Heterogeneous formation of particulate nitrate under ammonium-rich regimes during the high-PM 2.5 events in Nanjing, China

Lin, Yu-Chi; Zhang, Yan-Lin; Fan, Mei-Yi; Bao, Mengying

Particulate nitrate (inline-formula M2inlinescrollmathml chem normal NO normal 3 - 25pt16ptsvg-formulamathimg4c315b3ea451cf26923ad12993612b33 acp-20-3999-2020-ie00001.svg25pt16ptacp-20-3999-2020-ie00001.png ) not only influences regional climates but also contributes to the acidification of terrestrial and aquatic ecosystems. In 2016 and 2017, four intensive online measurements of water-soluble ions in PMinline-formula2.5 were conducted in Nanjing City in order to investigate the potential formation mechanisms of particulate nitrate. During the sampling periods, inline-formula M4inlinescrollmathml chem normal NO normal 3 - 25pt16ptsvg-formulamathimge16cba38499a6a16cb1a10e488ec56da acp-20-3999-2020-ie00002.svg25pt16ptacp-20-3999-2020-ie00002.png was the predominant species, accounting approximately for 35 % of the total water-soluble inorganic ions, followed by inline-formula M5inlinescrollmathml chem normal SO normal 4 normal 2 - 29pt17ptsvg-formulamathimg28cd4f8c12cf9ef751545712573a522a acp-20-3999-2020-ie00003.svg29pt17ptacp-20-3999-2020-ie00003.png (33 %) and inline-formula M6inlinescrollmathml chem normal NH normal 4 + 24pt15ptsvg-formulamathimg6ca56caa63735c5009fe6b299c1a126b acp-20-3999-2020-ie00004.svg24pt15ptacp-20-3999-2020-ie00004.png (24 %). Significant enhancements of nitrate aerosols in terms of both absolute concentrations and relative abundances suggested that inline-formula M7inlinescrollmathml chem normal NO normal 3 - 25pt16ptsvg-formulamathimga02883d0956e7dc256b9fe9fffa70b09 acp-20-3999-2020-ie00005.svg25pt16ptacp-20-3999-2020-ie00005.png was a major contributing species to high-PMinline-formula2.5 events (hourly PMinline-formula2.5≥150inline-formulaµg minline-formula−3). High inline-formula M12inlinescrollmathml chem normal NO normal 3 - 25pt16ptsvg-formulamathimg737339a8d3517116341490f01d8cfecf acp-20-3999-2020-ie00006.svg25pt16ptacp-20-3999-2020-ie00006.png concentrations mainly occurred under inline-formula M13inlinescrollmathml chem normal NH normal 4 + 24pt15ptsvg-formulamathimg9d8f7ee8bf88d657d75cdcf077dbe3e2 acp-20-3999-2020-ie00007.svg24pt15ptacp-20-3999-2020-ie00007.png -rich conditions, implying that the formation of nitrate aerosols in Nanjing involved inline-formulaNH3. During the high-PMinline-formula2.5 events, the nitrogen conversion ratios (inline-formulaFn) were positively correlated with the aerosol liquid water content (ALWC; inline-formulaR>0.72 and inline-formulap<0.05). Meanwhile, increasing inline-formula M19inlinescrollmathml chem normal NO normal 3 - 25pt16ptsvg-formulamathimg7248c728767abac31fc80ac33e5f4469 acp-20-3999-2020-ie00008.svg25pt16ptacp-20-3999-2020-ie00008.png concentrations regularly coincided with increasing ALWC and decreasing inline-formulaOx (inline-formula M21inlinescrollmathml chem normal O x = chem normal O normal 3 + chem normal NO normal 2 74pt13ptsvg-formulamathimg7aa02352661b2ab474ee3937394cf483 acp-20-3999-2020-ie00009.svg74pt13ptacp-20-3999-2020-ie00009.png ). These results suggested that the heterogeneous reaction was probably a major mechanism of nitrate formation during the high-PMinline-formula2.5 events. Moreover, the average production rate of inline-formula M23inlinescrollmathml chem normal NO normal 3 - 25pt16ptsvg-formulamathimg9d4fcb6817fb360ca45e4c6c5f1d67f7 acp-20-3999-2020-ie00010.svg25pt16ptacp-20-3999-2020-ie00010.png by heterogeneous processes was estimated to be 12.6 % hinline-formula−1 (4.1 inline-formulaµg minline-formula−3 hinline-formula−1), which was much higher than that (2.5 % hinline-formula−1; 0.8 inline-formulaµg minline-formula−3 hinline-formula−1) of gas-phase reactions. This can also explain the abrupt increases in nitrate concentrations during the high-PMinline-formula2.5 events. Utilizing the ISORROPIA II model, we found that nitrate aerosol formation in Nanjing during the high-PMinline-formula2.5 events was inline-formulaHNO3 limited. This indicated that control of inline-formulaNOx emissions will be able to efficiently reduce airborne particulate nitrate concentrations and improve the air quality in this industrial city.

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Lin, Yu-Chi / Zhang, Yan-Lin / Fan, Mei-Yi / et al: Heterogeneous formation of particulate nitrate under ammonium-rich regimes during the high-PM2.5 events in Nanjing, China. 2020. Copernicus Publications.

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