Inconsistent 3‐D Structures and Sources of Sulfate Ammonium and Nitrate Ammonium Aerosols During Cold Front Episodes.

Since the distinct thermostability difference of sulfate ammonium and nitrate ammonium aerosols, their distributions, evolutions and sources could be unpredictable on a long‐range transport condition. Here, we highlighted the 3‐D structures and sources of SO42−, NO3− and NH4+ (SNA) during two cold front episodes in east China. Cold fronts effectively uplift and transport PM2.5 and its precursors from upstream sources to the Yangtze River Delta (YRD). Specifically, in the YRD, surface SO42− is mostly imported from the upstreams, accounting for ∼48%, significantly higher than the contribution from the YRD itself (∼29%). In contrast, NH4NO3 is thermally unstable and more easily lost in the warmer and lower boundary layer (BL) ahead of cold front. Consequently, only 20% of the total NO3− originates from upstreams, while the YRD contributes 28%. In the upper BL, the contribution of SO42− from upstreams remain high (∼49%), with only 18% originating from the YRD. However, due to the intense thermostability of NH4NO3 in colder and wetter air, the YRD's contribution to NO3− is 27%, and upstreams contribute 20%. The physical processes exert relatively consistent effects on variations of PM2.5 and SNA concentrations. The aerosol chemical process (AERO) of (NH4)2SO4 consistently contributes positively throughout the entire BL. Conversely, the temperature‐sensitive NH4NO3 undergoes repeated dissociation/condensation and deposition, causing positive AERO contributions in upper BL and negative contributions in lower BL. Results indicate that one difference in physicochemical property of species could induce their distinct distributions and sources in large scale, and should be considered in regional air pollution control. Plain Language Summary: Long‐range transport of PM2.5 by cold frontal passage is an important haze formation mechanism. Previous field and modeling studies have revealed that the vertical distribution of particle sulfate and particle nitrate concentrations varied significantly. However, few studies have explored the 3‐D structures and sources of particle sulfate and nitrate under a long‐range transport conditions. In this study, we used combined observations and model simulations during cold front events to address the unrevealed inconsistent 3‐D structures and sources of sulfate and nitrate resulting from temperature stability differences. Cold front episodes redistribute SNA and precursors in three dimensions, driven by drastic meteorology changes and long range transport. Consequently, the distributions, variations and sources of SNA may exhibit complexity and unpredictablility during long‐range transport. Specificlly, (NH4)2SO4 is mostly imported from the upstream in the whole boundary layer (BL), but NH4NO3 is dominant by local source. Moreover, uplifting by cold front, temperature‐sensitive NH4NO3 form in the upper BL and dissociation in the lower BL, whereas (NH4)2SO4 forms uniformly across the whole BL. These findings suggest that one difference in physicochemical property of species can induce distinct distributions and sources in large scale, highlighting the importance of considering such differences in regional air pollution control. Key Points: Uplifted by cold fronts, the temperature‐sensitive NH4NO3 forms in the upper BL and dissociates in the lower BL(NH4)2SO4 forms in the whole BL because it is highly thermally stable and impervious to dissociationThe differences in thermostability and lifetime result in local sources being dominant for NH4NO3, while upstream sources dominate for (NH4)2SO4 [ABSTRACT FROM AUTHOR]