UC Berkeley

Nitrogen oxide (NO_x) abundances across the U.S. have fallen steadily over the last fifteen years. Patterns in anthropogenic sources result in two-fold lower NO_x on weekends than weekdays largely without co-occurring changes in other emissions. These trends taken together provide a near perfect NO_x constraint on the nonlinear chemistry of ozone (O₃), on the key oxidants hydroxyl radical (OH) and nitrate radical (NO₃), and on secondary aerosol formation. In this dissertation, I present three observationally driven analyses, each of which couples NO_x with temperature in new ways and so yields fresh insight into trends in chemical mechanisms of atmospheric oxidation. The study region is the San Joaquin Valley (SJV) of California, a location with both highly regular meteorology and severe O₃ in the summertime and the worst aerosol pollution in the U.S. in the wintertime. First, I analyze fifteen years of routine monitoring records of nitrogen oxides, ozone, and temperature to infer the relative import of NO_x and organic emission controls on the frequency of high O₃ days in three city-scale urban plumes. I show that chemical O₃ production (PO₃) rather than meteorology dominates the statistics of exceedances of the California 8-h O₃ standard, that temperature when combined with NO_x is a useful parameter for interpreting the total reactivity of organic emissions with OH, and that the sensitivity of PO₃ to its precursors has changed differently in response to controls as a function of temperature and location over the last decade. Second, I investigate the comprehensive CALNEX-SJV dataset (18 May−29 June, 2010). I classify components of daytime speciated organic reactivity as either independent of or exponential with temperature. I use this classification to consider the effects of changes in organic emissions over the last decade and to make predictions for the next ten years. I test the impacts of various emission controls on PO₃ with an analytical model constrained to CALNEX-SJV observations and show the effect of each reduction scenario is strongly temperature dependent. Third, I interpret trends in wintertime relationships between NO_x, ammonium nitrate (NH₄NO₃), and total aerosol mass over the last decade in the SJV. To do so, I combine the decade-long routine monitoring record of PM_2.5, nitrogen oxides, ozone, and meteorological conditions and the air- and ground-based DISCOVER-AQ-2013 dataset (14 January−14 February, 2013). I then construct observationally derived decadal trends in NH₄NO₃ production by daytime and nighttime chemical mechanisms and describe distinct circumstances for which NO_x reductions have increased and decreased NH₄NO₃ production over the record. The net effect has been a substantial reduction in aerosol NO₃⁻ due to decreased production in the nocturnal residual layer. I use this analysis to quantify the impacts of future NO_x controls on both NH₄NO₃ and total aerosol mass in the SJV. Due to NO_x decreases in the region, NH₄NO₃ formation will shift from being driven by NO₃-radical chemistry at night, as it has been for the last twelve years, to being dominated by daytime OH-initiated chemistry in the next decade.