In part 3 of this series of papers on a new 3-D global tropospheric chemical transport model, using an Integrated Modelling System (IMS), an evaluation of the model performance in simulating global distributions and seasonal variations for volatile organic compounds (VOCs) in the atmosphere, is presented. Comparisons of model OH concentrations with previous model studies show consistent modelled OH levels from the subtropics to midlatitudes, while more discrepancies occur over the tropical low latitudes, with IMS predicting the highest levels of OH. The close agreement between modelled OH concentrations over midlatitudes, where high surface NOx and VOC concentrations are also found, is indicative of the strong photochemical coupling between NOx, VOCs and O3 over these latitudes. IMS OH concentrations in the Northern Hemisphere (NH) midlatitudes during summer are generally lower than available measurements, implying that models in general are underestimating OH levels at this location and time of year. Substantial differences between model OH concentrations over low latitudes clearly highlight areas of uncertainty between models. IMS OH concentrations are the highest in general of the models compared, one possible reason is that biogenic emissions of species such as isoprene and monoterpenes are highest in IMS, leading to higher O3 levels and hence higher OH. Generally, the IMS VOC concentrations show a similar seasonality to the measurements at most locations. In general though, IMS tends to underestimate the NH wintertime VOC maximum and overestimate the NH summertime VOC minimum. Such an overestimate in summer could be due to IMS underestimating OH levels, or an overestimation of VOC emissions or possibly a problem with model transport, all of these possibilities are explored. Except for n-pentane, the model underprediction of a VOC maximum during the NH winter month strongly suggests a missing emission mechanism in the model or an underestimate of an existing one. It is very likely that there is a lack of time varying emission sources in the model to account for the seasonal change in emission behaviour such as increasing energy usage (e.g., electricity and gas), road transportation, engine performance, and other anthropogenic factors which show strong seasonal characteristics. The anomalous overprediction of wintertime n-pentane compared with its close summertime prediction with the measurements suggest that emissions in this case may be too high.
- Chemistry transport model