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

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.