The first in situ measurements of ClONO2 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, NO2, temperature, pressure, and the calculated photolysis rate coefficient (JClONO2) to examine the balance between production and loss of ClONO2. The observations demonstrate that the ClONO2 photochemical steady state approximation, 2>PSS=k¿¿2>/JClONO2, is in good agreement with the direct measurement, 2>MEAS. For the bulk of the data (80%), where T>220 K and latitudes >45 ¿N, 2>PSS=1.15¿0.36(1&sgr;)¿2>MEAS, while for T300 nm. These measurements confirm the mechanism by which active nitrogen (NOx=NO+NO2) controls the abundance of active chlorine (Clx=ClO+Cl) in the stratosphere. ¿ 1999 American Geophysical Union