Impacts of Foreign, Domestic, and State-Level Emissions on Ozone-Induced Vegetation Loss in the United States.

Exposure to elevated levels of ozone leads to yield reduction in agricultural crops and biomass loss in trees. Here, we quantify the impact of ozone pollution on two major U.S. crops, wheat and soybean, and two ozone-sensitive tree species, ponderosa pine and quaking aspen, using simulations with the GEOS-Chem model for 2010. Using previously established exposure-response functions, we estimate nationwide relative yield reductions of 4.9% for wheat and 6.7% for soybean, and relative biomass loss of 2.5% and 2.9% for ponderosa pine and aspen seedlings, respectively. Adjoint model sensitivities are used to estimate the impact of emissions sources from different locations, species, and sectors. We find that the nationwide relative loss in each vegetation type is influenced most by domestic anthropogenic NOx (>75%). Long-range transport from foreign sources is small relative to domestic influences. More than half of the anthropogenic NOx responsible for vegetation damage originates from outside the states where the damage occurs. Texas and Missouri are the highest contributors to the nationwide loss of wheat and soybean, respectively. California "exports" ozone damage for all types of vegetation studied, due to its location, high share of anthropogenic NOx, and a relatively low share of vegetation.