Significant concentration changes of chemical components of PM₁ in the Yangtze River Delta area of China and the implications for the formation mechanism of heavy haze–fog pollution

Since the winter season of 2013, a number of persistent haze–fog events have occurred in central-eastern China. Continuous measurements of the chemical and physical properties of PM₁ at a regional background station in the Yangtze River Delta area of China from 16 Nov. to 18 Dec., 2013 revealed several haze–fog events, among which a heavy haze–fog event occurred between 6 Dec. and 8 Dec. The mean concentration of PM₁ was 212 μg m⁻³ in the heavy haze–fog period, which was about 10 times higher than on clean days and featured a peak mass concentration that reached 298 μg m⁻³. Organics were the largest contributor to the dramatic rise of PM₁ on heavy haze–fog days (average mass concentration of 86 μg m⁻³), followed by nitrate (58 μg m⁻³), sulfate (35 μg m⁻³), ammonium (29 μg m⁻³), and chloride (4.0 μg m⁻³). Nitrate exhibited the largest increase (~ 20 factors), associated with a significant increase in NO_x. This was mainly attributable to increased coal combustion emissions, relative to motor vehicle emissions, and was caused by short-distance pollutant transport within surrounding areas. Low-volatility oxidized organic aerosols (OA) (LV-OOA) and biomass-burning OA (BBOA) also increased sharply on heavy haze–fog days, exhibiting an enhanced oxidation capacity of the atmosphere and increased emissions from biomass burning. The strengthening of the oxidation capacity during the heavy pollution episode, along with lower solar radiation, was probably due to increased biomass burning, which were important precursors of O₃. The prevailing meteorological conditions, including low wind and high relative humidity, and short distance transported gaseous and particulate matter surrounding of the sampling site, coincided with the increased pollutant concentrations mainly from biomass-burning mentioned above to cause the persistent haze–fog event in the YRD area. - Highlights: • Formation mechanism of a heavy haze-fog event that occurred in the YRD region of china was discussed. • Nitrate exhibited the largest increase on the heavy haze-fog days. • Increased biomass burning and atmospheric oxidizing capacity were the major reasons for the significant increase of OA. • Short-distance transport, low wind speed and high RH were highly responsible for the formation of the pollution event.