Haze Occurrence Caused by High Gas-to-Particle Conversion in Moisture Air under Low Pollutant Emission in a Megacity of China

Inverse effects of aerosol radiative forcing on heavy PM2.5 pollution of local accumulation and regional transport over Central China

Regional transport of air pollutants is a crucial factor influencing atmospheric environment, and aerosol radiative forcing (ARF) feedback to atmospheric boundary layer (ABL) structure and ambient air pollution is yet to be comprehensively understood over the receptor region of regional transport. By simulating meteorology and air pollutants during a heavy PM2.5 pollution event with WRF-Chem model, we quantitatively investigated the ARF and ABL interaction for PM2.5 pollution over the Twain-Hu Basin (THB), a key receptor region of regional transport over central China. Driven by northerly winds, PM2.5 was transported from upstream north China to downstream THB accompanied by high PM2.5 levels in the free troposphere. The ARF exacerbated local PM2.5 accumulation by up to 20 μg m-3 and inhibited the impact of regional transport on PM2.5 levels in the ABL with reducing near-surface PM2.5 concentrations of 5 μg m-3 over the THB. The ARF-intensified air temperature inversion at the top of ABL was unfavorable for the transported air pollutants crossing the ABL top to the near-surface layer, thus weakening the impact of regional PM2.5 transport on air quality in the receptor region. Meanwhile, the ARF of transported PM2.5 induced updrafts in the free troposphere, promoting vertical mixing of air pollutants with positive feedback on increasing secondary PM2.5 concentrations in the free troposphere. The ARF induced more and less secondary PM2.5 formations respectively in the free troposphere and the near-surface layer during the regional transport period of air pollution. These results enhance our comprehension of aerosol-meteorology feedback in regional changes of atmospheric environment with inverse effects of ARF on PM2.5 pollution of local accumulation and regional transport.

Critical contribution of moisture to the air quality deterioration in a warm and humid weather

The deterioration of air quality that threatens human health is recognized as focal compound hazard. Here, decisive thermodynamic conditions for activation of secondary aerosol formation have been investigated focused on Korea. In a dry environment with relative humidity < 60%, gas phase reaction to form fine particles depended largely on surface temperature. In a wet environment (relative humidity ≥ 60%), however, aqueous phase reaction of secondary inorganic aerosols, which are sulfate, nitrate, and ammonium, accounting for 67% of the total aerosol mass, was more activated. Thus, humidity is as important as temperature in the secondary production of aerosol; air quality often worsened when a low-pressure system was predominant over the Korean Peninsula. It is rather different from the general synoptic conditions of high concentrations of particulate matters characterized by high pressure and atmospheric stagnation. This study suggests additional favorable condition and responsible mechanism of air quality hazards that may be frequent in future.

Haze caused by NOx oxidation under restricted residential and industrial activities in a mega city in the south of North China Plain

The formation of secondary aerosol species, including nitrate and sulfate, induces severe haze in the North China Plain. However, despite substantial reductions in anthropogenic pollutants due to severe restriction of residential and industrial activities in 2020 to stop the spread of COVID-19, haze still formed in Zhengzhou. We compared ionic compositions of PM_2.5 during the period of the restriction with that immediately before the restriction and in the comparison period in 2019 to investigate the processes that caused the haze. The average concentration of PM_2.5 was 83.9 μg m^-3 in the restriction period, 241.8 μg m^-3 before the restriction, and 94.0 μg m^-3 in 2019. Nitrate was the largest contributor to the PM_2.5 in all periods, with an average mass fraction of 24%-30%. The average molar concentration of total nitrogen compounds (NO_x + nitrate) was 0.89 μmol m^-3 in the restriction period, which was much lower than that in the non-restriction periods (1.85-2.74 μmol m^-3). In contrast, the concentration of sulfur compounds (SO₂ + sulfate) was 0.34-0.39 μmol m^-3 in all periods. The conversion rate of NO_x to nitrate (NOR) was 0.35 in the restriction period, significantly higher than that before the restriction (0.26) and in 2019 (0.25). NOR was higher with relative humidity in 40-80% in the restriction period than in the other two periods, whereas the conversion rate of SO₂ to sulfate did not, indicating nitrate formation was more efficient during the restriction. When O₃ occupied more than half of the oxidants (O_x = O₃ + NO₂), NOR increased rapidly with the ratio of O₃ to O_x and was much higher in the daytime than nighttime. Therefore, haze in the restriction period was caused by increased NO_x-to-nitrate conversion driven by photochemical reactions.