Nitrogen dioxide (NO2) plays an important role in the troposphere. As part of NOx (= NO2 + NO) it is one of the key players in the formation of photochemical smog during pollution episodes and contributes to acid rain. Locally, it may also add to radiative forcing. As an example, this picture shows tropospheric excess NO2 above Europe from the SCIAMACHY project for August 2002.
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