[Show abstract][Hide abstract] ABSTRACT: 1. Background and Significance. Wind stress on the ocean surface results in the direct injection of sea-spray aerosols into the atmosphere through breaking waves as well as indirect injection through bursting bubbles at the sea surface (Blanchard and Woodcock, 1957; see Lewis and Schwartz, 2004, for a historical review). As a result, the oceans are the largest source of aerosols by mass to the atmosphere (Warneck, 1988). The non-water mass of marine aerosols is dominated by super-µm particles composed primarily of sea salt that have relatively short lifetimes (hours to several days) against gravitational settling (Prospero, 1981). The number production flux of marine aerosols is dominated by sizes less than 200 nm, as determined from atmospheric measurements over the open ocean (e.g. O'Dowd et al., 1997) and from the surf zone in Hawaii (Clarke et al., 2006), and from aerosols produced artificially by bubbling zero air through flowing seawater (Mårtensson et al., 2003; Keene et al., 2007a). Consequently, sea salt can be a significant source of condensation nuclei and cloud condensation nuclei in marine regions. In addition to the inorganic ions associated with sea salt (e.g., Na + , Mg +2 , Ca +2 , K + , Cl -, SO 4 -2), ocean-derived aerosols can contain inorganic and organic species produced through primary or secondary (gas to particle conversion) processes. Numerous studies in coastal and productive waters (e.g., O'Dowd et al., 2004) as well as oligotrophic waters (Keene et al., 2007a) have found that marine aerosols can be highly enriched in organic matter (OM) relative to bulk seawater, with enrichments increasing with decreasing particle size. Organic matter can account for more than 50% of the non-water mass of particles <500 nm diameter (e.g., Hoffman and Duce, 1977; Keene et al., 2007a; Facchini et al., 2008). Production mechanisms, composition, and chemical properties, including solubility, of this material are not well understood. In addition, the significance of ocean-derived OM, both in terms of aerosol mass concentration and climate impacts, versus that of OM transported from continental and anthropogenic sources is not known, primarily due to the lack of data and the highly variable results from a small number of studies. There is ample evidence to suggest that ocean-derived aerosols play an important role in controlling the Earth's radiation balance, cloud formation and properties, and chemistry of the marine atmosphere (SOLAS Implementation Plan Focus I Report, and references therein). Model estimates of top-of-atmosphere, global annual radiative forcing due to sea salt range from -1.51 to -5 W m -2 , depending on the emission scenario employed (IPCC, 2001). Models also indicate that sea salt aerosols can increase cloud condensation nuclei concentrations by up to 50% (Pierce and Adams, 2006). However, sea salt source functions used within different global models vary significantly, by up to a factor of five (e.g., Textor et al., 2006; Lewis and Schwartz, 2004). Emissions and chemical processing of primary ocean-derived particulate OM are now being integrated into marine aerosol source functions (O'Dowd et al., 2008; Long et al., 2009), and their incorporation into global aerosol climate models is being explored as well (E. Vignati, M. Kanakidou, personal comm.) 2 Several studies have been conducted since publication of the SOLAS Science Plan and accompanying SOLAS Implementation Plan Focus I report (2003) that have focused on marine aerosols including their emission and production mechanisms (e.g., Clarke et al., 2006; Keene et al., 2007a; de Leeuw et al., 2007; Nilsson et al., 2007; O'Dowd et al., 2008; Ceburnis et al., 2008; Bigg and Leck, 2008; Hultin et al., 2009), their chemical composition in coastal and high productivity regions (e.g., O'Dowd et al., 2004; Yoon et al., 2007; Facchini et al., 2008; Matrai et al., 2008), and their impact on climate (e.g., Clarke et al., 2006) and marine boundary layer chemistry (e.g., Keene et al., 2007b; Osthoff et al., 2008; Zhou et al., 2008). In spite of these efforts and related studies (e.g., McFiggins et al., 2005; Nilsson et al., 2007; O'Dowd et al., 2004; O'Dowd and de Leeuw, 2007; Sellegri et al., 2008; Witek et al., 2007), there are several fundamental questions that remain unanswered including some that were raised several years ago in the 2003 SOLAS Science Report and the Focus I Implementation Plan. 2. Questions to be addressed.
