This work presents an overview of 1 yr measurements of ozone (O₃) and fine particular matter (PM_2.5) and related trace gases at a recently developed regional background site, the Station for Observing Regional Processes of the Earth System (SORPES), in the western part of the Yangtze River Delta (YRD) in eastern China. Ozone and PM_2.5 showed strong seasonal cycles but with contrast patterns: O₃ reached a maximum in warm seasons but PM_2.5 in cold seasons. Correlation analysis suggests a VOC-sensitive regime for O₃ chemistry and a formation of secondary aerosols under conditions of high O₃ in summer. Compared with the National Ambient Air Quality Standards in China, our measurements report 15 days of O₃ exceedance and 148 days of PM_2.5 exceedance during the 1 yr period, suggesting a severe air pollution situation in this region. Case studies for typical O₃ and PM_2.5 episodes demonstrated that these episodes were generally associated with an air mass transport pathway over the mid-YRD, i.e., along the Nanjing–Shanghai axis with its city clusters, and showed that synoptic weather played an important role in air pollution, especially for O₃. Agricultural burning activities caused high PM_2.5 and O₃ pollution during harvest seasons, especially in June. A calculation of potential source contributions based on Lagrangian dispersion simulations suggests that emissions from the YRD contributed to over 70% of the O₃ precursor CO, with a majority from the mid-YRD. North-YRD and the North China Plain are the main contributors to PM_2.5 pollution in this region. This work shows an important environmental impact from industrialization and urbanization in the YRD region, and suggests an urgent need for improving air quality in these areas through collaborative control measures among different administrative regions

Abstract. The influence of air pollutants, especially aerosols, on regional and global climate has been widely investigated, but only a very limited number of studies report their impacts on everyday weather. In this work, we present for the first time direct (observational) evidence of a clear effect of how a mixed atmospheric pollution changes the weather with a substantial modification in the air temperature and rainfall. By using comprehensive measurements in Nanjing, China, we found that mixed agricultural burning plumes with fossil fuel combustion pollution resulted in a decrease in the solar radiation intensity by more than 70 %, a decrease in the sensible heat by more than 85 %, a temperature drop by almost 10 K, and a change in rainfall during both daytime and nighttime. Our results show clear air pollution-weather interactions, and quantify how air pollution affects weather via air pollution-boundary layer dynamics and aerosol-radiationcloud feedbacks. This study highlights cross-disciplinary needs to investigate the environmental, weather and climate impacts of the mixed biomass burning and fossil fuel combustion sources in East China.

Investigating sulfate formation processes is important not only for air pollution control but also for understanding the climate system. Although the mechanisms of secondary sulfate production have been widely studied, in situ observational evidence implicating an important role of NO2 in SO2 oxidation in the real atmosphere has been rare. In this study, we report two unique cases, from an intensive campaign conducted at the Station for Observing Regional Processes of the Earth System (SORPES) in East China, showing distinctly different mechanisms of sulfate formation by NO2 and related nitrogen chemistry. The first case occurred in an episode of mineral dust mixed with anthropogenic pollutants and especially high concentrations of NOx. It reveals that NO2 played an important role, not only in surface catalytic reactions of SO2 but also in dust‐induced photochemical heterogeneous reactions of NO2, which produced additional sources of OH radicals to promote new particle formation and growth. The second case was caused by aqueous oxidation of S(IV) by NO2 under foggy/cloudy conditions with high NH3 concentration. As a by‐product, the formed nitrite enhanced HONO formation and further promoted the gas‐phase formation of sulfate in the downwind area. This study highlights the effect of NOx in enhancing the atmospheric oxidizing capacity and indicates a potentially very important impact of increasing NOx on particulate pollution formation and regional climate change in East Asia.

