Sciences, University of California
Los Angeles, Los Angeles, CA, USA
4Environmental Chemical Processes Laboratory, Department of Chemistry,
University of Crete, Heraklion, Greece
5Center for the Study of Air Quality and Climate Change, Foundation for Research
and Technology – Hellas, Patras, Greece
6Institute of Environmental Physics, University of Bremen, Bremen, Germany
7Laboratory of Atmospheric Processes and Their Impacts, Ecole Polytechnique
Fédérale de Lausanne, Switzerland
Correspondence: Vlassis A. Karydis (v.karydis@fz-juelich.de)
Received: 27 May 2024 – Discussion started: 3 July 2024
Revised: 18 October 2024 – Accepted: 29 November 2024 – Published: 31 January 2025
Abstract. Nitrate (NO−
3) aerosol is projected to increase dramatically in the coming decades and may become
the dominant inorganic particle species. This is due to the continued strong decrease in SO2emissions, which is
not accompanied by a corresponding decrease in NOxand especially NH3emissions. Thus, the radiative effect
(RE) of NO−
3aerosol may become more important than that of SO2−
4aerosol in the future. The physicochemical
interactions of mineral dust particles with gas and aerosol tracers play an important role in influencing the overall
RE of dust and non-dust aerosols but can be a major source of uncertainty due to their lack of representation in
many global climate models. Therefore, this study investigates how and to what extent dust affects the current
global NO−
3aerosol radiative effect through both radiation (REari) and cloud interactions (REaci) at the top of the
atmosphere (TOA). For this purpose, multiyear simulations nudged towards the observed atmospheric circulation
were performed with the global atmospheric chemistry and climate model EMAC, while the thermodynamics of
the interactions between inorganic aerosols and mineral dust were simulated with the thermodynamic equilib-
rium model ISORROPIA-lite. The emission flux of the mineral cations Na+, Ca2+, K+, and Mg2+is calculated
as a fraction of the total aeolian dust emission based on the unique chemical composition of the major deserts
worldwide. Our results reveal positive and negative shortwave and longwave radiative effects in different regions
of the world via aerosol–radiation interactions and cloud adjustments. Overall, the NO−
3aerosol direct effect
contributes a global cooling of −0.11 W m−2, driven by fine-mode particle cooling at short wavelengths. Re-
garding the indirect effect, it is noteworthy that NO−
3aerosol exerts a global mean warming of +0.17 W m−2.
While the presence of NO−
3aerosol enhances the ability of mineral dust particles to act as cloud condensation
nuclei (CCN), it simultaneously inhibits the formation of cloud droplets from the smaller anthropogenic parti-
cles. This is due to the coagulation of fine anthropogenic CCN particles with the larger nitrate-coated mineral
dust particles, which leads to a reduction in total aerosol number concentration. This mechanism results in an
overall reduced cloud albedo effect and is thus attributed as warming.
Published by Copernicus Publications on behalf of the European Geosciences Union.
1334 A. Milousis et al.: Impact of mineral dust on the global nitrate aerosol
1 Introduction
Atmospheric aerosols are among the most complex compo-
nents of the Earth’s climate system. This is due not only to
the diversity of their origins, with many natural and anthro-
pogenic emission sources, but also to their extremely varied
chemical composition and properties. The many mechanisms
by which they interact with each other and with physical en-
tities such as radiation, clouds, land, and oceans add to their
complexity and play a critical role in the energy balance of
the planet (Arias et al., 2021). The most direct way in which
aerosols affect the Earth’s energy balance is through their
interactions with solar shortwave (SW) and terrestrial long-
wave (LW) radiation (IPCC, 2013). Overall, the radiative ef-
fect due to aerosol–radiation interactions (REari) is mainly
dominated by the scattering of SW radiation back to space
(negative radiative effect, generating a cooling of the climate
system) and the absorption of LW radiation (positive radia-
tive effect, generating a warming of the climate system) (Gao
et al., 2018; Tsigaridis and Kanakidou, 2018). Aerosols be-
longing to the black and/or brown carbon family, together
with mineral dust particles, contribute to absorption (Kanaki-
dou et al., 2005; Zhang et al., 2017; Wong et al., 2019),
while the main inorganic aerosol components, such as sulfate
and nitrate, as well as a significant amount of organic car-
bon, contribute mainly to scattering (Kirchstetter et al., 2004;
(Bond and Bergstrom, 2006; Klingmüller et al., 2019; Zhang,
2020). However, mineral dust can also influence the behavior
of the REari of anthropogenic pollution. Dust particles alter
the anthropogenic radiative effect of aerosol–radiation inter-
actions by reducing the loading of anthropogenic aerosols
(either by coagulating with them or by adsorption of their
precursor inorganic trace gases), leading to less scattering of
solar radiation and thus a warming effect (Kok et al., 2023).
Atmospheric aerosols can also indirectly affect the Earth’s
energy balance by forming clouds, controlling cloud optical
thickness and scattering properties, and altering their pre-
cipitation and lifetime (IPCC, 2013). Atmospheric aerosols
act as cloud condensation nuclei (CCN), providing a suitable
surface for water vapor to condense, leading to the forma-
tion of liquid droplets that develop into a corresponding liq-
uid cloud (Lance et al., 2004). Such clouds are referred to
as warm clouds and are typically found in the lower tropo-
sphere (Khain and Pinsky, 2018). However, there is constant
competition between small and large particles for the avail-
able amount of water vapor (Barahona et al., 2010; Morales
Betancourt and Nenes, 2014). Under the same humidity con-
ditions, the presence of small particles will lead to the for-
mation of small droplets with high number concentrations,
while the presence of larger particles will lead to the forma-
tion of large droplets but with lower number concentrations.
Depending on the size characteristics of its particle popu-
lation, a warm cloud will exhibit different optical proper-
ties, with a population dominated by smaller particles gen-
erally being more reactive in the SW spectrum. The change
in cloud reflectivity due to the presence of aerosols is re-
ferred to as the first radiative effect due to aerosol–cloud in-
teractions (REaci) and was first described by Twomey (1977).
The small size of anthropogenic aerosols results in an overall
smaller cloud droplet size, which reduces precipitation ef-
ficiency and thus increases cloud lifetime. This contributes
to cloud reflectivity and is referred to as the second ra-
diative effect of aerosol–cloud interactions, first described
by Albrecht (1989). These two indirect effects are consid-
ered equally important for the total indirect radiative ef-
fect of aerosols (Lohmann and Feichter, 2005). Atmospheric
aerosols exert a net cooling effect that can partially mask the
warming effect of greenhouse gases; therefore, the recent de-
cline in anthropogenic aerosol concentrations may accelerate
global warming (Urdiales-Flores et al., 2023). Overall, the
radiative effect due to aerosol–cloud interactions is consid-
ered the main source of existing uncertainty in the effective
(total) radiative effect of aerosols in the atmosphere (Myhre
et al., 2014; Seinfeld et al., 2016).
Mineral dust influences the anthropogenic radiative effect
through aerosol–cloud interactions in several ways that can
result in either a net warming or net cooling effect. Dust par-
ticles can increase the cloud droplet number concentration
(CDNC) in remote areas since through chemical aging by
pollutants (Nenes et al., 2014; Karydis et al., 2017), dust par-
ticles become more hygroscopic and require lower supersat-
uration thresholds for activation (Karydis et al., 2011). This
is caused by the transfer of anthropogenic pollutants towards
remote desert regions, which enhances the solubility of dust
particles. In such regions, this mostly results in increased
cloud albedo and a net cooling effect. However, dust particles
also tend to reduce the availability of smaller anthropogenic
CCN. This is due to intrusions of aged dust particles into
polluted environments, which reduce the numbers of smaller
aerosols through increased coagulation with them. This re-
sults in lower cloud reflectivity (albedo) and thus a net warm-
ing effect (Klingmüller et al., 2020). Furthermore, when dust
is above or below low-level clouds, the resulting effect of lo-
cal heating is an increase in total cloud cover due to enhanced
temperature inversion or enhanced upward vertical motion,
respectively (Kok et al., 2023). On the other hand, when dust
is present inside low-level clouds, local heating enhances in-
cloud evaporation, resulting in an overall decrease in cloud
cover. Kok et al. (2023) showed that the amount of desert
dust in the atmosphere has increased since the mid-19th cen-
tury, causing an overall cooling effect on the Earth that masks
up to 8 % of the warming caused by greenhouse gases. If the
increase in dust were halted, the previously hidden additional
warming potential of greenhouse gases could lead to slightly
faster climate warming.
