Satellite evidence for significant anthropogenic emission trends in Asian cities

Long-term (2005-2019) satellite records of formaldehyde measured with the OMI sensor aboard the Aura payload reveal strong positive trends over the Middle East (3.6%/year) and the Indian subcontinent (2.4%/year), where regulations of volatile organic compound (VOC) emissions from human activities are currently limited. Weaker trends are observed over Chinese cities, where such regulations were only recently introduced. Mostly negative trends are observed above cities located in Taiwan and Japan, where legislation has been in place since the early 2000s.

From formaldehyde observations to emission trends

Anthropogenic non-methane volatile organic compounds (AVOCs) are key atmospheric constituents affecting air quality and climate through their role in the formation of ozone and organic aerosols. Emissions of AVOCs are significant in urban and industrialized areas and their major sources include fossil fuel, industrial processes, solvent use, and road transport.

The derivation of AVOC emissions and their trends using satellite observations of formaldehyde (HCHO) is challenging due to the generally weak HCHO signal from anthropogenic sources. This work shows that emission trends can be accurately monitored above many large cities in Asia, based on HCHO column data from the OMI satellite instrument spanning 15 years, from 2005 to 2019.

Of the 133 studied cities, significant trends are found for 77 cities. Positive HCHO trends are found for most urban areas (cf. Figure 1 and Figure 2).

  • The observed trends over China are relatively weak and mostly positive.
  • In the Indian subcontinent, strong positive trends are observed, estimated on average at 1.4% per year.
  • Positive trends are also found over the Middle East and central Asia reaching up to 3.6% per year (Karaj, Iran).
  • Significant negative trends are found in Japan, estimated on average at −1.5% per year for all considered cities.
  • In Taiwan, the HCHO trends are also mostly negative.
     

Contrasting NO2 and HCHO trends

Since Chinese VOC regulations were only recently implemented, HCHO trends calculated between 2005 and 2018 do not yet validate these efforts. Nevertheless, the effect of AVOC regulations is clearly seen in two important megacities (Dongguan and Chengdu). In Japan and Taiwan, negative trends are found, demonstrating that air quality regulations targeting AVOC emissions are effective in these countries.

The contrast between the observed evolution of NO2 and HCHO columns over Asian cities (cf. Figure 1 and Figure 3) illustrates well that emission controls targeting NOx sources have little or no impact on AVOC emissions, in particular over China, as the major sources of NOx and AVOC are markedly different.

The important differences between HCHO and NO2 trends highlight that legislation targeting specifically VOC emissions are needed to improve air quality. The limited number of initiatives to regulate AVOC emissions is made clear by the detected positive HCHO column trends in many Asian countries, contrasting with the weaker, or even negative trends in countries were policy measures are in place.

 

Reference

Bauwens, M., B. Verreyken, T. Stavrakou, J.-F. Müller and I. De Smedt: Spaceborne evidence for significant anthropogenic VOC trends in Asian cities over 2005-2019. Environ. Res. Lett. 17, 015008, https://doi.org/10.1088/1748-9326/ac46eb, 2022.

Map HCHO trends 133 large Asian cities
Figure 1. HCHO trends (in % per year) over 133 large Asian cities considered in this study. Colored circles denote cities for which significant trends are detected (77), white circles denote cities with no significant trends (56).
Table with bars HCHO trends Asian countries
Figure 2. HCHO trends expressed in % per year for large cities in (a) China, (b) the Indian subcontinent, (c) the Middle East and (d) other Asian countries. Error bars indicate the uncertainty on the calculated trends.
Maps NO2 trends 133 cities year 2011
Figure 3. Satellite OMI NO2 trends over the 133 cities studied here over 2005-2019 (upper panel) and over two shorter periods showing the change in trend around the year 2011 (lower panels).