Case study of spring haze in Beijing: Characteristics, formation processes, secondary transition, and regional transportation

Continuous haze monitoring was conducted from 12:00 3 April to 12:00 8 April 2016 in Beijing, China to develop a more detailed understanding of spring haze characteristics. The PM₂.₅ concentration ranged from 6.30 to 165 μg m⁻³ with an average of 63.8 μg m⁻³. Nitrate was the most abundant species, accounting for 36.4% of PM₂.₅, followed by organic carbon (21.5%), NH₄⁺ (19.3%), SO₄²⁻ (18.8%), and elemental carbon (4.10%), indicating the key role of nitrate in this haze event. Species contribution varied based on the phase of the haze event. For example, sulfate concentration was high during the haze formation phase, nitrate was high during the haze, and secondary organic carbon (SOC) had the highest contribution during the scavenging phase. The secondary transition of sulfate was influenced by SO₂, followed by relative humidity (RH) and Oₓ (O₃+NO₂). Nitrate formation occurred in two stages: through NO₂ oxidation, which was vulnerable to Oₓ; and by the partitioning of N (+5) which was susceptible to RH and temperature. SOC tended to form when Oₓ and RH were balanced. According to hourly species behavior, sulfate and nitrate were enriched during haze formation when the mixed layer height decreased. However, SOC accumulated prior to the haze event and during formation, which demonstrated the strong contribution of secondary inorganic aerosols, and the limiting contribution of SOC to this haze case. Investigating backward trajectories showed that high speed northwestern air masses following a straight path corresponded to the clear periods, while southwesterly air masses which traversed heavily polluted regions brought abundant pollutants to Beijing and stimulated the occurrence of haze pollution. Results indicate that the control of NO₂ needs to be addressed to reduce spring haze. Finally, the correlation between air mass trajectories and pollution conditions in Beijing reinforce the necessity of inter-regional cooperation and control.