Srinivas Bikkina

High abundances of oxalic, azelaic, and glyoxylic acids and methylglyoxal in the open ocean with high biological activity: Implication for secondary OA formation from isoprene

Atmospheric dicarboxylic acids (DCA) are a ubiquitous water-soluble component of secondary organic aerosols (SOA), which can act as cloud condensation nuclei (CCN), affecting the Earths climate. Despite the high abundances of oxalic acid and related compounds in the marine aerosols, there is no consensus on what controls their distributions over the open ocean. Marine biological productivity could play a role in the production of DCA, but there is no substantial evidence to support this hypothesis. Here we present latitudinal distributions of DCA, oxoacids and α-dicarbonyls in the marine aerosols from the remote Pacific. Their concentrations were found several times higher in more biologically influenced aerosols (MBA) than less biologically influenced aerosols. We propose isoprene and unsaturated fatty acids as sources of DCA as inferred from significantly higher abundances of isoprene-SOA tracers and azelaic acid in MBA. These results have implications toward the reassessment of climate forcing feedbacks of marine-derived SOA.

Stable carbon and nitrogen isotopic composition of fine mode aerosols (PM2.5) over the Bay of Bengal: impact of continental sources

This study reports on stable carbon (δ13CTC) and nitrogen (δ15NTN) isotopic composition of total carbon and nitrogen (TC and TN) in the fine mode aerosols (PM2.5; N=31) collected over the Bay of Bengal (BoB). The samples represent two distinct wind regimes during the cruise (27 December 2008–28 January 2009); one from the Indo-Gangetic Plain (referred as IGP-outflow) and another from Southeast Asia (SEA-outflow). The PM2.5 samples from the IGP-outflow show higher δ13CTC (−25.0 to −22.8 ‰; −23.8±0.6 ‰) than those from the SEA-outflow (−27.4 to −24.7 ‰; −25.3±0.9 ‰). Similarly, δ15NTN varied from +11.8 to +30.6 ‰ (+20.4±5.4 ‰) and +10.4 to +31.7 ‰ (+19.4±6.1 ‰) for IGP- and SEA-outflows, respectively. Based on the literature data, MODIS-derived fire hotspots and back trajectories, we infer that higher δ13CTC in the IGP-outflow is predominantly associated with fossil fuel and biofuel combustion. In contrast, contribution of primary organic aerosols from the combustion of C3 plants or secondary organic aerosol (SOA) formation from biomass/biofuel-burning emissions (BBEs) can explain the lower δ13CTC values in the SEA-outflow. This inference is based on the significant linear correlations among δ13CTC, water-soluble organic carbon and non-sea-salt potassium (nss-K+, a proxy for BBEs) in the SEA-outflow. A significant linear relationship of δ15N with and equivalent mass ratio of / is evident in both the continental outflows. Since abundance dominates the TN over the BoB (>90 %), atmospheric processes affecting its concentration in fine mode aerosols can explain the observed large variability of δ15NTN.

Latitudinal distributions of atmospheric dicarboxylic acids, oxocarboxylic acids, and α-dicarbonyls over the western North Pacific : Sources and formation pathways

The present study aims to assess the molecular distributions of water-soluble dicarboxylic acids (diacids: C2–C12), oxocarboxylic acids (C2–C9), and α-dicarbonyls (glyoxal and methylglyoxal) in aerosols collected over the western North Pacific (WNP) during a summer cruise (August to September 2008). The measured water-soluble organics show pronounced latitudinal distributions with higher concentrations in the region of 30°N–45°N (average 63 ng m−3) than 10°N–30°N (18 ng m−3). Mass fraction of oxalic acid (C2) in total aliphatic diacids (ΣC2–C12) showed higher values (72 ± 10%) in lower latitude (10°N–30°N) than that (56 ± 16%) in higher latitude (30°N–45°N), suggesting a photochemical production of C2 due to an increased insolation over the tropical WNP. A similar trend was found in other diagnostic ratios such as oxalic to succinic (C2/C4) and oxalic to glyoxylic acid (C2/ωC2), which further corroborate an enhanced photochemical aging over the WNP. In addition, relative abundances of oxalic acid in total diacids showed a marked increase as a function of ambient temperature, supporting their photochemical production. Constantly low concentration ratios of adipic and phthalic acids relative to azelaic acid suggest a small contribution of anthropogenic sources and an importance of oceanic sources during the study period. Significant production of C2 through oxidation of biogenic volatile organic compounds emitted from the sea surface is also noteworthy, as inferred from the strong linear correlations among water-soluble organic carbon, methanesulphonic acid, and oxalic acid. Sea-to-air emission of unsaturated fatty acids also contributes to formation of diacids over the WNP.

Dual‐carbon‐isotope characterization of total organic carbon in wintertime carbonaceous aerosols from northern India

Large-scale emissions of carbonaceous aerosols (CA) from South Asia impact both regional climate and air quality, yet their sources are not well constrained. Here we use source-diagnostic stable an ...

