Ozone profile retrievals from TROPOMI: Implication for the variation of tropospheric ozone during the outbreak of COVID-19 in China

Abstract

During the outbreak of the coronavirus disease 2019 (COVID-19) in China in January and February 2020, production and living activities were drastically reduced to impede the spread of the virus, which also caused a strong reduction of the emission of primary pollutants. However, as a major species of secondary air pollutant, tropospheric ozone did not reduce synchronously, but instead rose in some region. Furthermore, higher concentrations of ozone may potentially promote the rates of COVID-19 infections, causing extra risk to human health. Thus, the variation of ozone should be evaluated widely. This paper presents ozone profiles and tropospheric ozone columns from ultraviolet radiances detected by TROPOospheric Monitoring Instrument (TROPOMI) onboard Sentinel 5 Precursor (S5P) satellite based on the principle of optimal estimation method. We compare our TROPOMI retrievals with global ozonesonde observations, Fourier Transform Spectrometry (FTS) observation at Hefei (117.17°E, 31.7°N) and Global Positioning System (GPS) ozonesonde sensor (GPSO₃) ozonesonde profiles at Beijing (116.46°E, 39.80°N). The integrated Tropospheric Ozone Column (TOC) and Stratospheric Ozone Column (SOC) show excellent agreement with validation data. We use the retrieved TOC combining with tropospheric vertical column density (TVCD) of NO₂ and HCHO from TROPOMI to assess the changes of tropospheric ozone during the outbreak of COVID-19 in China. Although NO₂ TVCD decreased by 63%, the retrieved TOC over east China increase by 10% from the 20-day averaged before the lockdown on January 23, 2020 to 20-day averaged after it. Because the production of ozone in winter is controlled by volatile organic compounds (VOCs) indicated by monitored HCHO, which did not present evident change during the lockdown, the production of ozone did not decrease significantly. Besides, the decrease of NO_x emission weakened the titration of ozone, causing an increase of ozone.

Keywords: COVID-19; Lockdown; Ozone profile retrievals; Soft calibration; TROPOMI.

Graphical abstract

Fig. 1

(a) Soft correction spectrum derived from ratio between TROPOMI measured radiances and simulated radiances at initial iteration, as a function of ranging from 305 nm to 360 nm. The vertical solid line indicates 305 nm. (b) Standard deviations of fitting residuals. The 450 cross-track positons are shown in different colors.

Fig. 2

Comparison of fitting residuals of band1–2 on 22 June 2018 with (a) and without (b) soft calibration for 3 cases: high-latitudes (green), mid-latitudes (blue), and tropics (red) for cloud fraction <0.3. (c), (d) similar to (a) and (b) but for band-3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Maps of (a) total ozone column, (b) stratospheric ozone column, (c) tropospheric ozone column and (d) cloud fraction on 1 June 2018. The data are grided on 0.1° latitude × 0.1° longitude.

Fig. 4

(top) An orbit of ozone profiles in DU at center cross-track position (224) on 1 June 2018. The red line indicates the NCEP tropopause height. (bottom) Cloud fraction (black), surface albedo (read) and cloud top pressure (blue) used in the retrievals. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 5

(a) Scatter plots of FTS TOC with (red) and without (blue) retrieval averaging kernels vs retrieved TOC and a priori (black). The dashed line denotes the linear regression and 1:1 relationship. Mean biases and standard deviations, the linear regression and correlation coefficients, and the number of coincident pairs are also shown in legends. (b) Time series of TROPOMI (green), a priori (black), and FTS (red) TOC at Hefei, China. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 6

Similar to Fig. 5, but for ozonesonde observations in Beijing, China.

Fig. 7

(left) Comparisons of retrieved ozone profiles (red), ozonesonde (blue) and the a priori (black) profiles. (right) Mean biases (solid lines) and standard deviations (dashed lines) between TROPOMI-a priori and ozonesondes with TROPOMI averaging kernels at five selected stations. The station information is shown in the legends.

Fig. 8

20-day average TROPOMI TOC before (a) and after (b) the lockdown of Wuhan on January 23, 2020. (c), (d), (e) and (f) similar to (a), (b) but for HCHO and NO₂ TVCD respectively. The black rectangle-lines present the Beijing-Tianjin-Hebei (BTH) regions.