Direct measurements of ozone and nitrogen dioxide photolysis rates in the troposphere

Direct measurements of the rate of O3 photolysis to O2(1Δg) and O(1D) and of NO2 photolysis to NO and O(3P) are reported as photolysis frequencies j(O3) and j(NO2). The effects of solar zenith angle, total ozone column, cloud cover, aerosol loading, temperature, pressure, and altitude are examined. For a clear sky, zero albedo, and zenith angle between 0° and 65°, the expression j (NO2) = 1.67 × 10−2 exp (−0.575 sec θ) (s−1) gives NO2 photolysis frequencies (photolysis rates per unit reactant) to within about 10% of the measured values, where θ is the solar zenith angle. Pressure has no measurable effect on j(NO2) between 0.15 and 1.2 bars. Temperature has only a small effect between 230 and 400 K, which can be described in terms of an effective activation energy of 0.48±0.05 kJ mol−1. Frequencies of ozone photolysis to O(1D) depend strongly on overhead ozone column and temperature as well as zenith angle; for 300 K, a clear sky, zero albedo, 45° zenith angle, 0.345-cm ozone column, and altitude between 1.6 and 6 km, j(O3) = 1.6(±0.25) × 10−5 s−1. From 300 to 273 K, j(O3) drops by 40±3% for a 65° zenith angle and a 0.306-cm ozone column; this value is not valid for different effective ozone columns. Measured photolysis frequencies show only weak dependence on altitude and aerosol loading. These frequencies were measured with instruments sensitive to radiation from the upward 2π sr only. Clouds dramatically reduce both photolysis frequencies, but they reduce the total radiation by a significantly larger factor. Airborne UV radiometers, calibrated against direct measurements of j(NO2), were used to measure albedos of many surfaces. While natural surfaces such as vegetation and oceans have albedos near zero, dense clouds and snow have an albedo near unity. With increasing altitude, molecular and particulate scattering often increase the effective albedo with respect to photolysis frequencies.