Water-soluble and water-insoluble organic aerosol (WSOA and WIOA) constitute a large fraction of fine particles in winter in northern China, yet our understanding of their sources and processes are still limited. Here we have a comprehensive characterization of WSOA in cold season in Beijing. Particularly, we present the first mass spectral characterization of WIOA by integrating online and offline organic aerosol measurements from high-resolution aerosol mass spectrometer. Our results showed that WSOA on average accounted for 59 % of the total OA and comprised dominantly secondary OA (SOA, 69 %). The WSOA composition showed significant changes during the transition season from autumn to winter. While the photochemical-related SOA dominated WSOA (51 %) in early November, the oxidized SOA from biomass burning increased substantially from8 % to 29 % during the heating season. Comparatively, local primary OA dominantly from cooking aerosol contributed the major fraction of WSOA during clean periods. WIOA showed largely different spectral patterns from WSOA which were characterized by prominent hydrocarbon ions series and low oxygen-to-carbon ( O / C   =  0.19) and organic mass-to-organic carbon ( OM / OC   =  1.39) ratios. The nighttime WIOA showed less oxidized properties ( O / C   =  0.16 vs. 0.24) with more pronounced polycyclic aromatic hydrocarbons (PAHs) signals than daytime, indicating the impacts of enhanced coal combustion emissions on WIOA. The evolution process of WSOA and WIOA was further demonstrated by the triangle plot of f 44 (fraction of m / z  44 in OA) vs. f 43 , f 44 vs. f 60 , and the Van Krevelen diagram ( H / C vs. O / C ). We also found more oxidized WSOA and an increased contribution of SOA in WSOA compared with previous winter studies in Beijing, indicating that the changes in OA composition due to clean air act have affected the sources and properties of WSOA.

The radiative forcing of black carbon (BC) dependsstrongly on its mixing state in different chemical environments. Here weanalyzed the chemical composition and mixing state of BC-containingparticles by using a single-particle aerosol mass spectrometer andinvestigated their impact on light absorption enhancement ( E abs ) at anurban (Beijing) and a rural site (Gucheng) in the North China Plain. Whilethe BC was dominantly mixed with organic carbon (OC), nitrate, and sulfate atboth the urban and rural sites, the rural site showed a much higher fraction ofBC coated with OC and nitrate (36 % vs. 15 %–20 %). Moreover, the BCmixing state evolved significantly as a function of relative humidity (RH),with largely increased coatings of OC–nitrate and nitrate at high RH levels.By linking with an organic aerosol (OA) composition, we found that the OCcoated on BC comprised dominantly secondary OA in Beijing, while primary andsecondary OA were similarly important in Gucheng. Furthermore, E abs washighly dependent on secondary inorganic aerosol coated on BC at both sites,while the coated primary OC also resulted in an E abs of ∼  1.2 for relatively fresh BC particles at the rural site. A positive matrixfactorization analysis was performed to quantify the impact of differentmixing states on E abs . Our results showed a small E abs (1.06–1.11)for BC particles from fresh primary emissions, while the E abs increasedsignificantly above 1.3 when BC was aged rapidly with increased coatings ofOC–nitrate or nitrate; it can reach above 1.4 as sulfate was involved inBC aging.

Surface ozone concentrations typically peak during the daytime, driven by active photochemical production, and decrease gradually after sunset, due to chemical destruction and dry deposition. Here, we report that nocturnal ozone enhancement (NOE, defined as an ozone increase of more than 5 ppbv h −1 in 1 of any 2 adjacent hours between 20:00 and 06:00 LT, local time) events are observed at multiple monitoring sites in China at a high frequency, which has not been recognized in previous studies. We present an overview of the general characteristics of NOE events in China and explore the possible mechanisms based on 6 years of observations from the national monitoring network. We find that the mean annual frequency of NOE events is 41±10  % (i.e., about 140 d would experience an NOE event per year)averaged over all 814 Chinese sites between 2014 and 2019, which is 46 % largerthan that over Europe or the United States. The NOE event frequency is higher inindustrialized city clusters ( >50  %) than in regions with lighterozone pollution, and it is higher in the warm season (46 %) than in the cold season(36 %), consistent with the spatiotemporal evolution of ozone levels. The mean ozone peak during NOE events reaches 37±6  ppbv in the warmseason. The ozone enhancements are within 5–15 ppbv h −1 during 85 % of the NOE events; however, in about 10 % of cases, the ozone increases can exceed 20 ppbv h −1 . We propose that high photochemistry-induced ozone during the daytime provides a rich ozone source in the nighttime residual layer, determining the overall high frequency of NOE events in China, and that enhanced atmospheric mixing then triggers NOE events by allowing the ozone-rich air in the residual layer to mix into the nighttime boundary layer. This is supported by our analyses which show that 70 % (65 %) of the NOE events are associated with increases in friction velocity (planetary boundary layer height), indicative of enhanced atmospheric mixing, and also supported by the observed sharp decreases in surface NO 2 and CO concentrations with ozone increases in NOE events, a typical signal of mixing with air in the residual layer. Three case studies in Beijing and Guangzhou show that synoptic processes such as convective storms and low-level jets can lead to NOE events by aggravating vertical mixing. Horizontal transport of ozone-rich plumes may also be a supplementary driver of NOE events. Our results summarize, for the first time, the characteristics and mechanism of NOE events in China based on nationwide and long-term observations, and our findings emphasize the need for more direct measurements and modeling studies on the nighttime ozone evolution from the surface to the residual layer.