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Environmental context The export of various man-made pollutants from northern India has a large impact on aerosol formation processes, their transformations and regional environmental chemistry over tropical peninsular India. The quantitative source apportionment of organic aerosols performed in this study provides a better understanding of their sources and implications for climate and air-quality management policies in South Asia. Abstract This study highlights seasonal characteristics, sources, daytime (sea-breeze) and night-time (land-breeze) variations of carbonaceous and water-soluble ionic components in PM10 (

Levoglucosan has been widely used to quantitatively assess biomass burning's contribution to ambient aerosols, but previous such assessments have not accounted for levoglucosan's degradation in the atmosphere. We develop the first global simulation of atmospheric levoglucosan, explicitly accounting for its chemical degradation, to evaluate the impacts on levoglucosan's use in quantitative aerosol source apportionment. Levoglucosan is emitted into the atmosphere from the burning of plant matter in open fires (1.7 Tg yr-1) and as biofuels (2.1 Tg yr-1). Sinks of atmospheric levoglucosan include aqueous-phase oxidation (2.9 Tg yr-1), heterogeneous oxidation (0.16 Tg yr-1), gas-phase oxidation (1.4 × 10-4 Tg yr-1), and dry and wet deposition (0.27 and 0.43 Tg yr -1). The global atmospheric burden of levoglucosan is 19 Gg with a lifetime of 1.8 days. Observations show a sharp decline in levoglucosan's concentrations and its relative abundance to organic carbon aerosol (OC) and particulate K+ from near-source to remote sites. We show that such features can only be reproduced when levoglucosan's chemical degradation is included in the model. Using model results, we develop statistical parametrizations to account for the atmospheric degradation in levoglucosan measurements, improving their use for quantitative aerosol source apportionment.

To better understand the molecular distributions, seasonality and source indices of organic aerosols in central Alaska, we measured homologous series of n-alkanes (C19-C35), fatty acids (C12-C32) and n-alcohols (C8-C30) in total suspended aerosols collected during June 2008 to 2009 using a gas chromatography/mass spectrometry (GC-MS). The whole sampling period was divided as warm (early May to late September; summer) and cold (late September to early May; winter) periods. Molecular distribution was characterized in both periods by the predominance of C25 for n-alkanes and C24:0 for fatty acids. However, we noticed a difference in molecular distribution of n-alcohols between warm and cold periods, which was characterized by the predominance of C22 in warm season while C26 in cold period. Except for fatty acids, n-alkanes and n-alcohols showed higher concentrations in warm period than in cold period. We found significantly higher ratios of C18:1/C18:0 in warm period than those in cold period, suggesting the fresh biogenic and aged anthropogenic aerosols in warm and cold periods, respectively. This inference was consistent with significantly higher ratios of WSOC/OC, a proxy for photochemical aging, in cold period. Based on the carbon preference index (CPI), average chain length (ACL), low-to-high molecular weight (LMW/HMW) ratios, wax n-alkanes (%WNA) and estimated fossil fuel concentrations, we demonstrate that higher plant waxes, biomass burning from wildfires are two important sources in warm period while combustion derived anthropogenic emissions are major sources in cold period in central Alaska. This finding was further supported by higher ratios of nss-K+/elemental carbon (EC) and methanesulfonate (MSA−)/EC ratios in warm period.

The present study reports on long-term trends of carbonaceous aerosols in total suspended particulate (TSP) samples collected at Chichijima in the western North Pacific during 2001–2012. Seasonal variations of elemental carbon (EC), organic carbon (OC), and water-soluble organic carbon (WSOC) concentrations showed maxima in winter to spring and minima in summer. These seasonal differences in the concentrations of carbonaceous aerosols were associated with the outflows of polluted air masses from East Asia, which are clearly distinguishable from pristine air masses from the central Pacific. The higher concentrations of carbonaceous aerosols during winter to spring are associated with long-range atmospheric transport of East Asian continental polluted air masses, whereas lower concentrations may be due to pristine air masses from the central Pacific in summer. The annual trends of OC ∕ EC (+0.46 % yr−1), WSOC (+0.18 % yr−1) and WSOC ∕ OC (+0.08 % yr−1) showed significant (p −1) in nss-K+ ∕ EC ratios, demonstrating that concentrations of biomass-burning-derived carbonaceous aerosols have increased, while those of primary fossil-fuel-derived aerosols have decreased over the western North Pacific. Further, secondary biogenic emissions are also important over the western North Pacific as inferred from a significant increase (+0.14 % yr−1) in the concentrations of methanesulfonate (MSA−, a tracer for biogenic sources). This point was further supported by a moderate correlation (r = 0.40) between WSOC and MSA−. We also found a significant increase in OC ∕ TC (total carbon) and WSOC ∕ TC ratios, further suggesting that photochemical formation of WSOC and its contributions to SOAs have increased over the western North Pacific during 2001–2012 via long-range atmospheric transport from East Asia.

