Emissions of biogenic volatile organic compounds and ozone balance under future climate conditions

Ozone (O3) is a phytotoxic trace gas in the troposphere where it is photochemically produced from volatile organic compounds (VOCs) and nitrogen oxides (NOx = NO + NO2). The dominant sink of O3 in the air over areas with dense plant cover is dry deposition on plant surfaces. However, plants can also contribute to photochemical O3 formation because they emit biogenic VOCs (BVOCs). In this study, the role of vegetation for tropospheric ozone balance was investigated by considering the following processes: O3 depletion by dry deposition on plant surfaces, O3 depletion by gas phase reactions with plant emitted BVOCs, and photochemical O3 production from BVOCs. Furthermore, drought and heat stress were applied to the plants, and possible impacts of these stresses on plant performance and on the tropospheric ozone balance were investigated. Dry deposition of O3 was dominated by O3 uptake through the plants stomata with negligible losses on cuticle and stem. For strong BVOC emitter, O3 destruction by gas phase reactions with BVOCs was significant at low NOx conditions. Switching from low NOx to high NOx conditions led to O3 production. A ratio of O3 formation rates over BVOC loss rates was measured for α-pinene as single BVOC and for BVOC mixtures emitted from real plants. For O3 formation under BVOC limited conditions, this ratio was in the range of 2–3 ppb/ppb. The ratio of O3 uptake/BVOC emission reflects the capability of a plant as a potential source of O3, while NOx concentration and BVOC/NOx ratio determine whether the emitted BVOCs act as an additional sink or a source of O3. O3 uptake rates and BVOC emission rates are affected by environmental variables such as temperature, light intensity and stresses to plants. The impacts of them are different and thus the capability of a plant to be a source of O3 is also affected by these variables. The focus of this work was the evaluation of the impact of drought and heat stress because future climate change will bring more and intense heat waves and elongated drought periods. With the application of moderate drought, the capability of a plant to be a source of O3 increased; under conditions of severe drought the impact of plants in the O3 balance decreased to almost zero. Heat stress also changes the capability of the plant to be a source of O3. However, this change depends on the basic emission mechanisms of BVOCs and the severity of stress.