In situ measurements of NOx in the Airborne Arctic Stratospheric Expedition

In situ measurements of nitric oxide (NO) and nitrogen dioxide (NO 2 ) were made simultaneously from the NASA DC‐8 aircraft as part of the Airborne Arctic Stratospheric Expedition. Mixing ratios of NO x (NO + NO 2 ) were typically higher in the Arctic troposphere than in the stratosphere, with median values of 59 and 40 pptv, respectively. In the stratosphere, there tended to be a positive correlation between NO x and water vapor (H 2 O) and negative correlations between NO x and ozone (O 3 ) and between NO x and total reactive odd‐nitrogen (NO y ). The ratio of NO x to NO y , in conjunction with NO y , appears to be an excellent tracer of tropospheric or stratospheric air at northern latitudes during winter. Tropospheric NO x was typically 10 to 50% of gas‐phase NO y , while in the stratosphere, NO x was typically < 10%, and frequently <5% of gas‐phase NO y .