Oxalic acid (OxA) is an end product in the oxidation of many organic compounds, and therefore is ubiquitous in the atmosphere and is often the most abundant organic species in ambient aerosols. To better understand the hygroscopic properties of OxA under sub- and supersaturated conditions in the atmosphere, we investigated the hygroscopic growth and cloud condensation nuclei (CCN) activation ability of pure OxA and its salts using a hygroscopic tandem differential mobility analyzer (HTDMA) and cloud condensation nuclei counter (CCNC), respectively. OxA particles absorb water under >45% RH, suggesting that the initial phase state might be an amorphous solid. The measured hygroscopic growth factor (HGF) of OxA at 90% RH was 1.47. We found that the HGF of ammonium oxalate (NH4-Ox) was larger than that of OxA, whereas HGFs of sodium, calcium, and magnesium oxalates (Na-Ox, Ca-Ox, and Mg-Ox) were smaller than that of OxA particles. Potassium oxalate (K-Ox) behaved like a typical water-soluble inorganic salt, exhibiting deliquescence and efflorescence transitions at around 85% and 50% RH, respectively. Na-Ox exhibited strong activation capabilities among all the investigated salts, followed by NH4-Ox and K-Ox as inferred from the activation ratios (CCN/CN) against supersaturations (SS). On the other hand, Ca-Ox showed moderate activation ability and Mg-Ox showed poor CCN activation ability. We also observed significantly higher κCCN values compared to κHTDMA for pure OxA and its salts (NH4-Ox and Na-Ox), suggesting that the condensation of OxA into the aqueous phase occurs during water uptake. These findings improve the fundamental understanding of hygroscopic behaviors and phase states of oxalic acid and its salts under sub- and supersaturated conditions in the atmosphere and impacts of hygroscopicity on the direct and indirect effects of aerosol particles.

Low molecular weight monocarboxylic acids (LMW monoacids) are most abundant volatile organic compounds (VOCs) in the atmosphere and often act as important contributors to the acidity of precipitation in addition to inorganic acids. However, there is a large uncertainty in the sources and secondary formations of these acids in the atmosphere. This study reports homologous series of LMW monoacids, including normal (C1-C10), branched chain (iC4-iC6), aromatic (benzoic acid) and hydroxyacids (lactic and glycolic acids) in the fine-mode (PM2.5) aerosols collected over the Bay of Bengal (BoB) during a winter cruise (December 2008 to January 2009). The samples were associated with two distinct continental air masses arriving from the Indo-Gangetic Plain (IGP-outflow) and Southeast Asia (SEA-outflow). The molecular distributions of organic acids are characterized by the dominance of formic acid (C1) followed by acetic acid (C2) and nonanoic acid (C9) in the IGP-outflow, whereas dominance of C1 or C9 was observed in the SEA-outflow followed by C2. Formic-to-acetic acid (C1/C2) ratios were higher than unity (mean: 1.3 ± 0.3) in the IGP-outflow, whereas they were less than unity (0.9 ± 0.5) in the SEA-outflow. These results suggest that secondary formation of organic acids is largely important in the IGP-outflow whereas primary emission is a major source of organic acids in the SEA-outflow. Based on the correlation coefficient matrix analysis and C1/C2 and C4/C3 ratios, we consider that the sources of C1 are probably associated with the secondary formation via the oxidation of biogenic VOCs, while C2 has both primary and secondary formations associated with anthropogenic sources in the IGP-outflow. On the other hand, C1 and C2 have similar sources (both primary and secondary) originated from biomass burning and bacterial activities via long-range atmospheric transport in the SEA-outflow, as inferred from the MODIS fire spot data, significant concentrations of isovaleric acid (iC5), and a significant correlation (r = 0.67) between nss-K+ and total LMW monoacids.

