Modelling tropospheric ozone and related tracers in a 3-D chemical transport model: An investigation of biogenic nitrogen oxides and VOC modelling techniques and changing emissions scenarios

The York Chemical Transport Model (YCTM) has been improved to include a meteorologically sensitive biogenic emissions processing scheme for biogenic VOCs (including isoprene) and soil NOx sources. The performance of the YCTM has been assessed against trace gas tropospheric measurements and other published CTMs. Although there are model weaknesses (e.g., overprediction of tropospheric CO in the Southern Hemisphere) the model nonetheless proves to be a useful tool within which to examine tropospheric constituents as it generally performs adequately on a global basis as tested in a contemporary control scenario. The YCTM is a semi-Lagrangian global scale chemical transport model including 38 species and 72 gas phase chemical reactions; meteorology is preprocessed separately. Inclusion of the meteorologically sensitive biogenic VOC emissions processing scheme has revealed that due to most of the O 3 being in a NOx limited regime, biogenic VOCs induce only a moderate effect on tropospheric ozone levels, but a more significant effect on CO levels in the troposphere. At a global scale updating emission contributions from biogenic sources on a 6 hourly basis results in very significant differences (up to 50%) in tropospheric ozone mixing ratios at short (3 hourly) time scales. At longer timescales (10 day averages) these effects are more moderate (up to 15%).; The YCTM has also been applied to examine a preindustrial and a future scenario and resulting impacts on ozone and related tropospheric trace gases have been examined. Results from this aspect of the study indicate that ozone levels have increased at least 60% in the Northern Hemisphere and 20% in the Southern Hemisphere since preindustrial times. Use of a conservative future scenario indicates that ozone levels are expected to increase 25 to 85% in the Northern Hemisphere and 15 to 55% in the Southern Hemisphere. Application of representative scenario climate effects (including temperature and precipitation) appears to have little impact on results. Emissions seem more critical than CO2 related climate changes such as precipitation and temperature. Investigation of changes in biogenic emissions due to climate feedbacks including temperature and precipitation was not carried out in this study and warrants further exploration.