During the 2020 Spring Festival, public activities were cancelled and people tried their best to stay at home due to COVID-19.

However, during this period, severe fog-haze events occurred over the North China Plain. What was the leading factor that caused these severe smog incidents? And what were the individual impacts of meteorological conditions and emission reductions?

Researchers led by Prof. ZHANG Meigen and his team from the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences evaluated the impacts of meteorological conditions and emission reduction measures on the near-surface PM_2.5 during the COVID-19 lockdown.

They conducted three numerical experiments with different meteorological fields and emission sources with a coupled meteorology and aerosol/chemistry model (WRF-Chem).

The findings were published in Atmospheric and Oceanic Science Letters.

The study showed that compared with the same period in 2019, the PM_2.5 concentration in the Beijing-Tianjin-Hebei region increased by 50-70 μg m^-3 from 7 to 14 February 2020.

Results from sensitivity tests showed that the main cause was that the increase in PM_2.5 caused by meteorological conditions was greater than the decrease in PM_2.5 caused by emission reductions.

"Higher temperatures and relative humidity usually hasten the formation of secondary aerosols by accelerating chemical reactions," said Prof. ZHANG. "Meanwhile, the lower wind speed in the Beijing-Tianjin-Hebei region inhibits the diffusion of air pollutants and the lower planetary boundary layer height enhances atmospheric stability. These unfavorable meteorological conditions led to these haze events in the Beijing-Tianjin-Hebei region."

According to this study, it is necessary to consider meteorological conditions when assessing the effectiveness of emission control policies on changes in air pollutants.

Aerosol, emission and meteorology (Image by GAO Yi)