لخّصلي

At the end of 2019, there was a sudden outbreak of the Coronavirus
Disease 2019(COVID-19) in China, which, as it did across most of the
globe, went on to reach pandemic level, posing serious threats to public
life and health. In order to prevent the spread of COVID-19, a first-level
public health emergency response (ELPHER) was triggered throughout
China in January 2020. During the ELPHER period, except for some
mainstay sectors that maintained the operation of ordinary human life
and society, most sectors, including traffic, construction, commerce, catering, tourism and industrial activities, were subjected to an extreme
or complete standstill (Huang et al., 2020; Sun et al., 2020; Li et al.,
2020; Zheng et al., 2020). Therefore, the ELPHER period is also called
the “COVID-19 lockdown” period. Since various types of anthropogenic
sources are important contributors to air pollutants, especially in urban
areas, all control measures during the lockdown period not only effectively limited the spread of the epidemic, but also made great changes in
the emissions of air pollutants from anthropogenic sources. Therefore,
this lockdown provides a unique opportunity to investigate the response
of air pollution to a drastic reduction in emissions from anthropogenic
sources.
Accordingly, a large number of studies over the past year from across
the globe have already evaluated the variations in air pollutants during
the COVID-19 lockdown period. In North America, Berman and Ebisu
(2020) found that the concentration of nitrogen dioxide (NO2) decreased
by 25.5%, while the change in fine particulate matter (PM2.5) (decreased
of 4.5%) was not as large as that of NO2 in the USA, which may be
because multiple non-transportation sources contributed PM2.5 concentrations. In Europe, Menut et al. (2020) found that the concentrations
of NO2 and PM2.5 decreased by 30%–50% and 5%–10%, respectively, in
all of western Europe during the lockdown periods, while the ozone (O3)
concentration increased. In Asia, the results reported by Sharma et al.
(2020) demonstrated that the concentrations of coarse particulate
matter (PM10) and PM2.5 witnessed maximum reductions (>50.0%)
compared to the period before lockdown. As the first country to experience a massive outbreak of COVID-19, the characteristics of pollutant
changes in China during the lockdown period have been reported in
many cities, such as Beijing (Huang et al., 2020; Sun et al., 2020),
Shanghai (Li et al., 2020), Wuhan (Zheng et al., 2020) and Suzhou
(Wang et al., 2021a). For gaseous pollutants, most of the results from
these studies are consistent; that is, nitric oxide (NO), NO2, carbon
monoxide (CO) and sulfur dioxide (SO2) decreased, while O3 increased.
For PM2.5, although its concentration decreased in some cities, such as
Hangzhou (Liu et al., 2021), Nanjing (Wang et al., 2021b) and Wuhan
(Zheng et al., 2020), serious pollution episodes still occurred in some
cities or regions, such as Shanghai (Chang et al., 2020) and the Beijing–Tianjin–Hebei region (Zhao et al., 2020), and unfavorable meteorological conditions, local emission sources, regional transmission and
the formation of secondary pollutants were important causes of these
episodes.
Sichuan province, located in southwest China, with a population of
over 83 million (https://data.stats.gov.cn), has also been seriously
affected by the COVID-19. Sichuan began its ELPHER period on January
25, 2020, and the whole province went into lockdown on the same day.
Many previous studies have pointed out that air pollution in Sichuan
province is affected by the topography and climate of the basin and the
pollution formation mechanism is obviously different from that of cities
in North China (Gao et al., 2018; Ji et al., 2016; Luo et al., 2020; Xu
et al., 2018b). Although many studies have reported the effects of
COVID-19 lockdown on the air pollution in many cities in China, studies
on Sichuan province or southwest China are fewer. At the same time,
those studies on Sichuan province that have been conducted mainly
focused on the variations in air pollution indexes (PM2.5, PM10, SO2,
NO2, CO and O3) before and during the lockdown period (Benchrif et al.,
2021; Deng et al., 2021; Fan et al., 2021; Wang et al., 2020c; Zhang
et al., 2020), and there is almost no in-depth research on the chemical
composition, source and pollution formation mechanism of particulate
matter. Therefore, in order to obtain more information about the effects
of the COVID-19 lockdown on air pollution in southwest China, single
particle aerosol mass spectrometer (SPAMS) observations, as well as
those from other types of online observation equipments, were used to
measure the atmospheric single aerosol particles and gaseous pollutants
in Chengdu, the capital city of Sichuan province, before and during the
lockdown period. Then, the composition and sources of single aerosol
particles and pollution formation mechanisms in different periods were
analyzed, which is of great significance for evaluating the contribution
and emissions reduction potential of different types of anthropogenic
sources to air pollution in this area