Staggered-peak production is a mixed blessing in the control of particulate matter pollution

Staggered-peak production (SP)-a measure to halt industrial production in the heating season-has been implemented in North China Plain to alleviate air pollution. We compared the variations of PM₁ composition in Beijing during the SP period in the 2016 heating season (SP_hs) with those in the normal production (NP) periods during the 2015 heating season (NP_hs) and 2016 non-heating season (NP_nhs) to investigate the effectiveness of SP. The PM₁ mass concentration decreased from 70.0 ± 54.4 μg m^-3 in NP_hs to 53.0 ± 56.4 μg m^-3 in SP_hs, with prominent reductions in primary emissions. However, the fraction of nitrate during SP_hs (20.2%) was roughly twice that during NP_hs (12.7%) despite a large decrease of NO_x, suggesting an efficient transformation of NO_x to nitrate during the SP period. This is consistent with the increase of oxygenated organic aerosol (OOA), which almost doubled from NP_hs (22.5%) to SP_hs (43.0%) in the total organic aerosol (OA) fraction, highlighting efficient secondary formation during SP. The PM₁ loading was similar between SP_hs (53.0 ± 56.4 μg m^-3) and NP_nhs (50.7 ± 49.4 μg m^-3), indicating a smaller difference in PM pollution between heating and non-heating seasons after the implementation of the SP measure. In addition, a machine learning technique was used to decouple the impact of meteorology on air pollutants. The deweathered results were comparable with the observed results, indicating that meteorological conditions did not have a large impact on the comparison results. Our study indicates that the SP policy is effective in reducing primary emissions but promotes the formation of secondary species.