Abstract
Abstract. The San Joaquin Valley (SJV) experiences some of the worst ozone air quality in the US, frequently exceeding the California 8 h standard of 70.4 ppb. To improve our understanding of trends in the number of ozone violations in the SJV, we analyze observed relationships between organic reactivity, nitrogen oxides (NOx), and daily maximum temperature in the southern SJV using measurements made as part of California at the Nexus of Air Quality and Climate Change in 2010 (CalNex-SJV). We find the daytime speciated organic reactivity with respect to OH during CalNex-SJV has a temperature-independent portion with molecules typically associated with motor vehicles being the major component. At high temperatures, characteristic of days with high ozone, the largest portion of the total organic reactivity increases exponentially with temperature and is dominated by small, oxygenated organics and molecules that are unidentified. We use this simple temperature classification to consider changes in organic emissions over the last and next decade. With the CalNex-SJV observations as constraints, we examine the sensitivity of ozone production (PO3) to future NOx and organic reactivity controls. We find that PO3 is NOx-limited at all temperatures on weekends and on weekdays when daily maximum temperatures are greater than 29 °C. As a consequence, NOx reductions are the most effective control option for reducing the frequency of future ozone violations in the southern SJV.
Highlights
California’s San Joaquin Valley suffers extremely poor air quality and has so for decades; violations of health-based ozone standards remain frequent despite statewide ozone control measures (Cox et al, 2009)
Using CalNex-San Joaquin Valley (SJV) observations of organic molecules, OH reactivity, O3, and nitrogen oxides we describe relationships between temperature, iVOCRi, VOCR, NOx, and P O3
We find the iVOCRi in the southern San Joaquin Valley (SJV) has a temperature-independent component with a reactivity of 2.1 s−1 and a temperature-dependent component that increases exponentially with temperature
Summary
California’s San Joaquin Valley suffers extremely poor air quality and has so for decades; violations of health-based ozone standards remain frequent despite statewide ozone control measures (Cox et al, 2009). During the California at the Nexus of Air Quality and Climate Change experiment (CalNex-SJV), 18 May–29 June 2010, we measured nitrogen oxides, OH reactivity, HOx source molecules, and a wide suite of individual volatile organic compounds (VOCs) at the San Joaquin Valley supersite allowing new insight into the production rate of ozone in the region We use these observations to test the response of P O3 to variations in temperature, VOCR, and NOx. We begin our analysis by describing the temperature dependence of the daytime organic reactivity and categorizing one component of the VOCR that is independent of temperature and a second that increases exponentially with temperature. We test various emission reduction scenarios and show both the magnitude and sign of the effects of controls on emissions are temperature dependent
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