Sources contributing to background surface ozone in the US Intermountain West

Abstract

Abstract. We quantify the sources contributing to background surface ozone concentrations in the US Intermountain West by using the GEOS-Chem chemical transport model with 1/2 • × 2/3 • horizontal resolution to interpret the Clean Air Status and Trends Network (CASTNet) ozone monitoring data for 2006-2008. We isolate contributions from lightning, wildfires, the stratosphere, and California pollution. Lightning emissions are constrained by observations and wildfire emissions are estimated from daily fire reports. We find that lightning increases mean surface ozone in summer by 10 ppbv in the Intermountain West, with moderate variability. Wildfire plumes generate high-ozone events in excess of 80 ppbv in GEOS-Chem, but CASTNet ozone observations in the Intermountain West show no enhancements during these events nor do they show evidence of regional fire influence. Models may overestimate ozone production in fresh fire plumes because of inadequate chemistry and gridscale resolution. The highest ozone concentrations observed in the Intermountain West (> 75 ppbv) in spring are associated with stratospheric intrusions. The model captures the timing of these intrusions but not their magnitude, reflecting numerical diffusion intrinsic to Eulerian models. This can be corrected statistically through a relationship between model bias and the model-diagnosed magnitude of stratospheric influence; with this correction, models may still be useful to forecast and interpret high-ozone events from stratospheric intrusions. We show that discrepancy between models in diagnosing stratospheric influence is due in part to differences in definition, i.e., whether stratospheric ozone is diagnosed as produced in the stratosphere (GEOS-Chem definition) or as transported from above the tropopause. The latter definition can double the diagnosed stratospheric influence in surface air by labeling as "stratospheric" any ozone produced in the troposphere and temporarily transported to the stratosphere. California pollution influence in the Intermountain West frequently exceeds 10 ppbv but is generally not correlated with the highest ozone events.

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

“…At the high-altitude sites in summer, the GEOS-Chem overestimate of observed O 3 has been attributed previously to an overestimate of O 3 produced from lightning NO x when prescribing a higher production of NO x from flashes at mid-latitudes and spatially scaling the source to match LIS-OTD climatological flash counts (Murray et al, 2012), which may lead to regional errors for a specific year (Zhang et al, 2013). The larger difference between the NAB estimates from the two models in August than between the simulated and observed total O 3 implies that the agreement with observations, while a necessary condition, does not sufficiently constrain the NAB estimates.…”

“…Inspection of Figure 7 (top two panels) shows that the episodic enhancements in the O3Se90 tracer can explain much of the episodic enhancements in NAB. A caveat is that the magnitude of the stratospheric contribution is an upper limit due to the definition of the O3Se90 tracer, which could be tagging O 3 in the lower stratosphere that originated in the troposphere (estimated to be approximately half of the O3Se90 during spring by Zhang et al, 2013).…”

“…The models differ markedly in their NAB estimates over this region in summer (e.g., Figures 1 and 2). This source has been reduced in a newer version of GEOS-Chem, decreasing simulated NAB O 3 in these regions (Zhang et al, 2013).…”

“…We note that the inter-annual variability may be underestimated in AM3 in some regions due its use of climatological inventories for soil NO x and wildfire emissions. (Fiore et al, 2002), the 2001 O 3 season (Fiore et al, 2003;Wang et al, 2009Wang et al, ), and the 2006Wang et al, -2008 seasons Zhang et al, 2013) including extensive evaluation with in situ and satellite observations. The AM3 model has previously been applied at ~50 km horizontal resolution globally to estimate the impacts of Asian pollution and stratospheric intrusions on surface O 3 over the WUS during March through June of 2010.…”

Responses of surface ozone air quality to anthropogenic nitrogen deposition in the Northern Hemisphere

“…The GEOS-Chem model has been applied in a number of studies to simulate atmospheric nitrogen deposition Ellis et al, 2013;Zhao et al, 2015Zhao et al, , 2017, surface ozone air quality (Zhang et al, , 2014, and recently impacts of land use changes on atmospheric composition through biosphere-atmosphere exchange processes (Fu and Tai, 2015;Fu et al, 2016;Geddes et al, 2016;Heald and Geddes, 2016). It includes a detailed simulation of tropospheric NO x -VOC-O 3 -aerosol chemistry (Park et al, 2004;Mao et al, 2010).…”