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 SO 4 2− , NO 3 − and NH 4 + (SNA) during two cold front episodes in east China. Cold fronts effectively uplift and transport PM 2.5 and its precursors from upstream sources to the Yangtze River Delta (YRD). Specifically, in the YRD, surface SO 4 2− is mostly imported from the upstreams, accounting for ∼48%, significantly higher than the contribution from the YRD itself (∼29%). In contrast, NH 4 NO 3 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 NO 3 − originates from upstreams, while the YRD contributes 28%. In the upper BL, the contribution of SO 4 2− from upstreams remain high (∼49%), with only 18% originating from the YRD. However, due to the intense thermostability of NH 4 NO 3 in colder and wetter air, the YRD’s contribution to NO 3 − is 27%, and upstreams contribute 20%. The physical processes exert relatively consistent effects on variations of PM 2.5 and SNA concentrations. The aerosol chemical process (AERO) of (NH 4 ) 2 SO 4 consistently contributes positively throughout the entire BL. Conversely, the temperature‐sensitive NH 4 NO 3 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.