Changes in Ozone Chemical Sensitivity in the United States from 2007 to 2016

Anthropogenic nitrogen oxide (NO_x) and volatile organic compound (VOC) emissions in the U.S. have declined substantially over the last decade, altering the NO_x-VOC chemistry and ozone (O₃) production characteristics of many areas. In this work we use multiple air quality analysis tools to assess how these large reductions in NO_x and VOC have affected O₃ production regimes across the U.S. between 2007 and 2016. We first compare observed and modeled evolution of NO_x-limited and NO_x-saturated O₃ formation regimes using a day-of-week (DOW) analysis. This comparison builds confidence in the model's ability to qualitatively capture O₃ changes due to chemistry and meteorology both within years and across periods of large emissions decreases. DOW analysis, however, cannot definitively differentiate between emissions and meteorology impacts. We therefore supplement this analysis with sensitivity calculations from CAMx-HDDM to characterize modeled shifts in O₃ formation chemistry between 2007 and 2016 in different regions of the U.S. We also conduct a more detailed investigation of the O₃ chemical behavior observed in Chicago and Detroit, two complex urban areas in the Midwest. Both the ambient and modeling data show that more locations across the U.S. have shifted towards NO_x-limited regimes between 2007 and 2016. The model-based HDDM sensitivity analysis shows only a few locations remaining NO_x-saturated on high-O₃ days in 2016 including portions of New York City, Chicago, Minneapolis, San Francisco and Los Angeles. This work offers insights into the current state of O₃ production chemistry in large population centers across the U.S., as well as how O₃ chemistry in these areas may evolve in the future.

Biogenic volatile organic compound emissions (BVOCs). I. Identifications from three continental sites in the U.S

Vegetation composition and biomass were surveyed for three specific sites in Atlanta, GA; near Rhinelander, WI; and near Hayden, CO. At each research site emissions of biogenic volatile organic compounds (BVOCs) from the dominant vegetation species were sampled by enclosing branches in bag enclosure systems and sampling the equilibrium head space onto multi-stage solid adsorbent cartridges. Analysis was performed using a thermal desorption technique with gas chromatography (GC) separation and mass spectrometry (MS) detection. Identification of BVOCs covering the GC retention index range (stationary phase DB-1) from approximately 400 to 1400 was achieved (volatilities C4-C14).

REGIONAL PHOTOCHEMICAL AIR QUALITY MODELING:Model Formulations, History, and State of the Science

▪ Abstract  Regional air quality models have been used for scientific investigation of trace species dynamics for over two decades and are now beginning to take a central position in air quality management. In particular, they have been used for studying the transport and fate of atmospheric acids, photochemical oxidants (e.g. ozone), and more recently, aerosols. Such models are based on numerically solving the mass conservation equations for a chemically interacting system of species and are applied to horizontal domains of 1000s of kms. Primary applications include assessing the response of pollutant concentrations to emissions controls, quantifying the flux of pollutants across and out of a region, and understanding the impact of specific processes on pollutant concentrations.