Theoretical, actinometric, and radiometric determinations of the photolysis rate coefficient of NO₂ during the Mauna Loa Observatory Photochemistry Experiment 2

Abstract

Measurements of the photolysis rate coefficient of NO 2 ( j NO 2 ) were made using a chemical actinometer during the fall (September 15 to October 23, 1991), winter (January 15 to February 15, 1992), spring (April 15 to May 15, 1992), and summer (July 15 to August 15, 1992) intensives during the second Mauna Loa Observatory Photochemistry Experiment (MLOPEX 2), Hawaii. The measured clear sky j NO 2 values of MLOPEX 2 for all four intensives have substantially increased over the measured j NO 2 values of MLOPEX 1 (May, 1988); a 37% increase occurs in midday clear sky j NO 2 from the spring intensive of MLOPEX 1 to the spring intensive of MLOPEX 2. The ratio of actinometer measurements to theoretical calculations of j NO 2 are 1.44 ± 0.03, 1.45 ± 0.02, 1.43 ± 0.03, and 1.34 ± 0.02 for the fall, winter, spring, and summer intensives, respectively. Inclusion of stratospheric aerosols or aerosols of any reasonable optical parameters in a detailed discrete ordinate model cannot account for the discrepancy between measurements and model. The photolysis rate coefficient of NO 2 is also derived from simultaneous measurements of the ultraviolet irradiance measured with an Eppley radiometer using a semiempirical relationship for each of the four intensives. A simplified cloud model is proposed to explain j NO 2 values that exceed clear sky values during days that have partial cloud coverage.