Summary
Heterogeneous reactions of NO<sub>2</sub> on particulate matter have been considered an important source of HONO (Nitrous acid) in the troposphere, whereas its contribution is controversial due to the lack of uptake coefficient of NO<sub>2</sub> (γ<sub>NO2</sub>) on the surfaces of ambient particulate matter (PM). Here we investigate the the γ<sub>NO2</sub> to form HONO and its evolution based on long-term comprehensive field observations (2019-2023) in Beijing and a random forest model with Shapley additive explanations. The γ<sub>NO2</sub> on ambient PM is on the order of 10<sup>-6</sup>, decreasing markedly from 3.07 ± 5.99 × 10<sup>-6</sup> in 2019 to 1.43 ± 3.22 × 10<sup>-6</sup> in 2023. This decrease is driven by the increase in aerosol pH, linked to increased ratio of NH<sub>4</sub
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