[Multi-time Scale Variation of Atmospheric Ammonia Concentration and Dry Deposition in a Paddy Rice Region in...

Meteorological factors and anthropogenic activities significantly affect atmospheric ammonia （NH₃） concentration and its dry deposition. Former studies have examined the spatial and temporal variability in atmospheric NH₃ concentrations at monthly scales. However, the characteristics of atmospheric concentrations at finer time scales such as hourly and daily scales and the influencing factors remain unclear. In this study, atmospheric NH₃ concentration and related meteorological factors were continuously monitored online for one year in a double cropping rice region in subtropical China, and atmospheric NH₃ concentration and its meteorological influencing factors as well as dry deposition were analyzed at different time scales （hourly, daily, and monthly）. The main results were as follows： The annual average daily concentration of NH₃ in the rice area varied from 0.01 to 58.0 μg·m^-3 （in N, same below）, and the annual average concentration was 5.3 μg·m^-3. On the hourly scale, the 24-hour dynamics of atmospheric NH₃ concentration showed a unimodal pattern, and the time of the NH₃ peak appearance in different seasons was different； the time of the peak that appeared in winter lagged behind that in the other seasons. From the perspective of daily scale, NH₃ concentration was mainly affected by fertilization in the paddy fields, peaking at 1-3 days after fertilization and then gradually decreasing. On the monthly scale, NH₃ concentration peaked at 12.8 μg·m^-3 in July and was the lowest in October at 1.6 μg·m^-3. On the hourly scale, NH₃ concentration varied seasonally due to the influences of meteorological factors, mainly as follows： NH₃ concentration showed significant positive correlations with air temperature and solar radiation in all four seasons and with wind speed in spring and summer, whereas it showed significant negative correlations with relative humidity except in winter. On the daily scale, NH₃ concentration showed a significant positive correlation with air temperature, rainfall, and solar radiation, whereas it showed a significant negative correlation with relative humidity. On the monthly scale, no significant correlation existed between each meteorological factor and NH₃ concentration. The annual dry deposition flux （in N） calculated from the hourly average NH₃ concentration was 8.5 kg·（hm²·a）^-1, which was 11.6% higher than the annual flux calculated from the daily average and 12.4% higher than the annual flux calculated from the monthly average. In summary, there were significant daily and seasonal variations in atmospheric NH₃ concentration in the paddy rice region in subtropical China, and conducting hourly-scale observations of NH₃ concentration can help to reveal the multi-time scale variations in NH₃ concentration and to quantify NH₃ dry deposition more accurately.