[show abstract][hide abstract] ABSTRACT: Though many global aerosols models prognose surface deposition, only a few models have been used to di- rectly simulate the radiative effect from black carbon (BC) deposition to snow and sea ice. Here, we apply aerosol de- position fields from 25 models contributing to two phases of the Aerosol Comparisons between Observations and Mod- els (AeroCom) project to simulate and evaluate within-snow BC concentrations and radiative effect in the Arctic. We ac- complish this by driving the offline land and sea ice com- ponents of the Community Earth System Model with dif- ferent deposition fields and meteorological conditions from
2004 to 2009, during which an extensive field campaign of BC measurements in Arctic snow occurred. We find that models generally underestimate BC concentrations in snow in northern Russia and Norway, while overestimating BC amounts elsewhere in the Arctic. Although simulated BC distributions in snow are poorly correlated with measure- ments, mean values are reasonable. The multi-model mean (range) bias in BC concentrations, sampled over the same grid cells, snow depths, and months of measurements, are −4.4 (−13.2 to +10.7) ng g−1 for an earlier phase of Aero- Com models (phase I), and +4.1 (−13.0 to +21.4) ng g−1
for a more recent phase of AeroCom models (phase II), com- pared to the observational mean of 19.2 ng g−1. Factors de- termining model BC concentrations in Arctic snow include Arctic BC emissions, transport of extra-Arctic aerosols, pre- cipitation, deposition efficiency of aerosols within the Arc- tic, and meltwater removal of particles in snow. Sensitivity studies show that the model–measurement evaluation is only weakly affected by meltwater scavenging efficiency because most measurements were conducted in non-melting snow. The Arctic (60–90◦ N) atmospheric residence time for BC in phase II models ranges from 3.7 to 23.2 days, imply- ing large inter-model variation in local BC deposition effi- ciency. Combined with the fact that most Arctic BC depo- sition originates from extra-Arctic emissions, these results suggest that aerosol removal processes are a leading source of variation in model performance. The multi-model mean (full range) of Arctic radiative effect from BC in snow is 0.15 (0.07–0.25) W m−2 and 0.18 (0.06–0.28) W m−2 in phase I and phase II models, respectively. After correcting for model biases relative to observed BC concentrations in different re- gions of the Arctic, we obtain a multi-model mean Arctic radiative effect of 0.17 W m−2 for the combined AeroCom ensembles. Finally, there is a high correlation between mod- eled BC concentrations sampled over the observational sites and the Arctic as a whole, indicating that the field campaign provided a reasonable sample of the Arctic.
Atmospheric Chemistry and Physics 03/2014; 14:2399-2417. · 4.88 Impact Factor
[show abstract][hide abstract] ABSTRACT: Most estimates of the global mean indirect effect of anthropogenic aerosol on the Earth's energy balance are from simulations by global models of the aerosol lifecycle coupled with global models of clouds and the hydrologic cycle. Extremely simple models have been developed for integrated assessment models, but lack the flexibility to distinguish between primary and secondary sources of aerosol. Here a simple but more physically-based model expresses the aerosol indirect effect using analytic representations of cloud and aerosol distributions and processes. Although the simple model is able to produce estimates of aerosol indirect effects that are comparable to those from some global aerosol models using the same global mean aerosol properties, the estimates by the simple model are sensitive to preindustrial cloud condensation nuclei concentration, preindustrial accumulation mode radius, width of the accumulation mode, size of primary particles, cloud thickness, primary and secondary anthropogenic emissions, the fraction of the secondary anthropogenic emissions that accumulates on the coarse mode, the fraction of the secondary mass that forms new particles, and the sensitivity of liquid water path to droplet number concentration. Estimates of present day aerosol indirect effects as low as −5 W m-2 and as high as −0.3 W m-2 are obtained for plausible sets of parameter values. Estimates are surprisingly linear in emissions. The estimates depend on parameter values in ways that are consistent with results from detailed global aerosol-climate simulation models, which adds to understanding of the dependence on aerosol indirect effect uncertainty on uncertainty in parameter values.
