Remote Marine Aerosols Research Articles

Decadal Variations in Hydroxy Fatty Acids Over Chichijima Island in the North Pacific: Long‐Term Seasonal Variability in Plant and Microbial Markers

AbstractTo understand the sources and transport pathways of organic compounds associated with soil microbes and higher plant waxes in the East Asian outflow, we assessed source‐specific tracers such as α‐, β‐ and ω‐hydroxy fatty acids (FAs) in remote marine aerosols collected at Chichijima Island in the western North Pacific (WNP) during 2001–2003. Molecular distributions of hydroxy FAs are characterized by strong even‐carbon numbered predominance, indicating biogenic sources. Hydroxy FAs showed a strong seasonality with higher loadings during winter/spring than summer/autumn. Cluster analysis of backward air mass trajectories, satellite‐based fire counts and dust extinction data reveal an impact of the East Asian outflow over the WNP in winter/spring. In the spring, there are larger relative abundances of short‐chain β‐hydroxy C10‐C18 FAs (a proxy for soil microbes), consistent with the higher loadings of non‐sea‐salt Ca2+ (dust tracer). The molecular distributions of β‐hydroxy FAs in spring are in agreement with those of the reference materials of Chinese loess (CJ‐1) and simulated Asian mineral dust (CJ‐2), suggesting their probable sources in East Asia. A comparison of relative abundances of short‐chain β‐hydroxy C10‐C18 FAs and long‐chain ω‐hydroxy C20‐C32 FAs (a proxy for higher plant metabolites) in Chichijima aerosols between this (2001–2003) and previous (1990–1993) studies have unveiled an increment of 20% and 30%, respectively. Such an increase was likely caused by the changes in source strength on a decadal‐scale and warrants further investigation. Furthermore, cluster analysis of trajectories and the overall distributions of hydroxy FAs between both datasets have shown their similar provenance in winter/spring.

The value of remote marine aerosol measurements for constraining radiative forcing uncertainty

Abstract. Aerosol measurements over the Southern Ocean are used to constrain aerosol–cloud interaction radiative forcing (RFaci) uncertainty in a global climate model. Forcing uncertainty is quantified using 1 million climate model variants that sample the uncertainty in nearly 30 model parameters. Measurements of cloud condensation nuclei and other aerosol properties from an Antarctic circumnavigation expedition strongly constrain natural aerosol emissions: default sea spray emissions need to be increased by around a factor of 3 to be consistent with measurements. Forcing uncertainty is reduced by around 7 % using this set of several hundred measurements, which is comparable to the 8 % reduction achieved using a diverse and extensive set of over 9000 predominantly Northern Hemisphere measurements. When Southern Ocean and Northern Hemisphere measurements are combined, uncertainty in RFaci is reduced by 21 %, and the strongest 20 % of forcing values are ruled out as implausible. In this combined constraint, observationally plausible RFaci is around 0.17 W m−2 weaker (less negative) with 95 % credible values ranging from −2.51 to −1.17 W m−2 (standard deviation of −2.18 to −1.46 W m−2). The Southern Ocean and Northern Hemisphere measurement datasets are complementary because they constrain different processes. These results highlight the value of remote marine aerosol measurements.

Seasonal Variations in Western North Atlantic Remote Marine Aerosol Properties

AbstractThe impact of ocean ecosystems on marine boundary layer aerosols and clouds has been the subject of much research but remains uncertain. Five experiments were recently conducted in the western North Atlantic to assess if the seasonally recurring phytoplankton bloom affects aerosol properties. These experiments include the second Western Atlantic Climate Study and four North Atlantic Aerosols and Marine Ecosystem Study cruises. Measurements of unheated and heated number size distributions, cloud condensation nucleus (CCN) concentrations, and aerosol composition were used to identify primary and secondary aerosol components that could be related to the state of the bloom. Only periods of clean marine air, as defined by radon, particle number concentrations, aerosol light absorption coefficient, and back trajectories, were included in the analysis. Nonvolatile material was found to be prevalent in the Aitken mode size range after heating to 230°, likely due to downward mixing from the free troposphere. CCN concentrations at 0.1% supersaturation were best correlated (r2 = 0.73) with accumulation mode nss SO4=. Sea spray aerosol was only correlated with CCN during November when bloom accumulation had not yet occurred and dimethylsulfide concentrations were at a minimum. The fraction of CCN attributable to sea spray aerosol was less than 20% during March, May/June, and September, indicating the limited contribution of sea spray aerosol to the CCN population of the western North Atlantic atmosphere. The strongest link between the plankton bloom and aerosol and cloud properties appears to be due to biogenic non‐sea salt SO4=.

