Ground-based max-doas observations of tropospheric no₂ and hcho during covid-19 lockdown and spring festival over Shanghai, China

Reduced mobility and less anthropogenic activity under special case circumstances over various parts of the world have pronounced effects on air quality. The objective of this study is to investigate the impact of reduced anthropogenic activity on air quality in the mega city of Shanghai, China. Observations from the highly sophisticated multi-axis differential optical absorption spectroscope (MAX-DOAS) instrument were used for nitrogen dioxide (NO₂ ) and formaldehyde (HCHO) column densities. In situ measurements for NO₂, ozone (O₃ ), particulate matter (PM_2.5 ) and the air quality index (AQI) were also used. The concentration of trace gases in the atmosphere reduces significantly during annual Spring Festival holidays, whereby mobility is reduced and anthropogenic activities come to a halt. The COVID-19 lockdown during 2020 resulted in a considerable drop in vertical column densities (VCDs) of HCHO and NO₂ during lockdown Level-1, which refers to strict lockdown, i.e., strict measures taken to reduce mobility (43% for NO₂; 24% for HCHO), and lockdown Level-2, which refers to relaxed lockdown, i.e., when the mobility restrictions were relaxed somehow (20% for NO₂; 22% for HCHO), compared with pre-lockdown days, as measured by the MAX-DOAS instrument. However, for 2019, a reduction in VCDs was found only during Level-1 (24% for NO₂; 6.62% for HCHO), when the Spring Festival happened. The weekly cycle for NO₂ and HCHO depicts no significant effect of weekends on the lockdown. After the start of the Spring Festival, the VCDs of NO₂ and HCHO showed a decline for 2019 as well as 2020. Backward trajectories calculated using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model indicated more air masses coming from the sea after the Spring Festival for 2019 and 2020, implying that a low pollutant load was carried by them. No impact of anthropogenic activity was found on O₃ concentration. The results indicate that the ratio of HCHO to NO₂ (RFN) fell in the volatile organic compound (VOC)-limited regime.