Atmos. Chem. Phys., 25, 1333–1351, 2025 https://doi.org/10.5194/acp-25-1333-2025
A. Milousis et al.: Impact of mineral dust on the global nitrate aerosol 1335
NO−
3is expected to dominate the global aerosol composi-
tion in the coming decades due to the predicted limited avail-
ability of SO−2
4following the abrupt decline in SO2emis-
sions, which will not necessarily be accompanied by propor-
tional reductions in NOxand NH3emissions (Bellouin et al.,
2011; Hauglustaine et al., 2014). Excess NO−
3is expected to
exert a cooling REari by scattering SW radiation (Bauer et
al., 2007a; Xu and Penner, 2012; Myhre et al., 2013; IPCC,
2013; Li et al., 2015), but the REaci is much more complex
and complicated and can lead to both cooling and warm-
ing. Mineral dust thus becomes a key factor, as it is one of
the main promoters of NO−
3aerosol formation, providing a
very suitable surface for gaseous HNO3condensation to the
aerosol phase (Karydis et al., 2011; Trump et al., 2015). In
addition to HNO3adsorption, heterogeneous reactions on the
surface of dust particles are known to promote nitrate forma-
tion (Krueger et al., 2004; Hodzic et al., 2006). The most im-
portant pathway through which this occurs is N2O5hydroly-
sis, with a yield for aerosol nitrate of ∼2 (Seisel et al., 2005;
Tang et al., 2012). At the same time, other reactions, such as
NO2oxidation, contribute to much slower nitrate production
and are of major importance mainly during short periods of
dust pollution events (Li et al., 2024). These processes affect
not only the optical properties of dust aerosols, which will in-
fluence their overall REari, but also how they can alter cloud
formation and microphysics. NO−
3aerosols increase the hy-
groscopicity of mineral dust (Kelly et al., 2007) by providing
layers of soluble material on their surface, thus increasing
their ability to act as CCN (Karydis et al., 2017). In doing
so, they also increase the size of dust particles through hy-
groscopic growth and therefore their coagulation efficiency.
Thus, nitrate–dust interactions are a complex mechanism that
ultimately affects climatology in a variety of ways. The role
of mineral dust in modifying the influence of NO−
3aerosols
in the global REaci is not yet well understood. This study
aims to focus on the extent of the REari and REaci of NO−
3
aerosols and on how interactions with mineral dust regulate
both on a global scale.
This study is organized as follows: in Sect. 2, details of
the modeling setup for conducting the global simulations as
well as the treatment of dust–nitrate interactions in the model
are discussed and the methodology for calculating the global
REari and REaci of NO−
3aerosols is explained. Section 3
presents the main results for the global REari for coarse and
fine NO−
3aerosols for the base case simulation and the sen-
sitivity cases listed in Table 1. Section 4 presents the results
for the global REaci of total NO−
3aerosols, while Sect. 5 in-
cludes the feedback mechanism of dust–nitrate interactions
with cloud microphysics. Finally, the main conclusions and
a general discussion on the scope of the study are presented
in Sect. 6.
2 Methodology
2.1 Model setup
The simulations were performed with the global atmospheric
chemistry and climate model EMAC (ECHAM/MESSy)
(Jöckel et al., 2006), which includes several submod-
els describing atmospheric processes and their interactions
with oceans, land, and human influences. These submod-
els are linked through the Modular Earth Submodel System
(MESSy) (Jöckel et al., 2005) to a base model, the 5th Gener-
ation European Center Hamburg General Circulation Model
(ECHAM) (Roeckner et al., 2006). The submodel system
used in this work includes the MECCA submodel, which
performs the gas-phase chemistry calculations (Sander et al.,
2019). The SCAV submodel is responsible for the in-cloud
liquid-phase chemistry and wet deposition processes (Tost
et al., 2006, 2007b), while DRYDEP and SEDI are used to
compute the dry deposition of gases and aerosols and grav-
itational settling, respectively (Kerkweg et al., 2006). All
aerosol microphysical processes are calculated by the GMXe
submodel (Pringle et al., 2010a, b), where aerosols are di-
vided into four lognormal size modes (nucleation, Aitken,
accumulation, and coarse). Each mode is defined in terms of
aerosol number concentration, mean dry radius, and geomet-
ric standard deviation (sigma). The mean dry radius for each
mode is allowed to vary within fixed bounds (0.5–6nm for
nucleation, 6–60 nm for Aitken, 60–700 nm for accumula-
tion, and above 700 nm for coarse) and the sigma is fixed and
equal to 1.59 for the first three size modes and 2 for the coarse
mode. The coagulation of aerosols is also handled by GMXe,
following Vignati et al. (2004), and the coagulation coeffi-
cients for Brownian motion are calculated according to Fuchs
and Davies (1964). The partitioning between the gas and
aerosol phases is calculated using the ISORROPIA-lite ther-
modynamic module (Kakavas et al., 2022) as implemented
in EMAC by Milousis et al. (2024). The optical properties of
the aerosols and the radiative transfer calculations are sim-
ulated by the submodels AEROPT (Dietmüller et al., 2016)
and RAD (Dietmüller et al., 2016), respectively. AEROPT
can be called several times within a model time step with
different settings for the aerosol properties. More details are
given in Sect. 2.3.1. All cloud properties and microphysical
processes are simulated by the CLOUD submodel (Roeckner
et al., 2006) using the two-moment microphysical scheme of
Lohmann and Ferrachat (2010) for liquid and ice clouds. The
activation processes of liquid cloud droplets and ice crys-
tals follow the physical treatment of Morales Betancourt and
Nenes (2014) and Barahona and Nenes (2009), respectively,
as described by Karydis et al. (2017) and Bacer et al. (2018).
More details are given in Sect. 2.3.2.
The meteorology for each of the simulations was nudged
by ERA5 reanalysis data (C3S, 2017); thus, this study esti-
mates the radiative effect of nitrate aerosols with respect to
REari and REaci separately, rather than the effective (total)
https://doi.org/10.5194/acp-25-1333-2025 Atmos. Chem. Phys., 25, 1333–1351, 2025
1336 A. Milousis et al.: Impact of mineral dust on the global nitrate aerosol
Table 1. Differences between the base case and all sensitivity simulations performed.
Simulation name Conditions applied
Base case Mineral dust ion composition according to Karydis et al. (2016)1
Sensitivity 1: chemically inert dust Mineral dust emitted exclusively as a chemically inert bulk particle
Sensitivity 2: homogeneous ion composition Global homogeneous ionic composition of mineral dust particles
according to Sposito (1989)2
Sensitivity 3: half dust scenario 50 % reduced dust emission flux
Sensitivity 4: increased dust scenario 50 % increased dust emission flux
1The ionic composition of the dust particles with respect to the mineral ions Ca2+, Mg2+, K+, and Na+depends on the chemical composition of the
soil in each grid cell, which is estimated from the desert soil composition maps of Klingmüller et al. (2018) based on the fraction of mineral ions found
in Karydis et al. (2016). 2The ionic composition of the dust particles is homogeneous and held constant in all grid cells where dust is present. The dust
particles are a mixture of bulk species and the mineral ions Na+, K+, Ca2+, and Mg2+with mass fractions of 94 %, 1.2%, 1.5 %, 2.4 %, and 0.9%,
respectively.
radiative effect, as this would require multiple free-run sim-
ulations with prescribed sea surface temperatures for each
case separately. The spectral resolution used for each simu-
lation was T63L31, which corresponds to a grid resolution of
1.875° ×1.875 and 31 vertical layers up to 25 km in height.
The period covered by the simulations is from 2007 to 2018,
with the first year representing the model spin-up period.
Anthropogenic aerosol and trace gas emissions were taken
from the CMIP6 database (O’Neill et al., 2016) according
to the SSP370 scenario. Natural NH3emissions (from land
and ocean) were based on the GEIA database (Bouwman et
al., 1997), and natural volcanic SO2emissions were taken
from the AEROCOM database (Dentener et al., 2006). Bio-
genic NO emissions from soils were calculated online ac-
cording to the algorithm of Yienger and Levy (1995), while
lightning-produced NOxwas also calculated online by the
LNOx submodel (Tost et al., 2007a) using the parameteriza-
tion of Grewe et al. (2001). Dimethyl sulfide (DMS) emis-
sions from the oceans are calculated online by the AIRSEA
submodel (Pozzer et al., 2006). Sea salt emissions are based
on the AEROCOM database (Dentener et al., 2006) follow-
ing the chemical composition reported by Seinfeld and Pan-
dis (2016), i.e., 30.6 % Na+, 3.7% Mg2+, 1.2 % Ca2+, 1.1 %
K+, and 55 % Cl−. Dust emissions are calculated online
using the parameterization of Astitha et al. (2012). In this
scheme, while the surface friction velocity is the most im-
portant parameter for the amount of the emitted dust flux, the
meteorological information for each grid cell is also taken
into account. Dust particles are emitted in the accumulation
and coarse size modes of the insoluble fraction but can be
transferred to the soluble fraction after coagulation with other
soluble species and/or by condensation of soluble material
on their surface. Both processes are treated and calculated
by GMXe and ISORROPIA-lite. The emissions of mineral
ions (Ca2+, Mg2+, K+, and Na+) are estimated as a frac-
tion of the total dust emission flux based on the soil chemical
composition of each grid cell. This is done using desert soil
composition maps from Klingmüller et al. (2018), which are
based on the mineral ion fractions from Karydis et al. (2016).
