Overestimated role of sulfate in haze formation over Chinese megacities due to improper simulation of heterogeneous reactions

Heterogeneous reactions involving aerosol water, i.e., haze chemistry, are responsible for high aerosol concentrations observed during severe haze events in Chinese megacities. Attempts were made to incorporate these reactions into air quality models, by using reactive uptake coefficients for gaseous precursors of secondary aerosols. Nonetheless, the revised models were most frequently evaluated using bulk particle concentrations, mainly due to the lack of observational constraints on aerosol compositions. Here, we tested such a revised model using data from primary and secondary aerosol components measured during the winter of 2019‒2020 in Harbin, an unexplored megacity located in the severe cold climate region in Northeast China. We studied reasons for discrepancies between observed and modeled results. Our findings show that despite the reasonable consistency between the observed and simulated aerosol concentrations, large discrepancies occur for sulfate and secondary-organic-carbon under humid conditions, e.g., with average overprediction of ~ 20 μg/m³ for sulfate, and underprediction of ~ 10 μgC/m³ for secondary-organic-carbon. These discrepancies point to improper simulation of heterogeneous reactions. Therefore, we suggest that previous modeling has exaggerated the role of sulfate in haze formation over Chinese megacities.