Satellites reveal changes in the NO2 weekly cycle over cities

Air quality
Human activities, the dominant source of NOx into the atmosphere, induce a weekly cycle of NO2 abundances over cities. Comprehensive analysis of NO2 observations from two satellite sensors (OMI and TROPOMI) reveals significant weekly cycles in 115 of the 274 cities considered. The data show a clear weakening of the weekly cycle over European and U.S. cities, whereas the opposite tendency is observed in regions undergoing rapid emission growth. Models capture the observed weekly cycles as well as their trends over large cities.

Anthropogenic activities, by far the largest source of NOx into the atmosphere, induce a weekly cycle of NO2 abundances in and around cities. Here we use NO2 column data from two nadir-viewing satellite sensors:

  1. the Ozone Monitoring Instrument (OMI) launched in July 2004
  2. the high-resolution Tropospheric Monitoring Instrument (TROPOMI), single payload of the Sentinel-5 Precursor (S-5P) launched in October 2017

Detailed analysis of the 2005–2017 OMI dataset reveals significant weekly cycles in 115 of the 274 cities considered. These results are corroborated by a full year of high-resolution TROPOMI NO2 observations (cf. Figure 1).

Observed and modelled changes in weekly cycles

The OMI dataset makes possible to identify trends in the weekly cycle resulting from NOx emissions changes. The data show a clear weakening of the weekly cycle over European and U.S. cities, an evolution attributed to the decline in anthropogenic emissions and the resulting growing importance of background NO2. The opposite tendency is observed in regions undergoing rapid emission growth.

Multiyear simulations over the U.S. and the Middle East using the MAGRITTEv1.1 chemistry-transport model succeed in capturing the observed weekly cycles over the largest cities, as well as the observed long-term trends in the weekly cycle. Over the U.S., the NOx emission decline over 2005–2017 has led to a significant weakening of the weekly cycle, as shown by the comparison of Sunday-to-week column ratios for the periods 2005–2007 and 2015–2017 (cf. Figure 2).

On average for all U.S. cities, the ratio has increased by 22% (from 0.72 to 0.88), in excellent agreement with the model prediction (+20%). This result reflects the non-linear relationship between anthropogenic NOx emissions and NO2 columns, due to the contribution of natural emissions (either local or imported) to the NO2 column above cities, and to chemical feedbacks of the NOx-OH system.

Reference:

Stavrakou, T., Müller, J., Bauwens, M., Boersma, K.F., and Van Geffen, J. (2020). Satellite evidence for changes in the NO₂ weekly cycle over large cities. Scientific Reports, 10, A10066. https://doi.org/10.1038/s41598-020-66891-0 Open Access Logo

NO2 data world maps
Figure 1. (a) Location of studied cities. The colour indicates the day of minimum according to OMI NO2 data over 2005–2017. Open circles indicate cities for which the OMI minimum is not significant. (b) NO2 column on the day of the minimum normalised by the average, according to OMI data. (c,d) Same as (a) and (b) for TROPOMI data over May 2018-April 2019.
NO2 US cities New York Los Angeles Chicago
Figure 2. (a) Sunday-to-week column ratios averaged over 2005–2007 and over 2015–2017 for 18 U.S. cities, and for New York, Los Angeles and Chicago as observed by OMI. (b) Same for ratios calculated with the MAGRITTE model. (c) Average observed and modelled weekly cycle for U.S. cities over 2005–2007 and 2015–2017.