Hygroscopic growth of particles nebulized from water-soluble extracts of PM2.5 aerosols over the Bay of Bengal: Influence of heterogeneity in air masses and formation pathways

Hygroscopic properties of water-soluble matter (WSM) extracted from fine-mode aerosols (PM2.5) in the marine atmospheric boundary layer of the Bay of Bengal (BoB) have been investigated during a cruise from 27th December 2008 to 30th January 2009. Hygroscopic growth factors were measured on particles generated from the WSM using an H-TDMA system with an initial dry size of 100 nm in the range of 5-95% relative humidity (RH). The measured hygroscopic growth of WSM at 90% RH, g(90%)WSM, were ranged from 1.11 to 1.74 (mean: 1.43 ± 0.19) over the northern BoB and 1.12 to 1.38 (mean: 1.25 ± 0.09) over the southern BoB. A key finding is that distinct hygroscopic growth factors are associated with the air masses from the Indo-Gangetic plains (IGP), which are clearly distinguishable from those associated with air masses from Southeast Asia (SEA). We found higher (lower) g(90%)WSM over the northern (southern) BoB, which were associated with an IGP (SEA) air masses, probably due the formation of high hygroscopic salts such as (NH4)2SO4. On the other hand, biomass burning influenced SEA air masses confer the low hygroscopic salts such as K2SO4, MgSO4, and organic salts over the southern BoB. Interestingly, mass fractions of water-soluble organic matter (WSOM) showed negative and positive correlations with g(90%)WSM over the northern and southern BoB, respectively, suggesting that the mixing state of organic and inorganic fractions could play a major role on the g(90%)WSM over the BoB. Further, WSOM/SO4(2-) mass ratios suggest that SO4(2-) dominates the g(90%)WSM over the northern BoB whereas WSOM fractions were important over the southern BoB. The present study also suggests that aging process could significantly alter the hygroscopic growth of aerosol particles over the BoB, especially over the southern BoB.

Seasonal and longitudinal distributions of atmospheric water‐soluble dicarboxylic acids, oxocarboxylic acids, and α‐dicarbonyls over the North Pacific

In order to assess the seasonal variability of atmospheric abundances of dicarboxylic acids, oxocarboxylic acids, and α-dicarbonyls over the North Pacific and Sea of Japan, aerosol samples were collected along the longitudinal transacts during six cruises between Canada and Japan. The back trajectory analyses indicate that aerosol samples collected in winter and spring are influenced by the East Asian outflow, whereas summer and fall samples are associated with the pristine maritime air masses. Molecular distributions of water-soluble organics in winter and spring samples show the predominance of oxalic acid (C2) followed by succinic (C4) and malonic acids (C3). In contrast, summer and fall marine aerosols are characterized by the predominance of C3 over C4. Concentrations of dicarboxylic acids were higher over the Sea of Japan than the North Pacific. With a lack of continental outflow, higher concentrations during early summer are ascribed to atmospheric oxidation of organic precursors associated with high biological activity in the North Pacific. This interpretation is further supported by the high abundances of azelaic acid, which is a photochemical oxidation product of biogenic unsaturated fatty acids, over the Bering Sea in early summer when surface waters are characterized by high biological productivity. We found higher ratios of oxalic acid to pyruvic and glyoxylic acids (C2/Pyr and C2/ωC2) and glyoxal and methylglyoxal (C2/Gly and C2/MeGly) in summer and fall than in winter and spring, suggesting a production of C2 from the aqueous-phase oxidation of oceanic isoprene. In this study, dicarboxylic acids account for 0.7–38% of water-soluble organic carbon.

Hydroxy fatty acids in snow pit samples from Mount Tateyama in central Japan: Implications for atmospheric transport of microorganisms and plant waxes associated with Asian dust

We report here the source apportionment of atmospheric soil microorganisms and higher plant metabolites based on chemical markers (hydroxy fatty acids: FAs) in the snowpack samples collected from Mt. Tateyama in central Japan during spring 2009 (N = 6) and 2011 (N = 7). A homologues series of β-hydroxy FAs (C9 − C20), constituents of Gram-negative bacteria (GNB), in snowpacks clearly suggest a long-range atmospheric transport of dust-associated bacteria followed by scavenging by snowflakes. Similarly, higher atmospheric abundances of α-(C16 − C32) and ω-(C9 − C30)-hydroxy FAs in the snow layers containing Asian dust revealed contributions from soil microbes and higher plant epicuticular waxes. Moreover, co-variation between the concentrations of hydroxy FAs and water-soluble Ca2+ (dust tracer), together with calculated air mass backward-trajectories, demonstrated their source regions such as the Taklamakan Desert, Gobi Desert and Loess Plateau. A close match of molecular distributions of hydroxy FAs (with the predominance of ω- and β-isomers) is noteworthy between snowpack (present study) and springtime aerosols from Chichijima Island in the western North Pacific (WNP). This observation suggests a “below-cloud scavenging” of transported dust particles and associated soil microbes in the East Asian outflow by snowflakes. These distributions are, however, contrary to those observed in the fresh snow samples from Sapporo, northern Japan (predominance of α-hydroxy FAs), which could be explained by “in-cloud” microbial oxidation processes. This comparison, therefore, provides additional insights regarding the aeolian transport of soil microbes in the East Asian outflow to the WNP, which has not been available.

