Actinometer and Eppley radiometer measurements of the NO2 photolysis rate coefficient during the Mauna Loa Observatory photochemistry experiment

Abstract

Measurements of the apparent first‐order rate coefficient for NO₂ photodissociation (j_NO2) were made using a chemical actinometer during the month of May 1988 at the Mauna Loa Observatory, Hawaii. Simultaneous measurements of the ultraviolet irradiance (E), obtained with an Eppley radiometer, allowed extensive testing of the semi‐empirical relationships between E and j_NO2 proposed by Madronich (1987a). More than 3700 simultaneous measurements of j_NO2 and E were obtained for solar zenith angles ranging from 4–90 degrees, and for different sky conditions (including clear skies, partial cloud cover, arid valley clouds below the horizon). For overhead clear skies, the NO₂ photodissociation rate coefficient derived from Eppley radiometer data, here denoted j′, was in good agreement with actinometric measurements, j′/j_NO2=1.01±0.05(1σ). The actinometer‐radiometer relationship holds reasonably well even when low‐lying valley clouds are present. For the periods of overhead intermittent clouds, the j′values track j_NO2 values well, but the observed ratio shows significantly more scatter and the average is somewhat less than unity: 0.93 ± 0.09 (1σ). Measurements taken with and without upward scattered and reflected radiation show that valley clouds can contribute to the j_NO2 values for the conditions encountered during the Mauna Loa Observatory Photochemistry Experiment.