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

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 PM_2.5 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-NH_x during the three different pollution periods suggested that the rural site was always in the NH_x-rich atmosphere where high levels of NH_x increased the particle pH inefficiently. Nevertheless, the particle pH still elevated by 1.5-2.2 units at the excessive NH_x 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.