Chemical Characterization of Isoprene- and Monoterpene-Derived Secondary Organic Aerosol Tracers in Remote Marine Aerosols over a Quarter Century

Isoprene and monoterpenes are ubiquitous precursors of biogenic secondary organic aerosol (SOA) over continental regions; however, their contributions to organic aerosol (OA) mass and chemical proc...

The North Atlantic Aerosol and Marine Ecosystem Study (NAAMES): Science Motive and Mission Overview

The North Atlantic Aerosols and Marine Ecosystems Study (NAAMES) is an interdisciplinary investigation to improve understanding of Earth's ocean ecosystem-aerosol-cloud system. Specific overarching science objectives for NAAMES are to (1) characterize plankton ecosystem properties during primary phases of the annual cycle and their dependence on environmental forcings, (2)determine how these phases interact to recreate each year the conditions for an annual plankton bloom, and (3) resolve how remote marine aerosols and boundary layer clouds are influenced by plankton ecosystems. Four NAAMES field campaigns were conducted in the western subarctic Atlantic between November 2015 and April 2018, with each campaign targeting specific seasonal events in the annual plankton cycle. A broad diversity of measurements were collected during each campaign, including ship, aircraft, autonomous float and drifter, and satellite observations. Here, we present an overview of NAAMES science motives, experimental design, and measurements. We then briefly describe conditions and accomplishments during each of the four field campaigns and provide information on how to access NAAMES data. The intent of this manuscript is to familiarize the broad scientific community with NAAMES and to provide a common reference overview of the project for upcoming publications.

Hydroxy Fatty Acids in Remote Marine Aerosols over the Pacific Ocean: Impact of Biological Activity and Wind Speed

Current parametrization of cloud–aerosol interactions of nascent sea-spray aerosols (SSA) in the climate models is hampered by our limited understanding of the constituents of organic aerosols. We investigated here the mass concentrations, molecular distributions, and relative abundances of hydroxy fatty acids (FAs), ubiquitous lipid compounds in the SSA collected over the western Pacific Ocean (35°N–40°S). Detectable levels of methanesulfonic acid (an oxidation product of dimethyl sulfide) and high enrichment factors of Mg2+ and Ca2+ relative to Na+ emphasize the impact of the marine biological activity on the organic fraction of SSA. The molecular distributions of β-hydroxy FAs with characteristic “odd-C” predominance (C9 > C10 and C11 > C12) among short-chain homologues (<C14) reveal their formation from the photochemical oxidation of marine-derived organic matter. Molecular distributions and the carbon preference index of n-fatty acids and n-alkanes in the SSA also indicate their oceanic origin. A rap...

Molecular distribution and compound-specific stable carbon isotopic composition of dicarboxylic acids, oxocarboxylic acids and &amp;lt;i&amp;gt;α&amp;lt;/i&amp;gt;-dicarbonyls in PM&amp;lt;sub&amp;gt;2.5&amp;lt;/sub&amp;gt; from Beijing, China

