ON THE USE OF HYDROCARBONS FOR THE DETERMINATION OF TROPOSPHERIC OH CONCENTRATIONS

This paper explores a new approach to estimating atmospheric hydroxyl radical concentrations from regional measurements of a suite of hydroc arbons. The approach is guided by the study of a suite of synthetic tr acers, i, with uniform continental sources and constant but different lifetimes of 1, 2, 5, 20, 50, and 100 days, whose global distributions are calculated from a three-dimensional chemical tracer model. With t he help of the model we show that in a grid box the standard deviation ai divided by the average concentration (M) over bar(i) is a unique f unction of the chemical lifetime sigma(i). In favorable cases, for ins tance, in surface air within a specific region sampled by the Pacific Exploratory Mission (PEM) West B campaign, that function takes a simpl e form: sigma(i)/(M) over bar(i) = A x tau(i)(-alpha), with alpha = 0. 48, very close to 1/2. An analogous relation is found for the alkanes, ethane through n-hexane, measured during the PEM West B campaign in t he same domain, with their reaction rate constant with OH, k(OH). That relation has the form sigma(i)/(M) over bar(i) = B x (k(OH,) (i))(alp ha'), with alpha' = 0.49. Using the alkenes, for example propene, whic h also react with O-3, the dependence on k(OH, i) can be related to a dependence on tau(i). This allows us to estimate the OH concentration, 6 x 10(5) cm(-3), with an error of roughly a factor of 2 for this reg ion (boundary layer, 30 degrees N-40 degrees N latitude, and 135 degre es E-155 degrees E longitude in March). This estimate is essentially b ased on empirical relations only and the assumption that the considere d hydrocarbons have the same source distribution. As a by-product, we show that the alpha defined above is related to the slope in the (loga rithmic) correlation plot, between the mixing ratios of two trace gase s with different lifetimes. We also show that the global distribution of a appears to be a useful tool to diagnose fast regional transport.