Abstract. The chemical mechanisms responsible for rapid sulfate production, an important
driver of winter haze formation in northern China, remain unclear. Here, we
propose a potentially important heterogeneous hydroxymethanesulfonate (HMS)
chemical mechanism. Through analyzing field measurements with aerosol mass
spectrometry, we show evidence for a possible significant existence in haze
aerosols of organosulfur primarily as HMS, misidentified as sulfate in
previous observations. We estimate that HMS can account for up to about
one-third of the sulfate concentrations unexplained by current air quality
models. Heterogeneous production of HMS by SO2 and formaldehyde is
favored under northern China winter haze conditions due to high aerosol water
content, moderately acidic pH values, high gaseous precursor levels, and low
temperature. These analyses identify an unappreciated importance of
formaldehyde in secondary aerosol formation and call for more research on
sources and on the chemistry of formaldehyde in northern China winter.
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