[Show abstract][Hide abstract] ABSTRACT: The domain of the surface ocean and lower atmosphere is a complex, highly dynamic component of the Earth system. Better understanding of the physics and biogeochemistry of the air-sea interface and the processes that control the exchange of mass and energy across that boundary define the scope of the Surface Ocean-Lower Atmosphere Study (SOLAS) project. The scientific questions driving SOLAS research, as laid out in the SOLAS Science Plan and Implementation Strategy for the period 2004-2014, are highly challenging, inherently multidisciplinary and broad. During that decade, SOLAS has significantly advanced our knowledge. Discoveries related to the physics of exchange, global trace gas budgets and atmospheric chemistry, the CLAW hypothesis (named after its authors, Charlson, Lovelock, Andreae and Warren), and the influence of nutrients and ocean productivity on important biogeochemical cycles, have substantially changed our views of how the Earth system works and revealed knowledge gaps in our understanding. As such SOLAS has been instrumental in contributing to the International Geosphere Biosphere Programme (IGBP) mission of identification and assessment of risks posed to society and ecosystems by major changes in the Earth́s biological, chemical and physical cycles and processes during the Anthropocene epoch. SOLAS is a bottom-up organization, whose scientific priorities evolve in response to scientific developments and community needs, which has led to the launch of a new 10-year phase. SOLAS (2015–2025) will focus on five core science themes that will provide a scientific basis for understanding and projecting future environmental change and for developing tools to inform societal decision-making.
[Show abstract][Hide abstract] ABSTRACT: High wintertime ozone levels have been observed in the Uintah Basin, Utah, a sparsely populated rural region with intensive oil and gas operations. The reactive nitrogen budget plays an important role in tropospheric ozone formation. Measurements were taken during three field campaigns in the winters of 2012, 2013, and 2014, which experienced varying climatic conditions. Average concentrations of ozone and total reactive nitrogen were observed to be 2.5 times higher in 2013 than 2012, with 2014 an intermediate year in most respects. However, photochemically active NOx(NO+NO2), remained remarkably similar all three years. Roughly half of the more oxidized forms of nitrogen were composed of nitric acid in 2013, with nighttime nitric acid formation through heterogeneous uptake of N2O5 contributing approximately 6 times more than daytime formation. The nighttime N2O5 lifetime between the high-ozone year 2013 and the low-ozone year 2012 is lower by a factor 2.6, and much of this is due to higher aerosol surface area in the high ozone year of 2013. A box-model simulation supports the importance of nighttime chemistry on the reactive nitrogen budget, showing a large sensitivity of NOx and ozone concentrations to nighttime processes.
Preview · Article · Aug 2015 · Atmospheric Chemistry and Physics
[Show abstract][Hide abstract] ABSTRACT: Aerosol particles influence global climate by determining cloud droplet number concentrations, brightness, and lifetime. Primary aerosol particles, such as those produced from breaking waves in the ocean, display large particle− particle variability in chemical composition, morphology, and physical phase state, all of which affect the ability of individual particles to accommodate water and grow into cloud droplets. Despite such diversity in molecular composition, there is a paucity of methods available to assess how particle−particle variability in chemistry translates to corresponding differences in aerosol hygroscopicity. Here, an approach has been developed that allows for characterization of the distribution of aerosol hygroscopicity within a chemically complex population of atmospheric particles. This methodology, when applied to the interpretation of nascent sea spray aerosol, provides a quantitative framework for connecting results obtained using molecular mimics generated in the laboratory with chemically complex ambient aerosol. We show that nascent sea spray aerosol, generated in situ in the Atlantic Ocean, displays a broad distribution of particle hygroscopicities, indicative of a correspondingly broad distribution of particle chemical compositions. Molecular mimics of sea spray aerosol organic material were used in the laboratory to assess the volume fractions and molecular functionality required to suppress sea spray aerosol hygroscopicity to the extent indicated by field observations. We show that proper accounting for the distribution and diversity in particle hygroscopicity and composition are important to the assessment of particle impacts on clouds and global climate.