To better understand the impact of agricultural waste burning on the air quality of free troposphere over the North China Plain (NCP), we collected total suspended particles (TSP) at the summit of Mt. Tai, located in the NCP using a high volume air sampler during 29 May to 28 June 2006, when the field burning of agricultural residue was intense. Temporal variations of all measured species showed that their concentration increases from late May to mid June (major BB period), peaking during 12–14 June, and then significantly decreased towards late June (minor BB period). We noticed that a significant reduction in the concentrations of carbonaceous aerosols during the period of 8–11 June, when the wind direction shifted from southerly to northerly. We found that concentrations of carbonaceous aerosols and some major ions showed several times higher during major BB period than those of minor BB period. We also found that nighttime concentrations are higher than daytime during major BB period, suggesting that a long-range atmospheric transport of biomass burning plumes in the free troposphere, which arrived at the summit of Mt. Tai. In contrast, daytime concentrations are higher than nighttime during minor BB period. We found higher concentrations of secondary organic carbon (SOC) during major BB period, suggesting that formation of secondary organic aerosols through aqueous phase chemistry under high NOx conditions during a long-range atmospheric transport. nss-K+ showed about four times higher concentrations during major BB than those of minor BB. Concentrations of nss-Ca2+ are higher in nighttime during major BB period, implying that a significant long-range atmospheric transport of mineral dust over the sampling site. These results are further supported by the positive matrix factorization (PMF) analysis, which showed that biomass burning was a major source for the carbonaceous aerosols followed by mineral dust sources over the summit of Mt. Tai.

The sources and formation processes of dicarboxylic acids are still under investigation. Size-segregated aerosol (9-sizes) samples collected in the urban site (Raipur: 21.2°N and 82.3°E) in eastern central India during summer of 2013 were measured for water-soluble diacids (C2-C12), ω-oxoacids (ωC2-ωC9), α-dicarbonyls (C2-C3), and inorganic ions to better understand their sources and formation processes. Diacids showed the predominance of oxalic acid (C2), whereas ω-oxoacids showed the predominance of glyoxylic acid (ωC2), and glyoxal (Gly) was a major α-dicarbonyl in all the sizes. Diacids, ω-oxoacids, and α-dicarbonyls as well as SO42-, NO3-, and NH4+ were enriched in coarse mode, where Ca2+ peaked, suggesting that they are preferentially produced in coarse mode via adsorption as well as heterogeneous and aqueous-phase oxidation reaction of precursors on the surface of water-soluble mineral dust particles having more alkaline species. Strong correlations of diacids and related compounds with NO3- (r = 0.66-0.91) and aerosol water content (AWC) (r = 0.63-0.93) further suggest an importance of heterogeneous and aqueous-phase production in coarse mode. We found strong correlations of C2/(C2-C12), C2/ωC2, and C2/Gly ratios with AWC in coarse mode (r = 0.83, 0.86, and 0.85, respectively), indicating that enhanced AWC is favorable for the production of C2 diacid through aqueous-phase oxidation of its higher homologous diacids, ωC2, and Gly. These results demonstrates unique reactivity of water-soluble mineral dust particles for the enhanced production of diacids and related compounds in aqueous-phase, having implications on the aerosol-cloud interaction, solubility, and hygroscopicity of a dominant fraction of water-soluble organic aerosol mass. [250]

To better understand the distributions and sources of low-molecular-weight (LMW) monocarboxylic acids (monoacids) in the forest atmosphere, we conducted simultaneous collection of gaseous and particulate samples at a deciduous broadleaf forest site in northern Japan. LMW normal (C1–C10), branched (iC4–iC6), hydroxyl (glycolic and lactic) and aromatic (benzoic) monoacids were detected in the gas and particle phases. The dominant LMW monoacids in gas phase were formic (mean: 953 ng m−3) and acetic (528 ng m−3) acids followed by propionic (37 ng m−3) or isopentanoic (42 ng m−3) acid. In the particle phase, isopentanoic (159 ng m−3) was dominant, followed by acetic (104 ng m−3) and formic (71 ng m−3) or lactic (65 ng m−3) acids. Concentrations of LMW monoacids did not show correlations with anthropogenic tracers such as nss-SO42- and NO3-, indicating that anthropogenic contribution is not important. Concentrations of C1–C6 monoacids in the gas phase showed positive correlations (r2=0.21–0.91) with isobutyric acid (iC4), which may be produced by microbial activity in soil. The forest soil may be a source of gaseous C1–C6 monoacids in the forest atmosphere. Acetic acid in the particle phase positively correlated with nonanoic acid (C9) (r2=0.63), suggesting that formation of acetic and nonanoic acids is associated with the oxidation of biogenic unsaturated fatty acids in the aerosol phase, in addition to photochemical oxidation of biogenic volatile organic compounds. The particle-phase fractions (Fp) of formic and acetic acids showed negative correlation with ambient temperature (C1: r2=0.49, C2: r2=0.60) but showed positive correlation with relative humidity (C1: r2=0.30, C2: r2=0.55) in daytime, suggesting that these meteorological parameters are important for the gas and particle portioning of monoacids in the forest atmosphere.

