{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,9,29]],"date-time":"2026-09-29T14:07:51Z","timestamp":1790690871930,"version":"4.1.0"},"reference-count":43,"publisher":"American Chemical Society (ACS)","issue":"38","license":[{"start":{"date-parts":[[2026,9,18]],"date-time":"2026-09-18T00:00:00Z","timestamp":1789689600000},"content-version":"stm-asf","delay-in-days":0,"URL":"https:\/\/doi.org\/10.15223\/policy-029"},{"start":{"date-parts":[[2026,9,18]],"date-time":"2026-09-18T00:00:00Z","timestamp":1789689600000},"content-version":"stm-asf","delay-in-days":0,"URL":"https:\/\/doi.org\/10.15223\/policy-037"},{"start":{"date-parts":[[2026,9,18]],"date-time":"2026-09-18T00:00:00Z","timestamp":1789689600000},"content-version":"stm-asf","delay-in-days":0,"URL":"https:\/\/doi.org\/10.15223\/policy-045"}],"funder":[{"name":"National Basic Research Program of China (973 Program)","award":["2023YFC3709302"],"award-info":[{"award-number":["2023YFC3709302"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":[],"published-print":{"date-parts":[[2026,9,29]]},"abstract":"<jats:title>Abstract<\/jats:title>\n                  <jats:p>Secondary nitrate is a major driver of wintertime PM2.5 pollution in China, yet the coupled photochemical and heterogeneous controls on its formation remain poorly constrained in humid, topographically confined basins. Here, we combined wintertime field observations with a box model and thermodynamic calculations to quantify nitrate formation pathways, radical chemistry, and precursor sensitivities. During pollution episodes, nitrate became the dominant PM2.5 component, systematically co-varied with elevated NO2, Ox, and aerosol liquid water content (ALWC). Pathway-resolved modeling revealed near-equal contributions from the two dominant nitrate formation pathways, daytime OH+NO2 reactions (52%) and nighttime heterogeneous N2O5 hydrolysis (46%), with the latter increasing sharply during polluted episodes. The modeled OH budget indicated efficient HOx recycling, dominated by HO2+NO recycling. Binning regressions identified critical thresholds of RH = 87% and ALWC = 76.5 \u03bcg m\u20133, above which nitrate formation efficiency increased abruptly. Sensitivity analyses indicated that VOCs exerted a slightly stronger control on nitrate formation than NOx, with oxygenated VOCs and alkenes as the most influential groups, and solvent usage and vehicle exhaust as dominant sources. These results reveal a humidity-modulated dual-pathway mechanism for wintertime nitrate formation sustained by VOC-influenced radical cycling. Effective nitrate mitigation therefore requires coordinated reductions in NOx and reactive VOCs, together with measures that limit aerosol water accumulation under high-humidity conditions.<\/jats:p>","DOI":"10.1021\/acs.est.6c08621","type":"journal-article","created":{"date-parts":[[2026,9,18]],"date-time":"2026-09-18T16:02:21Z","timestamp":1789747341000},"page":"27370-27382","source":"Crossref","is-referenced-by-count":0,"title":["Nonlinear\nNitrate\nFormation Governed by Aerosol Water\nand VOC-Sensitive Oxidation in a Humid Urban Basin"],"prefix":"10.1021","volume":"60","author":[{"given":"Rui","family":"Sun","sequence":"first","affiliation":[{"name":"Peking University , , ,","place":["Beijing, China, 100871"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Guiying","family":"You","sequence":"additional","affiliation":[{"name":"Peking University , , ,","place":["Beijing, China, 100871"]},{"name":"Peking University , , ,","place":["Beijing, China, 100871"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jiaxin","family":"Wei","sequence":"additional","affiliation":[{"name":"Peking University , , ,","place":["Beijing, China, 100871"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Danlin","family":"Song","sequence":"additional","affiliation":[{"name":"Chengdu Academy of Environmental Science , ,","place":["Chengdu, PR China, 610015"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Miao","family":"Feng","sequence":"additional","affiliation":[{"name":"Chengdu Academy of Environmental Science , ,","place":["Chengdu, PR China, 610015"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-3673-1289","authenticated-orcid":true,"given":"Shaodong","family":"Xie","sequence":"additional","affiliation":[{"name":"Peking University , , ,","place":["Beijing, China, 100871"]},{"name":"Tianfu Yongxing Laboratory , , ,","place":["Chengdu, Sichuan, China, 610213"]}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"316","published-online":{"date-parts":[[2026,9,18]]},"reference":[{"issue":"49","key":"2026092909073670800_cit1","doi-asserted-by":"publisher","first-page":"17373","DOI":"10.1073\/pnas.1419604111","article-title":"Elucidating severe\nurban haze formation in China","volume":"111","author":"Guo","year":"2014"},{"key":"2026092909073670800_cit2","doi-asserted-by":"publisher","first-page":"113345","DOI":"10.1016\/j.envpol.2019.113345","article-title":"Response of\naerosol chemistry to clean air action in Beijing, China: Insights\nfrom two-year ACSM measurements and model simulations","volume":"255","author":"Zhou","year":"2019","journal-title":"Environ. 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Int."},{"key":"2026092909073670800_cit6","doi-asserted-by":"publisher","first-page":"476","DOI":"10.1016\/j.jes.2022.09.019","article-title":"Formation\nmechanism and control strategy for particulate nitrate in China","volume":"123","author":"Wang","year":"2023","journal-title":"J. Environ. Sci."},{"issue":"9","key":"2026092909073670800_cit7","doi-asserted-by":"publisher","first-page":"428A","DOI":"10.1021\/es00115a727","article-title":"Acid generation in the troposphere\nby gas-phase chemistry","volume":"17","author":"Calvert","year":"1983","journal-title":"Environ. Sci. Technol."},{"issue":"15","key":"2026092909073670800_cit8","doi-asserted-by":"publisher","first-page":"9243","DOI":"10.1021\/acs.est.0c00972","article-title":"Field Determination of Nitrate Formation\nPathway in Winter Beijing","volume":"54","author":"Chen","year":"2020","journal-title":"Environ. Sci. 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Phys."},{"issue":"6","key":"2026092909073670800_cit28","doi-asserted-by":"publisher","first-page":"1461","DOI":"10.5194\/acp-4-1461-2004","article-title":"Evaluated\nkinetic and photochemical data for atmospheric chemistry: Volume I\n- gas phase reactions of Ox, HOx, NOx","volume":"4","author":"Atkinson","year":"2004","journal-title":"Atmos.\nChem. Phys."},{"issue":"16","key":"2026092909073670800_cit29","doi-asserted-by":"publisher","first-page":"11581","DOI":"10.5194\/acp-18-11581-2018","article-title":"Production\nof N2O5 and ClNO2 in summer in urban Beijing, China","volume":"18","author":"Zhou","year":"2018","journal-title":"Atmos. Chem. Phys."},{"issue":"7","key":"2026092909073670800_cit30","doi-asserted-by":"publisher","first-page":"4367","DOI":"10.5194\/acp-20-4367-2020","article-title":"Heterogeneous N2O5 reactions on atmospheric\naerosols at four Chinese sites: improving model representation of\nuptake parameters","volume":"20","author":"Yu","year":"2020","journal-title":"Atmos. Chem. Phys."},{"issue":"5","key":"2026092909073670800_cit31","doi-asserted-by":"publisher","first-page":"2686","DOI":"10.1021\/acs.est.7b05222","article-title":"Role of Nitrogen Dioxide in the Production\nof Sulfate during Chinese Haze-Aerosol Episodes","volume":"52","author":"Li","year":"2018","journal-title":"Environ. Sci. Technol."},{"key":"2026092909073670800_cit32","doi-asserted-by":"publisher","DOI":"10.1029\/2021JD035623","article-title":"An Observational Based\nModeling of the Surface Layer Particulate Nitrate in the North China\nPlain During Summertime","volume":"126","author":"Tan","year":"2021","journal-title":"J. Geophys. Res.: atmos."