The Henry's law constants of glyoxal, glycolic acid and glyoxylic acid in pure water were determined over the range of 278 and 308 K for the first time by a bubble column technique. These compounds were chosen because of their perceived involvement in the formation of secondary organic aerosol through in‐cloud processing pathways. The experimentally determined Henry's law constants are: glyoxal, KH = 4.19 × 105 × exp[(62.2 × 103/R) × (1/T − 1/298)]; glycolic acid, KH = 2.83 × 104 × exp[(33.5 × 103/R) × (1/T − 1/298)]; and glyoxylic acid, KH = 1.09 × 104 × exp[(40.0 × 103/R) × (1/T − 1/298)]. The Henry's law constants of glyoxal in the presence of sodium chloride and sodium sulfate were also determined at 298 K. While the glyoxal KH is enhanced by less than three times in the presence of chloride in the range of 0.05–4.0 M ionic strength, the presence of sulfate at 0.03 M ionic strength increases the glyoxal KH by 50 times.

PM 2.5 filter sampling was conducted on a daily basis at the HKUST Air Quality Research Supersite (AQRS) for one year from March 2011 to February 2012. Approximately one fifth of the filter samples were subjected to full chemical analysis including major ions, elements, organic carbon (OC), elemental carbon (EC), and non-polar organic compounds (NPOCs). The major ions (sulfate, nitrate, and ammonium) were compared with those measured online by a MARGA system and the two sets of data were found in agreement within 25% or better. The major PM 2.5 components (crustal materials, organic matter, soot, ammonium sulfate, ammonium nitrate, and non-crustal trace elements) accounted for 90% of the measured mass with sulfate being the most abundant (32.0%), followed by organic matter (23.5%) and ammonium (11.8%). The monthly variation patterns for different components suggested variable regional/super-regional sources, reflecting variation of transport contribution caused by shifts in synoptic weather conditions.Receptor modeling analysis by Positive Matrix Factorization revealed that secondary sulfate formation process (annual average of 31%), biomass burning (23%), and secondary nitrate formation process (13%) were the three dominant contributing sources to the observed PM 2.5 at HKUST AQRS throughout the sampling year. The PM 2.5 mass concentrations of all the individual sampling days were within the recently-proposed AQOs standards by the Hong Kong government (35 µg/m 3 for annual average and 75 µg/m 3 for 24-hr average) while approx. 52% of the sampling days were recorded with PM 2.5 concentrations exceeding the WHO health 24-hr standards of 25 µg/m 3 . Major composition and source analysis showed that the increased mass concentrations on high PM days were mainly caused by air pollutant transport from the outside-Hong Kong regions. Results from this study indicate the importance of regional/super-regional strategies such as reduction in SO 2 , NO 2 (precursors for secondary inorganic aerosols) and restricting biomass burning for lowering PM 2.5 in Hong Kong.

Environmental context Nitroaromatic compounds constitute an important portion of brown carbon and thereby contribute to the light-absorbing properties of atmospheric aerosols. We report their abundance in Hong Kong over 3 years and show that they were mainly associated with aged biomass burning particles. Knowledge of the abundance and sources of nitroaromatic compounds could assist in evaluating their contribution to brown carbon and in apportioning secondary organic aerosols from biomass burning sources. Abstract Biomass burning is a major source of atmospheric aerosols on both global and regional scales. Among the large number of unidentified organic compounds related to biomass burning, nitroaromatic compounds (NACs) have drawn attention because of their UV light-absorbing ability. In this study, an analytical method based on liquid chromatography–mass spectrometry was used to quantify a group of NACs (nitrophenol, methylnitrophenols, dimethylnitrophenol, nitrocatechol and methylnitrocatechols) in aerosol samples. The nitrocatechol–metal complex interference, sample matrix effects, sample stability, precision and reproducibility were investigated. The method detection limits ranged from 0.10 to 0.23ngmL–1 and the recoveries for the target NACs were in the range of 96–102%. The method was applied to a total of 184 ambient PM2.5 samples (particulate matter of 2.5µm or less in aerodynamic diameter) collected at an urban site in Hong Kong over 3 years (2010–2012). The NACs quantified showed a distinct seasonal variation with higher concentrations in autumn and winter (3.6–21.0ngm–3), coinciding with more biomass burning activities coming from the regions west and north-east to Hong Kong, and lower levels during spring and summer (0.3–3.8ngm–3). The good correlations between NACs and levoglucosan (R=0.82), a known biomass burning tracer compound, support the common origin from biomass burning. Moderate to good correlations between NACs and nitrate suggest that they might be products of secondary formation processes involving the same precursor gases (e.g. NOx). Additional lines of circumstantial evidence were also found and presented in the paper to support secondary formation derived from biomass burning as the main contributing source of NACs.

