Heterogeneous formation of particulate nitrate under ammonium-rich regimes during the high-PM _2.5 events in Nanjing, China

Particulate nitrate (inline-formula $M2inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 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-formula_2.5 were conducted in Nanjing City in order to investigate the potential formation mechanisms of particulate nitrate. During the sampling periods, inline-formula $M4inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 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 $M5inlinescrollmathmlchem{\mathrm{normal\; SO}}_{\mathrm{normal\; 4}}^{\mathrm{normal\; 2}-}$ 29pt17ptsvg-formulamathimg28cd4f8c12cf9ef751545712573a522a acp-20-3999-2020-ie00003.svg29pt17ptacp-20-3999-2020-ie00003.png (33 %) and inline-formula $M6inlinescrollmathmlchem{\mathrm{normal\; NH}}_{\mathrm{normal\; 4}}^{+}$ 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 $M7inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimga02883d0956e7dc256b9fe9fffa70b09 acp-20-3999-2020-ie00005.svg25pt16ptacp-20-3999-2020-ie00005.png was a major contributing species to high-PMinline-formula_2.5 events (hourly PMinline-formula_2.5≥150 inline-formulaµg minline-formula⁻³). High inline-formula $M12inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimg737339a8d3517116341490f01d8cfecf acp-20-3999-2020-ie00006.svg25pt16ptacp-20-3999-2020-ie00006.png concentrations mainly occurred under inline-formula $M13inlinescrollmathmlchem{\mathrm{normal\; NH}}_{\mathrm{normal\; 4}}^{+}$ 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-formula_2.5 events, the nitrogen conversion ratios (inline-formulaF_n) were positively correlated with the aerosol liquid water content (ALWC; inline-formulaR>0.72 and inline-formulap<0.05). Meanwhile, increasing inline-formula $M19inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimg7248c728767abac31fc80ac33e5f4469 acp-20-3999-2020-ie00008.svg25pt16ptacp-20-3999-2020-ie00008.png concentrations regularly coincided with increasing ALWC and decreasing inline-formulaO_x (inline-formula $M21inlinescrollmathmlchem{\mathrm{normal\; O}}_x=chem{\mathrm{normal\; O}}_{\mathrm{normal\; 3}}+chem{\mathrm{normal\; NO}}_{\mathrm{normal\; 2}}$ 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-formula_2.5 events. Moreover, the average production rate of inline-formula $M23inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 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-formula_2.5 events. Utilizing the ISORROPIA II model, we found that nitrate aerosol formation in Nanjing during the high-PMinline-formula_2.5 events was inline-formulaHNO₃ limited. This indicated that control of inline-formulaNO_x emissions will be able to efficiently reduce airborne particulate nitrate concentrations and improve the air quality in this industrial city.