Extreme haze pollution in Beijing during January 2013: chemical characteristics, formation mechanism and role of fog processing

Abstract. Severe haze hovered over large areas of China in January 2013 right after the public release of PM_2.5 data of major cities in China at the very first time. This historical severe haze emerged over the northern China with monthly average concentrations of PM_2.5, SO₂, and NO₂ exceeding 225, 200, and 80 μg m⁻³, respectively. Surface aerosol mean concentration of Beijing in January 2013 reached record high (only slightly lower than 2006) compared to historical data from 2003–2012, but with the largest daily fluctuation. Anomalous meteorological conditions in 2013 compared to the mean climatology from 2007–2012 were especially favorable for the formation of haze, such as higher humidity, lower temperature, lower PBL height, lower wind speed, and the high frequency of fog occurrences. The field campaign in Beijing showed an extremely high PM_2.5 average concentration of 299.2 ± 79.1μg m⁻³ with extremely low visibility of 0.92 ± 0.82 km during an episode of high relative humidity with fog events. High AOD (Aerosol Optical Depth) was observed during fog days but with relatively low Angstrom exponent (< 1.0), suggesting the modification of fog processing on the particle size. Major aerosol chemical species, such as SO₄²⁻, NO₃⁻, NH₄⁺, Cl⁻, K⁺, and C₂O₄²⁻ presented an explicit exponential growth relationship with relative humidity, suggesting the significant impact of aerosol hygroscopicity on the visibility impairment. SO₄²⁻ increased ∼5 folds while NO₃⁻, NH₄⁺, and C₂O₄²⁻ increased ∼3 folds in the fog days compared to the non-fog days. Aerosol in fog days was much more acidic than that in non-fog days. The in situ aerosol pH ranged from −0.78 to 0.14 in fog days based on the E-AIM model simulation. Bisulfate (HSO₄²⁻) accounted for 52% of the total sulfate and free hydrogen ion (H⁺_Aq) accounted for 27% of the total acids in average. Enhanced coal combustion during the winter heating season along with traffic and industrial emissions were recognized to be the major causes for this severe haze. Fog processing was found to be the major pathway of producing extremely high yields of secondary inorganic aerosol and impacting the neutralization process (i.e. aerosol acidity) in this study.