Measurement of ultraviolet radiation intensity in photochemical smog studies

Abstract

The most widely used technique of measuring ultraviolet radiation intensity in photochemical smog studies is to irradiate NO₂ in a nitrogen atmosphere. In the past a fictitious first-order rate constant, k_d, has been used to characterize the nonlinear NO₂-time behavior in such a system. The inadequacy in the use of k_d stems from the need to extrapolate the NO₂-time curve to zero time. The actual photolysis rate constant for NO₂ dissociation, k₁, provides an unambiguous measure of uv intensity. If the NO₂ photolysis is performed in the usual fashion, we show that k₁ can be computed directly from the NO₂-time data by the equation, 
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\nk₁ = 1/(2Δt){(1 + R₁ -R₂) ln [NO₂]₀/[NO₂] + R₂[[NO₂]₀/[NO₂] - 1] 
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\nwhere the constants R_i are defined in the text. The validity of this equation has been established both experimentally and by computer simulation.