Separately resolving NOx and VOC contributions to ozone formation

Ozone (O 3 ) is widely recognized as a significant air pollutant that affects public health across the globe. O 3 is formed from precursor emissions of oxides of nitrogen (NOx) and volatile organic compounds (VOCs) that react in the atmosphere, making it complex to identify the major source contributions to O 3 concentrations. O 3 source apportionment calculations within chemical transport models (CTMs) provide a specialized approach to resolve source contributions. Traditional O 3 apportionment techniques track source contributions based on the chemical formation regime, but they do not separately distinguish between NOx and VOC source contributions. In this study, a new O 3 source apportionment technique was developed to explicitly resolve the contributions from both NOx and VOC sources in order to provide a more detailed view on O 3 source origins so that policy makers can design more effective emission control strategies. The new technique is flexible and can be configured to identify the original source of precursors that contribute to O 3 formation or the most recent source depending on the choice of the tagging method. The detailed features of the new technique are demonstrated during a peak O 3 event in September 2010 in Los Angeles, while trends in O 3 source contributions over time are evaluated during two additional simulations in July 2005 and August 2015. Quality control checks show that the new source apportionment methodology does not alter predicted total O 3 concentrations, and the detailed source apportionment information can be averaged to yield results that are consistent with traditional O 3 source apportionment calculations. The detailed O 3 source apportionment results during Sept 2010 show that, among NOx sources, on-road gasoline, on-road diesel, off-road diesel, and soil NOx account for over 60% of the ground level O 3 concentrations. Among VOC sources, upwind boundary conditions and biogenic sources account for approximately 90% of the ground-level O 3 formed. The formaldehyde to NO 2 ratio suggests that the chemical regime in the year 2015 was VOC-limited, but given the uncontrollable nature of the VOC emissions, the results suggest that NOx emission controls would have been the preferred emission control strategy to reduce O 3 concentration in Los Angeles at that time, with the understanding that some period of O 3 disbenefits would need to be tolerated until the emissions control program shifts the atmospheric chemistry back into the NOx-limited regime. The NOx source apportionment results for O 3 identify the largest NOx sources that could be reduced in an effort to reach NOx-limited conditions. The chemical regime in the Los Angeles atmosphere is continuously evolving, and so these calculations would need to be repeated under current conditions to determine if we have arrived at this NOx limited regime. Future studies will undertake this analysis. • A new method is developed to fully tag NOx and VOC contributions to O 3 . • Tests carried out in LA in 2005, 2010, and 2015 show the method is applicable to any ozone episode. • Upwind sources and biogenic sources emit most of the VOC that contributes to O 3 formation in LA. • On-road vehicles and off-road diesel emit most of the NOx that contributes to O 3 formation in LA. • Given that dominant VOC sources cannot be controlled, NOx controls may be most practical.