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Oxygenated organic molecules (OOMs) are the crucial intermediates linking volatile organic compounds (VOCs) to secondary organic aerosols (SOAs) in the atmosphere, but comprehensive understanding of the characteristics of OOMs and their formation from VOCs is still missing. Ambient observations of OOMs using recently developed mass spectrometry techniques are still limited, especially in polluted urban atmospheres where VOCs and oxidants are extremely variable and complex. Here, we investigate OOMs, measured by a nitrate-ion-based chemical ionization mass spectrometer at Nanjing in eastern China, through performing positive matrix factorization on binned mass spectra (binPMF). The binPMF analysis reveals three factors about anthropogenic VOC (AVOC) daytime chemistry, three isoprene-related factors, three factors about biogenic VOC (BVOC) nighttime chemistry, and three factors about nitrated phenols. All factors are influenced by NOx in different ways and to different extents. Over 1000 non-nitro molecules have been identified and then reconstructed from the selected solution of binPMF, and about 72 % of the total signals are contributed by nitrogen-containing OOMs, mostly regarded as organic nitrates formed through peroxy radicals terminated by nitric oxide or nitrate-radical-initiated oxidations. Moreover, multi-nitrates account for about 24 % of the total signals, indicating the significant presence of multiple generations, especially for isoprene (e.g., C5H10O8N2 and C5H9O10N3). Additionally, the distribution of OOM concentration on the carbon number confirms their precursors are driven by AVOCs mixed with enhanced BVOCs during summer. Our results highlight the decisive role of NOx in OOM formation in densely populated areas, and we encourage more studies on the dramatic interactions between anthropogenic and biogenic emissions. [ABSTRACT FROM AUTHOR]

Nitrous acid (HONO), an important precursor of the hydroxyl radical (OH), has long been recognized as of significance to atmospheric chemistry, but its sources are still debated. In this study, we conducted continuous measurement of HONO from November 2017 to November 2018 at the SORPES station in Nanjing of eastern China. The yearly average mixing ratio of observed HONO was 0.69±0.58 ppb, showing a larger contribution to OH relative to ozone with a mean OH production rate of 1.16 ppb h -1. To estimate the effect of combustion emissions of HONO, the emitted ratios of HONO to NOx were derived from 55 fresh plumes (NO/NOx > 0.85), with a mean value of 0.79 %. During the nighttime, the chemistry of HONO was found to depend on RH, and heterogeneous reaction of NO2 on an aerosol surface was presumably responsible for HONO production. The average nighttime NO2 -to-HONO conversion frequency (CHONO) was determined to be 0.0055±0.0032 h -1 from 137 HONO formation cases. The missing source of HONO around noontime seemed to be photo-induced, with an average Punknown of 1.04 ppb h -1 , based on a semi-quantitative HONO budget analysis. An over-determined system of equations was applied to obtain the monthly variations in nocturnal HONO sources. Besides the burning-emitted HONO (accounting for about 23 % of the total concentration), the contribution of HONO formed heterogeneously on ground surfaces to measured HONO was an approximately constant proportion of 36 % throughout the year. The soil emission revealed clear seasonal variation and contributed up to 40 % of observed HONO in July and August. A higher propensity for generating HONO on aerosol surfaces occurred in severe hazes (accounting for 40 % of the total concentration in January). Our results highlight ever-changing contributions of HONO sources and encourage more long-term observations to evaluate the contributions from varied sources. [ABSTRACT FROM AUTHOR]

Despite frequent haze pollution in China in recent years, our knowledge of regional pollution episodes associated with air transport and synoptic weather systems is limited. In this study, we conducted two intensive campaigns simultaneously to measure the highly time-resolved chemical composition of fine particles (PM 2.5) in suburban Shanghai and Nanjing during the winter of 2017 and the summer of 2018. The average PM 2.5 mass concentrations were 53.9 (65.7) μg m−3 and 32.8 (37.3) μg m−3 in Shanghai (Nanjing) in winter and summer, respectively. In winter, extreme haze episodes were observed synchronously with enhanced contributions of nitrate at both sites and of low-volatile oxidized organic aerosol (LV-OOA) in Shanghai. Long-range transport from Northern China was demonstrated to play an important role in the episodes, which occurred simultaneously at both sites. Influenced by the cold fronts, Nanjing had a relatively longer pollution duration, whereas Shanghai exhibited faster PM increases. In summer, air masses passing through the city-clusters of the YRD were responsible for the pollution episodes. Low wind speeds, which favored the accumulation of primary aerosols, and strong photochemical activity indicated by high ozone level, which promoted the formation of secondary aerosols, resulted in elevated contributions of nitrate, Hydrocarbon-like organic aerosol (HOA) and semi-volatile oxidized organic aerosol (SV-OOA) to PM in Shanghai. In addition, a pollution episode dominated by increases of nitrate and organic aerosols was observed in Nanjing two days later despite the clean situation in Shanghai. Our results highlight the importance of regional or sub-regional emission control to mitigate haze pollution in city clusters, such as the YRD in Eastern China. • Particulate nitrate is the major driver of haze pollution during summer and winter in the YRD. • Regional air transport was responsible for severe haze episodes in winter. • Local air masses within the YRD with secondary processes are responsible for particulate matter pollution in summer. [ABSTRACT FROM AUTHOR]

Particulate organic nitrates (pON) are important species in the atmosphere that influence reactive nitrogen cycling and ozone (O 3) formation. Previous studies on pON focused on forest regions involving anthropogenic NO x emissions. Therefore, the investigation and quantitation of pON in polluted urban regions remain limited. Here, we conducted sampling campaigns in four seasons using an Aerodyne time-of-flight aerosol chemical speciation monitor in the SORPES station in Nanjing, East China. pON were calculated using a constrained-PMF method, which was validated using high-resolution aerosol mass spectrometer-based estimates. The results reveal that pON contribute significantly to the aerosol nitrate in the polluted Yangtze River Delta, and the fractions of pON to total nitrate are >21% in the summer, spring, and autumn, and >13% in the winter. The pON contribution to the total nitrate decreases as the fine particulate matter (PM 2.5) loading increases but remaines significant at the peak of pollution. This indicates that the pON formation is controlled by the gas-phase chemistry, which is pronounced during low PM 2.5 pollution. pON are also associated with organic matter (OM), and its contribution to the OM is independent of the PM 2.5 loading. The contributions to OM are 36.2%, 25.6%, 24.1%, and 13.9% during the winter, autumn, spring, and summer, respectively. Our study demonstrates that pON are potentially important components of atmospheric PM 2.5 in urban areas. ● Particulate organic nitrates (pON), estimated by constrained-PMF with the time-of-flight aerosol chemical speciation monitor, contributed significantly to aerosol nitrate and organic matter in the Yangtze River Delta. ● The pON contribution to the total nitrate revealed strong seasonal and diurnal variation. ● The pON contribution to the total nitrate decreased as thePM 2.5 loading increased, while its contribution to organic matter was independent of the PM 2.5 loading. [ABSTRACT FROM AUTHOR]