},{"issue":"5","key":"2026092909073670800_cit33","doi-asserted-by":"publisher","first-page":"1183","DOI":"10.1016\/j.atmosenv.2010.09.003","article-title":"Nighttime enhancement\nof PM2.5 nitrate\nin ammonia-poor atmospheric conditions in Beijing and Shanghai: Plausible\ncontributions of heterogeneous hydrolysis of N2O5 and HNO3 partitioning","volume":"45","author":"Pathak","year":"2011","journal-title":"Atmos. Environ."},{"issue":"4","key":"2026092909073670800_cit34","doi-asserted-by":"publisher","first-page":"2379","DOI":"10.5194\/acp-23-2379-2023","article-title":"Elucidate the formation\nmechanism of particulate nitrate based on\ndirect radical observations in the Yangtze River Delta","volume":"23","author":"Zhai","year":"2023","journal-title":"Atmos. Chem. Phys."},{"issue":"4","key":"2026092909073670800_cit35","doi-asserted-by":"publisher","first-page":"2649","DOI":"10.5194\/acp-23-2649-2023","article-title":"O3\u2013precursor\nrelationship over multiple patterns of timescale: a case study in\nZibo, Shandong Province, China","volume":"23","author":"Zheng","year":"2023","journal-title":"Atmos. Chem.\nPhys."},{"key":"2026092909073670800_cit36","doi-asserted-by":"publisher","DOI":"10.1029\/2021JD034560","article-title":"Secondary\nFormation of Aerosols Under Typical High-Humidity Conditions in Wintertime\nSichuan Basin, China: A Contrast to the North China Plain","volume":"126","author":"Wang","year":"2021","journal-title":"J. Geophys. Res.: atmos."},{"issue":"3","key":"2026092909073670800_cit37","doi-asserted-by":"publisher","first-page":"160","DOI":"10.1021\/acs.estlett.8b00021","article-title":"Aerosol Liquid Water Driven by Anthropogenic\nInorganic Salts: Implying Its Key Role in Haze Formation over the\nNorth China Plain","volume":"5","author":"Wu","year":"2018","journal-title":"Environ. Sci. Technol. Lett."},{"issue":"9","key":"2026092909073670800_cit38","doi-asserted-by":"publisher","first-page":"5703","DOI":"10.5194\/acp-17-5703-2017","article-title":"Fine particle pH\nand gas\u2013particle phase partitioning of inorganic species in\nPasadena, California, during the 2010 CalNex campaign","volume":"17","author":"Guo","year":"2017","journal-title":"Atmos. Chem. Phys."},{"key":"2026092909073670800_cit39","doi-asserted-by":"publisher","first-page":"118787","DOI":"10.1016\/j.atmosenv.2021.118787","article-title":"PM2.5 pH estimation\nin Seoul during the KORUS-AQ campaign using different thermodynamic\nmodels","volume":"268","author":"Kim","year":"2022","journal-title":"Atmos. Environ."},{"key":"2026092909073670800_cit40","doi-asserted-by":"publisher","first-page":"14675","DOI":"10.1021\/acs.est.5c18827","article-title":"Gas-to-Particle\nPhase Transformation of Atmospheric\nOrganics Amplified by Low Temperature and High Aerosol Water in Winter\nHaze of China","volume":"60","author":"Li","year":"2026","journal-title":"Environ. Sci. Technol."},{"issue":"10","key":"2026092909073670800_cit41","doi-asserted-by":"publisher","first-page":"7129","DOI":"10.5194\/acp-19-7129-2019","article-title":"Experimental budgets\nof OH, HO2,\nand RO2 radicals and implications for ozone formation in the Pearl\nRiver Delta in China 2014","volume":"19","author":"Tan","year":"2019","journal-title":"Atmos. Chem. Phys."},{"key":"2026092909073670800_cit42","doi-asserted-by":"publisher","first-page":"D7","DOI":"10.1029\/2009JD012714","article-title":"(O3 + NO2) production processes and formation regimes\nin Beijing","volume":"115","author":"Lu","year":"2010","journal-title":"J. Geophys. Res.: atmos"},{"issue":"19","key":"2026092909073670800_cit43","doi-asserted-by":"publisher","first-page":"15135","DOI":"10.5194\/acp-21-15135-2021","article-title":"Nonlinear responses of particulate\nnitrate to NOx emission controls in the megalopolises of China","volume":"21","author":"Li","year":"2021","journal-title":"Atmos. Chem. 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