UC Berkeley

The atmospheric chemistry of cities depends strongly on the concentration of the hydroxyl radical (OH). OH removes organic molecules (τ = minutes - days), nitrogen oxides (NO_x: NO + NO₂; τ = 2 - 20 h), and sulfur dioxide (τ = 2 - 3 d) from the atmosphere and forms ozone and particulate matter in the process. The spatial and temporal variability of OH is not fully understood due in large part to its strong nonlinear dependence on the concentration of NO_x. In this dissertation, I use measurements of the NO₂ column from the Ozone Monitoring Instrument (OMI), a satellite-based UV/Visible spectrometer, and calculations from chemical transport models to investigate the relationship of OH and NO_x in urban regions. I characterize the instrumental and model performance necessary to accurately infer OH concentration from space-based measurements of the NO₂ column. I use the OMI observations and the WRF Chem model to investigate the spatial variability of the NO₂ column over a variety of emission sources. I find, not surprisingly, that a result of the nonlinear dependence of OH on NO_x is that the lifetime of NO_x depends on the spatial distribution of NO_x. Where the nonlinear NO_x-OH feedbacks are most important, I find that a spatial resolution of 4 - 8 km is necessary for both model simulations and measurements to accurately capture variations of the NO₂ column. Although this result is expected, quantitative analyses of the nonlinear coupling were not previously available and the calculations presented in this dissertation serve as a guide to interpretation of coarse-resolution measurements and models. I investigate the response of the NO₂ column observed by OMI to variations of wind speed and day-of-week emission patterns. I find that these high spatial resolution measurements (13 × 24 km² at nadir) contain independent information on both NO_x emissions and removal.