Actinic Flux and Atmospheric Photolysis Frequency Measurements during TRACE P

R. E. Shetter and B. L. Lefer

Atmospheric Chemistry Division

National Center for Atmospheric Research

Photochemical reactions provide the driving force for much of the chemistry in the atmosphere.  The in situ rates of these photolysis reactions are important in understanding production and loss terms for the key atmospheric species odd hydrogen radicals and ozone.  The objective of the proposed research is to deploy dual scanning actinic flux spectroradiometer (SAFS) systems to determine the in situ total spectral actinic flux as a function of wavelength and calculate the in situ photolysis frequencies of 11 or more atmospherically important molecules.  These SAFS systems will be deployed on both the NASA DC-8 and P3-B aircraft platforms during the TRACE P mission in the spring of 2001.  Photolysis frequencies for O₃, NO₂, CH₂O, HONO, HNO₃, PAN, H₂O₂, CH₃OOH, CH₃ONO₂, CH₃CH₂ONO₂, and CH₃COCH₃ will be calculated from the measured spectral actinic flux.  PIs will use their expertise gained in making actinic flux and photolysis frequency measurements during MLOPEX I and II, the 1993 OH Photochemistry Experiment, PEM Tropics A and B, SONEX, and SOLVE missions.  The instrumentation, which has flown on the NASA DC-8 during the PEM-Tropics A, SONEX, PEM-Tropics B, and SOLVE missions, will have a detection limit of less than 0.1 mw/nm/cm^-2 resulting in photolysis frequency detection limits of 2 X 10^-7 sec^-1 for jO(¹D) and 1 X 10^-7 sec^-1 for jNO₂, all with a time response of £15 seconds.  Data for the spectral actinic flux as a function of wavelength and the photolysis frequencies for 11 molecules will be submitted to the final data archive.

Photolysis frequencies will also be calculated using the NCAR TUV clear sky radiative transfer model. These modeled photolysis frequencies will be compared to the measurements for clear skies and different complex atmospheres including aerosol and water cloud episodes to investigate effects pollution plumes and clouds on photolysis frequencies.