(THREAD) Important caveats about the recent NO2 decrease articles circulating on NYTimes and CNN (and maybe elsewhere). Personally, I think these articles are a bit deceiving and here’s why… nytimes.com/interactive/20… cnn.com/2020/03/23/hea…
1. I have seen some differences in #TROPOMI NO2 between years in non-COV19 months, perhaps related to other emission reduction strategies, or maybe other technical reasons. A 5% change unrelated to COVID-19 matters a lot! That is not accounted in articles above(and no colorbars!)
2. There should be some accounting for meteorology... NO2 concentrations are larger functions of temperature, wind speed, clouds, air mass origin, than small-scale (10-20%) changes in emissions. For example, warmer temperatures would lower NO2 amounts with identical NOx emissions
In my (preliminary) analysis of #TROPOMI NO2, I am NOT seeing large-scale reductions in NO2 that can be directly tied into a reduction of NOx emissions. My analysis looks at pre- and post-March 13. Varying trends in U.S cities. 
My hot take on this plot is that in northern cities, there hasn’t been much Arctic influence recently, it’s been relatively cloudy, and winds have been light, so NO2 is increasing! Transport is important! @weathergaige
In LA & Phoenix, it was wetter than normal for a few days, which may have cleaned things out. And in Houston, perhaps there's been a tropical influence of relatively clean air. Again transport & meteorology is important!
We will need more time and more data to make any definitive estimates of the magnitude of NOx reductions. Is it 5% or 50%? Neither article really sheds light on this. And that’s really the multi-billion dollar question.
I am concerned because a >50% reduction in NOx emissions would overwhelm any meteorological factors (e.g., China, Italy), but I am just not seeing that in U.S. analyses. We are looking for dramatic drops in NO2, and there haven't been any so far.
I think the real story is that while it’s likely there have been NOx reductions, it's disappointing they aren't obvious, and therefore may not be as large as one might expect. I hope for the sake of our environment, this last point will be proven wrong over time. (END)
