Abstract. Brown carbon (BrC), a certain group of organic carbon (OC) with strong absorption from the visible (VIS) to ultraviolet (UV) wavelengths, makes a considerable contribution to light absorption on both global and regional scales. A high concentration and proportion of OC has been reported in China, but studies of BrC absorption based on long-term observations are rather limited in this region. In this study, we reported 3-year results of light absorption of BrC based on continuous measurement at the Station for Observing Regional Processes of the Earth System (SORPES) in the Yangtze River Delta, China, combined with Mie theory calculation. Light absorption of BrC was obtained using an improved absorption Ångström exponent (AAE) segregation method. The AAE of non-absorbing coated black carbon (BC) at each time step is calculated based on Mie theory simulation, together with single particle soot photometer (SP2) and aethalometer observations. By using this improved method, the variation of the AAE over time is taken into consideration, making it applicable for long-term analysis. The annual average light absorption coefficient of BrC (b abs_BrC ) at 370 nm was 6.3 Mm −1 at the SORPES station. The contribution of BrC to total aerosol absorption (P BrC ) at 370 nm ranged from 10.4 to 23.9 % (10th and 90th percentiles, respectively), and reached up to ∼ 33 % in the openbiomass-burning-dominant season and winter. Both b abs_BrC and P BrC exhibited clear seasonal cycles with two peaks in later spring/early summer (May-June, b abs_BrC ∼ 6 Mm −1 , P BrC ∼ 17 %) and winter (December, b abs_BrC ∼ 15 Mm −1 , P BrC ∼ 22 %), respectively. Lagrangian modeling and the chemical signature observed at the site suggested that open biomass burning and residential coal/biofuel burning were the dominant sources influencing BrC in the two seasons, respectively.

Secondary organic aerosol (SOA) forms through the oxidation of various volatile organic compounds (VOCs) and consists of a large proportion of the total aerosol mass (

<p><strong>Abstract.</strong> Haze pollution caused by PM<sub>2.5</sub> is the largest air quality concern in China in recent years. Long-term measurements of PM<sub>2.5</sub> and the precursors and chemical speciation is crucially important for evaluating the efficiency of emission control, understanding formation and transport of PM<sub>2.5</sub> associated with the change of meteorology and for accessing the impact of human activities to regional climate change. Here we reported long-term continuous measurements of PM<sub>2.5</sub>, chemical components, and their precursors at a regional background station, the Station for Observing Regional Processes of the Earth System (SORPES), in Nanjing eastern China since 2011. We found that PM<sub>2.5</sub> at the station has experienced a substantial decrease (−9.1 %/yr), accompanied with even much significant reduction of SO<sub>2</sub> (−16.7 %/yr), since the national "Ten measures" for air took action in 2013. Control of open biomass burning and fossil-fuel combustion are the two dominant factors that influence the PM<sub>2.5</sub> reduction in early summer and winter, respectively. In cold season (November–January), increased nitrate fraction was observed with more NH<sub>3</sub> available from a substantial reduction of sulfate, and the change of year-to-year meteorology contributed to 24 % of the PM<sub>2.5</sub> decrease since 2013. This study highlights several important implications on air pollution control policy in China.</p>

Abstract. Oxygenated organic molecules (OOMs) are the crucial intermediates linking volatile organic compounds (VOCs) to secondary organic aerosol (SOA) in the atmosphere, but understandings on the characteristics of OOMs and their formations from VOCs are very limited. Ambient observations of OOMs using recently developed mass spectrometry techniques are still limited, especially in polluted urban atmosphere 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 VOCs (AVOCs) daytime chemistry, three isoprene-related factors, three factors about biogenic VOCs (BVOCs) 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 OOMs concentration on carbon number confirm their precursors driven by AVOCs mixed with enhanced BVOCs during summer. Our results highlight the decisive role of NOx on OOMs formation in densely populated areas, and encourage more studies on the dramatic interactions between anthropogenic and biogenic emissions.

Aerosol nitrate has become the most abundant compound during aerosol pollution in eastern China. The Chinese government implemented a stringent policy during 2013–2017 to tackle aerosol pollution. However, the response of nitrate to nitrogen oxides (NOx) reduction is unclear owing to the limitation of long‐term measurement. Here, we performed a 9‐year continuous measurement of aerosol compositions in Shanghai and confirmed a decrease in most species except nitrate. The contribution of nitrate to fine particulate matter (PM2.5) increased significantly, reaching up to 35% in pollution episodes after 2017. This is in contrast to the evident reduction in NOx emissions. We found that the elevated dinitrogen pentoxide (N2O5) hydrolysis is responsible for the observed nitrate trend. Increased ozone and decreased nitrogen dioxide (NO) facilitated the formation of N2O5, and increased nitrate proportion promoted the uptake of N2O5 and eventually enhanced the conversion efficiency of NO2 to nitrate. Our results highlight the importance of synergic control of aerosol and ozone pollution.