The coupling of ClONO₂, ClO, and NO₂ in the lower stratosphere from in situ observations using the NASA ER‐2 aircraft

Abstract

The first in situ measurements of ClONO 2 in the lower stratosphere, acquired using the NASA ER‐2 aircraft during the Polar Ozone Loss in the Arctic Region in Summer (POLARIS) mission, are combined with simultaneous measurements of ClO, NO 2 , temperature, pressure, and the calculated photolysis rate coefficient (J ClONO2 ) to examine the balance between production and loss of ClONO 2 . The observations demonstrate that the ClONO 2 photochemical steady state approximation, [ClONO 2 ] PSS = k × [ClO] × [NO 2 ] / J ClONO2 , is in good agreement with the direct measurement, [ClONO 2 ] MEAS . For the bulk of the data (80%), where T>220 K and latitudes >45°N, [ClONO 2 ] PSS = 1.15±0.36 (1σ) × [ClONO 2 ] MEAS , while for T<220 K and latitudes <45°N the result is somewhat less at 1.01±0.30. The cause of the temperature and/or latitude trend is unidentified. These results are independent of solar zenith angle and air density, thus there is no evidence in support of a pressure‐dependent quantum yield for photodissociation of ClONO 2 at wavelengths >300 nm. These measurements confirm the mechanism by which active nitrogen (NO x = NO + NO 2 ) controls the abundance of active chlorine (Cl x = ClO + Cl) in the stratosphere.