A month-long filter-based campaign in Longyou, a stagnation-prone basin city in Zhejiang, China (11 January–5 February 2018), attributed a severe haze episode mainly to rapid secondary inorganic aerosol formation: nitrate was inferred to grow predominantly via homogeneous neutralisation of nitric acid by ammonia, and sulfate via heterogeneous reactions of SO2, NO2 and NH3, with both conclusions resting on compositional correlations rather than direct rate or pathway measurements. The problem posed here is to replace these correlation-level inferences with quantitative, observation-constrained attribution: the fractional contributions of daytime homogeneous HNO3+NH3 formation versus nocturnal heterogeneous N2O5 hydrolysis to nitrate; the oxidants and rates controlling sulfate production, with the aerosol-water feedback as a testable mechanistic hypothesis; and the emission-source contributions to episode PM2.5 in this basin setting.

Background

PM2.5 (particulate matter with aerodynamic diameter below 2.5 micrometres) haze in eastern China is driven by the interplay of secondary aerosol formation, stagnant meteorology, and regional transport. Secondary inorganic aerosols (SIA) — sulfate, nitrate, and ammonium — often dominate PM2.5 mass during winter haze episodes. Nitrate forms through two competing routes: (i) the homogeneous pathway, in which nitric acid (HNO3) produced mainly by daytime oxidation of NO2 by the hydroxyl radical (OH) reacts with ammonia (NH3) to form ammonium nitrate; and (ii) heterogeneous nocturnal chemistry, in which the nitrate radical (NO3, formed from NO2+O3) combines with NO2 to give dinitrogen pentoxide (N2O5), which is then hydrolysed on aqueous aerosol surfaces to nitrate. Sulfate forms by oxidation of dissolved SO2 (S(IV)) in aqueous particles, cloud, or fog droplets by oxidants such as H2O2, O3, NO2, or transition-metal-ion catalysis (O2 with Fe3+/Mn2+); which oxidant dominates in ammonia-rich, high-humidity, low-photochemistry winter haze remains one of the most debated questions in Chinese haze chemistry (Cheng et al. 2016 proposed reactive nitrogen chemistry in aerosol water, i.e. NO2 oxidation, as a major sulfate source during haze). A hypothesised positive feedback — more sulfate and nitrate means more hygroscopic aerosol liquid water, which accelerates further aqueous production — makes these pathways self-amplifying but hard to quantify from composition alone. The sulfur oxidation ratio (SOR, the fraction of total measured sulfur present as sulfate) and its nitrate analogue (NOR) are commonly used correlation-level indicators of such secondary conversion. Bao et al. (2019) studied a severe January 2018 haze episode in Longyou, a small basin city in western Zhejiang province with an industrial area to its northeast, using discrete filter sampling of PM2.5 composition, wind analysis, and backward-trajectory methods — potential source contribution function (PSCF) and concentration-weighted trajectories (CWT), which map pollutant concentrations onto air-parcel histories to indicate likely source regions. They found nitrate to be the most abundant ion and inferred that the episode was driven mainly by homogeneous nitrate formation, with sulfate from heterogeneous reactions of NO2, SO2 and NH3, under stagnant winds below 2 m/s. Because only discrete compositional data were available, these pathway assignments rest on correlations (ion balances, SOR/NOR, humidity), and the study itself flags the quantitative questions it could not answer — the very questions that later regional work (e.g. isotopic nitrate studies in the Yangtze River Delta reporting N2O5 hydrolysis as a major nocturnal winter nitrate source, and continued N2O5 parameterization uncertainty in chemical transport models, i.e. regional models that simulate emissions, atmospheric chemistry, and pollutant transport) shows are still method-limited and contested.