Journal of Geophysical Research 06/2013; · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report on the AeroCom Phase II direct aerosol effect (DAE) experiment where 16 detailed global aerosol
models have been used to simulate the changes in the aerosol distribution over the industrial era. All 16 models have estimated the radiative forcing (RF) of the anthropogenic DAE, and have taken into account anthropogenic sulphate, black carbon (BC) and organic aerosols (OA) from fossil fuel, biofuel, and biomass burning emissions. In addition several models have simulated the DAE of anthropogenic nitrate and anthropogenic influenced secondary organic aerosols (SOA). The model simulated all-sky RF of the DAE from
total anthropogenic aerosols has a range from −0.58 to −0.02 Wm−2, with a mean of −0.27 Wm−2 for the 16 models. Several models did not include nitrate or SOA and modifying the estimate by accounting for this with information from the other AeroCom models reduces the range and slightly strengthens the mean. Modifying the model estimates for missing aerosol components and for the time period 1750 to 2010 results in a mean RF for the DAE of −0.35 Wm−2. Compared to AeroCom Phase I (Schulz et al., 2006) we find very similar spreads in both total DAE and aerosol component RF. However, the RF of the total DAE is stronger negative and RF from BC from fossil fuel and bio-fuel emissions are stronger positive in the present study than in the previous AeroCom study. We find a tendency for models having a strong (positive) BC RF to also have strong (negative) sulphate or OA RF. This relationship leads to smaller uncertainty in the total RF of the DAE compared to the RF of the sum of the individual aerosol components. The spread in results for the individual aerosol components is substantial, and can be divided into diversities in burden, mass extinction coefficient (MEC), and normalized RF with respect to AOD. We find that these three factors give similar contributions to
the spread in results.
ATMOSPHERIC CHEMISTRY AND PHYSICS 02/2013; 13:1853-1877. · 5.51 Impact Factor
[show abstract][hide abstract] ABSTRACT: The anthropogenic increase in aerosol concentrations since preindustrial
times and its net cooling effect on the atmosphere is thought to mask
some of the greenhouse gas-induced warming. Although the overall effect
of aerosols on solar radiation and clouds is most certainly negative,
some individual forcing agents and feedbacks have positive forcing
effects. Recent studies have tried to identify some of those positive
forcing agents and their individual emission sectors, with the hope that
mitigation policies could be developed to target those emitters.
Understanding the net effect of multisource emitting sectors and the
involved cloud feedbacks is very challenging, and this paper will
clarify forcing and feedback effects by separating direct, indirect,
semidirect and surface albedo effects due to aerosols. To this end, we
apply the Goddard Institute for Space Studies climate model including
detailed aerosol microphysics to examine aerosol impacts on climate by
isolating single emission sector contributions as given by the Coupled
Model Intercomparison Project Phase 5 (CMIP5) emission data sets
developed for Intergovernmental Panel on Climate Change (IPCC) AR5. For
the modeled past 150 years, using the climate model and emissions from
preindustrial times to present-day, the total global annual mean aerosol
radiative forcing is -0.6 W/m2, with the largest contribution
from the direct effect (-0.5 W/m2). Aerosol-induced changes
on cloud cover often depends on cloud type and geographical region. The
indirect (includes only the cloud albedo effect with -0.17
W/m2) and semidirect effects (-0.10 W/m2) can be
isolated on a regional scale, and they often have opposing forcing
effects, leading to overall small forcing effects on a global scale.
Although the surface albedo effects from aerosols are small (0.016
W/m2), triggered feedbacks on top of the atmosphere (TOA)
radiative forcing can be 10 times larger. Our results point out that
each emission sector has varying impacts by geographical region. For
example, the single sector most responsible for a net positive radiative
forcing is the transportation sector in the United States, agricultural
burning and transportation in Europe, and the domestic emission sector
in Asia. These sectors are attractive mitigation targets.
