<p>Severe winter hazes accompanied by high concentrations of fine particulate matter (PM<sub>2.5</sub>) occur frequently in the North China Plain and threaten public health. Organic matter (OM) and sulfate are recognized as major components of PM<sub>2.5</sub>, while atmospheric models often fail to predict their high concentrations during severe winter hazes due to incomplete understanding of secondary aerosol formation mechanisms. By using a novel combination of single particle mass spectrometer and optimized ion chromatography measurement, here we show that hydroxymethanesulfonate (HMS), formed by the reaction between formaldehyde (HCHO) and dissolved SO<sub>2</sub> in aerosol water, is ubiquitous in Beijing winter. The HMS concentration and the molar ratio of HMS to sulfate increased with the deterioration of winter haze. High concentrations of precursors (SO<sub>2</sub> and HCHO) coupled with low oxidant levels, low temperature, high relative humidity, and moderately acid pH facilitate the heterogeneous formation of HMS, which could account for up to 15 % of OM in winter haze and lead to 36 % overestimates of sulfate when using traditional ion chromatography measurements. Despite the clean air actions have substantially reduced SO<sub>2</sub> emissions, HMS concentration and molar ratio of HMS to sulfate during severe winter hazes increased from 2015 to 2016 with the growth of HCHO concentration. Our findings illustrate the significant contribution of heterogeneous HMS chemistry to severe winter hazes in Beijing, which help to improve the prediction of OM and sulfate, and suggest that the reduction in HCHO can help to mitigate haze pollution.</p>