Problem Statement

For winter haze episodes in the Longyou basin (Zhejiang, eastern China), determine quantitatively, with observation-constrained field measurements and/or chemical transport or chamber modelling: (1) the fractional contributions of the homogeneous pathway (daytime NO2+OH oxidation to HNO3 followed by NH3 neutralisation) and the nocturnal heterogeneous pathway (N2O5 formation and hydrolysis on aqueous aerosol) to nitrate formation during haze evolution, with explicitly stated uncertainties and their basis; (2) the dominant oxidant(s) and effective production rates of sulfate formation under the observed ammonia-rich, high-humidity, low-photochemistry conditions, with the hypothesised aerosol liquid water-sulfate positive feedback examinable as a mechanistic hypothesis where the data allow rather than a required result; and (3) quantitative attribution of episode PM2.5 (and its secondary inorganic components) to emission sources and source regions, going beyond the trajectory-level PSCF/CWT indication, by any quantitative apportionment approach — for example receptor modelling (PMF, CMB, or aerosol mass spectrometry factor analysis), isotope-based apportionment, chemical-transport-model source tagging (tracking emissions by sector within a model), or inverse modelling (fitting emissions to reproduce observed concentrations) — with the sector resolution determined by the data and method used (which may include local and regional industrial, combustion, agricultural ammonia and biomass-burning sources and regional transport). These deliverables are linked by one objective — replacing correlation-level with quantitative mechanistic and source attribution of Longyou winter haze.

References

  1. Bao Z, Chen L, Li K, Han L, Wu X, Gao X, Azzi M, Cen K (2019) Meteorological and chemical impacts on PM2.5 during a haze episode in a heavily polluted basin city of eastern China. Environmental Pollution 250:520-529. doi:10.1016/j.envpol.2019.04.045, https://doi.org/10.1016/j.envpol.2019.04.045
  2. Bao Z, Zhang X, Li Q, Zhou J, Shi G, Zhou L, Yang F, Xie S, Zhang D, Zhai C, Li Z, Peng C, Chen Y (2023) Measurement report: Intensive biomass burning emissions and rapid nitrate formation drive severe haze formation in the Sichuan Basin, China – insights from aerosol mass spectrometry. Atmospheric Chemistry and Physics 23:1147-1167. doi:10.5194/acp-23-1147-2023, https://doi.org/10.5194/acp-23-1147-2023
  3. Cheng Y, Zheng G, Wei C, Mu Q, Zheng B, Wang Z, Gao M, Zhang Q, He K, Carmichael G, Poschl U, Su H (2016) Reactive nitrogen chemistry in aerosol water as a source of sulfate during haze events in China. Science Advances 2:e1601530. doi:10.1126/sciadv.1601530, https://doi.org/10.1126/sciadv.1601530
  4. Gao J, Wei Y, Shi G, Yu H, Zhang Z, Song S, Wang W, Liang D, Feng Y (2020) Roles of RH, aerosol pH and sources in concentrations of secondary inorganic aerosols, during different pollution periods. Atmospheric Environment 241:117770. doi:10.1016/j.atmosenv.2020.117770, https://doi.org/10.1016/j.atmosenv.2020.117770
  5. Guo H, Otjes R, Schlag P, Kiendler-Scharr A, Nenes A, Weber RJ (2018) Effectiveness of ammonia reduction on control of fine particle nitrate. Atmospheric Chemistry and Physics 18:12241-12256. doi:10.5194/acp-18-12241-2018, https://doi.org/10.5194/acp-18-12241-2018
  6. Li J, Chen L, Bao Z, Zhang X, Xu H, Gao X, Cen K (2022) Seasonal Variation of Aerosol Composition and Sources of Water-Soluble Organic Carbon in an Eastern City of China. Atmosphere 13:1968. doi:10.3390/atmos13121968, https://doi.org/10.3390/atmos13121968
  7. Qu Y, Yang J, Guo Y, Chen Y, Liu X, Chen H, Ran H, Zhang J, Li Y, An J (2025) Evaluating parameterization schemes of dinitrogen pentoxide heterogeneous processes in the presence of organic coatings for size-resolved nitrate simulations over North China. Atmospheric Environment 358:121335. doi:10.1016/j.atmosenv.2025.121335, https://doi.org/10.1016/j.atmosenv.2025.121335
  8. Zhao ZY, Cao F, Fan MY, Zhai XY, Yu HR, Hong Y, Ma YJ, Zhang YL (2021) Nitrate aerosol formation and source assessment in winter at different regions in Northeast China. Atmospheric Environment 267:118767. doi:10.1016/j.atmosenv.2021.118767, https://doi.org/10.1016/j.atmosenv.2021.118767
  9. Zhou G, Yu Z, Qu Y (2021) Real-time numerical source apportionment of PM2.5 concentrations over the Yangtze River Delta region, China. Atmospheric Environment 246:118104. doi:10.1016/j.atmosenv.2020.118104, https://doi.org/10.1016/j.atmosenv.2020.118104