Variations of OH radical in an urban plume inferred from NO2 column measurements

The evolution of atmospheric composition downwind of a city depends strongly on the concentration of OH within the plume. We use space-based observations of NO2, a molecule that affects both the sources and sinks of OH, to examine the functional dependence of OH concentration on the speed of the wind over Riyadh, Saudi Arabia. These observations illustrate the nonlinear dependence of the OH concentration on NO2 and on the rate of atmospheric mixing. We derive a range of NOx lifetimes of 5.5-8.0 h, lifetimes that correspond to an effective plume-averaged OH concentration of 7.6 x 106 molecules cm(-3) at fast (26 km h(-1)) and 5.2 x 10(6) molecules cm(-3) at slow (4 km h(-1)) wind speeds.