Abstract. This study investigates the seasonal variation, molecular distribution and stable carbon isotopic composition of diacids, oxocarboxylic acids and α-dicarbonyls to better understand the sources and formation processes of fine aerosols (PM2.5) in Beijing. The concentrations of total dicarboxylic acids varied from 110 to 2580 ng m−3, whereas oxoacids (9.50–353 ng m−3) and dicarbonyls (1.50–85.9 ng m−3) were less abundant. Oxalic acid was found to be the most abundant individual species, followed by succinic acid or occasionally by terephthalic acid (tPh), a plastic waste burning tracer. Ambient concentrations of phthalic acid (37.9 ± 27.3 ng m−3) and tPh (48.7 ± 51.1 ng m−3) were larger in winter than in other seasons, illustrating that fossil fuel combustion and plastic waste incineration contribute more to wintertime aerosols. The year-round mass concentration ratios of malonic acid to succinic acid (C3 ∕ C4) were relatively low by comparison with those in other urban aerosols and remote marine aerosols. The values were less than or equal to unity in Beijing, implying that the degree of photochemical formation of diacids in Beijing is insignificant. Moreover, strong correlation coefficients of major oxocarboxylic acids and α-dicarbonyls with nss-K+ suggest that biomass burning contributes significantly to these organic acids and related precursors. The mean δ13C value of succinic acid is the highest among all species, with values of −17.1 ± 3.9 ‰ (winter) and −17.1 ± 2.0 ‰ (spring), while malonic acid is more enriched in 13C than others in autumn (−17.6 ± 4.6 ‰) and summer (−18.7 ± 4.0 ‰). The δ13C values of major species in Beijing aerosols are generally lower than those in the western North Pacific atmosphere, the downwind region, which indicates that stable carbon isotopic compositions of diacids depend on their precursor sources in Beijing. Therefore, our study demonstrates that in addition to photochemical oxidation, high abundances of diacids, oxocarboxylic acids and α-dicarbonyls in Beijing are largely associated with anthropogenic primary emissions, such as biomass burning, fossil fuel combustion and plastic waste burning.

Seasonal variations of low molecular weight hydroxy-dicarboxylic acids and oxaloacetic acid in remote marine aerosols from Chichijima Island in the western North Pacific (December 2010–November 2011)

Concentrations of homologous hydroxy-dicarboxylic acids (diacids) (hC3-hC6) and keto-diacid (oxaloacetic acid) were measured in the atmospheric aerosols collected at Chichijima Island (27.04° N, 142.13° E) in the western North Pacific from December 2010 to November 2011. The monthly averaged concentrations of hydroxy-diacids and oxaloacetic acid were significantly higher in spring followed by winter and autumn. Molecular distributions of hydroxy-diacids demonstrated that malic acid was the most abundant species in all four seasons, followed by tartronic acid in winter and spring and 3- and 2-hydroxyglutaric acids in summer and autumn. Hydroxy-diacids and keto-diacid maximized in spring and winter when air masses originated from the Asian continent with westerly winds. The concentrations of total hydroxy-diacids and oxaloacetic acid ranged from 0.1 to 27.3 ng m−3 and <0.005 to 2 ng m−3, respectively. The enhanced concentrations of diacids and their intermediates in winter and spring are associated with a long-range atmospheric transport of pollutants from East Asia to remote Chichijima Island followed by photochemical processing of organic aerosols. Seasonal molecular distribution of hydroxy-diacids and oxaloacetic acid was found to be dependent on the source strengths and plausible photochemical processing to form smaller diacids. Moderate to strong correlations among hydroxy-diacids, oxaloacetic acid and low molecular weight (LMW) diacids suggest that hydroxy-diacids and oxaloacetic acid are the intermediates in the photochemical oxidation of LMW diacid. Hence, photochemical formation of the most abundant LMW diacids, i.e., oxalic acid, could be produced from hydroxy- and keto-diacid as intermediates.

Hygroscopic growth of water-soluble matter extracted from remote marine aerosols over the western North Pacific: Influence of pollutants transported from East Asia

