The influence of climate on ozone risk for vegetation

Sammanfattning: Ground-level ozone (O3) is a harmful air pollutant causing reduced crop yield and quality, reduced forest growth and negative effects on human health in large parts of the world. O3 is generally seen as a regional scale air pollution problem, but O3 concentration ([O3]) variation on a smaller geographical scale can be considerable. Knowledge of the size of this local scale variation and the underlying causes is important in environmental monitoring and assessments of O3 exposure. The local scale variation in [O3] in Sweden was investigated and described in relation to local climate and site characteristics such as altitude, topography, vicinity to the coast and local NO emissions based on measurements of [O3] and meteorology with a mobile monitoring station. In addition, [O3] and [NO2] were measured with passive diffusion samplers and [O3] data from permanent monitoring stations were analysed. The strength of nocturnal temperature inversions was found to be crucial in determining the differences in average [O3] and diurnal [O3] range (DOR) at rural sites in southern Sweden. Inland low sites experienced stronger nocturnal temperature inversions, lower average [O3] and larger DOR compared to inland high and coastal sites. In addition, the underlying surface (important for the deposition rate), advection of O3-rich marine air and local NO emissions also influence the local scale variation of [O3]. The negative effects of O3 on vegetation are more closely related to the plant uptake of O3 through the stomata than to the [O3] in the ambient air. Environmental factors such as humidity, temperature and light, influence the degree of stomatal opening and thus the stomatal O3 flux into the leaf interior. The flux-based PODY-index (phytotoxic O3 dose above a flux threshold Y) was used to assess the O3 risk for vegetation. It allows modification of O3 uptake by climatic conditions to be incorporated in O3 risk assessment for vegetation. A large part of the local scale variation in [O3] in southern Sweden occurs during night-time. At night the stomatal O3 uptake by vegetation is low and the risk of O3 damage is therefore not greatly influenced. Thus, plant stomatal O3 uptake and O3 risk for vegetation are less influenced by the site position in the landscape than 24-hour average [O3]. At the coastal sites the [O3] were higher also during daytime, which implies an increased risk of negative effects of O3 on vegetation compared to inland sites. The influence of potential future climate change on the flux-based risk of negative effects of O3 on vegetation in Europe was investigated with modelled future [O3] from the chemistry transport model MATCH and meteorology from the regional climate model RCA3. The future plant O3 uptake and risk of O3 damage to vegetation was predicted to remain unchanged or decrease in Europe, despite substantially increased modelled [O3] in Central and Southern Europe. The expected reduction in stomatal conductance with rising atmospheric [CO2] is of large importance for this result. However, the magnitude of the CO2 effect is uncertain, especially for trees. If the CO2 effect will turn out to be small, future climate change has the potential to dramatically increase the flux-based O3 risk for vegetation in Northern and Central Europe.