These mineral ions are treated as individual species that are
part of the aerosol in each size mode and are assumed to
be well mixed with the rest of the aerosol species consid-
ered (i.e., dust, black carbon, organics, inorganic ions). The
aerosol composition within each of the seven modes consid-
ered is uniform in size (internally mixed) but may vary be-
tween modes (externally mixed).
To assess the impact of changes in mineral dust chemistry
and emissions on the global NO−
3aerosol REari and REaci,
four additional sensitivity simulations were performed (Ta-
ble 1). In the first sensitivity simulation, mineral dust is de-
scribed only by a bulk, chemically inert species. In this case,
there is no uptake of HNO3by the dust particles due to acid–
base interactions with the non-volatile cations (NVCs), so
it remains in the gas phase. In the second sensitivity case,
the chemical composition of the mineral dust was assumed
to be spatially uniform, with a percentage distribution for
bulk dust, Na+, K+, Ca2+, and Mg2+particles assumed to be
94 %, 1.2 %, 1.5 %, 2.4 %, and 0.9 %, respectively, according
to Sposito (1989). Finally, two additional simulations were
performed to assess the impact of the global mineral dust
budget on the results, where the dust emission fluxes were
first halved and then increased by 50 % to account for the his-
torical increase in global dust mass load since pre-industrial
times, as reconstructed by Kok et al. (2023). The particle size
distribution of the emitted dust mass remained unchanged in
all sensitivity simulations.
Overall, the EMAC model is well established in the lit-
erature for its ability to accurately predict organic and inor-
ganic aerosol concentrations and compositions, aerosol op-
tical depth, acid deposition, gas-phase mixing ratios, cloud
properties, and meteorological parameters (de Meij et al.,
2012; Pozzer et al., 2012, 2022; Tsimpidi et al., 2016, 2017;
Karydis et al., 2016, 2017; Bacer et al., 2018; Milousis et
al., 2024); factually replicate dust emissions (Astitha et al.,
Atmos. Chem. Phys., 25, 1333–1351, 2025 https://doi.org/10.5194/acp-25-1333-2025
A. Milousis et al.: Impact of mineral dust on the global nitrate aerosol 1337
2012; Abdelkader et al., 2015; Klingmüller et al., 2018); and
provide realistic estimates for CCN and CDNC (Chang et al.,
2017; Karydis et al., 2017; Fanourgakis et al., 2019). Here,
a comparison of the performance of the model in estimat-
ing the surface mass concentrations of PM2.5NO−
3and total
PM10 aerosols is provided in the Supplement (Figs. S2, S3
and Tables S1, S2). In addition, the ability of the model to es-
timate CDNCs is evaluated (Fig. S4 and Table S3). The com-
parison is made with observations of PM2.5nitrate aerosols
from regional networks in the polluted Northern Hemisphere
covering the regions of East Asia (EANET, the Acid De-
position Monitoring Network in East Asia, 2024), Europe
(EMEP, European Monitoring and Evaluation Programme,
2024), and the USA for urban (EPA-CASTNET, U.S. Envi-
ronmental Protection Agency Clean Air Status and Trends
Network, 2024) and rural (IMPROVE, Interagency Monitor-
ing of Protected Visual Environments, 2024) locations. The
comparison with observations of surface mass PM10 aerosols
also covers the aforementioned monitoring networks, with
the exception of the EPA. Finally, the CDNCs estimated by
the base case simulation are compared with the CDNCs ob-
served in different regions of the planet (continental, pol-
luted, and clean marine) over different time periods, but also
altitudes, as found in Karydis et al. (2017) and all relevant
references therein.
2.2 Treatment of dust–nitrate interactions
The interactions between mineral dust and nitrate aerosols
play a crucial role in altering the size distribution and op-
tical properties of both species and can also strongly in-
fluence cloud microphysical processes (Fig. 1). Therefore,
these interactions affect both the REari and the REaci of
both nitrate and dust aerosols. First, the adsorption of HNO3
onto the surface of dust particles is a process that strongly
promotes the formation of nitrate aerosols on dust (Kary-
dis et al., 2016). We treat this condensation process using
the GMXe submodel. Specifically, the quantity of gas-phase
species that are able to kinetically condense within a model
time step (equal to 10 min in this study) is calculated accord-
ing to the diffusion-limited condensation theory of Vignati et
al. (2004). The diffusive flux of gas on a single particle sur-
face for each size mode iis described by the condensation
coefficient Ciaccording to Fuchs and Davies (1964) and is
estimated from the following function as found in Vignati et
al. (2004).
Ci=4πDrgi
4D
svrgi +rgi
rgi +1
,
where rgi is the geometric mean radius of the size mode i,D
is the diffusion coefficient, and sis an accommodation coeffi-
cient for each gas species treated that has the assigned values
of 1 for H2SO4(Vignati et al. 2004), 0.1 for HNO3, 0.064
for HCl, and 0.09 for NH3(Pringle et al., 2010a, b). vis
the mean thermal velocity of the molecule and 1is the mean
free path length of the gas molecule (the distance from which
the kinetic regime applies with respect to the particle).This
information is then passed to the ISORROPIA-lite thermo-
dynamic module to calculate the gas–aerosol partitioning.
Specifically, the module receives as input the ambient tem-
perature and humidity along with the diffusion-limited con-
centrations of H2SO4, NH3, HNO3, and HCl; the concen-
trations of the non-volatile cations (NVCs) Na+, K+, Ca2+,
and Mg2+; and the concentrations of the ions SO2−
4, NO−
3,
NH+
4, and Cl−present in the aerosol phase from the previ-
ous time step. The module then calculates the equilibrium
reactions of the NO−
3anion with the NVCs, depending on
their abundance with respect to the SO2−
4anion, taking into
account mass conservation, electroneutrality, water activity
equations, and precalculated activity coefficients for specific
ionic pairs (Fountoukis and Nenes, 2007; Kakavas et al.,
2022). Therefore, in all cases where mineral dust is consid-
ered chemically active, all reactions of nitrate aerosols with
NVCs are treated. The salts that may be formed are assumed
to be completely deliquesced as follows.
Ca(NO3)2→Ca2+
(aq) +2NO−
3(aq)
NaNO3→Na+
(aq)+NO−
3(aq)
KNO3→K+
(aq) +NO−
3(aq)
Mg(NO3)2→Mg2+
(aq) +2NO−
3(aq)
Salt deliquescence over a range of relative humidities
is treated by the mutual deliquescence relative humidity
(MDRH) approach of Wexler and Seinfeld (1991). In a mul-
ticomponent salt mixture, the MDRH determines the humid-
ity value above which all salts are considered to be satu-
rated. In this study, if the wet aerosol is below the MDRH,
it does not crystalize and remains in a supersaturated aque-
ous solution (Kakavas et al., 2022), with all salts completely
deliquesced. More information on equilibrium reactions and
equilibrium constants as well as the corresponding thermo-
dynamic equilibrium calculations can be found in Fountoukis
and Nenes (2007). It should be noted that in this study ni-
trate production on dust particles occurs not only via the
thermodynamic equilibrium between gas-phase HNO3and
particulate nitrate, but also via heterogeneous chemistry by
hydrolysis of N2O5on the dust surface. This chemical for-
mation pathway is the most dominant for heterogeneous ni-
trate production (Seisel et al., 2005; Tang et al., 2012), while
others, such as NO2oxidation during dust pollution events
over polluted regions (Li et al., 2024), do not show such high
yields under normal conditions. On the other hand, consid-
eration of sulfate production by heterogeneous chemistry on
dust would theoretically result in slightly reduced amounts
of particulate nitrate in some cases due to acidification of
dust particles inhibiting partitioning of HNO3to the aerosol
https://doi.org/10.5194/acp-25-1333-2025 Atmos. Chem. Phys., 25, 1333–1351, 2025
1338 A. Milousis et al.: Impact of mineral dust on the global nitrate aerosol
phase (Nenes et al., 2020). Overall, full consideration of het-
erogeneous chemistry on dust could change simulated ni-
trate aerosol concentrations only slightly and episodically,
and therefore changes to radiative effect estimates are not ex-
pected to be critical.