We examined the hygroscopic properties of water-soluble matter (WSM) nebulized from water extracts of total suspended particles (TSP) collected at Chichijima Island in the western North Pacific during January to September 2003. The hygroscopic growth factor g(RH) of the aerosol particles was measured using a hygroscopic tandem differential mobility analyzer (HTDMA) with an initial dry particle diameter of 100nm and relative humidity (RH) of 5–95%. The measured growth factor at 90% RH, g(90%), ranged from 1.51 to 2.14 (mean: 1.76±0.15), significantly lower than that of sea salts (2.1), probably owing to the heterogeneous reactions associated with chloride depletion in sea-salt particles and water-soluble organic matter (WSOM). The g(90%) maximized in summer and minimized in spring. The decrease in spring was most likely explained by the formation of less hygroscopic salts or particles via organometallic reactions during the long-range transport of Asian dust. Cl− and Na+ dominate the mass fractions of WSM, followed by nss-SO42− and WSOM. Based on regression analysis, we confirmed that g(90%) at Chichijima Island largely increased due to the dominant sea spray; however, atmospheric processes associated with chloride depletion in sea salts and WSOM often suppressed g(90%). Furthermore, we explored the deviation (average: 18%) between the measured and predicted g(90%) by comparing measured and model growth factors. The present study demonstrates that long-range atmospheric transport of anthropogenic pollutants (SO2, NOx, organics, etc.) and the interactions with sea-salt particles often suppress the hygroscopic growth of marine aerosols over the western North Pacific, affecting the remote background conditions. The present study also suggests that the HCl liberation leads to the formation of less hygroscopic aerosols over the western North Pacific during long-range transport.

Identification of hydroxy- and keto-dicarboxylic acids in remote marine aerosols using gas chromatography/quadruple and time-of-flight mass spectrometry.

The identification of hydroxy- and keto-dicarboxylic acids (diacids) in remote marine aerosol samples is important for a better understanding of the composition of organic particulate matter, as this chemical composition is essential for predicting the effects on climate, air quality, and human health. Molecular characterization of these compounds provides insights into sources and formation pathways of organic aerosols. The method of chemical derivatization followed by gas chromatography-flame ionization detection (GC-FID), gas chromatography/quadruple mass spectrometry (GC/QMS) and gas chromatography/time-of-flight mass spectrometry (GC/TOFMS) was used to identify hydroxy- and keto-diacids in remote marine aerosols. Atmospheric samples were collected at Chichijima Island in the western North Pacific and the diacids and related compounds were extracted with organic-free ultrapure water. A two-step derivatization technique was employed, using 14% BF3 /n-butanol for the butylation of carboxyl groups and acidic ketones followed by N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) for the trimethylsilylation of hydroxyl groups. Several new peaks were detected in the gas chromatogram after trimethylsilylation of the dibutyl ester fraction. Based on mass spectral interpretation with authentic standards, we successfully identified and quantified a homologous series of hydroxydiacids, including tartaric and oxaloacetic acids. In addition, transformation of oxaloacetic acid into its enol form was elucidated. Utilizing GC-FID, GC/QMS and GC/TOFMS, hydroxy- and keto-diacids were identified in the remote marine aerosols. A complete structural characterization was achieved with extensive mass spectral analysis. Molecular distributions of hydroxydiacids generally showed the predominance of malic acid followed by tartronic acid. We consider that these hydroxydiacids are important intermediates in the atmospheric oxidation of organic aerosols to result in smaller diacids. Copyright © 2016 John Wiley & Sons, Ltd.

Bacterial Communities in Marine Aerosols Revealed by 454 Pyrosequencing of the 16S rRNA Gene*

Abstract Although bacteria are an important biological component of aerosol particles, studies of bacterial communities in remote marine aerosol are largely lacking. In this study, aerosol samples were collected over the western Pacific Ocean, the northern Pacific Ocean, the Arctic Ocean, and the Norwegian Sea during the Fifth Chinese National Arctic Research Expedition (CHINARE 5). The diversity and structure of aerosol bacterial communities, based on 454 pyrosequencing, were explored in these samples. The bacterial community in the aerosols collected over the Pacific Ocean was more diverse than over the Norwegian Sea. Both temporal and spatial variations in aerosol bacterial communities were observed based on phylogenetic analysis. These results suggest that the source of air masses shape bacterial communities in aerosol particles over remote marine regions. Aerosols are clearly important for long-range transport of bacteria. Since potential human pathogens (e.g., Streptococcus sp.) were retrieved in this study, further investigation is needed to evaluate the potential for their long-distance migration via aerosol.

