Abstract
Abstract. Tropospheric ozone is an air pollutant that substantially harms vegetation and is also strongly dependent on various vegetation-mediated processes. The interdependence between ozone and vegetation may constitute feedback mechanisms that can alter ozone concentration itself but have not been considered in most studies to date. In this study we examine the importance of dynamic coupling between surface ozone and leaf area index (LAI) in shaping ozone air quality and vegetation. We first implement an empirical scheme for ozone damage on vegetation in the Community Land Model (CLM) and simulate the steady-state responses of LAI to long-term exposure to a range of prescribed ozone levels (from 0 to 100 ppb). We find that most plant functional types suffer a substantial decline in LAI as ozone level increases. Based on the CLM-simulated results, we develop and implement in the GEOS-Chem chemical transport model a parameterization that computes fractional changes in monthly LAI as a function of local mean ozone levels. By forcing LAI to respond to ozone concentrations on a monthly timescale, the model simulates ozone–LAI coupling dynamically via biogeochemical processes including biogenic volatile organic compound (VOC) emissions and dry deposition, without the complication from meteorological changes. We find that ozone-induced damage on LAI can lead to changes in ozone concentrations by −1.8 to +3 ppb in boreal summer, with a corresponding ozone feedback factor of −0.1 to +0.6 that represents an overall self-amplifying effect from ozone–LAI coupling. Substantially higher simulated ozone due to strong positive feedbacks is found in most tropical forests, mainly due to the ozone-induced reductions in LAI and dry deposition velocity, whereas reduced isoprene emission plays a lesser role in these low-NOx environments. In high-NOx regions such as the eastern US, Europe, and China, however, the feedback effect is much weaker and even negative in some regions, reflecting the compensating effects of reduced dry deposition and reduced isoprene emission (which reduces ozone in high-NOx environments). In remote, low-LAI regions, including most of the Southern Hemisphere, the ozone feedback is generally slightly negative due to the reduced transport of NOx–VOC reaction products that serve as NOx reservoirs. This study represents the first step to accounting for dynamic ozone–vegetation coupling in a chemical transport model with ramifications for a more realistic joint assessment of ozone air quality and ecosystem health.
Highlights
Tropospheric ozone (O3) is an important greenhouse gas with an estimated radiative forcing of 0.40 ± 0.20 W m−2 (IPCC, 2013)
We find that monthly γraw for a given location generally decreases as ozone concentration increases, but its decrease per unit of ozone increase becomes smaller at higher ozone concentrations (Fig. 1) because of the progressive closure of stomata as represented by the ozone damage scheme
We develop a parameterized function for an ozone impact factor on leaf area index (LAI) by conducting various land surface–biogeochemical model simulations using Community Land Model (CLM) with an empirical scheme of ozone damage on vegetation to represent the impact of long-term ozone exposure on monthly mean LAI
Summary
Tropospheric ozone (O3) is an important greenhouse gas with an estimated radiative forcing of 0.40 ± 0.20 W m−2 (IPCC, 2013). As vegetation variables such as stomatal resistance, LAI, and plant functional type (PFT) distribution all play important roles shaping surface ozone, dynamic changes in these variables following ozone damage may induce a cascade of feedbacks that affect ozone itself The impact of such ozone–vegetation coupling on ozone air quality has only recently been examined by Sadiq et al (2017), who found that by implementing synchronous ozone–vegetation coupling in the Community Earth System Model (CESM), simulated present-day surface ozone concentrations can be higher by 4–6 ppb over North America, Europe, and China. This effort allows ozone–vegetation coupling to be considered dynamically within an atmospheric model without the complication from meteorological changes and feedbacks, and renders the incorporation of ozone-induced biogeochemical feedbacks and air quality–ecosystem coevolution more computationally affordable in regional climate and air quality models
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