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

The first in situ measurements of ClONO2 in the lower stratosphere, acquire d using the NASA ER-2 aircraft during the Polar Ozone Loss in the Arctic Re gion in Summer (POLARIS) mission, are combined with simultaneous measuremen ts of ClO, NO2, temperature, pressure, and the calculated photolysis rate c oefficient (J(CLONO2)) to examine the balance between production and loss o f ClONO2. The observations demonstrate that the ClONO2 photochemical steady state approximation, [ClONO2](PSS) = k x [ClO] x [NO2] / J(ClONO2) is in g ood agreement with the direct measurement, [ClONO2](MEAS). For the bulk of the data (80%), where T > 220 K and latitudes > 45 degrees N, [ClONo2](PSS) = 1.15 +/- 0.36 (1 sigma) x [ClONO2](MEAS), while for T < 220 K and latitu des < 45 degrees 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 eviden ce in support of a pressure-dependent quantum yield for photodissociation o f ClONO2 at wavelengths >300 nm. These measurements confirm the mechanism b y which active nitrogen (NOx = NO + NO2) controls the abundance of active c hlorine (Cl-x = ClO + Cl) in the stratosphere.