Atmospheric in situ measurement of nitrate radical (NO₃) and other photolysis rates using spectroradiometry and filter radiometry

[1] We describe field measurements of nitrate radical photolysis rates, j(NO₃), conducted during the International Consortium for Atmospheric Transport and Transformation (ICARTT) study in the summer of 2004 in the northeastern United States on board the NOAA research vessel Ronald H. Brown (RHB) and the NOAA aircraft WP-3. The photolysis rates of 17 other atmospherically important compounds were also measured. Direct measurements of spectral actinic fluxes using spectroradiometers were conducted on board the WP-3, which were then converted into photolysis rates. On board RHB, we used filter radiometers that specifically measured j(NO₃) and were calibrated before and after the campaign by the spectroradiometers. NO₃ photolysis rates ranged from below the detection limit of 10⁻⁵ s⁻¹ at twilight to peak values of 0.5 s⁻¹ over clouds at midday. The measurement uncertainties were 9% for the spectroradiometers and 14% for the filter radiometers. A field intercomparison between ship and aircraft instruments showed general agreement, indicating that aircraft data can be used to calculate the ship nadir radiation from the ocean surface. The measurements were used to evaluate the importance of photolysis of nitrate radicals in the troposphere. One result was that because of its spatial correlation with NO, NO₃ daytime loss is dominated by reaction with NO in the free troposphere and the marine boundary layer. The tropospheric branching ratio between the two NO₃ photolysis channels producing NO and NO₂, was found to be (10.8 ± 1.2)% for NO in the lower troposphere.