Background ozone over the United States in summer: Origin, trend, and contribution to pollution episodes

Abstract

[1] Observations indicate that ozone (O 3 ) concentrations in surface air over the United States in summer contain a 20-45 ppbv background contribution, presumably reflecting transport from outside the North American boundary layer. We use a threedimensional global model of tropospheric chemistry driven by assimilated meteorological observations to investigate the origin of this background and to quantify its contribution to total surface O 3 on both average and highly polluted summer days. The model simulation is evaluated with a suite of surface and aircraft observations over the United States from the summer of 1995. The model reproduces the principal features in the observed distributions of O 3 and its precursors, including frequency distributions of O 3 concentrations and the development of regional high-O 3 episodes in the eastern United States. Comparison of simulations with 1995 versus 1980 global fossil fuel emissions indicates that the model captures the previously observed decrease in the high end of the O 3 probability distribution in surface air over the United States (reflecting reduction of domestic hydrocarbon emissions) and the increase in the low end (reflecting, at least in the model, rising Asian emissions). In the model, background O 3 produced outside of the North American boundary layer contributes an average 25-35 ppbv to afternoon O 3 concentrations in surface air in the western United States. and 15-30 ppbv in the eastern United States during the summer of 1995. This background generally decays to below 15 ppbv during the stagnation conditions conducive to exceedances of the 8-hour 0.08 ppmv (80 ppbv) National Ambient Air Quality Standard (NAAQS) for O 3 . A high background contribution of 25-40 ppbv is found during 9% of these exceedances, reflecting convective mixing of free tropospheric O 3 from aloft, followed by rapid production within the U.S. boundary layer. Anthropogenic emissions in Asia and Europe are found to increase afternoon O 3 concentrations in surface air over the United States by typically 4-7 ppbv, under both average and highly polluted conditions. This enhancement is particularly large (up to 14 ppbv) for O 3 concentrations in the 50-70 ppbv range, and would represent a major concern if the NAAQS were to be tightened.

New exposure-based metric approach for evaluating O3 risk to North American aspen forests

“…With increasing expansion of city centers and urban sprawl, unmanaged forest areas will increasingly become recipients of long-range transport of secondary oxidation products of primary urban VOC and NO x emissions. Modeled estimates (Fiore et al, 2002) indicate that anthropogenic emissions in Asia and Europe may increase afternoon O 3 concentrations in surface air over the United States by between 4 and 7 ppb, an enhancement that may be particularly large for O 3 concentrations in the mid range (50e70 ppb). Any rise in background O 3 concentration may, therefore, offset the positive benefits for North American forests accrued from the recent downward trend in peak O 3 concentrations.…”

Impact of the assimilation of ozone from the Tropospheric Emission Spectrometer on surface ozone across North America

“…The model includes a detailed description of tropospheric O 3 -NO x -hydrocarbon chemistry. The model has been used in a range of studies, including the analysis of the impact of background O 3 on surface O 3 over North America [e.g., Fiore et al, 2002] and interpretation of in situ aircraft observations from the International Consortium on Atmospheric Transport and Transformation (ICARTT) aircraft campaign over North America [e.g.. Hudman et al, 2007]. We use version v7-02-04 of GEOS-Chem with a horizontal resolution of 2°Â 2.5°and 55 levels in the vertical from the surface to 0.01 hPa, with 5 levels in the PBL.…”

“…Precise estimates of the tropospheric O 3 budget are desirable in an air quality context as O 3 produced in the free troposphere and the intercontinental transport of O 3 provides a background contribution to O 3 abundances at the surface. It has been estimated [e.g., Fiore et al, 2002;Vingarzan, 2004] that this background source contributes as much as 20-45 ppbv to surface O 3 . Fiore et al [2002] showed that Asian and European anthropogenic emissions increase surface O 3 in North America by 4 -7 ppbv.…”

“…It has been estimated [e.g., Fiore et al, 2002;Vingarzan, 2004] that this background source contributes as much as 20-45 ppbv to surface O 3 . Fiore et al [2002] showed that Asian and European anthropogenic emissions increase surface O 3 in North America by 4 -7 ppbv. Jacob et al [1999] and Lin et al [2008] have suggested that future increases in Asian anthropogenic emissions could provide an additional increase of 1 -7 ppbv in surface O 3 in North America.…”

Resolution Dependency of Numerically Simulated Stratosphere-to-Troposphere Transport Associated with Mid-Latitude Closed Cyclones in Early Spring around Japan

“…Therefore, it is important to determine the spatial distribution of STT and its process for each region, in order to understand the regional characteristics of STT. Many previous studies have focused on STT events in North America using both observations and global models with a horizontal resolution of 50−300 km (Lefohn et al 2001(Lefohn et al , 2011Fiore et al 2002;Langford et al 2009;Bourqui and Trepanier 2010;Lin et al 2012). Some studies have focused on STT events in the Mediterranean, using a global model with a relatively low resolution (~300 km; e.g., Akritidis et al 2016), as well as on events in Europe, using global models with relatively higher resolutions (~100 km), and regional models with a resolution of ~14 km (e.g., Hoffmann et al 2016).…”