Abstract. Comprehensive measurements are vital to obtain big enough
datasets for better understanding the complex atmosphere and further
improving the air quality. To investigate the 4-year variation of air
quality and the influences of special events (Beijing Winter Olympics, COVID
lockdown and Chinese New Year) on it during the wintertime in polluted urban
air, we conducted comprehensive observations in Beijing, China, during
1 January–20 February, in the years from 2019 to 2022. The
mass concentration of PM2.5 and its composition (organics, nitrate,
sulfate, ammonium, chloride and black carbon) and the number size distributions of
particles (down to ∼1 nm) and ions, gaseous pollutants (CO,
NOx, SO2, O3) and condensable vapors (sulfuric acid and
oxygenated organic molecules), as well as meteorological parameters, were
simultaneously measured. The days before 22 January without any
special events in each year were selected to investigate the 4-year
variability of air quality. We found that the concentrations of CO,
NOx, total oxygenated organic molecules (OOMs), total PM2.5,
organics, chloride and black carbon and the number concentration of sub-3 nm
particles (N1.3−3) showed similar variations, decreasing from 2019 to
2021 and then increasing in 2022. For SO2, however, its concentration
decreased year by year due to the significant emission reduction, further
leading to the decrease of gaseous sulfuric acid and particulate sulfate
from 2019 to 2022. O3 concentration showed an opposite 4-year
variation compared with NOx. Meanwhile, both the oxygen and nitrogen
contents of oxygenated organic molecules increased year by year, implying
that not only the oxidation state of those compounds increased, but also
NOx was involved more efficiently in their formation processes. With
higher sulfuric acid concentrations and new particle formation (NPF) frequencies in 2021 than in
2022, and with the lowest concentrations of background aerosols and the
lowest ambient temperatures in 2021, N1.3−3 was still the lowest in
2021. Unlike N1.3−3, the ion concentrations in both 0.8–2 and
2–4 nm size ranges were higher in 2021 than in the other years. Then, the
days after 4 February were chosen to explore the influence of special
events. The non-event days within this date range in 2019 and 2021 were
chosen as the reference period. Due to the favorable meteorological
conditions together with reductions in anthropogenic emissions, there were
basically no haze events during the Olympics. Therefore, CO, NOx,
SO2, total OOMs, accumulation-mode particles (N100−1000), and total
PM2.5 and its composition were much lower, while ion concentrations
were much higher compared with the reference period. Although there was also
emission reduction during COVID, especially for NOx, the
enhancement of secondary inorganic aerosol formation, together with
unfavorable meteorological conditions, caused severe haze events during this
period. Hence, CO, total OOMs and all PM2.5 compositions during
COVID increased dramatically compared with the reference period. Influenced
by SO2, condensation sink and sunlight, sulfuric acid concentration was
found to be comparable between the Olympics and the reference period but was
lower during COVID and Chinese New Year. Additionally, N1.3−3 was
almost at the same level during different periods, indicating that the
special events only had little impact on the NPF
processes. These results provide useful information to the development of
more targeted pollution control plans.