The coating of dust particles by nitrate aerosols during
gas–aerosol partitioning calculations is an important process
that leads to an increase in dust solubility and hygroscop-
icity (Laskin et al., 2005). Therefore, after these processes
have taken place, a large fraction of the originally insoluble
dust particles has become soluble (Fig. 1a), which leads to
changes in their optical properties, as their increased abil-
ity to absorb water makes them more efficient in extinguish-
ing SW radiation and absorbing and emitting LW radiation
(Fig. 1a, b) (Kok et al., 2023). The transfer to the soluble
fraction after coating with soluble material is handled by the
GMXe submodel, which also provides key aerosol attributes
necessary for the calculation of the dust optical properties
(see Sect. 2.3).
In general, the changes in the properties of dust particles
through their interactions with nitrate aerosols will result in
more efficient removal rates, mainly through wet deposition,
due to their higher hygroscopicity and increased size (Fan
et al., 2004). The reduced number of dust particles that can
act as ice nuclei (IN) and their increased size can lead to
an optical thinning of cirrus clouds (Fig. 1c) (Kok et al.,
2023). Furthermore, the changes induced by dust–nitrate in-
teractions reduce the activation of smaller aerosols in warm
clouds (Fig. 1d). In particular, the enhanced hygroscopicity
of dust particles will lead to a faster depletion of the avail-
able supersaturation, as they act as giant CCN that absorb
large amounts of water vapor to activate into cloud droplets
(Karydis et al., 2017). In addition, the population of smaller
aerosols will also be depleted by increased coagulation with
the large dust particles. As a consequence of the different de-
grees of complexity of the dust–nitrate interactions, it is very
important to note that they do not always result in a linear
response in terms of how they affect climate through their
subsequent interactions with radiation, clouds, or both.
2.3 Radiative effect calculation
To calculate the global REari and REaci of NO−
3aerosols, the
optical properties from the AEROPT submodel and the radia-
tive transfer calculations from the RAD submodel were used.
First, AEROPT provides the aerosol extinction (absorption
and scattering) coefficients, the single-scattering albedo, and
the aerosol asymmetry factor for each grid cell with a ver-
tical distribution analogous to the vertical resolution used.
The GMXe submodel is used to provide input of aerosol
attributes for the calculation of aerosol optical properties,
which is done online using 3D look-up tables. The tables
provide information on the real and imaginary parts of the
refractive index and the Mie size parameter per size mode
(Dietmüller et al., 2016). Then, the radiative scheme of RAD
uses the particle number weighted average of the extinction
cross section, the single-scattering albedo, and the asymme-
try factor as input for the radiative transfer calculations. In
addition to AEROPT, RAD takes input from the submod-
els ORBIT (Earth orbital parameters), CLOUDOPT (cloud
optical properties) (Dietmüller et al., 2016), and IMPORT
(import of external datasets) to calculate the radiative trans-
fer properties for longwave and shortwave radiation fluxes
separately. Both the AEROPT and RAD submodels can be
invoked multiple times within a model time step, each time
with different settings for the aerosol optical properties, al-
lowing radiative transfer estimates for identical climatologi-
cal conditions. This is of paramount importance for the cal-
culation of the REari of aerosols since any effects due to pos-
sibly different climatological conditions must be eliminated.
Henceforth, all references to RE estimates, as well as net,
longwave, and shortwave flux quantities, will refer to the top
of the atmosphere (TOA) only.
2.3.1 Radiative effect from aerosol–radiation
interactions (REari)
To estimate the global REari of all aerosols as well as that of
total, coarse, and fine NO−
3aerosols, three simulations were
performed for each sensitivity case in Table 1. In the first
simulation all aerosol species are present. In the second sim-
ulation NO−
3aerosols are completely removed by turning off
their formation by removing the pathway of HNO3formation
through both NO2oxidation and N2O5hydrolysis, leaving no
available HNO3to condense on the aerosol via equilibrium
partitioning and form nitrate. In the third simulation, coarse-
mode NO−
3aerosols are removed by allowing HNO3to con-
dense only on the fine mode (i.e., the sum of the three smaller
lognormal size modes: nucleation, Aitken, and accumula-
tion). For each of these three simulations, the radiative trans-
fer routines are called twice for each time step. One call uses
the normal aerosol optical properties of the existing popula-
tion, and the other call uses an aerosol optical depth equal to
0 to emulate an atmosphere without aerosols. Essentially, the
global REari of each simulation can be calculated by taking
the difference between the net fluxes between the two calls.
More specifically, the first simulation will yield the REari of
the total aerosol load (F1,ari hereafter), the second simula-
tion will yield the REari of all aerosols except NO−
3(F2,ari
below), and the third simulation will yield the REari of all
aerosols except the coarse-mode NO−
3(F3,ari below). Since
the above estimates of the radiative effect were computed
using the exact same climatology, its effect was effectively
eliminated. However, in order to isolate the NO−
3aerosol ra-
diative effect, it is also essential to disable any aerosol–cloud
interactions; otherwise the cooling effect would be severely
underestimated because cloud scattering would make aerosol
scattering less relevant (Ghan et al., 2012). For this purpose,
the simplest cloud scheme available in the EMAC model is
used, which calculates the cloud microphysics according to
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A. Milousis et al.: Impact of mineral dust on the global nitrate aerosol 1339
Figure 1. Conceptual illustration of how dust–nitrate interactions affect the total NO−
3(left) REari and (right) REaci.(a) In dust-rich envi-
ronments, nitric acid transported from anthropogenic pollution and biomass burning regions interacts with mineral cations to form a soluble
coating on the surface of dust particles. The dominant effect of these interactions is an enhanced LW absorption (warming REari) by the
coarse dust particles. (b) In nitrate-rich environments, the intrusion of dust particles and their subsequent interaction with freshly formed ni-
tric acid lead to an overall increase in aerosol hygroscopicity and thus a stronger SW reflection (cooling REari). (c) In dust-rich environments,
the number of ice crystals in cirrus clouds is reduced while their size is increased due to the interaction of dust particles with the transported
HNO3. This results in an optical thinning of the ice clouds, which leads to less trapping of outgoing LW radiation (cooling REaci). (d) In
nitrate-rich environments, the increased wet radius of aged dust particles leads to enhanced coagulation with smaller particles, resulting in a
decrease in the number of smaller aerosols and, in turn, a decrease in the number of activated particles in cloud droplets by smaller aerosols,
which ultimately leads to a reduction in the backscattering of SW radiation by warm clouds (warming REari).
Lohmann and Roeckner (1996), who empirically relate the
cloud droplet number concentration to the sulfate aerosol
mass (Boucher and Lohmann, 1995) and specifically to its
monthly mean values as derived from the sulfur cycle of
the ECHAM5 circulation model (Feichter et al., 1996). The
cloud coverage is estimated according to Tompkins (2002)
with the use of prognostic equations for the water phases and
the distribution moments. To disable aerosol–cloud interac-
tions, no aerosol activation routines are used to avoid cou-
pling with the activation schemes. Overall, the global REari
values of total, coarse, and fine NO−
3aerosols are obtained as
follows:
FNO3,ari (FN,ari)=F1,ari -F2,ari,
FcoarseNO3,ari (FcN,ari)=F1,ari -F3,ari,
FfineNO3,ari (FfN,ari)=F3,ari -F2,ari.
2.3.2 Radiative effect from aerosol–cloud interactions
(REaci)
In this work we estimate the effect of total NO−
3aerosols
on the calculated global REaci. Climatology plays a crucial
role in aerosol–cloud interactions and simulating a “fine-only
NO−
3atmosphere”, as done for the REari calculations, would
produce an unrealistic climatological scenario, since coarse-
mode NO−
3is strongly associated with cations in mineral dust
particles (Karydis et al., 2016), making them quite effective
as CCN (Karydis et al., 2017). Therefore, the REaci calcula-
tions require two additional simulations for each sensitivity
case separately: one with all aerosols present and one with
the entire NO−
3aerosol load removed by turning off their for-
mation as described in the previous section. The global REaci
is then given by
FNO3,aci (FN,aci)=FFN-FN,ari ,
where FFNis the total NO−
3aerosol feedback radiative effect.
