Modelling interactions between vegetation and tropospheric ozone

In this thesis, the TOMCAT and HadGEM2-ES models are used to improve the understanding
of the complex interactions between tropospheric ozone and the global biosphere.
The focus is on the impacts of surface ozone on crops, impacts of land surface vegetation
changes on tropospheric ozone and the effects of vegetation ozone damage on tropospheric
chemistry through changes in dry deposition and BVOC emissions. This thesis presents
the first estimates of the impact of ozone precursor emissions from each of the northern
hemisphere’s major industrialised regions (N America, SE Asia and Europe) on crop yields
globally and in continents downwind of each region. Using a range of ozone crop damage
metrics, 100% reductions in SE Asian anthropogenic NOx emissions tend to produce the
greatest global reduction in crop production losses (42.3-95.2 %). A 100% reduction in
N American anthropogenic NOx emissions produces the largest transboundary impact,
resulting in European production loss reductions of between 14.2% and 63.2 %. Preindustrial
model tropospheric ozone distributions are shown to be sensitive to the model
vegetation coverage and representation of biosphere atmosphere processes. Simulations
with HadGEM2-ES indicate a pre-industrial ozone burden which ranges from 200.4 Tg to
220.9 Tg with the variation resulting from differences in the model land vegetation distribution,
the CO2 mixing ratio the vegetation was exposed to and the model climate
setup. The change in pre-industrial to present day ozone burden ranged from 71.9 Tg
to 92.5 Tg translating into an estimated radiative forcing of 0.227Wm−2 to 0.244Wm−2.
The response of ozone concentrations to land cover changes is greater than the response
to changes in the CO2 mixing ratio the vegetation is exposed to. The ozone distribution is
shown to be more sensitive to changes in the rates of isoprene emissions than to changes in
dry deposition processes. The effects of ozone damage have a self limiting effect on ozone
and NOy dry deposition leading to a rise in surface ozone concentrations. Depending on
plant sensitivity to ozone damage, mean total stomatal uptake was shown to decline by
0.22 to 0.42 mmolO3m−2 resulting in average surface ozone concentrations rising by 0.02
to 0.03 ppbv, with local increases of up to 14 ppbv. The effects of ozone damage on photosynthesis
is to reduce isoprene emissions, producing slight drops in ozone over the more
polluted regions offsetting the rise in concentrations due to the dry deposition effect. The
inclusion of both effects resulted in a net rise in surface ozone concentrations indicating the
dry deposition response to ozone damage dominates over the effects on isoprene emissions.