NO2 satellite retrievals biased by absorption in water

Localized tropospheric nitrogen oxides (NOx = NO + NO2) are mostly formed from emission sources, such as large cities, mineral mining sites, busy transportation routes, fuel delivery infrastructure and wildfires. Kong et al. recently reported anomalous tropospheric NO2 columns from spaceborne remote sensing observations of the TROPOspheric Monitoring Instrument (TROPOMI) over Tibetan Plateau lakes and attributed them to megacity-scale emissions from these lakes. Here we report serious anomalies in the NO2 retrievals over most of these lakes, possibly due to absorption in the water, which may have biased the NO2 retrieval results. Without addressing this potential absorption, it is premature to attribute any anomalies in tropospheric NO2 to emissions from Tibetan lakes, let alone estimate their magnitude.