Since FN,ari is calculated using the methodology described
in Sect. 2.3.1, it is only necessary to estimate FFN. This is
equal to the difference in net fluxes between the two addi-
tional simulations. There is no need to emulate an aerosol-
free atmosphere here since any differences induced by dif-
ferent climatologies must be included. The two simulations
performed for the calculation of FFNuse the cloud forma-
tion scheme as described in Lohmann and Ferrachat (2010),
which uses prognostic equations for the water phases and
the bulk cloud microphysics. In addition, the empirical cloud
cover scheme of Sundqvist et al. (1989) is used. For the cloud
droplet formation, the CDNC activation scheme of Morales
Betancourt and Nenes (2014) is used, which includes the ad-
https://doi.org/10.5194/acp-25-1333-2025 Atmos. Chem. Phys., 25, 1333–1351, 2025
1340 A. Milousis et al.: Impact of mineral dust on the global nitrate aerosol
Table 2. Net, longwave, and shortwave global mean TOA REari
of total, coarse, and fine NO−
3aerosols for the base case and each
sensitivity case simulation.
Simulation Aerosol TOA REari (W m−2)
Component Net LW SW
Base case Total NO−
3−0.11 +0.23 −0.34
Coarse NO−
3+0.17 +0.22 −0.05
Fine NO−
3−0.28 +0.01 −0.29
Chemically Total NO−
3−0.09 +0.11 −0.20
inert dust Coarse NO−
3+0.07 +0.10 −0.03
Fine NO−
3−0.16 +0.01 −0.17
Homogeneous ion Total NO−
3−0.09 +0.18 −0.27
Composition Coarse NO−
3+0.13 +0.17 −0.04
Fine NO−
3−0.22 +0.01 −0.23
Half dust scenario Total NO−
3−0.08 +0.19 −0.27
Coarse NO−
3+0.15 +0.18 −0.03
Fine NO−
3−0.23 +0.01 −0.24
Increased dust Total NO−
3−0.10 +0.27 −0.37
scenario Coarse NO−
3+0.20 +0.26 −0.06
Fine NO−
3−0.30 +0.01 −0.31
sorption activation of mineral dust as described in Karydis et
al. (2017). The effect of dust–nitrate interactions on clouds
presented here refers to the lowest level of cloud formation
at 940 hPa. For the ice crystal number concentration (ICNC),
the activation scheme of Barahona and Nenes (2009) is used,
which calculates the ice crystal size distribution through het-
erogeneous and homogeneous freezing as well as ice crystal
growth.
3 Radiative effect from aerosol–radiation
interactions (REari)
3.1 Base case
The global average REari of total NO−
3aerosols at the top
of the atmosphere was found to be −0.11 W m−2, which is
within the reported range of the estimated present-day all-sky
direct radiative effect of total NO−
3aerosols by other stud-
ies (Liao et al., 2004; Bauer et al., 2007a, b; Bellouin et al.,
2011; Xu and Penner, 2012; Heald et al., 2014) (Table S4).
The NO−
3cooling of the REari calculated by EMAC is driven
by the scattering of SW radiation (equal to −0.34 W m−2),
which outweighs the warming due to absorption of LW ra-
diation (equal to +0.23 W m−2) (Table 2). The REari of the
total NO−
3aerosol shows a clearly contrasting behavior with
respect to the size mode considered (Table 2; Fig. 2).
In particular, the coarse particles show a net warming
effect of +0.17 W m−2(Fig. 2i) and contribute to 96 %
of the LW warming of the total nitrate, while only con-
tributing 15 % of the radiative cooling in the SW spectrum
(−0.05 W m−2). The LW warming is strongest over the dust
belt zone and especially over the Sahara, the Middle East,
and the northern face of the Himalayan plateau, while the
contribution over other arid regions such as the Atacama,
Gobi, Taklimakan, and Mojave deserts is significant. These
regions are characterized by moderate to high concentrations
of coarse NO−
3aerosols due to the adsorption of HNO3on
desert soil particles (Karydis et al., 2016; Milousis et al.,
2024). Therefore, the warming due to absorption of terres-
trial LW radiation by coarse-mode nitrates interacting with
mineral dust is the strongest over these areas (see Fig. 1a),
ranging from +1.5 to +5 W m−2(Fig. 2iii). On the other
hand, the cooling exerted by coarse nitrate aerosol through
the SW REari is more pronounced over areas where it inter-
acts strongly with high concentrations of mineral dust par-
ticles (see Fig. 1b). Such areas include the Congo Basin,
where HNO3from tropical forest biomass burning interacts
with Saharan mineral dust particles; the Middle East and
northern Indian regions, where anthropogenic HNO3emis-
sions interact with mineral dust particles from the Sahara
and Taklimakan deserts, respectively; and the East Asian re-
gion, where HNO3emissions from Chinese megacities inter-
act with mineral dust particles from the Gobi Desert. These
regions can lead to an average cooling of up to −3.5 W m−2
(Fig. 2v).
Interestingly, there is no significant cooling from SW in-
teractions over the Sahara for the coarse mode. This phe-
nomenon can be attributed to two factors, the first related to
nitrate–dust interactions and the second related to the char-
acteristics of the region. Specifically, because the underly-
ing desert surface is very bright, its absorption in this part of
the spectrum is less than that of the particles above it, which
means that the desert surface can scatter radiation more ef-
fectively than the particles above it. This is further enhanced
by the growth of coarse-mode particles there (see Fig. 4x and
Sect. 5.1), which increases the absorption cross section of
the particles. All this leads to an overall attenuation of the
cooling effect over this region and sometimes even to local
warming (Fig. 2v).
In contrast to the radiative effect of coarse NO−
3parti-
cles, the REari of fine NO−
3particles is an overall cool-
ing of −0.28 W m−2(Fig. 2ii). Fine nitrates have a negli-
gible 4 % contribution to the warming in the LW spectrum
(Fig. 2iv) but account for 85 % of the net cooling of the to-
tal nitrate aerosols (Fig. 2vi). The cooling induced by fine
NO−
3aerosols from scattering of SW radiation is stronger
(up to −5 W m−2) over regions of high anthropogenic ac-
tivity, particularly the East Asian and Indian regions, where
fine nitrates dominate the total nitrate aerosol load. The re-
gions of West Africa and the Amazon Basin are characterized
by moderate fine nitrate concentrations, and the cooling ob-
served there is enhanced by HNO3associated with biomass
burning interacting with fresh and aged Saharan dust par-
ticles, respectively, which are dominated by accumulation-
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A. Milousis et al.: Impact of mineral dust on the global nitrate aerosol 1341
Figure 2. Global mean TOA net REari for (i) coarse and (ii) fine
NO−
3aerosols, longwave REari for (iii) coarse and (iv) fine NO−
3
aerosols, and shortwave REari for (v) coarse and (vi) fine NO−
3
aerosols as calculated by EMAC from the base case simulation.
mode sizes in the absence of coarse-mode nitrates. Finally,
other polluted regions such as North America and Europe
also show SW cooling up to −2 W m−2.
3.2 Sensitivity of REari estimates
The comparison of the calculated total NO−
3radiative effect
due to interactions with net, LW, and SW radiation for the
sensitivity cases listed in Table 1 can be found in Table 2,
which shows each of the estimates. Consideration of nitrate
interactions with mineral dust cations can greatly affect the
NO−
3REari estimates. Assuming that mineral dust particles
are inert, the estimated warming due to LW radiation interac-
tions for total nitrate aerosols is 52 % weaker than in the base
case where dust reactivity is considered. Similarly, the cool-
ing effect exerted by all nitrate aerosols through interactions
with SW radiation is estimated to be 41 % weaker under the
assumption that mineral dust is non-reactive. Both estimates
are lower when mineral dust is assumed to be chemically in-
ert, since HNO3is no longer effectively adsorbed on dust par-
ticles. However, since both the estimated warming and cool-
ing are weaker, the effects partially cancel each other out,
resulting in a net cooling effect (−0.09 W m−2) that is 18 %
weaker compared to the base case calculations. Assuming
a homogeneous ionic composition for the dust results in SW
cooling and LW warming for total nitrate aerosols being 21 %
and 22 % lower, respectively, weakening the estimate for the
net cooling REari by 18 % (−0.09 W m−2). The net direct ra-
diative effect of total NO−
3is the same for the cases where
dust is assumed to have a homogeneous chemical compo-
sition and where it has no chemical identity, indicating the
importance of both aspects for the impact of dust–nitrate in-
teractions on the direct radiative effect.
