Nitrate aerosol has become an increasingly important component of fine particles. While the formation and evolution of nitrate in airborne particles are extensively investigated, little is known about the formation of nitrate in clouds. Here we present a detailed investigation on the in-cloud formation of nitrate based on the size-resolved mixing state of nitrate in the individual cloud residual and cloud-free particles by single particle mass spectrometry, and the mass concentrations of nitrate in the cloud water and PM2.5 at a mountain site (1690 m a.s.l.) in southern China. The results show a significant enhancement of nitrate mass fraction in cloud water and relative intensity of nitrate in the mass spectra of the cloud residual particles, underlining a critical role of in-cloud processing in the formation of nitrate. Based on the size distribution of relative intensity of nitrate in individual particles, we exclude the gas phase scavenging of HNO3 and the facilitated activation of nitrate-containing particles as the major contribution for the enhanced nitrate. Regression analysis and theoretical calculations further reveal that nitrate is highly related (R2 = ~0.6) to the variation of [NOx][O3], temperature and droplet surface area in clouds. Accounting for droplet surface area greatly enhances the predictability of the observed nitrate compared with using [NOx][O3] and temperature. Our results indicate a critical role of in-cloud formation of nitrate via N2O5 hydrolysis, even during the daytime, attributed to the diminished light in clouds. The detailed observation would benefit future investigations of the evolution and oxidative impacts of nitrate.

Mixing state of black carbon (BC) with secondary species has been highlighted as a major uncertainty in assessing its radiative forcing. While recent laboratory simulation has demonstrated that BC could serve as a catalyst to enhance the formation of sulfate, its role in the formation and evolution of secondary aerosols in the real atmosphere remains poorly understood. In the present study, the mixing of BC with sulfate/nitrate in the atmosphere of Guangzhou (China) was directly investigated with a single particle aerosol mass spectrometer (SPAMS). The peak area ratios of sulfate to nitrate (SNRs) for the BC-containing particles are constantly higher than those of the BC-free particles (with negligible BC signals). Furthermore, the seasonal SNR peak is observed in summer and autumn, and the diurnal peak is found in the afternoon, consistent with the trends of radiation-related parameters (i.e., solar radiation and temperature), pointing to the BC-induced photochemical production of sulfate. Such hypothesis is further supported by the multilinear regression and random forest analysis, showing that the variation of SNRs associated with the BC-containing particles could be well explained (R2 = ~0.7-0.8) by the radiation-related parameters (> 30% of the variance) and the relative BC content (~20%) in individual particles, but with limited influence of precursors (SO2/NOx: < 5%). Differently, the radiation-related factors only explain < 10% of the SNR variation for the BC-free particles. These results provide ambient observational evidence pointing to a unique role of BC on the photochemical formation and evolution of sulfate, which merits further quantitative evaluations.