Polar Nitrated Aromatic Compounds in Urban Fine Particulate Matter: A Focus on Formation via an Aqueous-Phase Radical Mechanism.

Polar nitrated aromatic compounds (pNACs) are key ambient brown carbon chromophores; however, their formation mechanisms, especially in the aqueous phase, remain unclear. We developed an advanced technique for pNACs and measured 1764 compounds in atmospheric fine particulate matter sampled in urban Beijing, China. Molecular formulas were derived for 433 compounds, of which 17 were confirmed using reference standards. Potential novel species with up to four aromatic rings and a maximum of five functional groups were found. Higher concentrations were detected in the heating season, with a median of 82.6 ng m-3 for Σ17pNACs. Non-negative matrix factorization analysis indicated that primary emissions particularly coal combustion were dominant in the heating season. While in the non-heating season, aqueous-phase nitration could generate abundant pNACs with the carboxyl group, which was confirmed by their significant association with the aerosol liquid water content. Aqueous-phase formation of 3- and 5-nitrosalicylic acids instead of their isomer of 4-hydroxy-3-nitrobenzoic acid suggests the existence of an intermediate where the intramolecular hydrogen bond favors kinetics-controlled NO2• nitration. This study provides not only a promising technique for the pNAC measurement but also evidence for their atmospheric aqueous-phase formation, facilitating further evaluation of pNACs' climatic effects.

Major source categories of PM2.5 oxidative potential in wintertime Beijing and surroundings based on online dithiothreitol-based field measurements.

Fine particulate matter (PM2.5) causes millions of premature deaths each year worldwide. Oxidative potential (OP) has been proposed as a better metric for aerosol health effects than PM2.5 mass concentration alone. In this study, we report for the first time online measurements of PM2.5 OP in wintertime Beijing and surroundings based on a dithiothreitol (DTT) assay. These measurements were combined with co-located PM chemical composition measurements to identify the main source categories of aerosol OP. In addition, we highlight the influence of two distinct pollution events on aerosol OP (spring festival celebrations including fireworks and a severe regional dust storm). Source apportionment coupled with multilinear regression revealed that primary PM and oxygenated organic aerosol (OOA) were both important sources of OP, accounting for 41 ± 12 % and 39 ± 10 % of the OPvDTT (OP normalized by the sampled air volume), respectively. The small remainder was attributed to fireworks and dust, mainly resulting from the two distinct pollution events. During the 3.5-day spring festival period, OPvDTT spiked to 4.9 nmol min-1 m-3 with slightly more contribution from OOA (42 ± 11 %) and less from primary PM (31 ± 15 %). During the dust storm, hourly-averaged PM2.5 peaked at a very high value of 548 µg m-3 due to the dominant presence of dust-laden particles (88 % of total PM2.5). In contrast, only mildly elevated OPvDTT values (up to 1.5 nmol min-1 m-3) were observed during this dust event. This observation indicates that variations in OPvDTT cannot be fully explained using PM2.5 alone; one must also consider the chemical composition of PM2.5 when studying aerosol health effects. Our study highlights the need for continued pollution control strategies to reduce primary PM emissions, and more in-depth investigations into the source origins of OOA, to minimize the health risks associated with PM exposure in Beijing.

Organosulfur compounds in ambient fine particulate matter in an urban region: Findings of a nontargeted approach.

Organosulfur compounds (OSCs) are important components of fine particulate matter (PM2.5); however, little information is available on OSCs in urban regions due to their chemical complexity, especially for novel species such as aromatic sulfonates. To supplement the detection technique and systematically identify OSCs, in this study we developed a nontargeted approach based on gas chromatography and high-resolution mass spectrometry (GC-HRMS) to screen OSCs in PM2.5 of urban Beijing and provide field evidence for their source and formation mechanism. 76 OSCs were found through mass difference of sulfur isotopes and characteristic sulfur-containing fragments. 6 species were confirmed as aromatic sulfonates by authentic standards. 32 OSCs showed higher levels in the heating season, presumably because of the intensive emission, especially from coal combustion. While certain species, with 2-sulfobenzoic acid as the representative, were 2.6-times higher in the non-heating season than in the heating season. Such species were significantly correlated with ozone and aerosol liquid water content (r = 0.2-0.8, p < 0.05), implying an oxidation-involved aqueous-phase formation in the atmosphere. In addition, with an average proportion of ∼95 % of the total sulfobenzoic acids, the predominance of the 2-substitution product over its isomers of 3- or 4-sulfobenzoic acid suggests a more plausible mechanism of radical-initiated reaction of phthalic acid followed by sulfonation, with atmospheric reactivity indicated by ozone and temperature as the determining factor. This study provided not only a nontargeted approach for OSCs in ambient PM2.5, but also field evidence on their secondary formation proposed in previous simulation studies.