Trace elements in atmospheric particulate matter play a significant role in air quality, health and biogeochemical cycles. The present study reports on geochemical characteristics of size-resolved trace elements in PM10 aerosols collected under different air masses over a coastal urban location in peninsular India. A contrast in elemental distribution was observed for the particle size above 7.0 μm and below 1.1 μm under the influence of northeasterly air masses as characterized by Al > Fe > Zn and Fe > Al > Zn, respectively. The concentrations of the crustal elements (Al, Fe, Ti, P, Ba, Co) were high and illustrated by a unimodal size distribution with a peak in coarse mode (>2.0 μm) during northwesterly air masses. On the other hand, combustion-derived metals (Cu, Zn, Cd, Sb, and Pb) were maximized under northeasterly air masses, characterized by unimodal size distribution with a peak in fine mode ( 7.0 μm, whereas combustion related emissions such as vehicular and traffic sources are predominant for particles

The present study reports on homologous series of dicarboxylic acids (C2–C12), ω-oxocarboxylic acids (ωC2-ωC9), pyruvic acid, and α-dicarbonyls (glyoxal and methylglyoxal) in near source biomass burning influenced (PM2.5) aerosols collected at Doi Ang Khang (DAK), Thailand, during an intense forest fire (March 1 to April 13, 2015) occurred in the vicinity of the sampling site over Southeast Asia (SEA). The molecular distributions of diacids were characterized by a predominance of oxalic (C2) acid followed by succinic (C4) and malonic (C3) acids. The abundance of maleic acid (M) was almost three times higher than that of fumaric acid (F). The observed lower values of C3/C4 (mean: 0.54), C2/C4 (4.45) and F/M ratios (0.33), indicating the limited photochemistry of organic aerosols at DAK and these ratios were comparable to those reported for biomass burning influenced aerosols (C3/C4: 0.51–0.66, C2/C4: ~4.0, and F/M: 0.2–0.35). Further, strong positive correlations (R2 > 0.80) were observed for diacids and related organic compounds with levoglucosan (LG, a unique tracer for biomass burning emission), suggesting that enhanced biomass burning associated with forest fires are major sources of diacids at DAK. This inference was further supported by the air mass back trajectories and MODIS (moderate resolution imaging spectroradiometer) derived fire spots over the study region. The formation pathway of C2 was largely linked to biomass burning derived precursor compounds as inferred from the correlation coefficient matrix analysis. We also found that C3/C4 and C2/C4 ratios were negatively correlated with LG, although they have strongly correlated each other. This inference suggests that biomass burning affect not only the concentration levels of organic aerosols but also their formation processes in the atmosphere. The present study demonstrates that intensive biomass burning during forest fires in SEA largely control the levels of carbonaceous aerosols and seriously affect the climate in the outflow regions.

This paper presents the long – term observational studies on aerosol optical properties measured at Sri Krishnadevaraya University (SKU) campus (14° 62′ N, 77° 65′ E, 331m asl), Anantapur, in southern India during 2007–2013 using a ground based Multi – Wavelength solar Radiometer (MWR). Seasonal mean values of Aerosol Optical Depth (AOD) for the whole study period were observed to be 0.34 ± 0.03, 0.45 ± 0.04, 0.24 ± 0.04, and 0.31 ± 0.03 during the winter, summer, monsoon and post – monsoon, respectively. Annual mean values of Angstrom exponent (α) (turbidity coefficient (β)) varied from 0.68 ± 0.25 (0.18 ± 0.03) to 1.1 ± 0.12 (0.32 ± 0.12) during 2007–2013. However, high values of α in the range of 0.8–1.1 were observed during the winter, while low values in the range 0.3–0.7 were noticed during the monsoon. The frequency distribution of AODs during winter in the range of 0.2–0.4 is about 78%, while summer these were shifted from 0.3 to 0.6 around ∼70%, which indicated the dust strongly affects this region. The accumulated frequencies of Angstrom exponent (α) less than 1.0 and greater than 1.0 were about 69% and 31%, respectively, occurred in the summer months indicates the dominance of coarse particles. Columnar size distributions, retrieved from the spectral optical depths, in general, show a bimodal log normal distribution in the optically active size range. The seasonal mean effective radius (Reff) was found to be high in monsoon (∼0.61 μm) and low in winter (∼0.38 μm). The highest mean mass loading values are lies between 475 ± 36 to 769 ± 49 mg m−2 during the summer, whereas the lowest value in the range 204 ± 19 to 278 ± 23 mg m−2 during the monsoon. The difference between α and curvature effect have been studied as a function of AOD on seasonal basis. Finally, to understand the contribution of long range transported aerosols, we have investigated this analysis making use of back trajectories obtained from the HYSPLIT model for different seasons.