Monoterpenes, a major class of biogenic volatile organic compounds, are known to produce oxidation products that further react with sulfate to form organosulfates. The accurate quantification of monoterpene-derived organosulfates (OSs) is necessary for quantifying this controllable aerosol source; however, it has been hampered by a lack of authentic standards. Here we report a unified synthesis strategy starting from the respective monoterpene through Upjohn dihydroxylation or Sharpless asymmetric dihydroxylation followed by monosulfation with the sulfur trioxide-pyridine complex. We demonstrate the successful synthesis of four monoterpene-derived OS compounds, including α-pinene OS, β-pinene OS, limonene OS, and limonaketone OS. Quantification of OSs is commonly achieved using liquid chromatography-mass spectrometry (LC-MS) by either monitoring the [M-H] ion or through multiple reaction monitoring (MRM) of mass transitions between the [M-H] and m/z 97 ions. Comparison between the synthesized standards and previously adopted quantification surrogates reveals that camphor-10-sulfonic acid is a better quantification surrogate using [M-H] as the quantification ion, while the highly compound-specific nature of MRM quantification makes it difficult to choose a suitable surrogate. Both could be rationalized in accordance to their respective MS quantification mechanisms. The in-house availability of the authentic standards enables us to discover that β-pinene OS, due to the sulfate group at the primary carbon, partially degrades to a dehydrogenated OS compound during LC/MS analysis and a hydroperoxy OS over a prolonged storage period (>5 month) and forms a regioisomer through intermolecular isomerization. Limonene OS was positively identified for the first time in ambient samples and found to be more abundant than α-/β-pinene OS in the Pearl River Delta, China. This work highlights the critical importance of having authentic standards in advancing our understanding of the interactions between biogenic VOC emissions and anthropogenic sulfur pollution.

Environmental contextChina has been experiencing severe particulate pollution and frequent haze episodes in recent years. We compare the molecular composition of urban organic aerosols on clear and hazy days in Beijing by high-resolution mass spectrometry. The comparative study shows that oxidation, sulfation and nitrification processes actively involve precursors of anthropogenic origin in the Beijing polluted urban atmosphere. AbstractHaze has frequently affected many cities and threatened human health in China. Detailed knowledge of the chemical composition of secondary organic aerosol provides fundamental information in the study of the formation mechanism of haze and its adverse effects on human health. In our work, dichloromethane and water extracts of ambient aerosols collected on hazy and clear days in Beijing were characterised by negative-ion electrospray ionisation and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Formulae in four elemental compositional groups, namely CHOS, CHONS, CHO and CHON, were identified relying on the ultrahigh resolution and mass accuracy of FT-ICR MS. Significantly more compounds were detected and the peaks were much more intense in the hazy day samples, especially for the CHOS, CHONS and CHON formula groups. Organosulfates (OS) and nitrooxy-organosulfates (nitrooxy OS) were the major forms of CHOS and CHONS formulae respectively, and their numbers more than tripled on the hazy days. Under the severely polluted conditions in Beijing, the compositional distribution of the OS and nitrooxy OS exhibited distinct features such as intense peaks of low double-bond equivalent (DBE) (DBE=0, 1 for OS and DBE=1, 2 for nitrooxy OS) and low degree oxidation, of medium DBE (DBE=2, 3 for OS and DBE=3, 4 for nitrooxy OS), and of high DBE (DBE ≥ 4 for OS and DBE ≥ 5 for nitrooxy OS). The likely respective candidates for these could be aliphatic OS having a low degree of oxidation, biogenic OS and many aromatics and polycyclic aromatic hydrocarbon (PAH)-derived OS. The CHON formulae observed on hazy days were double those on clear days and had higher DBE values and larger O/N ratios. Slightly more CHO compounds were detected in the hazy-day samples and they had higher DBE values and more oxygen atoms. The comparative study suggests that oxidation, sulfation and nitrification processes actively involve precursors of anthropogenic origin in the Beijing polluted urban atmosphere.

