The influence of foreign vs. North American emissions on surface ozone in the US

Abstract

Abstract. As part of the Hemispheric Transport of Air Pollution (HTAP; http:// www.htap.org) project, we analyze results from 15 global and 1 hemispheric chemical transport models and compare these to Clean Air Status and Trends Network (CASTNet) observations in the United States (US) for 2001. Using the policy-relevant maximum daily 8-h average ozone (MDA8 O3) statistic, the multi-model ensemble represents the observations well (mean r2=0.57, ensemble bias = +4.1 ppbv for all US regions and all seasons) despite a wide range in the individual model results. Correlations are strongest in the northeastern US during spring and fall (r2=0.68); and weakest in the midwestern US in summer (r2=0.46). However, large positive mean biases exist during summer for all eastern US regions, ranging from 10–20 ppbv, and a smaller negative bias is present in the western US during spring (~3 ppbv). In nearly all other regions and seasons, the biases of the model ensemble simulations are ≤5 ppbv. Sensitivity simulations in which anthropogenic O3-precursor emissions (NOx + NMVOC + CO + aerosols) were decreased by 20% in four source regions: East Asia (EA), South Asia (SA), Europe (EU) and North America (NA) show that the greatest response of MDA8 O3 to the summed foreign emissions reductions occurs during spring in the West (0.9 ppbv reduction due to 20% emissions reductions from EA + SA + EU). East Asia is the largest contributor to MDA8 O3 at all ranges of the O3 distribution for most regions (typically ~0.45 ppbv) followed closely by Europe. The exception is in the northeastern US where emissions reductions in EU had a slightly greater influence than EA emissions, particularly in the middle of the MDA8 O3 distribution (response of ~0.35 ppbv between 35–55 ppbv). EA and EU influences are both far greater (about 4x) than that from SA in all regions and seasons. In all regions and seasons O3-precursor emissions reductions of 20% in the NA source region decrease MDA8 O3 the most – by a factor of 2 to nearly 10 relative to foreign emissions reductions. The O3 response to anthropogenic NA emissions is greatest in the eastern US during summer at the high end of the O3 distribution (5–6 ppbv for 20% reductions). While the impact of foreign emissions on surface O3 in the US is not negligible – and is of increasing concern given the recent growth in Asian emissions – domestic emissions reductions remain a far more effective means of decreasing MDA8 O3 values, particularly those above 75 ppb (the current US standard).

Modeling intercontinental transport of ozone in North America with CAMx for the Air Quality Model Evaluation International Initiative (AQMEII) Phase 3

“…In the GLO scenario US surface O 3 is more sensitive to domestic emission reductions than changes in the BCs. Our 2010 EAS contribution results are slightly higher than the estimates of 0.35-0.45 ppb from a multi-model experiment that also simulated the EAS scenario but for the year 2001 (Reidmiller et al, 2009). This is expected as East Asia emissions have increased over the last decade.…”

Estimating North American background ozone in U.S. surface air with two independent global models: Variability, uncertainties, and recommendations

“…More recent increases in Asian emissions may have additionally raised WUS NAB by up to 3 ppb in spring between (Zhang et al, 2008. This Asian component of NAB, as well as European contributions and global anthropogenic methane has received particular attention under the UNECE Task Force on Hemispheric Transport of Air Pollution Reidmiller et al, 2009;TFHTAP, 2010;Wild et al, 2012). Recent studies have further documented the mechanisms by which Asian pollution can reach surface air over the WUS (e.g., (Brown-Steiner and Hess, 2011;Lin et al, 2012b).…”

Tropospheric Ozone Assessment Report: Assessment of global-scale model performance for global and regional ozone distributions, variability, and trends

“…Model evaluation relies on the availability of high quality observations with high spatial and temporal coverage. Reasonably comprehensive "baseline" (TOARObservations) surface ozone observations over the U.S., Canada, Europe, and Japan augmented with data over numerous polluted sites in these regions have facilitated a thorough evaluation of global chemistry models over those regions (e.g., Fiore et al, 2009;Reidmiller et al, 2009;Schnell et al, 2015;Sofen et al, 2016b). Evaluation elsewhere is limited by poor data availability.…”

“…In practice the different formulation and chemical complexity in different models means that some processes remain poorly constrained (e.g., natural emissions are often left unspecified) and others may be omitted entirely (e.g., higher VOC chemistry). An important consequence is that there are first order differences in model implementation which need to be considered when comparing and evaluating model simulations (Shindell et al, 2008;Fiore et al, 2009;.…”