Combining nitrogen oxides and temperature to derive new observational constraints on the changing chemistry of ozone and aerosol

Nitrogen oxide (NOx) abundances across the U.S. have fallen steadily over the last fifteen years. Patterns in anthropogenic sources result in two-fold lower NOx on weekends than weekdays largely without co-occurring changes in other emissions. These trends taken together provide a near perfect NOx 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 NOx 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 NOx 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 NOx 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 NOx, 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 PM2.5, 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 NOx 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 NOx controls on both NH4NO3 and total aerosol mass in the SJV. Due to NOx 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.

Advisor: Cohen, Ronald C