Welcome to the Harvard Atmospheric Chemistry Modeling Group!

Our goal is to understand the chemical composition of the atmosphere, its perturbation by human activity, and the implications for climate change and life on Earth. We address this goal through global modeling of atmospheric chemistry and climate, aircraft measurement campaigns, satellite data retrievals, and analyses of atmospheric observations.

GROUP LEADERS: Daniel J. Jacob, Jennifer A. Logan, Loretta J. Mickley, Elsie Sunderland

May 2012: Improved estimate of the policy-relevant background ozone in the United States

The policy-relevant background (PRB) ozone is defined by the US Environmental Protection Agency (EPA) as the surface ozone concentration that would be present over the US in the absence of North American anthropogenic emissions. It is intended to provide a baseline for risk and exposure assessments used in setting the National Ambient Air Quality Standard (NAAQS). We present three-year statistics of PRB ozone over the US calculated using the GEOS-Chem model with 1/2 x 2/3 degree horizontal resolution over North America and adjacent oceans (2 x 2.5 degree for the rest of the world). We also provide estimates of the US background (no anthropogenic US emissions) and natural background (no anthropogenic emissions worldwide and pre-industrial methane). PRB is particularly high in the intermountain West due to high elevation, arid terrain, and large-scale subsidence. We show that the model is successful in reproducing ozone exceedances up to 70 ppbv. However, the model cannot reproduce PRB-relevant exceptional events associated with wildfires or stratospheric intrusions. The mean PRB estimates for spring-summer are 27 +/- 8 ppbv at low-altitude sites and 40 +/- 7 ppbv at high-altitude sites. Differences between the PRB simulation and the natural simulation indicate a mean enhancement from intercontinental pollution and anthropogenic methane of 9 ppbv at low-altitude sites and 13 ppbv at high-altitude sites. The PRB is higher than average when ozone exceeds 60 ppbv, particularly in the intermountain West. Our PRB estimates are on average 4 ppbv higher than previous GEOS-Chem studies and we attribute this to higher lighting, increasing Asian emissions, and improved model resolution.

The figure above shows North American background (PRB) ozone concentration in surface air for spring and summer 2006. The top panels show seasonal means while the bottom panels show the means for days with total ozone > 60 ppbv. Gray areas in the bottom panels had no days with total ozone >60 ppbv. For more information, see Zhang et al. (2011)

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