The organic composition of airborne fine particulate matter (PM 2.5 , aerodynamic diameter less than 2.5 μm) at a molecular level has yet to be achieved, hindering a full understanding of the climatic impacts and health effects of PM 2.5 . Compounds containing aromatic rings are closely associated with optically active brown carbon and toxicologically important quinones. In this work, a group of ten aromatic organic acids including three phthalic acids, four phenolic acids, and three benzene-tricarboxylic acids (BTCAs) in PM 2.5 were studied for their abundance and potential sources through quantifying their ambient concentrations at four sites in the Pearl River Delta (PRD) region in Southern China, where biomass burning and anthropogenic emissions are both significant PM sources. Average concentrations of individual aromatic acids in a total of 240 PM 2.5 samples collected throughout 2012 were in the order of 0.1−20 ng/m 3 with p-and o-phthalic acid being the most abundant. Interspecies correlation analysis with known PM source tracers reveals different source origins for the ten aromatic acids. The four phenolic acids, all possessing partial lignin structures, are highly correlated with levoglucosan, indicating their association with biomass burning emissions. Specific lignin tracer ratios characteristic of different types of biomass fuels (i.e., cinnamyl-to vanillyl-phenol ratio) revealed the significant influence of crop burning emissions in the PRD region. The three BTCAs have moderate correlation with sulfate but no correlation with levoglucosan, suggesting a strong association with secondary formation origins while negating a strong link with biomass burning. The three phthalic acids are moderately correlated with sulfate, levoglucosan, and a number of polycyclic aromatic hydrocarbons (PAHs), indicating multiple significant sources. This study provides a valuable data set toward establishing quantitative links between molecular composition of organic matter and the optical and toxicological properties of PM 2.5 as well as assisting identification of tracers for PM 2.5 sources.

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Oxalic acid is one of the most abundant dicarboxylic acids in the atmosphere, receiving a great deal of attention due to its potential influence on cloud condensation nucleus activities. In this work, we report 10 months of hourly oxalate measurements in particulate matter of less than 2.5 μm in aerodynamic diameter (PM 2.5 ) by a Monitor for Aerosols and Gases in ambient Air at a suburban coastal site in Hong Kong from April 2012 to February 2013. A total of more than 6000 sets of oxalate and inorganic ion data were obtained. The mean (±SD) oxalate concentration was 0.34 (±0.18) μg m À3 , accounting for 2.8% of the total ion mass and 1.5% of the PM 2.5 mass. Seasonal variation showed higher concentrations in fall and winter (0.54 and 0.36 μg m À3 , respectively) and lower concentrations in spring and summer (~0.26 μg m À3 ). Different from the inorganic ions, a shallow dip in the oxalate concentration consistently occurred in the morning after sunrise (around 9:00 A.M.) throughout all seasons. Our analysis suggests that this was likely due to photolysis of oxalate-Fe (III) complex under sunlight. In summer, a small daytime peak was discernable for oxalate and nitrate. This characteristic, together with a more evident diurnal variation of O 3 , indicates comparatively more active photochemical oxidation in summer than other seasons. High correlations were observed between oxalate and non-sea-salt SO 4 2À (NSS) (R 2 = 0.63) and O x (O 3 + NO 2 ) (R 2 = 0.48), indicating significant commonality in their secondary formation. Positive matrix factorization analysis of oxalate and other real-time gas and particle-phase component data estimates that secondary formation processes, including secondary gas or aqueous oxidation processes (49%), oxidation processes of biomass burning emissions (37%), accounted for the majority of PM 2.5 oxalate. A backward trajectories cluster analysis found that higher oxalate/NSS ratios were associated with low pollution samples under the influence of marine air masses while the ratios were lower in high pollution samples that were typically associated with continental air masses passing through areas of high anthropogenic emissions. Isolating the "low pollution marine" aerosols across the entire data set indicates that oxalate production increased in the summer compared to other seasons, suggesting either more active marine emissions of oxalate precursors or stronger photochemical processes in the summer.