In the half dust scenario, the total nitrate aerosol LW
warming estimate is 17 % weaker than in the base case, while
the total nitrate aerosol SW estimate is even more so (21 %),
resulting in a lower net cooling estimate of −0.08 W m−2. Fi-
nally, the increased dust scenario shows the strongest total ni-
trate aerosol LW warming effect (17 % increase over the base
case) due to an increase in coarse-mode nitrate. At the same
time, the cooling effect of total nitrate aerosols due to inter-
actions with SW radiation shows a smaller increase of 9 %.
Thus, accounting for the historical increase in mineral dust
emissions results in a net cooling estimate of −0.10 W m−2,
which is smaller than the base case. Interestingly, the behav-
ior of the global total NO−
3REari does not exhibit linearity
with respect to the global dust load. This is not surprising
since the nitrate–dust interactions themselves are not linearly
correlated, and a given increase or decrease in dust emissions
does not lead to an analogous change in nitrate aerosol levels.
For example, Karydis et al. (2016) have shown that moving
from a scenario in which nitrate–dust chemistry is not con-
sidered to one in which it is, but with half dust emissions, re-
sulted in a 39 % increase in the tropospheric burden of nitrate
aerosols. However, moving from a scenario with half to full
dust emissions, the corresponding increase was only 9 %. In
our case, moving from the chemically inert dust scenario to
the half dust scenario led to an 18 % increase in atmospheric
nitrate aerosol burden, while moving from the half dust sce-
nario to the base case led to an additional 8 % increase, and
finally moving from the base case to the increased dust sce-
nario led to an even smaller increase of 5 %.
There are several reasons for this nonlinearity between
changes in dust load and nitrate production. Firstly, since the
adsorption of HNO3onto dust particles is the main driver
of nitrate production on dust, over desert areas (where the
change in dust load takes place) the amount of nitric acid
present is the limiting factor for such production, rather
than the amount of dust itself. Secondly, when more dust is
present in the atmosphere, the combination of its increased
coating with the higher aerosol numbers tends to result in its
more efficient removal by wet deposition as well as coagu-
lation. This inherently affects nitrate production, which does
not increase in proportion to the increase in dust.
4 Radiative effect from aerosol–cloud interactions
(REaci)
4.1 Base case
The global average REaci of total NO−
3aerosols at the top
of the atmosphere was found to be +0.17 W m−2. In con-
trast, an estimate of the REaci of nitrate aerosols by Xu and
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1342 A. Milousis et al.: Impact of mineral dust on the global nitrate aerosol
Penner (2012) showed only a trivial cooling effect for par-
ticulate NO−
3(−0.01 W m−2). Similar to the REari, the net
REaci estimated by EMAC is driven by the effect on the
SW part of the spectrum, which causes a warming effect of
+0.27 W m−2, while the effect on the LW radiation causes an
average cooling of −0.10 W m−2(Table 3). Overall, the net
REaci of total NO−
3aerosols is reversed compared to the net
REari; i.e., REaci exerts a strong cooling effect over regions
where REari exerts a warming effect and vice versa (Fig. 3i).
The reason for this is that the regions contributing to a cool-
ing REari are dominated by smaller-sized nitrate aerosols and
vice versa. Therefore, the size characteristics of the domi-
nant nitrate aerosol population lead to different effects on the
cloud optical properties as discussed in Sect. 1. For example,
as the dominance of smaller nitrate aerosols decreases over
a particular region, the optical thinning of low-level clouds
will have an opposite effect on the REaci (Fig. 1d). Details
of the mechanism by which nitrate–dust interactions affect
cloud microphysical processes are discussed in Sect. 5. Over
North America and Europe, REaci causes a warming effect of
up to +3 W m−2, driven solely by the effect on SW radiation
(Fig. 3iii). Over the regions of East Asia and the Amazon
and Congo basins, REaci reaches a maximum of +5 W m−2,
driven by the effect on both the SW (up to +4 W m−2) and
LW (up to +1.5 W m−2) parts of the radiation spectrum. The
cooling effect of REaci (up to −2 W m−2) extends mainly be-
tween the equatorial line and the Tropic of Cancer, mainly
due to the interaction of nitrate aerosols with desert dust par-
ticles (e.g., from the Sahara) and their effect on the terrestrial
spectrum (LW) (Figs. 1c, 3ii). The cooling effect of dust in-
teractions with anthropogenic particles in the LW spectrum
corroborates the findings of Klingmüller et al. (2020) and is
attributed to the reduced ice-water path due to the depletion
of small aerosols, which in turn leads to less trapped outgo-
ing terrestrial radiation. In addition, Kok et al. (2023) note
how the presence of dust particles leads to an optical thin-
ning of cirrus clouds by reducing the number of ice crystals
while increasing their size, which also leads to less trapping
of outgoing LW radiation and thus a cooling effect (Fig. 1c).
On the other hand, the warming effect of dust interactions
with anthropogenic particles in the SW spectrum requires
further investigation and is therefore discussed in more de-
tail in Sect. 5.
4.2 Sensitivity of REaci estimates
Table 3 shows the comparison of the net, LW, and SW contri-
butions of total NO−
3to the REaci at the top of the atmosphere
as calculated by the base case simulation and all sensitivity
cases considered. By assuming chemically inert dust, the cal-
culated net REaci of nitrate decreases by 35 %, resulting in a
net warming of +0.11 W m−2. As with the REari estimate,
this sensitivity case produces the largest deviation from the
base case among all sensitivity simulations for both the SW
(37 % less warming) and LW (40 % less cooling) estimates.
Figure 3. Global mean TOA REaci for total NO−
3aerosols. Esti-
mates for (i) net, (ii) longwave, and (iii) shortwave, as calculated by
EMAC from the base case simulation.
This is due to the fact that the absence of dust–nitrate inter-
actions does not have such a large impact on the population
of aerosols and activated particles (see also Sect. 5). The as-
sumption of a homogeneous ionic composition of the mineral
dust leads to a weakened LW cooling estimate of 10 % and a
weakened SW warming estimate of 19 %, resulting in a net
NO−
3REaci of +0.13 W m−2(24 % lower than in the base
case).
The reduced dust emissions result in a 15 % weaker warm-
ing in the SW spectrum and a 20 % weaker cooling in the LW
spectrum, leading to an overall NO−
3REaci of +0.15 W m−2
(12 % weaker than the base case scenario). This is because
the reduced loading of nitrate aerosols, especially in the
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A. Milousis et al.: Impact of mineral dust on the global nitrate aerosol 1343
Table 3. Net, longwave, and shortwave global mean TOA REaci
of total NO−
3aerosols for the base case and each sensitivity case
simulation.
Simulation TOA REaci (W m−2)
Net LW SW
Base case +0.17 −0.10 +0.27
Chemically inert dust +0.11 −0.06 +0.17
Homogeneous ion composition +0.13 −0.09 +0.22
Half dust scenario +0.15 −0.08 +0.23
Increased dust scenario +0.14 −0.11 +0.25
coarse mode, in the half dust scenario results in less absorp-
tion of LW radiation (Fig. 1c) (hence less cooling). Simi-
larly, the effect of dust–nitrate interactions on the activation
of smaller particles (Fig. 1d) is less drastic and results in
a weaker inhibition of SW radiation scattering (hence less
warming; see also Sect. 5). Finally, increased dust emissions
in the increased dust scenario show a 10 % increase in the
LW cooling and an 8% decrease in the SW warming effect,
surprisingly resulting in a net warming (+0.14 W m−2) that
is lower than in the half dust scenario. The reason that this
scenario results in more LW cooling than the base case is
that the increased quantity of dust particles leads to even
more optical thinning of the ice clouds and therefore even
less trapping of LW radiation (more cooling). However, the
reason why the SW warming estimate is lower than the base
case is more complicated. First, the transition from the half
dust scenario to the base case and then to the increased dust
scenario does not lead to an analogous increase in the nitrate
aerosol burden (see Sect. 3.2). Moreover, since the number
of aerosols has increased from the increased dust scenario to
the base case, but the relative humidity has remained largely
the same, there is more competition for water vapor because
it is now distributed over a larger population. As a result, the
wet radius increase in the presence of nitrates is not as strong
in the increased dust scenario compared to the base case, and
the depletion of smaller-sized particles is also not as strong
(not shown). The implications of the depletion of the aerosol
population in the presence of nitrate aerosols for the micro-
physical processes of warm clouds, and consequently for SW
warming, are discussed in the next section.