A 12-year observation of water-soluble ions in TSP aerosols collected at a remote marine location in the western North Pacific: an outflow region of Asian dust

Abstract. In order to characterize the long-term trend of remote marine aerosols, a 12-year observation was conducted for water-soluble ions in TSP (total suspended particulate) aerosols collected from 2001 to 2012 in the Asian outflow region at Chichijima Island in the western North Pacific. We found a clear difference in chemical composition between the continentally affected and marine background air masses over the observation site. Asian continental air masses are delivered from late autumn to spring, whereas marine air masses were dominated in summer. Concentrations of non-sea salt (nss-) SO42−, NO3−, NH4+, nss-K+ and nss-Ca2+ are high in winter and spring and low in summer. On the other hand, MSA− (methanesulfonate) exhibits higher concentrations during spring and winter, probably due to springtime dust bloom or due to the direct continental transport of MSA− to the observation site. We could not find any clear decadal trend for Na+, Cl−, Mg2+ and nss-Ca2+ in all seasons, although there exists a clear seasonal trend. However, concentrations of nss-SO42− continuously decreased from 2007 to 2012, probably due to the decreased SO2 emissions in East Asia especially in China. In contrast, nss-K+ and MSA− concentrations continuously increased from 2001 to 2012 during winter and spring seasons, demonstrating that biomass burning and/or terrestrial biological emissions in East Asia are being increasingly transported from the Asian continent to the western North Pacific. This study also demonstrates that Asian dusts can act as an important source of nutrients for phytoplankton and thus sea-to-air emission of dimethyl sulfide over the western North Pacific.

Spatial distributions of dicarboxylic acids, ω-oxoacids, pyruvic acid and α-dicarbonyls in the remote marine aerosols over the North Pacific

To better understand the sources and atmospheric processing of organic aerosols in the remote marine atmosphere, aerosol (TSP) samples were collected over the North Pacific (13°14′N-53°37′N and 140°46′E-179°54′W) in May–July, 2010 during a cruise of T/V Oshoro Maru, and studied for dicarboxylic acids (C2–C11), ω-oxoacids, pyruvic acid and α-dicarbonyls as well as organic and elemental carbon, water-soluble organic carbon and total nitrogen. Over the North Pacific, diacids are most abundant followed by ω-oxoacids and α-dicarbonyls. Although the molecular compositions of diacids are generally characterized by the predominance of oxalic (C2) acid, we found a predominance of succinic (C4) acid in four samples, which were collected in the central northern North Pacific. We consider that photochemical degradation of unsaturated fatty acids emitted from the ocean surface is the main source of C4 over the central northern North Pacific, where Chlorophyll-a maximized during the cruise. Moreover, seven samples collected in the central northern North and western North Pacific shows predominance of malonic (C3) acid over C4, suggesting the photochemical degradation of C4 to C3. Spatial distributions of diacids, ω-oxoacids and α-dicarbonyls together with total carbon and water soluble total nitrogen contents showed higher abundances in the coastal western North Pacific followed by the central northern North and subtropical Pacific, signifying that continental aerosols are transported long distances to the remote marine atmosphere.

Aerosol size-resolved trace metal composition in remote northern tropical Atlantic marine environment: case study Cape Verde islands

