Aircraft measurements reveal vertical distribution of atmospheric ammonia over the North China Plain in early autumn

摘要

Ammonia (NH3) is an atmospheric contaminant contributing to the formation of fine particles, e.g., particulate matter (PM2.5), which induces respiratory diseases. In the atmosphere, horizontal NH(3)levels in the stratosphere, middle-upper troposphere, and planetary boundary layer have been studied using surface monitoring and satellite data, yet NH(3)vertical distribution above the planetary boundary layer within the lower troposphere is poorly known. In particular, vertical NH(3)data would provide information on the role of NH(3)in secondary aerosol formation in the lower troposphere. Here, two NH(3)profiles on September 2 and 3, 2017, were obtained by in situ aircraft measurement, plus temperature, dew point temperature, and relative humidity over the North China Plain. We also used synoptic analysis and WRF-CHEM modeling. We observed one pollution layer from 2800 to 3500 m on September 2, and two pollution layers, 1750-2250 m and 2500-3450 m, on September 3. Results show that the highest NH(3)concentration above the planetary boundary layer was 28.0 ppb at 2500-3450 m over Baoding, comparable to that observed in the planetary boundary layer, of 33.1 ppb, on September 3, 2017. The transport of clean air masses from the northwest of China led to a sharp decrease in NH(3)concentration between 2300 and 2800 m and highlighted a relatively high NH(3)concentration of 20.0 ppb at approximately 3000 m, which led to the formation of an elevated pollution layer on September 2. The highest pollution layer of 28.0 ppb at 2500-3450 m on September 3, 2017, appears to be driven by the westerly wind, whereas the lower pollution layer of 20.1 ppb at 1750-2250 m was influenced by both local pollutants of the North China Plain and a vertical entrainment effect.