Atmospheric ammonia in the rural North China Plain during wintertime: Variations, sources, and implications for HONO heterogeneous formation

Atmospheric ammonia (NH3) plays an important role in secondary inorganic aerosol formation. Understanding the temporal variations, sources, and environmental influences of NH3 is conducive to better formulate PM2.5 pollution control strategies for policy-makers. Here, we performed a comprehensive field campaign with the measurements of NH3 and related parameters at a rural site of the North China Plain (NCP) in winter of 2017. The results showed that residential coal combustion contributed dominantly to NH3 during the entire observation period, resulting in the obviously high average concentration of NH3 (31.2 ± 24.6 ppbv). The sensitivity tests of pH-NHx during the three different pollution periods suggested that the rural site was always in the NHx-rich atmosphere where high levels of NHx increased the particle pH inefficiently. Nevertheless, the particle pH still elevated by 1.5–2.2 units at the excessive NHx levels during the three pollution periods. In addition, the HONO/NO2 ratios were found to correlate linearly with NH3 concentrations, implying the acceleration effect of NH3 on HONO production from NO2 heterogeneous reactions. After considering the NH3-enhanced uptake coefficient of NO2 in the nocturnal HONO budget, the unknown source of HONO could be fully explained. Therefore, more attentions should be given for effective emission control of NH3 to improve air quality throughout the NCP, especially in the rural areas.