Ground-Based MAX-DOAS Observations of Tropospheric NO2 and HCHO During COVID-19 Lockdown and Spring Festival Over Shanghai, China

Aimon Tanvir,Zhou Bin,Shanshan Wang,Ruibin Xue,Zhu Jian,Zeeshan Javed,Muhammad Bilal,Sanbao Zhang

doi:10.3390/rs13030488

Abstract

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 (PM2.5) 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.

Highlights

IntroductionNitrogen dioxide (NO2 ) and formaldehyde (HCHO) are two important trace gas species in the atmosphere which play a key role in defining the atmospheric chemistry
MAX-differential optical absorption spectroscopy (DOAS) observations were conducted for NO2 and HCHO, while in situ measurements were obtained for NO2, PM2.5, O3 and air quality index (AQI)
Our results show higher reduction in the NO2 vertical column densities (VCDs) as compared to HCHO, owing to the fact that NO2 mainly comes from anthropogenic sources

Summary

Introduction

Nitrogen dioxide (NO2 ) and formaldehyde (HCHO) are two important trace gas species in the atmosphere which play a key role in defining the atmospheric chemistry. Their concentration may vary depending on certain physical conditions and chemical or photochemical processes. Meteorology is an important factor which plays a significant role in determining the chemical composition of the atmosphere as it largely impacts the residence time of trace gas species [1,2]. NO2 has detrimental impacts on air quality as it holds a key role in defining tropospheric chemistry [3]. As a precursor for secondary organic aerosols and a component of catalytic cycles that lead to the formation of tropospheric ozone (O3 ), this gas is a crucial atmospheric pollutant [4,5]. Nitric acid is the oxidation product of NO2 which can be deposited either in dry or precipitate form [6]

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