Quantifying photolysis rates in the troposphere and stratosphere

Solar radiation drives the chemistry and dynamics of the atmosphere. An accurate and precise knowledge of the rates of photochemical reactions throughout the atmosphere is essential to understanding the state of the atmosphere and its future changes (e.g., in photochemical smog, aerosol formation, and stratospheric ozone depletion) on all timescales. This dissertation uses state-of-the-art instruments and radiative transfer models to quantify photolysis rate coefficients (j-values) in the troposphere and stratosphere during recent field campaigns. A UMD chemical actinometer was used to measure jNO2 at the surface during the IPMMI intercomparison. The JHU/APL radiative transfer model was also used to calculate cloud-free jNO2 and jO3 (→ O2 + O(1D), or jO&parl0;D1&parr0; ). Model-spectroradiometer agreement is within 2% for both jNO2 and jO3 , and model-actinometer agreement is within 1% for jO3 and about 14% for jNO2 . Comparisons suggest that currently accepted NO2 absorption cross sections may be too small. Treatments of aerosols and the extraterrestrial solar irradiance are shown to have a significant effect on jNO2 and jO3 . During POLARIS, agreement of modeled jNO2 and jO3 with data from a spectroradiometer flown on the NASA ER-2 near 20 km is shown to be within 6% and 1% on average, respectively, depending on the input albedo and overhead ozone. The model is also used to explore the sensitivity of j-values to surface height, surface albedo, and ozone column, with the variability in j-value measurements related to variations in the geophysical environment. During SOLVE, modeled j-values are shown to agree with a spectroradiometer on the NASA DC-8 near 11 km to within 6% for jNO2 and 15% for jO3 , even at solar zenith angles beyond 85°. Intermittent attenuation of measured jNO2 at 11 km of up to 75% is presented, attributed to the optical effects of polar stratospheric clouds (PSCs). Inferred PSCs are shown to have a significant (up to 10%) impact on daily polar vortex-averaged ClOOCl photolysis in late January 2000. PSC attenuation of ClOOCl photolysis is estimated to lead to a maximum reduction in ozone loss of about 2%. Photochemical ozone loss rates in the Arctic during SOLVE are determined by MSX/UVISI satellite stellar occultation measurements combined with diabatic trajectory analyses of airmass subsidence within the polar vortex, with a maximum of ∼1 ppmv ozone loss in the lower stratosphere during January 23--March 4, 2000.