  Background: On March 19, 2020, the government of California ordered all 40
million Californians to stay at home in the coming weeks as the result of the
escalation of the coronavirus disease 2019 (COVID-19) pandemic. As lockdowns
were implemented, the significant changes caused by these restrictions brought
a dramatic improvement in air quality in metropolitan cities such as Los
Angeles (LA Basin).Methods: We use real-time data from The South Coast Air
Quality Management District (South Coast AQMD), and the California Department
of Transportation to evaluate the drivers of the pollution sources. We also
mapped monthly spatial variations and constructed hourly heatmaps of those
pollutants in 2020 to understand the impacts of the lockdown on different
locations and times of day in the LA Basin. Results: Compared to the same dates
in 2019, traffic flow on highways in the Los Angeles Basin dropped by 20.86 %
when the stay at home order was initiated and it continued to decrease along
with dramatic declines in NO2, CO, and PM2.5. The correlation (Pierson r)
between truck flow change and changes of NO2, CO, and PM2.5 is statistically
significant. Conclusion: The declines in truck flow are mainly responsible for
the drop of NO2 and CO, with traffic having a slightly smaller effect on PM2.5.
The lockdowns provided a large-scale experiment into air quality research. The
result of this research would provide an important reference for the policy
markers regarding truck management in light of air quality control to prepare
for the 2028 Summer Olympics in LA.
