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2020-02-24

It was to be expected that the effects of the corona virus on air quality in China would be more than noticeable. The lockdown of various cities in the Chinese province of Hubei, which started on January 23, 2020, halted an entire community. The impact on air quality, as a result of the strong reduction in the burning of fossil fuels, is clearly perceptible from space with the TROPOMI instrument. Chinese New Year and the holiday period that follows it cause a short-term decline in NO2 emissions every year, but the decline started earlier this year, went a lot faster and is lasting longer. The levels of NO2 air pollution above major cities in China have decreased by 30 to 50% compared to a comparable period last year.

Wuhan lockdown

Million city Wuhan is the capital of the Hubei Province, the industrial heart of China. On January 23, 2020, 2 days before the Chinese New Year celebration, the Chinese government imposed a lockdown in Wuhan and other cities in Hubei province, in an attempt to quarantine the epicenter of an outbreak of the 2019 corona virus disease (COVID-19). A large city with no fewer than 11 million people was paralyzed.

The Chinese New Year celebration causes a noticeable short-term fall in NO2 emissions every year. China is on holiday for a week, the industry is at a very low pace, traffic is reduced. The climate website CarbonBrief shows that coal-fired power generation typically drops by an average of 50% in the 10 days following the eve of Chinese New Year. But unlike other years, when coal production is typically back to its normal level after 20 days, the trend continues even a month later this year due to the lockdown.

Consequences for air quality

NO2 is a gas that enters the atmosphere during all kinds of combustion processes, such as in a car's engine, in heating boilers, in industry or in power stations. Tropospheric NO2 is therefore an important indicator for air pollution.

The TROPOMI sensor on board the Sentinel-5 Precursor platform measures the composition of the atmosphere worldwide and on a daily basis with an unprecedented spatial resolution. This also provides us with crucial information about air quality over China, and in particular above cities such as Wuhan or Nanjing, one of the cities in the pollution chain around Shanghai.

Evolution of NO2 emissions over three large Chinese cities in the period 2018-2020..
Time series of TROPOMI tropospheric NO2 above three major Chinese cities (Wuhan, Nanjing and Shanghai) over a 2-year period (orange / red for this year, 2019-2020; blue for last year 2018-2019). The concentrations are average values over a circle with a radius of 50 km around these cities, using a moderate cloud filter. The full lines are a rolling median in 30 days.

The observations of NO2 by the TROPOMI instrument confirm the downward trend in the use of fossil fuels. The time series of tropospheric NO2 concentrations above Wuhan and Nanjing shows that the concentrations up to the end of 2019 (when the first cases of the corona virus were reported) were still comparable to those in 2018. But whereas the NO2 concentration above Wuhan, the epicenter of the corona virus, only started to fall around the Chinese New Year on 5 February 2019 last year, they are already tumbling downwards as of the end of December 2019 onwards this year, at a very rapid pace. The minima finally reached are about 50% lower than last year. Whereas normal activities typically resume within 1 to 2 weeks following Chinese New Year and concentrations are slowly rising back to normal levels, the concentrations remain persistently low this year, until now already at least a month after Chinese New Year, due to the lockdown and other measures taken to reduce the spreading of the corona virus.

The impact of the corona virus on tropospheric NO2 emissions is of course not limited to these large cities, but is visible throughout China. In the week following the Chinese New Year holiday 2020, the average levels were more than 30% lower than in China in the same period in 2019.

Reduced NO2 emissions over China due to corona virus
(TOP) Evolution of tropospheric NO2 over China from (left) before the corona virus outbreak, (middle-left) during the corona virus outbreak period, (middle-right) a one-week period after the lockdown in the province Hubei takes effect and local governments strongly recommend to stay at home. This happens to coincide with the Chinese New Year holiday. To take into account the impact of the lower activity that exists anyway during the Chinese New Year's leave, the image on the right also shows the 20-day period after Chinese holiday.
TROPOMI clearly observes the rapid decline of tropospheric NO2 above China in the course of January as well as persistently low values also after the Chinese holiday period.
(BOTTOM) For comparison, we show the tropospheric NO2 columns for a comparable period last year. The Chinese New Year then fell on 5 February 2019.
Contains modified Copernicus Sentinel data (2018-2020), processed by BIRA-IASB

The Sentinel 5P satellite is part of the space component of the Copernicus program coordinated by the European Commission. The TROPOMI instrument on board this satellite provides us with information about air quality, stratospheric ozone, climate change and natural hazards such as forest fires and volcanic eruptions. This data is used to provide qualitative information to governments and policy makers. BIRA-IASB plays a key role in the development of prototype data processors for the operational production, validation and exploitation of the Sentinel-5p data.

Contact

Dr. Steven Compernolle, Scientist in the Research Group "Synergistic Exploitation of Atmospheric Data",  +32-(0)2 37 30 412
Dr. Karolien Lefever, Head of the Communication department, Karolien.Lefever_AT_aeronomie.be

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Illustration of the coronavirus pandemic in China.
Image by _freakwave_ from Pixabay.