Abstract
Abstract. Tropospheric ozone is one of the most hazardous air pollutants as it harms both human health and plant productivity. Foliage uptake of ozone via dry deposition damages photosynthesis and causes stomatal closure. These foliage changes could lead to a cascade of biogeochemical and biogeophysical effects that not only modulate the carbon cycle, regional hydrometeorology and climate, but also cause feedbacks onto surface ozone concentration itself. In this study, we implement a semi-empirical parameterization of ozone damage on vegetation in the Community Earth System Model to enable online ozone–vegetation coupling, so that for the first time ecosystem structure and ozone concentration can coevolve in fully coupled land–atmosphere simulations. With ozone–vegetation coupling, present-day surface ozone is simulated to be higher by up to 4–6 ppbv over Europe, North America and China. Reduced dry deposition velocity following ozone damage contributes to ∼ 40–100 % of those increases, constituting a significant positive biogeochemical feedback on ozone air quality. Enhanced biogenic isoprene emission is found to contribute to most of the remaining increases, and is driven mainly by higher vegetation temperature that results from lower transpiration rate. This isoprene-driven pathway represents an indirect, positive meteorological feedback. The reduction in both dry deposition and transpiration is mostly associated with reduced stomatal conductance following ozone damage, whereas the modification of photosynthesis and further changes in ecosystem productivity are found to play a smaller role in contributing to the ozone–vegetation feedbacks. Our results highlight the need to consider two-way ozone–vegetation coupling in Earth system models to derive a more complete understanding and yield more reliable future predictions of ozone air quality.
Highlights
Tropospheric ozone is one of the air pollutants of the greatest concern due to its significant harm to human respiratory health
This study investigates the impacts of ozone–vegetation coupling on ozone concentrations using the Community Earth System Model (CESM), which includes several different model components representing the atmosphere, land, ocean, and sea ice to be run independently or in various coupled configurations (Oleson et al, 2010; Lamarque et al, 2012; Neale et al, 2013)
The simulated increases in ozone levels due to ozone– vegetation coupling are significant when compared with the possible impacts of 2000–2050 climate and land cover changes on surface ozone, which are in the range of +1– 10 ppbv (Jacob and Winner, 2009; Tai et al, 2013; Val Martin et al, 2015)
Summary
Tropospheric ozone is one of the air pollutants of the greatest concern due to its significant harm to human respiratory health. Ozone exposure can reduce stomatal conductance and transpiration rate, which may modify the partition between latent and sensible heat fluxes and lead to a cascade of meteorological changes: lower humidity that reduces the chemical loss rate of ozone; a thicker boundary layer that dilutes all pollutants, but may enhance entrainment, which either increases or decreases surface ozone depending on the vertical ozone profile (Super et al, 2015); and higher temperature that enhances ozone mainly through increased biogenic emissions and higher abundance of NOx (Jacob and Winner, 2009) These transpiration-mediated pathways can be characterized as biogeophysical feedbacks, as they are commonly known in the context of climate change, but here we prefer to call them hydrometeorological or “meteorological feedbacks” to emphasize that they are effected through ozone-induced changes in the hydrometeorological variables that affect ozone. We perform sensitivity simulations to quantify the relative importance of different biogeochemical and meteorological feedback pathways, elucidate the larger sources of uncertainties, and make specific suggestions regarding Earth system model development
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