Characteristics and sources of atmospheric ammonia at the SORPES station in the western Yangtze river delta of China

Abstract

Ammonia (NH3) is an important air pollutant with crucial impacts on air quality, ecosystems and climate change. However, NH3 is not included in routine air quality monitoring and the availability of long-term NH3 measurements is still limited, resulting in large uncertainties in our knowledge of the spatial distribution and sources of NH3. Here we performed 1 year (March 2021–February 2022) of atmospheric NH3 measurements at a regional background station, the Station for Observing Regional Processes of the Earth System (SORPES) in the western Yangtze River Delta of China. We found that the annual mean NH3 concentration was 12.2 ± 4.6 ppb and exhibited apparent seasonal variations, with a maximum in June and a minimum in February, influenced by agricultural activities, air temperature, gas-particle partitioning and precipitation. Moreover, air temperature and absolute humidity correlated well with NH3, indicating that they are important factors in influencing NH3 levels. The diurnal variation of NH3 showed a single peak in the morning and higher concentrations during the day. In spring and autumn, the NH3 morning peak can be attributed to dew evaporation. The air mass backward trajectory, local wind direction and velocity analysis suggested that NH3 was influenced by both local emissions and regional transport from nearby cities. During the COVID-19 lockdown, a strong reduction of NOx (−77%) and a weak reduction of NH3 (−14%) were observed compared to the pre-lockdown, highlighting that traffic emissions have a minor impact on NH3 at the SORPES station. Our results provide more insights into the characteristics and sources of atmospheric NH3 in background regions influenced by mixed air pollution plumes.