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). SO₄^2-, 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.