Secondary Organic Aerosol Formation over Coastal Ocean: Inferences from Atmospheric Water-Soluble Low Molecular Weight Organic Compounds

A lack of consensus on the distributions and formation pathways of secondary organic aerosols (SOA) over oceanic regions downwind of pollution sources limits our ability to assess their climate impact globally. As a case study, we report here on water-soluble SOA components such as dicarboxylic acids, oxocarboxylic acids, and α-dicarbonyls in the continental outflows from the Indo-Gangetic Plain (IGP) and Southeast Asia (SEA) to the Bay of Bengal. Oxalic acid (C2) is the dominant species followed by succinic (C4) and glyoxylic acids (ωC2) in the outflow. Nonsea-salt SO42- also dominates (∼70%) total water-soluble inorganic constituents and correlates well with aerosol liquid water content (LWC) and C2, indicating their production through aqueous phase photochemical reactions. Furthermore, mass ratios of dicarboxylic acids (C2/C4, C2/ωC2), and their relative abundances in water-soluble organic carbon and total organic carbon are quite similar between the two continental (IGP and SEA) outflows, indicating the formation of SOA through aqueous phase photochemical reactions in LWC-enriched aerosols, largely controlled by anthropogenic SO42-.

Impact of biomass burning on soil microorganisms and plant metabolites: A view from molecular distributions of atmospheric hydroxy fatty acids over Mount Tai

Biomass burning events (BBEs) in the North China Plain is one of the principal sources of air-borne pollutants in China and also for the neighboring countries. To examine the impact of BBEs on soil bacteria and other higher plant metabolites, their tracer compounds, hydroxy fatty acids (FAs), were measured in the bulk particulate matter (TSP) over Mt. Tai during the period of wheat residue burning in June 2006. Higher inputs of epicuticular waxes and soil microorganisms during high BBEs (H; 6 − 14 and 27 June) relative to low BBEs (L; 15 − 26 and 28 June), were characterized by increased concentrations of homologous series of α-(C9 − C32), β-(C9 − C32) and ω-(C12 − C28) hydroxy FAs in TSP samples. However, their relative abundances were not significantly different between H-BBEs and L-BBEs, suggesting their common source/transport pathways. We also found higher concentrations of trehalose and mannitol (tracers of soil microbes), and levoglucosan (tracer of biomass combustion) during H-BBEs than L-BBEs. These results are consistent with hydroxy FAs, suggesting that they are associated with biomass combustion processes of agricultural wastes as well as re-suspension of mineral dust and plant pathogens. In addition, enhanced concentrations of endotoxin and mass loading of Gram-negative bacteria during H-BBEs (117 endotoxin units (EU) m-3 and 390 ng m-3, respectively) were noteworthy as compared to those in L-BBEs (22.5 EU m-3 and 75 ng m-3, respectively). Back-trajectory analysis and fire spots together with temporal variations of hydroxy FAs revealed an impact of biomass burning on emissions and atmospheric transport of bacteria and plant metabolites.

Anthropogenic fine aerosols dominate the wintertime regime over the northern Indian Ocean

Abstract This study presents and evaluates the most comprehensive set to date of chemical, physical and optical properties of aerosols in the outflow from South Asia covering a full winter (Nov. 2014 – March 2015), here intercepted at the Indian Ocean receptor site of the Maldives Climate Observatory in Hanimaadhoo (MCOH). Cluster analysis of air-mass back trajectories for MCOH, combined with AOD and meteorological data, demonstrate that the wintertime northern Indian Ocean is strongly influenced by aerosols transported from source regions with three major wind regimes, originating from the Indo-Gangetic Plain (IGP), the Bay of Bengal (BoB) and the Arabian Sea (AS). As much as 97 ± 3% of elemental carbon (EC) in the PM10 was also found in the fine mode (PM2.5). Other mainly anthropogenic constituents such as organic carbon (OC), non-sea-salt (nss) -K+, nss-SO42− and NH4+ were also predominantly in the fine mode (70–95%), particularly in the air masses from IGP. The combination at this large-footprint receptor observatory of consistently low OC/EC ratio (2.0 ± 0.5), strong linear relationships between EC and OC as well as between nss-K+ and both OC and EC, suggest a predominance of primary sources, with a large biomass burning contribution. The particle number-size distributions for the air masses from IGP and BoB exhibited clear bimodal shapes within the fine fraction with distinct accumulation (0.1 μm < d < 1 μm) and Aitken (0.025 μm < d < 0.10 μm) modes. This study also supports that IGP is a key source region for the wider South Asia and nearby oceans, as defined by the criteria that anthropogenic AODs exceed 0.3 and absorption AOD > 0.03. Taken together, the aerosol pollution over the northern Indian Ocean in the dry season is dominated by a well-mixed long-range transported regime of the fine-mode aerosols largely from primary combustion origin.