[Show abstract][Hide abstract] ABSTRACT: Organic nitrates in both gas and condensed (aerosol) phases were measured during the Uintah Basin Winter Ozone Study from January to February in 2012. A high degree of correlation between total aerosol volume at diameters less than 500 nm and the particulate organic nitrate concentration indicates that organic nitrates are a consistent, if not dominant, fraction of fine aerosol mass. In contrast, a similar correlation with sub 2.5 μm aerosol volume is weaker. The C : N atomic ratio inferred from field measurements of PM2.5 and particulate organic nitrate is 34 : 1. Calculations constrained by the observations indicate that both condensation of gas phase nitrates and heterogeneous reactions of NO3 / N2O5 are responsible for introducing organic nitrate functionality into the aerosol and that the source molecules are alkanes. Extrapolating the results to urban aerosol suggests organic nitrate production from alkanes may be a major secondary organic aerosol source.
No preview · Article · Apr 2015 · Atmospheric Chemistry and Physics
[Show abstract][Hide abstract] ABSTRACT: The concentrations of sulfate, black carbon (BC) and other aerosols in the Arctic are characterized by high values in late winter and spring (so-called Arctic Haze) and low values in summer. Models have long been struggling to capture this seasonality and especially the high concentrations associated with Arctic Haze. In this study, we evaluate sulfate and BC concentrations from eleven different models driven with the same emission inventory against a comprehensive pan-Arctic measurement data set over a time period of two years (2008–2009). The set of models consisted of one Lagrangian particle dispersion model, four chemistry-transport models (CTMs), one atmospheric chemistry-weather forecast model and five chemistry-climate models (CCMs), of which two were nudged to meteorological analyses and three were running freely. The measurement data set consisted of surface measurements of equivalent BC (eBC) from five stations (Alert, Barrow, Pallas, Tiksi and Zeppelin), elemental carbon (EC) from Station Nord and Alert and aircraft measurements of refractory BC (rBC) from six different campaigns. We find that the models generally captured the measured eBC/rBC and sulfate concentrations quite well, compared to past comparisons. However, the aerosol seasonality at the surface is still too weak in most models. Concentrations of eBC and sulfate averaged over three surface sites are underestimated in winter/spring in all but one model (model means for January-March underestimated by 59 and 37% for BC and sulfate, respectively), whereas concentrations in summer are overestimated in the model mean (by 88 and 44% for July–September), but with over- as well as underestimates present in individual models. The most pronounced eBC underestimates, not included in the above multi-site average, are found for the station Tiksi in Siberia where the measured annual mean eBC concentration is three times higher than the average annual mean for all other stations. This suggests an underestimate of BC sources in Russia in the emission inventory used. Based on the campaign data, biomass burning was identified as another cause of the modelling problems. For sulfate, very large differences were found in the model ensemble, with an apparent anti-correlation between modeled surface concentrations and total atmospheric columns. There is a strong correlation between observed sulfate and eBC concentrations with consistent sulfate/eBC slopes found for all Arctic stations, indicating that the sources contributing to sulfate and BC are similar throughout the Arctic and that the aerosols are internally mixed and undergo similar removal. However, only three models reproduced this finding, whereas sulfate and BC are weakly correlated in the other models. Overall, no class of models (e.g., CTMs, CCMs) performed better than the others and differences are independent of model resolution.
[Show abstract][Hide abstract] ABSTRACT: Freshly emitted, primary sea spray aerosol (SSA) generated at the ocean surface and factors affecting its composition and associated properties are reviewed. At greater wind speeds, breaking waves are formed on the ocean surface.5 As waves break, air bubbles are entrained into ocean surface waters. These bubbles rise to the surface and burst, with each bubble producing up to hundreds of film drops in the nanometer to micrometer size range. Film drops form due to fragmentation of the thin fluid cap film of each bubble. Cap films can be stabilized by surfactants, increasing their lifetime and affecting the bursting dynamics of collections of bubbles on the water surface. SSA impacts the earth's radiation balance directly by scattering incoming solar radiation and indirectly by acting as cloud condensation nuclei (CCN) and altering cloud microphysical properties. the enriched organic matter in freshly emitted SSA can react photochemically, resulting in a loss of particulate organic species and the production of volatile, low molecular-weight organic compounds. SSA chemical composition and related properties may also vary depending on the production method. However, a direct comparison of bubble bursting methods indicates that subsaturated water uptake properties show only slight differences.
