Derivation and validation of photolysis rates of O3, NO 2, and CH2O from a GUV-541 radiometer

Solar UV radiation plays a very significant role in tenns of atmospheric chemistry. The UV radiation that reaches the troposphere drives the photodissociation of tropospheric species whose photolysis energies fall in the range of these wavelengths. The relevant measure of radiation for atmospheric chemistry is actinic flux, but the great majority of UV measurements, not common in themselves, are UV irradiance (radiation on a flat surface). Spectral UV irradiance was measured in Manchester, UK (78 m altitude, 53.28°N, 2.14°W) using a moderate bandwidth filter radiometer (GLTV-541) that has five channels in the UV region with center wavelengths at 305, 313, 320, 340, and 380 nm and band-pass functions of approximately 10 nm. This study describes a new method of deriving the photolysis rate of O3, NO2, and CH 2O utilizing the GUV irradiance measurements. The measured irradiance was compared with independently measured photolysis rates determined from the actinic flux measured by a Bentham spectroradiometer on a clear sky day. On the reference day, a linear regression of GUV global irradiance to simultaneous photolysis rate measurements was performed to generate scale factors for each optical channel. The results compare favorably with the general uncertainty in photolysis measurements of about 20%. A further separate comparison is made with comparable filter radiometers. Copyright 2004 by the American Geophysical Union.