Actually, the paper goes on to explain that they discovered that a lot of the damage is caused by other pollutants that either just happen to exist on the same regions as NO2 or are created by its reaction with the air (they suspect ozone is a large guilty).

They measured NO2 because it's the best one to measure.

Those are summer numbers, they also given winter numbers of China: 25%, Western Europe: 10%, India: 6%.

The article is kind enough to link to the study: https://www.science.org/doi/10.1126/sciadv.abm9909

The data is coming from satellites, which are apparently able to directly measure NO2 levels (a good proxy for NOx levels), and they use near infrared reflectivity to estimate crop yield (which they say previous studies have shown to be a good way to estimate it).

Thank you for posting the actual quantitative data. All the sound and fury in the world pales in comparison to the awesome and terrifying power of Microsoft Excel!

The main source of NOx in the air would be ICE vehicles right?