[Show abstract][Hide abstract] ABSTRACT: High concentrations of volatile organic compounds (VOCs) associated with oil and natural gas extraction were measured during a strong temperature inversion in winter of 2013 at a rural site in the Uintah Basin, Utah. During this period, photochemistry enhanced by the stagnant meteorological conditions and concentrated VOCs led to high ozone mixing ratios (150 ppbv). A simple analysis of aromatic VOCs measured by proton-transfer-reaction mass-spectrometry (PTR-MS) is used to estimate (1) VOC emission ratios (the ratio of two VOCs at the time of emission) relative to benzene, (2) aromatic VOC emission rates, and (3) ambient OH radical concentrations. These quantities are determined from a best fit to VOC : benzene ratios as a function of time. The main findings are that (1) emission ratios are consistent with contributions from both oil and gas producing wells, (2) the emission rate of methane (27-57 × 103 kg methane h−1), extrapolated from the emission rate of benzene (4.1 ± 0.4 × 105 molecules cm−3 s−1), agrees with an independent estimate of methane emissions from aircraft measurements in 2012, and (3) calculated daily OH concentrations are low, peaking at 1× 106 molecules cm−3, and are consistent with Master Chemical Mechanism (MCM) modeling. The analysis is extended to photochemical production of oxygenated VOCs measured by PTRMS and is able to explain daytime variability of these species. It is not able to completely reproduce nighttime behavior, possibly due to surface deposition. Using results from this analysis, the carbon mass of secondary compounds expected to have formed by the sixth day of the stagnation event was calculated, then compared to the measured mass of primary and secondary compounds. Only 17% of the expected secondary carbon mass is accounted for by gas phase, aerosol, and snow organic carbon measurements. The disparity is likely due to substantial amounts of unquantified oxygenated products.
No preview · Article · Mar 2015 · Atmospheric Chemistry and Physics
[Show abstract][Hide abstract] ABSTRACT: Formic acid (HCOOH) is one of the most abundant carboxylic acids in the atmosphere. However, current photochemical models cannot fully explain observed concentrations and in particular secondary formation of formic acid across various environments. In this work, formic acid measurements made at an urban receptor site (Pasadena) in June-July 2010 during CalNex (California Research at the Nexus of Air Quality and Climate Change) and a site in an oil and gas producing region (Uintah Basin) in January-February 2013 during UBWOS 2013 (Uintah Basin Winter Ozone Studies) will be discussed. Although the VOC (volatile organic compounds) compositions differed dramatically at the two sites, measured formic acid concentrations were comparable: 2.3 +/- 1.3 in UBWOS 2013 and 2.0 +/- 1.0 ppb in CalNex. We determine that concentrations of formic acid at both sites were dominated by secondary formation (> 99 %). A constrained box model using the Master Chemical Mechanism (MCM v3.2) underestimates the measured formic acid concentrations drastically at both sites (by a factor of > 10). Compared to the original MCM model that includes only ozonolysis of unsaturated organic compounds and OH oxidation of acetylene, when we updated yields of ozonolysis of alkenes and included OH oxidation of isoprene, vinyl alcohol chemistry, reaction of formaldehyde with HO2, oxidation of aromatics, and reaction of CH3O2 with OH, the model predictions for formic acid were improved by a factor of 6.4 in UBWOS 2013 and 4.5 in CalNex, respectively. A comparison of measured and modeled HCOOH / acetone ratios is used to evaluate the model performance for formic acid. We conclude that the modified chemical mechanism can explain 19 and 45% of secondary formation of formic acid in UBWOS 2013 and CalNex, respectively. The contributions from aqueous reactions in aerosol and heterogeneous reactions on aerosol surface to formic acid are estimated to be 0-6 and 0-5% in UBWOS 2013 and CalNex, respectively. We observe that air-snow exchange processes and morning fog events may also contribute to ambient formic acid concentrations during UBWOS 2013 (similar to 20% in total). In total, 53-59 in UBWOS 2013 and 50-55% in CalNex of secondary formation of formic acid remains unexplained. More work on formic acid formation pathways is needed to reduce the uncertainties in the sources and budget of formic acid and to narrow the gaps between measurements and model results.
