Measurements of the HONO photodissociation constant

Measurements of the photodissociation constant for nitrous acid (j HONO) were made at an urban site in Toronto, Canada, during the months of May–July 2005, using an optically thin actinometer. Operating details of the j HONO monitor are reported, along with laboratory tests. Measurements of j HONO were obtained for solar zenith angles ranging from 20–75∘, under clear and cloudy skies. Maximum error estimates on j HONO under clear skies range from 11% at sunrise, to 4% at solar noon, with a minimum detection limit of 5.7 × 10−4/sec for our actinometer. Measured clear-sky values of j HONO were compared with values calculated by a four-stream discrete ordinate radiative transfer (RT) model (ACD TUV version 4.1), and were found to be within better than 10% agreement for solar zenith angles < 65∘. For conditions of scattered cloud, enhancement and suppression of the j HONO values occurred by as much as 16%–70%, and 59%–80%, respectively. The integrated band area of the nπ∗ transition for gas-phase nitrous acid yields an oscillator strength, f = (1.06 ± 0.044)×10−3 (based on clear-sky data), 19.1% higher than the value reported by Bongartz et al. (1991).