NO2 anomalies - economy attribution and rapid climate response

Abstract:

COVID-19 lockdowns in the winter of 2019/2020 and the spring of 2020 resulted in contracting economies around the world, and it has been tempting to attribute observed negative NO₂ anomalies to anticipated reduced anthropogenic emissions.

While indeed unambiguously observed on the ground and even from space, the anomalies still pose the question as to what extent they can be attributed to contracting economies. The problem is, observed NO₂ amounts over the biggest polluters, USA, China and Europe, have been declining steadily at least in the past 16 years when quality space observations exist. Also, all largest NO₂ anomalies in the satellite observation record appear in the winter months, but they are not correlated with the major economic indicators.

We first decompose 16-year monthly series of NO₂ anomalies (seasons removed) derived using data from the Ozone Monitoring Instrument (OMI) on board of NASA’s Aura satellite into Principal Components (PC). Then, using officially published indicators of the U.S. and China economic variability, we identify which modes of NO₂ variability can be attributed to economic variability. Our results show that it is the third PC (PC3) from the full hierarchy of principal modes of OMI NO₂ deseasonalized series that is best coupled with the economic indicators. This coupling is positive, i.e. PC3 and economic indicators manifest positive covariance. However, the economic variability can explain only 40% of the information in PC3. Furthermore, this mode by itself explains only 3% of the total deseasonalized NO₂ variability. We thus conclude that, while having an unambiguous impact, the economy can be awarded at best third order of importance in driving the deseasonalized NO₂ series, i.e. after seasonal variability is removed from the series.

Once we identified PC3 as the NO₂ mode that is coupled with economic variability, we use this mode as an indicator to look for rapid climate adjustments to that part of NO₂ variability that we are certain is coupled with the economic variability. We focus on observational data from the Atmospheric Infrared Sounder (AIRS) on board of NASA Aqua satellite, decompose series of surface skin temperature and clear-sky outgoing longwave radiances (OLR) into principal components, and identify impacts of NO₂ PC3 onto these climate variables.