Full-text · Article · Feb 2015 · Atmospheric Chemistry and Physics
[Show abstract][Hide abstract] ABSTRACT: In this paper laboratory work is documented establishing iodide ion chemical ionization mass spectrometry (I- CIMS) as a sensitive method for the unambiguous detection of peroxynitric acid (HO2NO2; PNA). A dynamic calibration source for HO2NO2, HO2, and HONO was developed and calibrated using a novel total NOy cavity ring-down spectroscopy (CaRDS) detector. Photochemical sources of these species were used for the calibration and validation of the I- CIMS instrument for detection of HO2NO2. Ambient observations of HO2NO2 using I- CIMS during the 2013 and 2014 Uintah Basin Wintertime Ozone Study (UBWOS) are presented. Strong inversions leading to a build-up of many primary and secondary pollutants as well as low temperatures drove daytime HO2NO2 as high as 1.5 ppbv during the 2013 study. A comparison of HO2NO2 observations to mixing ratios predicted using a chemical box model describing an ozone formation event observed during the 2013 wintertime shows agreement in the daily maxima HO2NO2 mixing ratio, but a differences of several hours in the timing of the observed maxima. Observations of vertical gradients suggest that the ground snow surface potentially serves as both a net sink and source of HO2NO2 depending on the time of day. Sensitivity tests using a chemical box model indicate that the lifetime of HO2NO2 with respect to deposition has a non-negligible impact on ozone production rates on the order of 10 %.
No preview · Article · Feb 2015 · Atmospheric Chemistry and Physics
[Show abstract][Hide abstract] ABSTRACT: We present mass spectrometry measurements of black-carbon containing particles made onboard the R/V Atlantis during the CalNex 2010 study using an Aerodyne Research Inc. soot particle aerosol mass spectrometer (SP-AMS). The R/V Atlantis was deployed to characterize air masses moving offshore the California coast and to assess emissions from sources in urban ports. This work presents a first detailed analysis of the size-resolved chemical composition of refractory black carbon (rBC) and of the associated coating species (NR-PMBC). A co-located standard high resolution aerosol mass spectrometer (HR-AMS) measured the total non-refractory submicron aerosol (NR-PM1). Our results indicate that, on average, 35% of the measured NR-PM1 mass (87% of the primary and 28% of the secondary NR-PM1, as obtained from the mass-weighted average of the NR-PMBC species) was associated with rBC. The peak in the average size distribution of the rBC-containing particles measured by the SP-AMS in vacuum aerodynamic diameter (dva) varied from ~100 nm to ~450 nm dva, with most of the rBC mass below 200 dva. The NR-PMBC below 200 nm dva was primarily organic, whereas inorganics were generally found on larger rBC-containing particles. Positive matrix factorization (PMF) analyses of both SP-AMS and HR-AMS data identified organic aerosol factors that were correlated in time, but had different fragmentation patterns due to the different instruments vaporization techniques. Finally, we provide an overview of the volatility properties of NR-PMBC and report the presence of refractory oxygen species in some of the air masses encountered.
