Intercomparison between actinic flux measurements during ICARTT 2004 on board the NOAA W-P3 and R.H. Brown

Tropospheric chemistry depends critically on photochemistry. Therefore, accurate measurement of all important photolysis rates are required to improve our understanding. We deployed two sets of instruments to measure photolysis rates during ICARTT 2004. On the NOAA aircraft W-P3, we measured photolysis rates using two spectroradiometers consisting of one single- and one dual-grating spectrometer and radiometer heads specifically designed to be independent of the incident light angle. On the NOAA research vessel R.H. Brown, we used filter radiometers that were specifically designed to measure individual photolysis rates of O3, NO2, and NO3 photolysis. We calibrated these filter radiometers before and after the campaign using the spectroradiometers described above. During the campaign, the aircraft and the ship were close enough together on 5 occasions to allow field-intercomparison. When the cloud conditions were comparable for the ship and the aircraft, the photolysis rates compared well. In other situations, the influence of clouds and aerosols were clearly noticeable in different photolysis rates. It is also desirable to retrieve other photolysis rates from the ship than those measured directly by the filter radiometers. Since the filter radiometers measure wavelength bands from the UV to the visible, simple mathematical combinations can be used to derive other photolysis rates than those directly measured. We will show results for HCO production from formaldehyde photolysis, and discuss the improvements that can be made on the accurate determination of this important photolysis rate. The method used to derive formaldehyde photolysis rates will also be employed for other molecules.