Abstract. Aerosol particles play important roles in regional air quality and global climate change. In this study, we analyzed 2 years (2011)(2012)(2013) of measurements of submicron particles (6-800 nm) at a suburban site in the western Yangtze River Delta (YRD) of eastern China. The number concentrations (NCs) of particles in the nucleation, Aitken and accumulation modes were 5300 ± 5500, 8000 ± 4400, 5800 ± 3200 cm −3 , respectively. The NCs of total particles are comparable to those at urban/suburban sites in other Chinese megacities, such as Beijing, but about 10 times higher than in the remote western China. Long-range and regional transport largely influenced number concentrations and size distributions of submicron particles. The highest and lowest accumulation-mode particle number concentrations were observed in air masses from the YRD and coastal regions, respectively. Continental air masses from inland brought the highest concentrations of nucleation-mode particles. New particle formation (NPF) events, apparent in 44 % of the effective measurement days, occurred frequently in all the seasons except winter. The frequency of NPF in spring, summer and autumn is much higher than other measurement sites in China. Sulfuric acid was found to be the main driver of NPF events. The particle formation rate was the highest in spring (3.6 ± 2.4 cm −3 s −1 ), whereas the particle growth rate had the highest values in summer (12.8 ± 4.4 nm h −1 ). The formation rate was typically high in relatively clean air masses, whereas the growth rate tended to be high in the polluted YRD air masses. The frequency of NPF events and the particle growth rates showed a strong year-to-year difference. In the summer of 2013, associated with a multi-week heat wave and strong photochemical processes, NPF events occurred with larger frequency and higher growth rates compared with the same period in 2012. The difference in the location and strength of the subtropical high pressure system, which influences the air mass transport pathways and solar radiation, seems to be the cause for year-to-year differences. This study reports, up to now, the longest continuous measurement records of submicron particles in eastern China and helps to achieve a comprehensive understanding of the main factors controlling the seasonal and year-to-year variation of the aerosol size distribution and NPF in this region.

Abstract. Nitrous acid (HONO) plays a key role in atmospheric chemistry by influencing the budget of hydroxyl radical (OH). In this study, a two-month measurement of HONO and related quantities were analyzed during a biomass burning season in 2012 at a suburban site in the western Yangtze River delta, eastern China. An overall high HONO concentration with the mean value of 0.76 ppbv (0.01 ppbv to 5.95 ppbv) was observed. During biomass burning (BB) periods, both HONO concentration and HONO/NO 2 ratio were enhanced significantly (more than a factor of 2, p<0.01) compared with non-biomass burning (non-BB) periods. A correlation analysis showed that the HONO in BB plumes was more correlated with nitrogen dioxide (NO 2 ) than that with potassium (a tracer of BB). Estimation by the method of potassium tracing suggests a maximum contribution of 17 ± 12 % from BB emission to the observed HONO concentrations, and the other over 80 % of the observed nighttime HONO concentrations during BB periods were secondarily produced by the heterogeneous conversion of NO 2 . The NO 2 -to-HONO conversion rate (C HONO ) in BB plumes was almost twice as that in non-BB plumes (0.0062 hr −1 vs. 0.0032 hr −1 ). Given that the residence time of the BB air masses was lower than that of non-BB air masses, these results suggest BB aerosols have higher NO 2 conversion potentials to form HONO than non-BB aerosols. A further analysis based on comparing the surface area at similar particle mass levels and HONO/NO 2 ratios at similar surface area levels suggested larger specific surface areas and higher NO 2 conversion efficiencies of BB aerosols. A mixed plume of BB and anthropogenic fossil fuel (FF) emissions was observed on 10 June with even higher HONO concentrations and HONO/NO 2 ratios. The strong HONO production potential (high HONO/NO 2 to PM 2.5 ratio) was accompanied with a high sulfate concentration in this plume, suggesting a promotion of mixed aerosols to the HONO formation. In summary, our study suggests an important role of BB in atmospheric chemistry by affecting the HONO budget. This can be especially important in eastern China, where agricultural burning plumes are inevitably mixed with urban and industrial pollution.