The goal of the project is to improve the understanding of factors affecting the spatial and temporal patterns of ozone, OH, and aerosol – especially their links to temperature and to the dramatic decreases occurring in anthropogenic NOx emissions through a series of linked research objectives. The U.S. is moving from a regime where urban mobile sources and large power plants dominate the spatial pattern of NOx to one where agricultural sources are prominent. This is bringing about dramatic changes in the chemical regime that affects O3, aerosol, and tropospheric oxidation of molecules that impact stratospheric ozone. To enhance understanding of this chemistry, an integrated analyses of field measurements from CalNex, ARCTAS-CA, and 15 plus years of routine monitoring in California is planned to address two lines of research: 1) describe and understand changes in the rate of ozone production, hydroxyl radical, and alkyl nitrates in response to emissions controls and 2) describe and explain field observations of aerosol organic nitrate and total secondary organic aerosol mass. Relationships between high temperatures and both stagnation and increases in anthropogenic and biogenic emissions are well known. It has been shown that, in the Sacramento, California Metropolitan Region, NOx controls have been extremely effective at reducing violations of health based ozone standards. It has also been shown that, in the San Joaquin Valley, California, not only is temperature a useful proxy for the organic reactivity (and therefore the chemical regime), but also that temperature must be considered in order to accurately predict responses to future emissions controls. This project will examine this issue more thoroughly, linking 15 plus years of the routine monitoring record with observations from the CalNex-Bakersfield supersite to study interannual and regional trends in ozone, organic reactivity, hydroxyl radical, and alkyl nitrate formation. Special attention will be paid to the chemistry of alkyl nitrates (RONO2) for three reasons: RONO2 formation suppresses ozone production, RONO2 concentrations provide an additional constraint on unmeasured organic reactivity, and RONO2 vary by location and their abundance will change (non-linearly) in response to both NOx and VOC emissions controls. Finally, the PIs recently developed a new approach to observing aerosol organic nitrates and deployed the instrument for the first time at the CalNex-Bakersfield site. The project will include analyses of the role of nitrates in SOA formation and comparisons of this dataset with independent observations of aerosol-N.