5 Effect of NO−
3aerosols on cloud microphysics
5.1 Maximum supersaturation, hygroscopicity, and wet
radius
To further investigate the cause of the positive REaci induced
by the NO−
3aerosols, their effect on the aerosol population
characteristics as well as on the cloud microphysics is in-
vestigated with respect to the lowest-forming cloud level of
940 hPa. For this purpose, a sensitivity simulation is per-
formed assuming a “nitrate-aerosol-free” (NAF) atmosphere,
in which the formation of NO−
3aerosols has been switched
off, but an advanced cloud scheme is considered which is
the same as the one described in Sect. 2.3.2. This is essen-
tially the same setup that was used for the estimation of
the total nitrate aerosol feedback radiative effect. This sim-
ulation is used to determine whether the presence of NO−
3
aerosols has a significant effect on the hygroscopicity and
size of atmospheric aerosols and ultimately on the maximum
supersaturation developed during cloud formation. Over pol-
luted areas affected by transported dust air masses from sur-
rounding arid areas, the presence of NO−
3aerosols can in-
crease the CCN activity of the large mineral dust particles,
resulting in a reduction of the maximum supersaturation and
inhibiting the activation of the small anthropogenic parti-
cles into cloud droplets (Klingmüller et al., 2020). Results
from the NAF sensitivity simulation support this hypothesis
over parts of eastern and central Asia, where the maximum
supersaturation decreases by up to 0.05 %. In contrast, the
presence of NO−
3aerosols increases maximum supersatura-
tion by up to 0.2 % over North America, Europe, the Mid-
dle East, and parts of southern Asia (Fig. 4ii). Therefore,
changes in maximum supersaturation caused by the presence
of NO−
3aerosols cannot explain their warming effect through
the REaci.
The presence of NO−
3has a significant effect on the hygro-
scopicity of both fine and coarse aerosols and consequently
on their wet radius, as shown in Fig. 1a and b and Fig. 4.
This is most evident for coarse desert dust particles, which
mix with NO−
3aerosols from urban and forest regions, in-
creasing their hygroscopicity by an order of magnitude (up
to 0.1), especially over the African–Asian dust belt and the
Atacama Desert in South America (Fig. 4vi). Aerosol hy-
groscopicity is similarly increased for the fine-mode parti-
cles both near arid regions and over the highly industrialized
region of Southeast Asia (Fig. 4iv). The low values of the
hygroscopic parameter of the fine-aerosol population, espe-
cially over the dust belt zone, are largely due to the higher
proportion of insoluble fine particles present over these re-
gions (Fig. S5). This is also observed over other regions with
similarly low fine-aerosol hygroscopicity (southern Africa,
South America, and the western US). Nevertheless, the esti-
mates of aerosol kappa values at 940 hPa are broadly consis-
tent with the results of Pringle et al. (2010c). On the other
hand, the aerosol hygroscopicity for the two size modes is
only slightly reduced, by up to 0.06 (or <10 %) over the
oceans and coasts of Europe and East Asia, due to interac-
tions of NO−
3with sea salt particles, reducing their hygro-
scopicity. The increased ability of both coarse dust aerosols
and smaller aerosols to absorb water leads to an increase in
their wet radius, but in different parts of the world. For ex-
ample, fine particle sizes increase by up to 0.04 µm (up to
40 %), mostly over regions of high anthropogenic activity
(North America, Europe, and East Asia) (Fig. 4viii). On the
other hand, coarse-mode particle sizes are increased by up
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1344 A. Milousis et al.: Impact of mineral dust on the global nitrate aerosol
Figure 4. (i) Global mean maximum supersaturation, fine-aerosol
(iii) hygroscopicity, and (v) wet radius, as well as coarse-aerosol
(vii) hygroscopicity and (ix) wet radius, as calculated by EMAC
from the base case simulation at the altitude of 940 hPa. Absolute
difference between the base case and nitrate-aerosol-free (NAF)
sensitivity simulation in (ii) maximum supersaturation, fine-aerosol
(iv) hygroscopicity, and (vi) wet radius, as well as coarse-aerosol
(viii) hygroscopicity and (x) wet radius at the altitude of 940 hPa.
Red indicates higher values calculated by the base case simulation
in the presence of NO−
3aerosols.
to 0.1 µm (up to 10 %) over the forests of central Africa and
the African–Asian dust belt zone (Fig. 4x), while showing
a similar decrease near the coasts of the polluted Northern
Hemisphere due to the effect of NO−
3on the hygroscopicity
of sea salt.
Figure 5. Global mean number concentration of (i) fine and
(iii) coarse aerosols as calculated by EMAC from the base case sim-
ulation at the altitude of 940 hPa. Absolute difference between the
base case and the nitrate-aerosol-free (NAF) sensitivity simulation
in the number concentration of (ii) fine and (iv) coarse aerosols at
the altitude of 940 hPa. Blue indicates that number concentrations
are lower in the presence of NO−
3aerosols.
5.2 Number concentrations of aerosol and activated
particles
Figure 5 shows the effect of NO−
3on the number concentra-
tion of fine and coarse aerosols between the base case and the
NAF sensitivity simulation, as well as the total aerosol pop-
ulation. The presence of NO−
3aerosols decreases the total
aerosol number concentration over forests and polluted re-
gions (see also Fig. 1d). This behavior is driven solely by the
decrease in smaller particle sizes, as the effect is minimal for
the coarser particles (Figs. 5ii, iv). The largest decrease is cal-
culated over East and South Asia (up to 1000 cm−3or 10 %),
while decreases of up to 200 cm−3on average (∼10 %) are
found over Europe, the USA, and central Africa. This effect
is directly related to the increased wet radius of the aerosol
population (Fig. 4viii) over these regions and thus to its depo-
sitional efficiency. In addition, coarse dust particles become
more hygroscopic due to interactions with NO−
3aerosols that
increase in size, resulting in increased coagulation with the
smaller anthropogenic particles, which reduces their abun-
dance.
The reduced aerosol number concentration in the presence
of NO−
3can lead to a reduction of particles that are also acti-
vated into cloud droplets. Such behavior can be seen in Fig. 6,
which shows the effect of NO−
3on the number concentration
of activated fine and coarse particles in cloud droplets be-
tween the base case and the NAF sensitivity simulation. The
reduction in the total number of activated cloud droplets is
almost entirely due to the reduction in smaller-sized parti-
cles (Figs. 6ii, iv). A reduction in the total number of acti-
vated droplets of up to 30 cm−3or 10 % is observed over the
USA, Amazon, Europe, central Africa, and parts of the Mid-
Atmos. Chem. Phys., 25, 1333–1351, 2025 https://doi.org/10.5194/acp-25-1333-2025
A. Milousis et al.: Impact of mineral dust on the global nitrate aerosol 1345
Figure 6. Global mean number concentration of activated (i) fine
and (iii) coarse aerosols as calculated by EMAC from the base case
simulation at the altitude of 940 hPa. Absolute difference between
the base case and the nitrate-aerosol-free (NAF) sensitivity simula-
tion in the number concentration of activated (ii) fine and (iv) coarse
aerosols at the altitude of 940 hPa. Blue indicates that number con-
centrations are lower in the presence of NO−
3aerosols.
dle East, while this reduction reaches up to 100 cm−3(10 %)
over Southeast Asia, where the largest reductions in aerosol
numbers are also calculated (Fig. 4ii). In turn, these are the
regions where the warming effect of NO−
3aerosols on the
calculated mean REaci is strongest (Fig. 3i). The small in-
crease in activated droplets (∼10 cm−3or 1%) over Beijing,
which concerns the fine-mode particles, is most likely be-
cause their number concentration decreases with increasing
size. The high aerosol number concentration there, which is
the global maximum (Fig. 5i), results in a hotspot of more
readily activated particles in the presence of NO−
3. On the
other hand, the CDNC decreases slightly over the Sahara due
to the more efficient deposition capacity of coarse dust par-
ticles due to their interactions with nitrate aerosols, which is
also reflected in the decrease in aerosol number (Fig. 6iv).
Overall, the lower particle number in the presence of NO−
3
aerosols hinders the ability of the smaller anthropogenic par-
ticles to activate into cloud droplets, leading to a reduced
cloud cover and thus a reduced cloud albedo effect. There-
fore, not only is less LW radiation absorbed, but also, more
importantly, less SW radiation is scattered back to space, re-
sulting in an overall warming of the net average REaci for
total NO−
3aerosols.
