Past and future ozone trends in California's South Coast Air Basin: Reconciliation of ambient measurements with past and projected emission inventories
ABSTRACT This paper updates the historic trends (1980-2010) in ambient ozone and ozone precursor concentrations in the South Coast Air Basin (SoCAB) and examines the evolution of the ozone-precursor relationship in the Basin. Whereas reductions in NOx (oxide of nitrogen) emissions have decreased nitrate and PM2.5 (particulate matter with an aerodynamic diameter < or = 2.5 microm) concentrations in the Basin during the past decade, ozone levels have increased at the central basin locations since about 2005 following a reversal in the decline of volatile organic compound (VOC)/NOx ratios during the previous two decades. A chemical box model was used to simulate the effects of changes in precursor concentrations on ozone formation using day-of-week-specific initial precursor concentrations that were derived from measurements and'projected to 2020 based on expected emission reductions from 2005 (-10% VOC and -50% NOx). Results show that peak ozone formation rates in 2020 will increase on weekdays by a factor of 3 relative to 2005 and will be comparable to 1995 weekday and 2005 Sunday rates. Ozone production will become precursor limited on Sundays in 2020, but with higher initial rates than 2005. Although a greater NOx reduction scenario in 2020 of -75% will result in even higher initial ozone formation rates, precursor limitation is reached quickly, leading to a further shift westward in the location of peak ozone levels. However ozone levels will likely be lower in downwind areas where transport is more important than local production of ozone. The ambient versus emission inventory reconciliation indicates a factor of 2 underestimation of VOC emissions in 2009 relative to NOx. Other analyses suggest that there is an overall increase in VOC emissions on hot days that is not fully accounted for by emission inventory estimates. Air quality models using emission inventories that underestimate VOC emissions relative to NOx may lead to inaccurate forecasting of the consequence of emission reductions. IMPLICATIONS: The rate and efficiency of ozone formation and accumulation in the SoCAB is more rapid than would be indicated by air quality model simulations based on the current inventory. Projected reductions in NOx emissions without concurrent reductions in VOC emissions will likely cause ozone to increase during the next decade within central regions of the SoCAB compared with a flat or slightly declining trend in far downwind locations. Air quality statistics that are commonly used to track progress toward attainment, such as basin-wide ozone design value and standard exceedances mask these varying trends within the Basin.
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ABSTRACT: Positive Matrix Factorization (PMF) was utilized to quantify sources of ambient PM 2.5 in central Los Angeles (LA) and Rubidoux, using the Speciation Trends Network data, collected between 2002 and 2013. Vehicular emissions (including gasoline and diesel vehicles) were the second major contributor to PM 2.5 , following secondary aerosols, with about 20% contribution to total mass in both sites. Starting in 2007, several major federal, state, and local regulations on vehicular emissions were implemented. To assess the effect of these regulations, daily-resolved vehicular source contributions from 2002 to 2006 were pooled together and compared to the combination of 2008 to 2012 datasets. Compared to the 2002 e2006 dataset, the median values of vehicular emissions in 2008e2012 statistically significantly decreased by 24 and 21% in LA and Rubidoux, respectively. These reductions were noted despite an overall increase or similarity in the median values of the daily flow of vehicles after 2007, at the sites.Environmental Pollution 07/2014; 193:54-64. DOI:10.1016/j.envpol.2014.06.012 · 3.90 Impact Factor
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ABSTRACT:  Decreases in ozone (O3) observed in California's South Coast Air Basin (SoCAB) over the past five decades have resulted from decreases in local emissions of its precursors, nitrogen oxides (NOx = NO + NO2) and volatile organic compounds (VOCs). Ozone precursors have been characterized in the SoCAB with measurements dating back to 1960. Here we compile an extensive historical data set using measurements in the SoCAB between 1960 and 2010. Faster rates of decrease have occurred in abundances of VOCs (−7.3 ± 0.7% yr−1) than in NOx (−2.6 ± 0.3% yr−1), which have resulted in a decrease in VOC/NOx ratio (−4.8 ± 0.9% yr−1) over time. Trends in the NOx oxidation products peroxyacetyl nitrate (PAN) and nitric acid (HNO3), measured in the SoCAB since 1973, show changes in ozone production chemistry resulting from changes in precursor emissions. Decreases in abundances of PAN (−9.3 ± 1.1% yr−1) and HNO3 (−3.0 ± 0.8% yr−1) reflect trends in VOC and NOx precursors. Enhancement ratios of O3 to (PAN + HNO3) show no detectable trend in ozone production efficiency, while a positive trend in the oxidized fraction of total reactive nitrogen (+2.2 ± 0.5% yr−1) suggests that atmospheric oxidation rates of NOx have increased over time as a result of the emissions changes. Changes in NOx oxidation pathways have increasingly favored production of HNO3, a radical termination product associated with quenching the ozone formation cycle.06/2013; 118(11). DOI:10.1002/jgrd.50472