Biomass burning related ammonia emissions promoted a self-amplifying loop in the urban environment in Kunming (SW China).

Particulate ammonium (NH 4 +) is one of the most important inorganic components in aerosol. The concentrations of NH 4 + in PM 2.5 significantly increased when PM 2.5 levels elevated. Increased understanding of the atmospheric processes and sources of ambient ammonia is an effective way to control atmospheric ammonia and tackle air pollution problems. This study focused on the concentration and nitrogen stable isotopic composition of particulate NH 4 + in PM 2.5 in a southwest typical plateau city, Kunming. The trend in NH 4 +concentrations was parallel to the trend in PM 2.5 levels, with obviously increased concentrations observed in November. Aerosol pH and liquid water content (ALWC) were synchronously simulated by the ISORROPIA-II model. And the ammonia gas-particle conversion ratio (f) was calculated for each day. Then, we proposed that a self-amplifying feedback mechanism of NH 4 + formation was associated with the variations of ALWC, pH, and the ammonia gas-particle conversion ratio. Based on the inverse analysis of the nitrogen isotopic composition of particulate NH 4 +, the corresponding δ15N values of initial ambient NH 3 were estimated to be −27.4‰–15.3‰, with an average of −8.1 ± 8.3‰. Results from Back trajectory analysis, PSCF analysis, and isotope-based source apportionment of NH 3 shed light on source compositions and potential source regions, indicating that, in the study area, ambient NH 3 during slightly polluted days were dominated by biomass burning emissions, which might have been originated from local emissions and regional transport process in late autumn. [Display omitted] • A self-amplifying loop was proposed when the concentration of NH 4 + in PM 2.5 elevated in late autumn in Kunming. • Biomass burning related ammonia emissions significantly enhanced during slightly polluted days. • Increased biomass burning emissions in Kunming were related to ammonia emissions from local area and regional transport. [ABSTRACT FROM AUTHOR]