[Show abstract][Hide abstract] ABSTRACT: Aerosol composition and concentration measurements along the coast of California were obtained using an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-AMS) onboard the research vessel Atlantis during the CalNex study in 2010. This paper focuses on the measurement of aerosol chloride using the HR-AMS that can be ambiguous in regions with significant quantities of sea salt aerosols. This ambiguity arises due to large differences in the sensitivity of the HR-AMS to refractory chloride species (i.e., NaCl) and non refractory chloride species (i.e., NH4Cl, HCl, etc.). Using the HR-AMS, the aerosol chloride signal is typically quantified using ion signals for 35Cl+, H35Cl+, 37Cl+ and H37Cl+ (HxCl+). During this study, the highest aerosol chloride signal was observed during sea sweep experiments when the source of the aerosol chloride was NaCl present in artificially generated sea salt aerosols even though the HR-AMS has significantly lower sensitivity to such refractory species. Other prominent ion signals that arise from NaCl salt were also observed at m/z 22.99 for Na+ and m/z 57.96 for Na35Cl+ during both sea sweep experiments and during periods of ambient measurements. Thus, refractory NaCl contributes significantly to the HxCl+ signal, interfering with attempts to quantify non sea salt chloride (nssCl). It was found that during ambient aerosol measurements, the interference in the HxCl+ signal from sea salt chloride (ssCl) was as high as 89%, but with a study wide average of 10%. The Na35Cl+ ion signal was found to be a good tracer for NaCl. We outline a method to establish nssCl in the ambient aerosols by subtracting the sea salt chloride (ssCl) signal from the HxCl+ signal. The ssCl signal is derived from the Na35Cl+ ion tracer signal and the HxCl+ to Na35Cl+ ratio established from the sea sweep experiments. Ambient submicron concentrations of ssCl were also established using the Na35Cl+ ion tracer signal and a scaling factor determined through simultaneous measurements of submicron aerosol chloride on filters. This scaling factor accounts for the low vaporization response of the AMS heater to ssCl, although regular calibration of this response is recommended in future applications. It follows that true total particulate chloride (pCl) is the sum of nssCl and ssCl. In this study, the median levels observed for the concentrations of pCl, nssCl and ssCl were 0.052, 0.017 and 0.024 μg m−3 respectively. The average contributions of nssCl and ssCl to pCl were 48 and 52% respectively, with nssCl dominating in periods of continental outflow and ssCl dominating during other periods. Finally, we propose a method to measure percentage chloride depletion of NaCl in ambient submicron sea salt aerosols, strictly using the AMS measurements of Na+ and Na35Cl+ ion signals. The median chloride depletion in submicron aerosols in this study was found to be 78%.
Full-text · Article · Jan 2015 · Atmospheric Chemistry and Physics
[Show abstract][Hide abstract] ABSTRACT: POLARCAT provided a wealth of data on the concentrations and sources of short-lived climate pollutants (aerosols, ozone) and their precursors in the Arctic. At the same time, significant discrepancies between simulated seasonal cycles of trace gases and aerosols and surface observations in the Arctic point to gaps in our knowledge about pollution origins and processing during long-range transport to the Arctic. Despite these observations, the extent to which aerosol haze layers aloft are associated with the seasonal Arctic haze phenomenon at the surface remains an open question. POLARCAT was recognized as an IPY activity, cosponsored by the International Geosphere Biosphere Program core projects International Global Atmospheric Chemistry (IGAC) and Integrated Land Ecosystem?Atmosphere Processes Study (iLEAPS) and the World Climate Research Program (WCRP) core project Stratosphere?Troposphere Processes and Their Role in Climate (SPARC).
Full-text · Article · Dec 2014 · Bulletin of the American Meteorological Society
[Show abstract][Hide abstract] ABSTRACT: The sources and composition of atmospheric marine aerosol particles (aMA) have been investigated with a range of physical and chemical measurements from open-ocean research cruises. This study uses the characteristic functional group composition (from Fourier transform infrared, or FTIR, spectroscopy) of aMA from five ocean regions to show that: (i) The organic functional group composition of aMA that can be identified as mainly atmospheric primary marine (ocean-derived) aerosol particles (aPMA) is 65 ± 12% hydroxyl, 21 ± 9% alkane, 6 ± 6% amine, and 7 ± 8% carboxylic acid functional groups. Contributions from photochemical reactions add carboxylic acid groups (15%-25%), shipping effluent in seawater and ship emissions add additional alkane groups (up to 70%), and coastal or continental emissions mix in alkane and carboxylic acid groups. (ii) The organic composition of aPMA is nearly identical to model generated primary marine aerosol particles from bubbled seawater (gPMA, which has 55 ± 14% hydroxyl, 32 ± 14% alkane, and 13 ± 3% amine functional groups), indicating that its overall functional group composition is the direct consequence of the organic constituents of the seawater source. (iii) While the seawater organic functional group composition was nearly invariant across all three ocean regions studied and the ratio of organic carbon to sodium (OC/Na+) in the gPMA remained nearly constant over a broad range of chlorophyll-a concentrations, the gPMA alkane group fraction appeared to increase with chlorophyll-a concentrations (r =0.66). gPMA from productive seawater had a larger fraction of alkane functional groups (42 ± 9%) compared to gPMA from non-productive seawater (22 ± 10%), perhaps due to the presence of surfactants in productive seawater that stabilize the bubble film and lead to preferential drainage of the more soluble (lower alkane group fraction) organic components. gPMA has a hydroxyl group absorption peak location characteristic of monosaccharides and disaccharides, where the seawater OM hydroxyl group peak location is closer to that of polysaccharides. This may result from the larger saccharides preferentially remaining in the seawater during gPMA and aPMA production.
