Humidity plays an important role in the PM2.5 pollution in Beijing

Heavily-polluted PM2.5 (fine particulate matter) episodes frequently impacting Beijing, especially during winter, have become a substantial concern. We found that during winter, the daily variation of PM2.5 in Beijing tracked the pattern of relative humidity (RH). With the increase of PM2.5 (or RH), water-soluble components (especially inorganic ions) became more abundant, and the water-soluble organic carbon to organic carbon ratios increased. The nitrate to sulfate ratios also exhibited dependence on RH, and were higher than those measured about a decade ago, consistent with the increasing trend of nitrogen oxides emissions. Surprisingly, the ratios of water-insoluble organic carbon to elemental carbon showed significant increase at high RH levels, presumably indicating the formation of secondary organic aerosol that is not soluble in water. In addition, humid winters were occasionally identified during 1996–2013 which are expected to be favorable for the formation of air pollution episodes with high PM2.5 concentrations. •The variation of PM2.5 during winter in Beijing tracked the pattern of RH.•Water-soluble components became more abundant with the increase of PM2.5 and RH.•Nitrate showed the highest enrichment ratio during PM2.5 episodes.•Nitrate to sulfate ratios were higher than those measured about a decade ago.•Humid winters were occasionally identified since 1996 in Beijing. Aqueous-phase reactions are highly responsible for the formation and evolution of PM2.5 episodes in Beijing.