Spatial distribution and seasonal variability in atmospheric ammonia measured from ground-based FTIR observations at Hefei, China

Abstract. Atmospheric ammonia (NH₃) plays an important role in the formation of fine particulate matter, leading to severe environmental degradation and human health issues. In this work, ground-based FTIR observations are used to obtain the total columns and vertical profiles of atmospheric NH₃ at a measurement site in Hefei, China, from December 2016 to November 2018. The spatial distribution and temporal variation, seasonal trend, emission sources and potential sources of NH₃ are analyzed. The time series of ammonia columns show that FTIR observations captured the seasonal cycle of NH₃ over the two years of measurement, with a 22.14 % yr^-1 annual increase rate over the Hefei site. We used IASI satellite data to compare with the FTIR data, and the correlation coefficients (R) between the two datasets are 0.86 and 0.78 for IASI-A and IASI-B, respectively. The results demonstrate the IASI data are in broad agreement with our FTIR data. To examine the contribution of traffic to NH₃ columns, we analyze the relationship of NH₃ columns with CO surface concentrations. NH₃ columns show high correlation (R = 0.77) with CO concentrations in summer, indicating that the elevated NH₃ columns are partly caused by urban emissions from vehicles. Further, high correlation of NH₃ columns with air temperature is obvious from their diurnal variation during the observation period. In addition, the clear correlation between NH₃ columns and air temperature in spring and autumn over Hefei, suggests that agriculture was indeed the main source of ammonia in spring and autumn. Furthermore, the back trajectories of air masses calculated by the HYSPLIT model confirmed that agriculture was the dominant source of ammonia in spring, autumn and winter, while urban anthropogenic emissions contributed to the high level of NH₃ in summer over the Hefei site. The potential source areas influencing the NH₃ columns were distributed in the local area of Hefei, the northern part of Anhui province, as well as Shangdong, Jiangsu and Henan provinces. This study helps to identify the emission sources and potential sources that contribute to NH₃ columns over Hefei, a highly populated and polluted area. This is the first time that ground-based FTIR remote sensing of NH₃ columns and comparison with satellite data are reported in China.