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 100 nm 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-SO 4 2 − 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 (SO 2 , NO x , 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.

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.

The present study reports on carbonaceous, water-soluble species and trace metals in PM10 aerosols collected at Bharati station during the austral summer (December 17, 2016 to February 2, 2017) over East Antarctica. Organic matter was the predominant among all measured species followed by nss-Ca2+ and nss-SO42-. Two distinct air masses that arrive from high-altitude-troposphere (HTAs) and low-altitude-troposphere (LTAs) regions which clearly showed significant differences in concentrations of measured chemical and metal species as inferred from the air mass backward trajectory analysis and mixed layer height variations. Based on significant positive correlations among chemical species, aerosol liquid water content (ALWC), and PM10 mass, we confirmed that the aqueous-phase formation of secondary aerosols followed by atmospheric processing are possible sources in HTAs while primary emissions associated with soil/dust from ice-free regions and in-situ emissions are major sources in LTAs. This result further supported by significant (p < 0.05) higher concentrations of specific trace metals (for example, Fe, Ti, and Pb) in LTAs. Moreover, higher cation-to-anion (Σ+/Σ−) ratios suggesting the alkaline nature of aerosols at Bharati. The present study demonstrates that ice-free regions over East Antarctica may act as major sources of particulate matter, thus significant implications toward climate change over the global environment.

To better understand the distributions of low molecular weight (LMW) monocarboxylic acids (monoacids) and their sources in the forest, we conducted simultaneous collection of gaseous and particulate samples at a deciduous broadleaf forest site in northern Japan. LMW normal (C1–C10), branched chain (iC4–iC6), hydroxyl (lactic and glycolic) and aromatic (benzoic) monoacids were detected in the gas and particle phases. The dominant LMW monoacids in gas phase were formic (mean: 953 ng m−3) and acetic (528 ng m−3) acids. In particle phase, we found that isopentanoic (159 ng m−3) and acetic (104 ng m−3) acids are dominant species together with lactic acid. Concentrations of LMW monoacids did not correlate with SO42− that was used as an anthropogenic tracer, indicating that LMW monoacids are derived from the local sources within the forest ecosystem. Concentrations of C1–C6 monoacids in gas phase showed positive correlations (r2 = 0.21–0.91) with isobutyric acid (iC4), which is produced by soil microorganisms. These monoacids are closely linked to the microbial process in soils. Isopentanoic acid in particle phase showed a positive correlation with lactic acid (r2 = 0.98), which is produced by soil microbes. The observed high abundances of isopentanoic acid are involved with soil microbial activity. We found that acetic acid in particle phase positively correlated with nonanoic acid (C9) (r2 = 0.63), suggesting that formation of acetic and nonanoic acids are associated with the oxidation of unsaturated fatty acids. We found that forest floor with soil microbes contributes to the emissions of gaseous and particulate LMW monoacids. Our results suggest that forest ecosystem is an important source of organic gases and aerosols in the atmosphere.

In this study, we investigated hygroscopic properties of water-soluble matter (WSM) in marine aerosols over the East China Sea, which were collected during a Natural Science Foundation of China (NSFC) sharing cruise in 2014. Hygroscopic growth factors (g) of WSM were measured by a hygroscopicity tandem differential mobility analyzer (H-TDMA) with an initial dry particle mobility diameter of 100nm. The observed g at 90% relative humidity (RH), g(90%)WSM, defined as the ratio of the particle diameter at 90% RH to that at RH

To better understand the impact of East Asian pollutants on the molecular composition of marine organic aerosols, we conducted long-term (2001–2013) observations of water-soluble dicarboxylic acids and related compounds in total suspended particulate samples collected at Chichijima Island in the western North Pacific (WNP). Seasonal variations of all the diacids and related compounds showed maxima in winter and spring and minima in summer, except for azelaic acid (C9), which maximized in summer to autumn. The overall annual concentrations of the total diacids, ω-oxoacids and α-dicarbonyls showed an increase during 2001–2013. We found a significant (p 0.05) acids, and methylglyoxal (MeGly). In contrast, phthalic acid (p < 0.05) and glyoxal (Gly) showed a decrease in their trends. We also found a significant decrease in the trend of the Gly/MeGly mass ratios. These results demonstrate that the enhanced concentrations of diacids over the WNP are majorly attributed to the aqueous-phase photooxidation of biogenic volatile organic compounds from East Asia followed by long-range atmospheric transport. Further, positive matrix factorization analysis showed a biogenic photochemical contribution (42%) was the dominant source of oxalic acid in the WNP.