NIGHTTIME FORMATION OF N2O5 INFERRED FROM THE HALOGEN OCCULATATION EXPERIMENT SUNSET SUNRISE NOX RATIOS/

The sunset/sunrise NOX ratios measured in 1994 by the Halogen Occultat ion Experiment (HALOE) onboard the Upper Atmosphere Research Satellite (UARS) peak at up to 2.8 in the tropics around 30 km and decrease to 1 at the stratopause. These ratios are interpreted by using a fully di urnal one-dimensional model, which considers gas phase reactive nitrog en chemistry, and a two-dimensional model, which includes a more compl ete set of chemical reactions. The diurnal cycle of nighttime N2O5 for mation followed by daytime photolysis entirely accounts for the observ ed ratios above about 35 km. In the lower to middle stratosphere, addi tional reactions involving ClONO2 and HNO3 also appear to contribute t o the observed diurnal variations in NOX. The limitations of a simple function of ozone, temperature, and length of night, which estimates d iurnal variations in NOX based on gas phase reactive nitrogen chemistr y, are tested through comparison to one-dimensional model results. The function approximates the one-dimensional model sunset/sunrise NOX ra tios well in the lower stratosphere but overestimates them by up to 10 % in the upper stratosphere above about 35 km, corresponding to a larg e overestimate of N2O5 formation. The function is extended to estimate sunrise N2O5 in the middle stratosphere.