Abstract. Size-resolved trace metal concentrations of 15 elements in aerosol particles at the Cape Verde Atmospheric Observatory (CVAO) under remote background conditions were investigated through analysis of aerosol samples collected during intensive field studies from January 2007 to November 2011 using total reflection x-ray fluorescence (TXRF). The identification of the main air mass origin that influence remote marine aerosol in the northern tropical Atlantic has been investigated. In total, 317 samples were collected. The dataset was analyzed according to the main air mass inflow at the station. We found that remote conditions make up about 45% of the meteorological conditions in a year at CVAO and thus the northern tropical Atlantic. Surprisingly, air masses from North America are often responsible for higher trace metal concentrations in this region. Elements such as Zn, Pb, Cu, Cr, Ni, and V were mostly found in the submicron size fractions, while elements with dominant crustal or oceanic origin such as Fe, Ti, Mn, Sr, and Rb were found in the coarse fractions (&gt;1 μm). The highest metal concentrations, especially for Zn (3.23 ng m−3), Cu (0.81 ng m−3), Sr (2.63 ng m−3), and Cr (0.53 ng m−3), were observed in air masses originating from North America and the concentrations were within the same concentration range to those reported previously in the literature for remote marine aerosols. Fe (12.26 ng m−3), Ti (0.91 ng m−3), and Mn (0.35 ng m−3) showed higher concentrations when air mass came from Europe and the Canary Islands. Pb concentration was low (&lt;0.20 ng m−3) and did not vary significantly with air mass direction. The low Pb concentration is indicative of the complete phase-out of leaded gasoline even in African countries. Crustal enrichment factor values decreased from fine to coarse-mode particles with low values (&lt;4) observed for Fe, Mn, and Rb, and high values (&gt;20) for Zn, Cu, Ni, Cr, Pb, and Se. The observed enrichment of the elements was attributed to crustal, marine, anthropogenic, and biogenic sources, as well as long-range transport and resuspension. Zn, Cu and Pb were indicators of anthropogenic activities, while Ti and Sr were indicators of crustal and marine origin, respectively. Oceanic and biogenic emissions might have contributed to most of the Se observed. This work provides the first long-term size-resolved trace metals study for remote tropical northern Atlantic marine aerosols and the dataset could serve as good initiation of yearly flux estimates.

Long-term observations of saccharides in remote marine aerosols from the western North Pacific: A comparison between 1990–1993 and 2006–2009 periods

Anhydrosugars (galactosan, mannosan and levoglucosan), sugars (xylose, fructose, glucose, sucrose and trehalose) and sugar alcohols (erythritol, arabitol, mannitol and inositol) were measured in the aerosol samples collected in a remote island (Chichi-Jima, Japan) in the western North Pacific from 1990 to 1993 and from 2006 to 2009. Total concentrations of anhydrosugars, the biomass burning tracers, were 0.01–5.57 ng m−3 (average 0.76 ng m−3) during 1990–1993 versus 0.01–7.19 ng m−3 (0.64 ng m−3) during 2006–2009. Their seasonal variations were characterized by winter/spring maxima and summer/fall minima. Such a seasonal pattern should be caused by the enhanced long-range atmospheric transport of biomass burning products and terrestrial organic matter (such as higher plant detritus and soil dust) from the Asian continent in winter/spring seasons, when the westerly or winter monsoon system prevails over the western North Pacific. Sugars and sugar alcohols showed different seasonal patterns. The monthly mean concentrations of erythritol, arabitol, mannitol, inositol, fructose, glucose and trehalose were found to be higher in spring/summer and lower in fall/winter during both 1990–1993 and 2006–2009 periods, indicating an enhanced biogenic emission of aerosols in warm seasons. Interestingly, saccharides showed a gradual decrease in their concentrations from 1991 to 1993 and an increase from 2006 to 2009. In addition, the monthly averaged concentrations of sugars and sugar alcohols showed maxima in early summer during 1990–1993, which occurred about 1–2 months earlier than those during 2006–2009. Such a clear seasonal shift may be attributable to the changes in the strength of westerly and trade wind systems during two periods.

海洋大気中のシュウ酸、マロン酸の安定炭素同位体比：夏季におけるδ13Cの増加と有機エアロゾルの光化学的エイジング

海洋大気中のシュウ酸、マロン酸の安定炭素同位体比：夏季におけるδ13Cの増加と有機エアロゾルの光化学的エイジング

Glycine in aerosol water droplets: a critical assessment of Köhler theory by predicting surface tension from molecular dynamics simulations

Abstract. Aerosol particles in the atmosphere are important participants in the formation of cloud droplets and have significant impact on cloud albedo and global climate. According to the Köhler theory which describes the nucleation and the equilibrium growth of cloud droplets, the surface tension of an aerosol droplet is one of the most important factors that determine the critical supersaturation of droplet activation. In this paper, with specific interest to remote marine aerosol, we predict the surface tension of aerosol droplets by performing molecular dynamics simulations on two model systems, the pure water droplets and glycine in water droplets. The curvature dependence of the surface tension is interpolated by a quadratic polynomial over the nano-sized droplets and the limiting case of a planar interface, so that the so-called Aitken mode particles which are critical for droplet formation could be covered and the Köhler equation could be improved by incorporating surface tension corrections.

