Impact of clouds and aerosols on photolysis frequencies and photochemistry during TRACE-P: 1. Analysis using radiative transfer and photochemical box models

This study examines the agreement between photolysis frequency measurements by the NCAR scanning actinic flux spectroradiometer (SAFS) and calculations from a cloud-free model (CFM) and investigates the impact of these differences on ozone photochemistry. Overall, the mean jNO₂ measurement to model ratio for all flights of TRACE-P was 0.943 ± 0.271. The sky conditions during the Transport and Chemical Evolution over the Pacific (TRACE-P) experiment were determined to be "cloud-free" 40% of the time; hence a CFM is frequently not representative of the local atmospheric conditions. Our analysis indicates that clouds have a larger impact on photolysis frequencies (from -90 to +200%) than do aerosols (maximum of ±20%). The CFM and SAFS jN0₂ and jO(¹D) values differed by 9% and 0-7%, respectively, during a vertical profile through a cloud-free and low AOD atmosphere. This suggests that measurement/model agreement to less than 10% may be difficult without better aerosol optical parameter inputs even under low-AOD conditions. For the TRACE-P chemical environment, OH, NO, and HO₂ were more sensitive than other compounds (e.g., CH₃C(O)O₂, CH₃OOH) to changes (or errors) in photolysis frequency inputs to a photochemical box model. Compounds including NO₂, PAN, and HCHO exhibited different relationships to j-value changes below and above the boundary layer. Ozone production and loss rates increased linearly with changes (or errors) in the photolysis frequency with the resulting net O₃ tendency increasing with a linear slope near unity. During the TRACE-P mission the net photochemical effect of clouds and aerosols was a large decrease in photochemical O₃ production in the boundary layer. Copyright by the American Geophysical Union.