Partitioning of reactive nitrogen in the midlatitude lower stratosphere

Balloon‐borne in situ measurements of total reactive nitrogen (NO y ), nitric oxide (NO), nitric acid (HNO 3 ), ozone (O 3 ), and aerosols were conducted over Aire sur l'Adour, France (44°N), between October 11 and 12, 1994. NO y and NO were measured by chemiluminescence NO detectors combined with a gold catalytic converter, and HNO 3 was measured by ion mass spectrometry. Two independent box models (Goddard Space Flight Center and Atmospheric and Environment Inc.) run on isentropic trajectories were used to interpret the observed partitioning of reactive nitrogen. The NO y partitioning calculated by the two models agreed within 10% of each other. The model‐calculated NO/NO y ratios increase by ∼25% when the updated reaction rates of NO 2 + OH, HNO 3 + OH, and NO 2 + O [ Portmann et al ., 1999] were used instead of those given by DeMore et al . [1997], leading to a better agreement with the observations by the same amount. At 21–25 km the calculated NO/NO y ratios depended sensitively on the estimate of aerosol surface area. Within the uncertainty of the estimate of aerosol surface area the NO/NO y and HNO 3 /NO y ratios calculated by the models reproduced those observed within 20%. The same trajectory models also reproduced the NO/NO y ratios measured below 25 km under high aerosol loading in October and November 1992 within ±20%, which is the accuracy level of the measurements.