6 Conclusions and discussion
This study presents the effects of interactions between min-
eral dust and NO−
3aerosols on the present-day global TOA
radiative effect of the latter. We investigate how the presence
of dust affects the radiative effect of NO−
3aerosols through
aerosol interactions with both radiation and separately with
clouds (REari and REaci, respectively). Sensitivity simula-
tions are also performed, varying both the mineral dust com-
position and its emissions, to assess the effect on the calcu-
lated NO−
3aerosol radiative effect.
It was found that the global average net REari of total NO−
3
aerosols is −0.11 W m−2, which is mainly due to the cool-
ing from the shortwave part of the radiation spectrum due
to scattering, equal to −0.34 W m−2. A warming from the
longwave part of the spectrum due to absorption was found
to be +0.23 W m−2on global average and was mainly lo-
cated over regions with high concentrations of coarse NO−
3
aerosols. SW cooling was also observed in these regions,
but also over regions of high anthropogenic activity, mainly
over the polluted Northern Hemisphere. The behavior of the
REari was opposite when considering different sizes of NO−
3
aerosols. Specifically, the coarse mode was responsible for
96 % of the estimated warming in the LW part of the spec-
trum but 15 % of the estimated cooling in the SW part of
the spectrum. On the other hand, the contribution of the fine
mode to the LW warming was negligible, but it was the main
contributor to the SW cooling, accounting for 85 % of the
net estimate. The sensitivity experiments revealed that the
chemistry of the mineral dust is the most important factor
in changing the estimated REari of the total NO−
3aerosols.
In particular, LW warming is most affected by this assump-
tion, being 52 % weaker after assuming chemically inert dust
emissions, while the SW cooling is reduced by 41 % com-
pared to the base case simulation, amounting to a net cool-
ing of −0.09 W m−2. A globally homogeneous ionic com-
position for mineral dust had a smaller effect in the LW
(22 % decrease) and SW (21 % decrease) but resulted in the
same net estimate of −0.09 W m−2. Halving the dust emis-
sions resulted in weaker estimates for LW and SW by 17 %
and 21 %, respectively, and the lowest overall net REari of
−0.08 W m−2. On the other hand, a 50 % increase in dust
emissions increased both LW warming and SW cooling by
17 % and 9 %, respectively, resulting in a net cooling REari
of −0.10 W m−2, indicating the strong nonlinear relationship
of nitrate–dust interactions and how they affect the radiative
effect estimates.
The global average net REaci of total NO−
3aerosols was
+0.17 W m−2due to the effect on the shortwave portion of
the spectrum. This was found to be +0.27 W m−2, while the
cooling from the longwave part was −0.10 W m−2. Spatially,
the net REaci is reversed compared to the net REari for total
NO−
3aerosols, where regions responsible for a strong SW
cooling of the REari contribute to a strong SW warming of
the REaci and vice versa. This is due to the fact that nitrate–
dust interactions challenge the dominance of smaller parti-
cles over heavily polluted regions, reducing the reflectivity of
warm clouds and thus having an opposite effect on the REaci.
The sensitivity experiments again showed that the consid-
eration of the mineral dust chemistry is the most impor-
tant aspect for the calculation of the REaci of the total NO−
3
aerosols. When the dust was assumed to be chemically inert,
https://doi.org/10.5194/acp-25-1333-2025 Atmos. Chem. Phys., 25, 1333–1351, 2025
1346 A. Milousis et al.: Impact of mineral dust on the global nitrate aerosol
the LW and SW estimates were up to 40% weaker, result-
ing in a net warming of +0.11 W m−2. Assuming a homoge-
neous ion composition resulted in a smaller weakening of the
estimates (up to 18 %) and a net warming of +0.13 W m−2.
When dust emissions were halved, the LW cooling was re-
duced slightly more than in the base case, resulting in a net
warming of +0.15 W m−2. The 50 % increase in dust emis-
sions had the largest effect on LW behavior (10 % increase),
but surprisingly the net estimate (+0.14 W m−2) was smaller
than in the half dust scenario. The reason for this is that the
SW estimate did not increase but decreased by 8 % due to the
fact that in this scenario the increased nitrate burden causes
increased competition for the available supersaturation and
the effect of dust–nitrate interactions on the smaller aerosol
populations is not as emphasized as in the base case.
The total NO−
3aerosol REaci shows a positive sign, which
is attributed to a reduced cloud albedo effect. More specifi-
cally, although the presence or absence of NO−
3aerosols in
the atmosphere did not significantly affect the total available
maximum supersaturation, it did alter both the hygroscopic-
ity and wet radii of the aerosols. In the presence of NO−
3,
the hygroscopicity of aerosols over deserts was increased by
up to an order of magnitude, leading to an increase in their
wet radius of up to 10 %, with an even larger increase of
up to 40 % for smaller particles over urban regions. There-
fore, in the presence of NO−
3aerosols, the depletion of fine
particles by coagulation with coarser particles (i.e., mineral
dust) is enhanced and further increases the size of the coarse
particles. The reduction in the number of aerosols is up to
10 % in some regions, with maximum reductions calculated
over Southeast Asia. This reduction in the number of fine
aerosols leads to a reduction in the number of cloud droplets
activated by fine aerosols (also up to 10%), which would oth-
erwise have absorbed more outgoing longwave radiation and,
more importantly, scattered more incoming shortwave radia-
tion. Thus, the reduced cloud albedo effect leads to a cooling
in the longwave part of the spectrum, which is offset by a
strong warming in the shortwave part, overall resulting in a
net warming of the atmosphere.
The chemistry–climate model simulations presented here
suggest that NO−
3aerosol–radiation interactions lead to a net
effect of −0.11 W m−2(cooling) driven by fine NO−
3aerosol,
while NO−
3aerosol–cloud interactions lead to a net effect
of +0.17 W m−2(warming) driven mainly by coarse-mode
NO−
3aerosol.
Code and data availability. The usage of MESSy (Modular
Earth Submodel System) and access to the source code are li-
censed to all affiliates of institutions which are members of
the MESSy Consortium. Institutions can become a member
of the MESSy Consortium by signing the MESSy Memoran-
dum of Understanding. More information can be found on the
MESSy Consortium website: http://www.messy-interface.org (last
access: 22 May 2024). The code used in this study is based on
MESSy version 2.55 and is archived with a restricted-access DOI
(https://doi.org/10.5281/zenodo.8379120, The MESSy Consortium,
2023). The data produced in the study are available from the authors
upon request.
Supplement. The supplement related to this article is available
online at: https://doi.org/10.5194/acp-25-1333-2025-supplement.
Author contributions. AM and VAK wrote the paper with con-
tributions from KK, APT, JFK, MK, and AN. VAK planned the
research with contributions from APT, MK, and AN. AM, KK, and
VAK designed the methodology for the radiative effect calculations.
AM performed the simulations and analyzed the results, assisted by
VAK and APT. All the authors discussed the results and contributed
to the paper.
Competing interests. At least one of the (co-)authors is a mem-
ber of the editorial board of Atmospheric Chemistry and Physics.
The peer-review process was guided by an independent editor, and
the authors also have no other competing interests to declare.
Disclaimer. Publisher’s note: Copernicus Publications remains
neutral with regard to jurisdictional claims made in the text, pub-
lished maps, institutional affiliations, or any other geographical rep-
resentation in this paper. While Copernicus Publications makes ev-
ery effort to include appropriate place names, the final responsibility
lies with the authors.
Special issue statement. This article is part of the special issue
“The Modular Earth Submodel System (MESSy) (ACP/GMD inter-
journal SI)”. It is not associated with a conference.
Acknowledgements. This work was supported by the project
FORCeS funded from the European Union’s Horizon 2020 research
and innovation program under grant agreement no. 821205. JFK
was funded by the National Science Foundation (NSF) Directorate
for Geosciences under grants 1856389 and 2151093. The work de-
scribed in this paper has received funding from the Initiative and
Networking Fund of the Helmholtz Association through the project
“Advanced Earth System Modelling Capacity (ESM)”. The authors
gratefully acknowledge the Earth System Modelling Project (ESM)
for funding this work by providing computing time on the ESM par-
tition of the supercomputer JUWELS (Alvarez, 2021) at the Jülich
Supercomputing Centre (JSC).
Financial support. This research has been supported by Horizon
2020 (grant no. 821205) and the Directorate for Geosciences (grant
nos. 1856389 and 2151093).
The article processing charges for this open-access
publication were covered by the Forschungszentrum Jülich.
Atmos. Chem. Phys., 25, 1333–1351, 2025 https://doi.org/10.5194/acp-25-1333-2025
A. Milousis et al.: Impact of mineral dust on the global nitrate aerosol 1347
Review statement. This paper was edited by Joshua Fu and re-
viewed by Kan Huang and one anonymous referee.
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