Journal of Geophysical Research 01/2012; 117(D1):1206-. · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: The impact of black carbon (BC) aerosols on the global radiation balance is not well
constrained. Here twelve global aerosol models are used to show that at least 20% of
the present uncertainty in modeled BC direct radiative forcing (RF) is due to diversity in
the simulated vertical profile of BC mass. Results are from phases 1 and 2 of the global
aerosol model intercomparison project (AeroCom). Additionally, a significant fraction of
the variability is shown to come from high altitudes, as, globally, more than 40% of the
total BC RF is exerted above 5 km. BC emission regions and areas with transported
BC are found to have differing characteristics. These insights into the importance of the
vertical profile of BC lead us to suggest that observational studies are needed to better
characterize the global distribution of BC, including in the upper troposphere.
Atmospheric Chemistry and Physics 01/2012; · 4.88 Impact Factor
[show abstract][hide abstract] ABSTRACT: Proposed mechanisms for solar variability to impact climate are legion
and range from straightforward (e.g. via total solar irradiance (TSI))
to very indirect (e.g. solar modulation of Galactic Cosmic Rays
affecting aerosols nucleation which affects cloud condensation nuclei
which affects clouds). We will present the latest results from the GISS
ModelE suite of simulations including the response to TSI change,
spectral solar irradiance changes (particularly in the UV), subsequent
tropospheric and stratospheric ozone changes and the impacts of
solar-modulated GCR changes on ionisation using a state-of-the-art
aerosol module (MATRIX) (Bauer et al.,2008). We will assess the relative
roles of these mechanisms and their collective match to observed
variability from the stratosphere to the surface. The impact of
considerable uncertainties in each of these processes and/or their
history will also be discussed.
[show abstract][hide abstract] ABSTRACT: 1] The CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) layer product is used for a multimodel evaluation of the vertical distribution of aerosols. Annual and seasonal aerosol extinction profiles are analyzed over 13 sub-continental regions representative of industrial, dust, and biomass burning pollution, from CALIOP 2007–2009 observations and from AeroCom (Aerosol Comparisons between Observations and Models) 2000 simulations. An extinction mean height diagnostic (Z a) is defined to quantitatively assess the models' performance. It is calculated over the 0–6 km and 0–10 km altitude ranges by weighting the altitude of each 100 m altitude layer by its aerosol extinction coefficient. The mean extinction profiles derived from CALIOP layer products provide consistent regional and seasonal specificities and a low inter-annual variability. While the outputs from most models are significantly correlated with the observed Z a climatologies, some do better than others, and 2 of the 12 models perform particularly well in all seasons. Over industrial and maritime regions, most models show higher Z a than observed by CALIOP, whereas over the African and Chinese dust source regions, Z a is underestimated during Northern Hemisphere Spring and Summer. The positive model bias in Z a is mainly due to an overestimate of the extinction above 6 km. Potential CALIOP and model limitations, and methodological factors that might contribute to the differences are discussed. Citation: Koffi, B., et al. (2012), Application of the CALIOP layer product to evaluate the vertical distribution of aerosols estimated by global models: AeroCom phase I results, J. Geophys. Res., 117, D10201, doi:10.1029/2011JD016858.
Journal of Geophysical Research 07/2011; 117(D10201):16858. · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: We use global models to explore the microphys-ical effects of carbonaceous aerosols on liquid clouds. Al-though absorption of solar radiation by soot warms the atmo-sphere, soot may cause climate cooling due to its contribu-tion to cloud condensation nuclei (CCN) and therefore cloud brightness. Six global models conducted three soot experi-ments; four of the models had detailed aerosol microphysi-cal schemes. The average cloud radiative response to biofuel soot (black and organic carbon), including both indirect and semi-direct effects, is −0.11 Wm −2 , comparable in size but opposite in sign to the respective direct effect. In a more idealized fossil fuel black carbon experiment, some mod-els calculated a positive cloud response because soot pro-vides a deposition sink for sulfuric and nitric acids and sec-ondary organics, decreasing nucleation and evolution of vi-able CCN. Biofuel soot particles were also typically assumed to be larger and more hygroscopic than for fossil fuel soot and therefore caused more negative forcing, as also found in previous studies. Diesel soot (black and organic carbon) experiments had relatively smaller cloud impacts with five Correspondence to: D. Koch (firstname.lastname@example.org) of the models <±0.06 Wm −2 from clouds. The results are subject to the caveats that variability among models, and re-gional and interrannual variability for each model, are large. This comparison together with previously published results stresses the need to further constrain aerosol microphysical schemes. The non-linearities resulting from the competition of opposing effects on the CCN population make it difficult to extrapolate from idealized experiments to likely impacts of realistic potential emission changes.
ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2011; 11:1051-1064. · 5.51 Impact Factor
[show abstract][hide abstract] ABSTRACT: Attention has been drawn to black carbon aerosols, as a target for short-term mitigation of climate warming. This measure seems attractive because soot is assumed to warm the atmosphere and at the same time has a lifetime of just a few days. Therefore regulating soot emissions could, as a short-term action, potentially buy time by slowing global warming until regulations for longer lived greenhouse gases are set in place. Currently the scientific community debates the impacts of such mitigation measures, especially when considering indirect effects. We tested with the GISS/MATRIX model, a global climate model including detailed aerosol microphysics, the effect of reducing fossil fuel emissions and bio-fuel emissions and found that opposite changes in cloud droplet number concentration lead to positive cloud forcing numbers in the bio-fuel reduction case and negative forcing numbers in the diesel mitigation case. Similar experiments have been carried out and have recently been published by other modeling groups, finding partly similar partly contradicting results to our study. In this presentation we want to explain the differences in black carbon research carried out with complex microphysical models, by focusing on the treatment of mixing state, and separation between forcings and feedbacks.
[show abstract][hide abstract] ABSTRACT: A much-cited bar chart provided by the Intergovernmental Panel on Climate Change displays the climate impact, as expressed by radiative forcing in watts per meter squared, of individual chemical species. The organization of the chart reflects the history of atmospheric chemistry, in which investigators typically focused on a single species of interest. However, changes in pollutant emissions and concentrations are a symptom, not a cause, of the primary driver of anthropogenic climate change: human activity. In this paper, we suggest organizing the bar chart according to drivers of change-that is, by economic sector. Climate impacts of tropospheric ozone, fine aerosols, aerosol-cloud interactions, methane, and long-lived greenhouse gases are considered. We quantify the future evolution of the total radiative forcing due to perpetual constant year 2000 emissions by sector, most relevant for the development of climate policy now, and focus on two specific time points, near-term at 2020 and long-term at 2100. Because sector profiles differ greatly, this approach fosters the development of smart climate policy and is useful to identify effective opportunities for rapid mitigation of anthropogenic radiative forcing.
Proceedings of the National Academy of Sciences 02/2010; 107(8):3382-7. · 9.74 Impact Factor
[show abstract][hide abstract] ABSTRACT: Recently, attention has been drawn towards black carbon aerosols as a short-term climate warming mitigation candidate. However the global and regional impacts of the direct, indirect and semi-direct aerosol effects are highly uncertain, due to the complex nature of aerosol evolution and the way that mixed, aged aerosols interact with clouds and radiation. A detailed aerosol microphysical scheme, MATRIX, embedded within the GISS climate model is used in this study to present a quantitative assessment of the impact of microphysical processes involving black carbon, such as emission size distributions and optical properties on aerosol cloud activation and radiative effects. Our best estimate for net direct and indirect aerosol radiative flux change between 1750 and 2000 is −0.56 W/m2. However, the direct and indirect aerosol effects are quite sensitive to the black and organic carbon size distribution and consequential mixing state. The net radiative flux change can vary between −0.32 to −0.75 W/m2 depending on these carbonaceous particle properties at emission. Taking into account internally mixed black carbon particles let us simulate correct aerosol absorption. Absorption of black carbon aerosols is amplified by sulfate and nitrate coatings and, even more strongly, by organic coatings. Black carbon mitigation scenarios generally showed reduced radiative fluxeswhen sources with a large proportion of black carbon, such as diesel, are reduced; however reducing sources with a larger organic carbon component as well, such as bio-fuels, does not necessarily lead to a reduction in positive radiative flux.
ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2010; · 5.51 Impact Factor
[show abstract][hide abstract] ABSTRACT: Desert dust plays an important role in the climate system through its impact on Earth's radiative budget and its role in the biogeochemical cycle as a source of iron in high-nutrient-low-chlorophyll regions. A large degree of diversity exists between the many global models that simulate the dust cycle to estimate its impact on climate. We present the results of a broad intercomparison of a total of 15 global aerosol models within the AeroCom project. Each model is compared to observations focusing on variables responsible for the uncertainties in estimating the direct radiative effect and the dust impact on the biogeochemical cycle, i.e., aerosol optical depth (AOD) and dust deposition. Additional comparisons to Angström Exponent (AE), coarse mode AOD and dust surface concentration are included to extend the assessment of model performance. These datasets form a benchmark data set which is proposed for model inspection and future dust model developments. In general, models perform better in simulating climatology of vertically averaged integrated parameters (AOD and AE) in dusty sites than they do with total deposition and surface concentration. Almost all models overestimate deposition fluxes over Europe, the Indian Ocean, the Atlantic Ocean and ice core data. Differences among the models arise when simulating deposition at remote sites with low fluxes over the Pacific and the Southern Atlantic Ocean. This study also highlights important differences in models ability to reproduce the deposition flux over Antarctica. The cause of this discrepancy could not be identified but different dust regimes at each site and issues with data quality should be considered. Models generally simulate better surface concentration at stations downwind of the main sources than at remote ones. Likewise, they simulate better surface concentration at stations affected by Saharan dust than at stations affected by Asian dust. Most models simulate the gradient in AOD and AE between the different dusty regions, however the seasonality and magnitude of both variables is better simulated at African stations than Middle East ones. The models also reproduce the dust transport across the Atlantic in terms of both AOD and AE; they simulate the offshore transport of West Africa throughout the year and limit the transport across the Atlantic to the summer months, yet overestimating the AOD and transporting too fine particles. However, most of the models do not reproduce the southward displacement of the dust cloud during the winter responsible of the transport of dust into South America. Based on the dependency of AOD on aerosol burden and size distribution we use model data bias with respect to AOD and AE and infer on the over/under estimation of the dust emissions. According to this we suggest the emissions in the Sahara be between 792 and 2271 Tg/yr and the one in the Middle East between 212 and 329 Tg/yr.
ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2010; · 5.51 Impact Factor
[show abstract][hide abstract] ABSTRACT: Recently, attention has been drawn towards black carbon aerosols as a short-term climate warming mitigation candidate. However the global and regional impacts of the direct, cloud-indirect and semi-direct forcing effects are highly uncertain, due to the complex nature of aerosol evolution and the way that mixed, aged aerosols interact with clouds and radiation. A detailed aerosol microphysical scheme, MATRIX, embedded within the GISS climate model is used in this study to present a quantitative assessment of the impact of microphysical processes involving black carbon, such as emission size distributions and optical properties on aerosol cloud activation and radiative forcing. Our best estimate for net direct and indirect aerosol radiative forcing between 1750 and 2000 is −0.56 W/m2. However, the direct and indirect aerosol effects are quite sensitive to the black and organic carbon size distribution and consequential mixing state. The net radiative forcing can vary between −0.32 to −0.75 W/m2 depending on these carbonaceous particle properties at emission. Assuming that sulfates, nitrates and secondary organics form a coating around a black carbon core, rather than forming a uniformly mixed particle, changes the overall net aerosol radiative forcing from negative to positive. Taking into account internally mixed black carbon particles let us simulate correct aerosol absorption. Black carbon absorption is amplified by sulfate and nitrate coatings, but even more strongly by organic coatings. Black carbon mitigation scenarios generally showed reduced radiative forcing when sources with a large proportion of black carbon, such as diesel, are reduced; however reducing sources with a larger organic carbon component as well, such as bio-fuels, does not necessarily lead to climate benefits.
