Efficient\nNitrate Formation in Fog Events Implicates\nFog Interstitial Aerosols as Significant Drivers of Atmospheric\nChemistry

Clouds\nand fogs, consisting of tiny water droplets formed by the\ncondensation of water in supersaturated air, are vital in atmospheric\nchemistry, as they facilitate multiphase reactions. While measuring\nhigh-altitude cloud is challenging, fog as ground-level clouds offer\na unique opportunity for direct observation. In this study, we explored\nradiation fogs in the North China Plain using an advanced aerosol-fog\nsampling system to measure the chemical and physical properties of\nboth inactivated interstitial aerosols and activated fog droplet residues.\nOur findings revealed that efficient nitrate formation primarily occurred\non fog interstitial aerosols rather than within fog droplets, with\nobserved fog interstitial aerosol nitrate net production rates reaching\nup to 3.6 μg m<sup>–3</sup> h<sup>–1</sup>. Box model simulations identified the hydrolysis\nof NO<sub>2</sub> and N<sub>2</sub>O<sub>5</sub> as key pathways for\nnitrate formation. NO<sub>2</sub> hydrolysis was often overlooked\nin previous studies, contributing 40–79 and 57–76% to\ntotal nitrate production during nighttime and daytime fog periods.\nThis oversight suggests that substantial nitrate formation through\nhydrolysis reactions involving interstitial aerosols may have been\nneglected. Our results highlight the need for further research into\nthe chemistry of cloud and fog interstitial aerosols and their inclusion\nin atmospheric chemistry models.