Enhanced Particulate Nitrate Formation in Residual Layer Exacerbates Near-Surface Pollution: Insights From Tethered Airship and Long-Term Ground Measurements

Abstract

Recent years have witnessed a surge in nitrate-driven aerosol pollution across China with N2O5 hydrolysis emerging as a critical formation pathway. Common surface measurements may misleadingly imply this process due to low nighttime ozone at surface level in winter. However, our study reveals a more complex picture by unveiling the vertical dynamics of nitrate formation through an integration of tethered airship campaign, long-term ground measurements, and model simulations. Interestingly, we observed rapid nitrate growth at approximately 400 m altitude, where the box model revealed optimal conditions for sustained nocturnal nitrate production. The nitrate accumulated overnight in the residual layer (RL) is transported downward in the next morning as the nocturnal boundary layer breaks down, substantially increasing surface-level nitrate and thus exacerbating pollution. Annual-averaged diurnal patterns of nitrate measured at the ground station clearly confirm this morning increase. The vertical mixing from the RL contributes up to 80% of the total surface nitrate at 10:00 LT with its influence persisting at 31% even after sunset. Air mass trajectory analysis further confirms that emissions from city-cluster significantly contribute to downwind pollution by transporting pollutants into the RL. This research underscores the important role of RL chemical processes, facilitated by elevated ozone, in shaping surface-level pollution. It highlights the indispensability of vertical profiling in understanding aerosol pollution and advocates for regionally coordinated control strategies across eastern China.