[Show abstract][Hide abstract] ABSTRACT: We investigate hygroscopic growth of marine aerosols from three research cruises: TexAQS-GoMACCS 2006, ICEALOT 2008 and CalNex 2010. Particle hygroscopic growth was characterized by measuring the effect of water uptake under sub-saturated conditions on the aerosol light extinction at 532 nm. Mie theory calculations were utilized to convert the observed optical growth factors (fext(RH)) into physical growth factors (GF) at 85% RH. GF is found to be a more robust measure of aerosol hygroscopic growth than fext(RH), which can be biased by changes in aerosol dry size. Consistent with previous observations, the overall GF(85%) for submicron aerosol depended on the fraction of organics. The submicron GFOM(85%) specifically was found to range from 1.0-1.3 for all three campaigns. A robust positive linear dependence of the overall supermicron GF(85%) on the mass fraction of sea salt was observed. During TexAQS, two types of dust particles with distinct hygroscopic properties were identified in the supermicron mode; one that originated from the Sahara desert was moderately hygroscopic (GFdust(85%) = ~1.4) and the other from continental sources was nearly hydrophobic. The GF(85%) of supermicron organics was estimated through hygroscopicity closure calculations. Supermicron organics that originated from marine sources were found to be substantially more hygroscopic than those from continental sources, with the latter having a GF(85%) similar to that of the submicron organics. This study demonstrates the potential of using aerosol optical measurements to retrieve hygroscopic growth factor and underlines the importance and need for future investigations on the hygroscopic properties of marine supermicron aerosols.
[Show abstract][Hide abstract] ABSTRACT: We present a sensitive, compact detector that measures total reactive nitrogen (NOy), as well as NO2, NO, and O3. In all channels, NO2 is directly detected by laser diode based cavity ring-down spectroscopy (CRDS) at 405 nm. Ambient O3 is converted to NO2 in excess NO for the O3 measurement channel. Likewise, ambient NO is converted to NO2 in excess O3. Ambient NOy is thermally dissociated at ∼700° to form NO2 or NO in a heated quartz inlet. Any NO present in ambient air or formed from thermal dissociation of other reactive nitrogen compounds is converted to NO2 in excess O3 after the thermal converter. We measured thermal dissociation profiles for six of the major NOy components, and compared ambient measurements with other instruments during field campaigns in Utah and Alabama. Alabama measurements were made in a rural location with high biogenic emissions, and Utah measurements were made in the wintertime in unusual conditions that form high ozone from emissions related to oil and gas production. The NOy comparison in Alabama, to an accepted standard measurement method (a molybdenum catalytic converter/chemiluminescence instrument), agreed to within 12%, which we define as an upper limit to the accuracy of the NOy channel. The 1σ precision is <30 pptv at 1 second and <4 pptv at 1 minute time resolution for all measurement channels. The accuracy is 3% for the NO2 and O3 channels, and 5% for the NO channel. The precision and accuracy of this instrument make it a versatile alternative to standard chemiluminescence-based NOy instruments.
[Show abstract][Hide abstract] ABSTRACT: Quinn, P.K., A. Stohl, A. Baklanov, M.G. Flanner, A. Herber, K.Kupiainen, K.S. Law, J.