Size distributions of organic nitrogen and carbon in remote marine aerosols: Evidence of marine biological origin based on their isotopic ratios

Size‐segregated aerosol samples were collected over the western North Pacific in summer 2008 for the measurements of organic nitrogen (ON) and organic carbon (OC). ON and OC showed bimodal size distributions. Their concentrations showed positive correlation with those of biogenic tracers, methanesulfonic acid (MSA) and azelaic acid (C9). We found that average ON and OC concentrations were twice greater in aerosols collected in the oceanic region with higher biological productivity than in the regions with lower productivity. The average ON/OC ratios are higher (0.49 ± 0.11) in more biologically influenced aerosols than those (0.35 ± 0.10) in less influenced aerosols. Stable carbon isotopic analysis indicates that marine‐derived carbon accounted for ∼46–72% of total carbon in more biologically influenced aerosols. These results provide evidence that organic aerosols in this region are enriched in ON that is linked to oceanic biological activity and the subsequent emissions to the atmosphere.

The role of sea-salt emissions in controlling the marine Aitken and accumulation mode aerosol: a model study

The remote marine aerosol and the cloud droplet number concentration (CDNC) are examined with an aerosol microphysics box model in an attempt to better understand the processes involved in the formation and transformation of the marine aerosol. Emission of submicrometre sea-salt and dimethylsulfide (DMS) have been included together with aerosol dynamics, gas and liquid phase chemistry and cloud processing representative for the marine boundary layer atmosphere. Our simulations are able to reproduce a bimodal submicrometre size distribution with realistic number concentrations even when new particle formation by nucleation is neglected. This indicates that ultrafine primary sea-salt flux is an important source of Aitken mode particles and CDNC. However, sulphate still constitutes 20–80% of the Aitken and accumulation mode masses. The temperature dependence of the sea-salt source function leads to a 23% decrease in total number concentration when the temperature increases from 12 to 20 ◦C. The influence of DMS emission on the aerosol and CDNC is minimal but the size distribution and mass concentration of sulphate is changed, mostly due to in-cloud processes. The wind speed is the dominant factor determining the CDNC, although entrainment of aerosols from free troposphere can have a substantial effect.

Evaluation of the global oceanic isoprene source and its impacts on marine organic carbon aerosol

Abstract. We have combined the first satellite maps of the global distribution of phytoplankton functional type and new measurements of phytoplankton-specific isoprene productivities, with available remote marine isoprene observations and a global model, to evaluate our understanding of the marine isoprene source and its impacts on organic aerosol abundances. Using satellite products to scale up data on phytoplankton-specific isoprene productivity to the global oceans, we infer a mean "bottom-up" oceanic isoprene emission of 0.31±0.08 (1σ) Tg/yr. By minimising the mean bias between the model and isoprene observations in the marine atmosphere remote from the continents, we produce a "top-down" oceanic isoprene source estimate of 1.9 Tg/yr. We suggest our reliance on limited atmospheric isoprene data, difficulties in simulating in-situ isoprene production rates in laboratory phytoplankton cultures, and limited knowledge of isoprene production mechanisms across the broad range of phytoplankton communities in the oceans under different environmental conditions as contributors to this difference between the two estimates. Inclusion of secondary organic aerosol (SOA) production from oceanic isoprene in the model with a 2% yield produces small contributions (0.01–1.4%) to observed organic carbon (OC) aerosol mass at three remote marine sites in the Northern and Southern Hemispheres. Based on these findings we suggest an insignificant role for isoprene in modulating remote marine aerosol abundances, giving further support to a recently postulated primary OC source in the remote marine atmosphere.