Ozone vegetation damage effects on gross primary productivity in the United States

We apply an off-line process-based vegetation model to assess the impacts of ozone vegetation damage on gross primary productivity (GPP) in the contiguous United States during the past decade (1998-2007). The semi-mechanistic parameterization of ozone-induced photosynthesis inhibition from Sitch et al. (2007) is implemented into the vegetation model framework. We first evaluate the model's GPP simulation at 40 sites of the North American Carbon Program (NACP). This ecosystem-scale site-level model is driven with hourly meteorological forcings from the Modern-Era Retrospective Analysis (MERRA) and site-based measurements. The model reproduces interannual variability and seasonality of GPP at most sites, especially in croplands. The annual mean GPP shows a correlation coefficient of 0.68 between simulations and observations. The inclusion of the ozone damage impact improves the simulated GPP at most NACP sites. The simulated annual GPP averaged over all NACP sites changes from 3.8 g C m-2 day-1 to 3.6 g C m-2 day-1, closer to the observations of 3.0 g C m-2 day-1. We then perform a regional gridded simulation at 1.3°×1° resolution over the contiguous U.S. The distributed model is driven with the MERRA meteorology and land cover from the International Satellite Land-Surface Climatology Project (ISLSCP). The simulation shows an average GPP of 5.9 g C m-2 day-1 in summer, with 9.2 g C m-2 day-1 in the East of 95°W and 3.7 g C m-2 day-1 in the West. Hourly surface ozone concentrations are output from simulations representative of the present climatic state performed using the Yale-E2 global carbon-chemistry-climate model. After evaluating the model's surface ozone based on ground observations from ~1200 sites, we probe the response of GPP over the United States to ozone vegetation damage. On average, the summer GPP declines by 2-5% in the contiguous U.S., depending on the sensitivity of GPP to ozone. A larger reduction of 4-7% is estimated in eastern U.S., where both GPP and ozone concentrations are high, with significant decreases of 13-17% in east coast hotspots. Simulated reduction fraction in GPP over contiguous U.S. due to (a) low and (b) high ozone sensitivity.