Flux of NH3 release from dew evaporation in downtown and suburban Changchun, China

Ammonia, as the only high-concentration alkaline gas in the atmosphere, plays an extremely important role in the initial nucleation process of aerosols. A rise in the concentration of NH3 after sunrise has been observed in many areas, known as the "morning peak phenomenon", which is likely related to the dew evaporation process because of the considerable amount of NH4+ present in dew. To investigate and compare the flux and rate of NH3 release from dew evaporation in downtown (WH) and suburban areas (SL), the dew amount and chemical makeup were measured and analyzed in Changchun, in northeastern China, from April to October 2021. The differences in the fraction of NH4+ released as NH3 gas and the NH3 emission flux and rate during the process of dew evaporation between SL and WH were identified. The results showed that the daily dew amount in WH (0.038 ± 0.017 mm) was lower than that in SL (0.065 ± 0.032 mm) (P < 0.01), and the pH in SL (6.58 ± 0.18) was approximately 1 pH unit higher than that in WH (5.60 ± 0.25). SO42−, NO3−, Ca2+ and NH4+ were the main ions in WH and SL. The ion concentration in WH was significantly higher than that in SL (P < 0.05), which was influenced by human activities and pollution sources. A total of 24%-48% NH4+ was released as NH3 gas during dew evaporation in WH, which was lower than the conversion fraction of SL dew (44%-57%). The evaporation rate of NH3 was 3.9–20.6 ng/m2·s (9.9 ± 5.7 ng/m2·s) in WH and 3.3–15.9 ng/m2·s (8.6 ± 4.2 ng/m2·s) in SL. The dew evaporation process makes an important contribution to the NH3 morning peak phenomenon, but it is not the only contributor.