Abstract
Abstract. High-ozone events, approaching or exceeding the National Ambient Air Quality Standard (NAAQS), are frequently observed in the US Intermountain West in association with subsiding air from the free troposphere. Monitoring and attribution of these events is problematic because of the sparsity of the current network of surface measurements and lack of vertical information. We present an Observing System Simulation Experiment (OSSE) to evaluate the ability of the future geostationary satellite instrument Tropospheric Emissions: Monitoring of Pollution (TEMPO), scheduled for launch in 2018–2019, to monitor and attribute high-ozone events in the Intermountain West through data assimilation. TEMPO will observe ozone in the ultraviolet (UV) and visible (Vis) bands to provide sensitivity in the lower troposphere. Our OSSE uses ozone data from the GFDL AM3 chemistry-climate model (CCM) as the "true" atmosphere and samples it for April–June 2010 with the current surface network (CASTNet –Clean Air Status and Trends Network– sites), a configuration designed to represent TEMPO, and a low Earth orbit (LEO) IR (infrared) satellite instrument. These synthetic data are then assimilated into the GEOS-Chem chemical transport model (CTM) using a Kalman filter. Error correlation length scales (500 km in horizontal, 1.7 km in vertical) extend the range of influence of observations. We show that assimilation of surface data alone does not adequately detect high-ozone events in the Intermountain West. Assimilation of TEMPO data greatly improves the monitoring capability, with little information added from the LEO instrument. The vertical information from TEMPO further enables the attribution of NAAQS exceedances to background ozone. This is illustrated with the case of a stratospheric intrusion.
Highlights
Harmful impacts of surface level ozone on both humans and vegetation is of increasing concern in areas formerly considered remote
We demonstrated the potential of future Tropospheric Emissions: Monitoring of Pollution (TEMPO) UV + Vis geostationary observations to monitor ozone exceedances in the Intermountain West and identify those exceedances caused by the North American background
Our goal was to inform the TEMPO observing strategy and develop methods for exploitation of its data. To accomplish this we performed an Observing System Simulation Experiment (OSSE) for assimilation of synthetic TEMPO data designed to best represent future observations based on current estimates of TEMPO instrument characteristics
Summary
Harmful impacts of surface level ozone on both humans and vegetation is of increasing concern in areas formerly considered remote. The US Environmental Protection Agency (EPA) is considering lowering the current National Ambient Air Quality Standard (NAAQS) of 75 ppbv (parts per billion by volume, fourth highest maximum daily 8 h average per year) to a value in the range of 60–70 ppbv (EPA, 2012) Ozone concentrations in this range are frequently observed at high-elevation sites in the Western US with minimal local pollution influence (Lefohn et al, 2001). We show that multispectral measurements from a configuration designed to represent the best current estimate of the NASA Tropospheric Emissions: Monitoring of Pollution (TEMPO) geostationary satellite mission over North America, scheduled for launch in 2018–2019, can provide a powerful ozone monitoring resource to complement surface sites, and can help to identify NAAQS exceedances caused by elevated background ozone. We describe below our OSSE framework including the simulation models (GFDL AM3 and GEOS-Chem), the observing system, and the data assimilation system
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