Atmospheric Chemistry and Physics 01/2010; · 4.88 Impact Factor
[show abstract][hide abstract] ABSTRACT: We evaluate black carbon (BC) model predictions from the AeroCom model intercomparison project by considering the diversity among year 2000 model simulations and comparing model predictions with available measurements. These model-measurement intercomparisons include BC surface and aircraft concentrations, aerosol absorption optical depth (AAOD) retrievals from AERONET and Ozone Monitoring Instrument (OMI) and BC column estimations based on AERONET. In regions other than Asia, most models are biased high compared to surface concentration measurements. However compared with (column) AAOD or BC burden retreivals, the models are generally biased low. The average ratio of model to retrieved AAOD is less than 0.7 in South American and 0.6 in African biomass burning regions; both of these regions lack surface concentration measurements. In Asia the average model to observed ratio is 0.7 for AAOD and 0.5 for BC surface concentrations. Compared with aircraft measurements over the Americas at latitudes between 0 and 50N, the average model is a factor of 8 larger than observed, and most models exceed the measured BC standard deviation in the mid to upper troposphere. At higher latitudes the average model to aircraft BC ratio is 0.4 and models underestimate the observed BC loading in the lower and middle troposphere associated with springtime Arctic haze. Low model bias for AAOD but overestimation of surface and upper atmospheric BC concentrations at lower latitudes suggests that most models are underestimating BC absorption and should improve estimates for refractive index, particle size, and optical effects of BC coating. Retrieval uncertainties and/or differences with model diagnostic treatment may also contribute to the model-measurement disparity. Largest AeroCom model diversity occurred in northern Eurasia and the remote Arctic, regions influenced by anthropogenic sources. Changing emissions, aging, removal, or optical properties within a single model generated a smaller change in model predictions than the range represented by the full set of AeroCom models. Upper tropospheric concentrations of BC mass from the aircraft measurements are suggested to provide a unique new benchmark to test scavenging and vertical dispersion of BC in global models.
ATMOSPHERIC CHEMISTRY AND PHYSICS 12/2009; · 5.51 Impact Factor
[show abstract][hide abstract] ABSTRACT: Evaluating multicomponent climate change mitigation strategies requires knowledge of the diverse direct and indirect effects of emissions. Methane, ozone, and aerosols are linked through atmospheric chemistry so that emissions of a single pollutant can affect several species. We calculated atmospheric composition changes, historical radiative forcing, and forcing per unit of emission due to aerosol and tropospheric ozone precursor emissions in a coupled composition-climate model. We found that gas-aerosol interactions substantially alter the relative importance of the various emissions. In particular, methane emissions have a larger impact than that used in current carbon-trading schemes or in the Kyoto Protocol. Thus, assessments of multigas mitigation policies, as well as any separate efforts to mitigate warming from short-lived pollutants, should include gas-aerosol interactions.
[show abstract][hide abstract] ABSTRACT: Aerosol indirect effects continue to constitute one of the most important uncertainties for anthropogenic climate perturbations. Within the international AEROCOM initiative, the representation of aerosol-cloud-radiation inter-actions in ten different general circulation models (GCMs) is evaluated using three satellite datasets. The focus is on stratiform liquid water clouds since most GCMs do not in-clude ice nucleation effects, and none of the model explicitly parameterises aerosol effects on convective clouds. We com-pute statistical relationships between aerosol optical depth (τ a) and various cloud and radiation quantities in a manner that is consistent between the models and the satellite data. It is found that the model-simulated influence of aerosols on Correspondence to: J. Quaas (email@example.com)
ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2009; 9:8697-8717. · 5.51 Impact Factor