Schmale, S. Sharma, V. Vestreng, and K. von Salzen, The Arctic, Radiative forcing by black
carbon in the Arctic in “State of the Climate in 2013”, Bull. Amer. Meteor, Soc., 95 (7) S124 –
Full-text · Article · Jul 2014 · Bulletin of the American Meteorological Society
[Show abstract][Hide abstract] ABSTRACT: Ship-based measurements of gas-phase hydrochloric acid (HCl), particulate chloride (pCl-), and reactive nitrogen oxides (NOy) were made in the polluted marine boundary layer along the California coastline during spring 2010. These observations are used to assess both the rate of Cl atom production from HCl and the role of direct HCl emissions and subsequent partitioning as a source for pCl-. Observations of HCl made in coastal Southern California are broadly correlated with NOz (NOz ≡ NOy – NOx), peaking at 11 AM. The observed median HCl mixing ratio in Southern California is 1.3 ppb (interquartile range: 0.53 − 2.7 ppb), as compared to 0.19 ppb (interquartile range: 0.10 − 0.38 ppb) measured along the Sacramento River between San Francisco and Sacramento. Concurrent measurements of aerosol ion chemistry indicate that aerosol particles sampled in Northern California are heavily depleted in Cl-, corresponding to a mean pCl- deficit of 0.05 ± 0.03 (1σ) ppb for sub-10 μm aerosol particles. In comparison, aerosols measured in Southern California indicate that over 25% of particles showed an addition of Cl- to the particle population. Observations presented here suggest that primary sources of HCl, or gas-phase chlorine precursors to HCl, are likely underestimated in the California ARB emissions inventory. These results highlight the need for future field observations designed to better constrain direct reactive halogen emissions.
[Show abstract][Hide abstract] ABSTRACT: Aerosol variations and trends over different land and ocean regions from 1980 to 2009 are analyzed with the Goddard Chemistry Aerosol Radiation and Transport (GOCART) model and observations from multiple satellite sensors and available ground-based networks. Excluding time periods with large volcanic influence, aerosol optical depth (AOD) and surface concentration over polluted land regions generally vary with anthropogenic emissions, but the magnitude of this association can be dampened by the presence of natural aerosols, especially dust. Over the 30-year period in this study, the largest reduction in aerosol levels occurs over Europe, where AOD has decreased by 40–60% on average and surface sulfate concentrations have declined by a factor of up to 3–4. In contrast, East Asia and South Asia show AOD increases, but the relatively high level of dust aerosols in Asia reduces the correlation between AOD and pollutant emission trends. Over major dust source regions, model analysis indicates that the change of dust emissions over the Sahara and Sahel has been predominantly driven by the change of near-surface wind speed, but over Central Asia it has been largely influenced by the change of the surface wetness. The decreasing dust trend in the North African dust outflow region of the tropical North Atlantic and the receptor sites of Barbados and Miami is closely associated with an increase of the sea surface temperature in the North Atlantic. This temperature increase may drive the decrease of the wind velocity over North Africa, which reduces the dust emission, and the increase of precipitation over the tropical North Atlantic, which enhances dust removal during transport. Despite significant trends over some major continental source regions, the model-calculated global annual average AOD shows little change over land and ocean in the past three decades, because opposite trends in different land regions cancel each other out in the global average, and changes over large open oceans are negligible. This highlights the necessity for regional-scale assessment of aerosols and their climate impacts, as global-scale average values can obscure important regional changes.
Full-text · Article · May 2014 · ATMOSPHERIC CHEMISTRY AND PHYSICS
[Show abstract][Hide abstract] ABSTRACT: Physical and biogeochemical processes in seawater controlling primary marine aerosol (PMA) production and composition are poorly understood and associated with large uncertainties in estimated fluxes into the atmosphere. PMA production was investigated in the biologically-productive NE Pacific Ocean and in biologically-productive and oligotrophic regions of the NW Atlantic Ocean. Physicochemical properties of model PMA, produced by aeration of fresh seawater under controlled conditions, were quantified. Diel variability in model PMA mass and number fluxes was observed in biologically productive waters, increasing following sunrise and decreasing to pre-dawn levels overnight. Such variability was not seen in oligotrophic waters. Surfactant scavenging by aeration in the aerosol generator without replenishing the seawater in the reservoir during daytime reduced model PMA production in productive waters to nighttime levels but had no influence on production from oligotrophic waters. Results suggest bubble-plume interactions with sunlight-mediated biogenic surfactants in productive seawater significantly enhanced model PMA production.