Growing seasons in a chemical transport model

Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Biogeosciences Discussions This discussion paper is/has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG if available. Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Abstract Chemical transport models (CTMs), used for the prediction of, for example, nitrogen deposition or air quality changes, require estimates of the growing season of plants for a number of reasons. Typically, the growing seasons are defined in a very simplified way in CTMs, using e.g. fixed dates or simple functions. In order to explore the importance 5 of more realistic growing season estimates, we have developed a new and simple method (the " T5 " method) for calculating the start of the growing season (SGS) of birch (which we use as a surrogate for deciduous trees), suitable for use in CTMs and other modelling systems. We developed the " T5 " method from observations, and here we compare with these and other methodologies, and show that with just two parameters 10 " T5 " captures well the spatial variation in SGS across Europe. We use the EMEP MSC-W chemical transport model to illustrate the importance of improved SGS estimates for ozone and two metrics associated with ozone-damage to vegetation. This study shows that although inclusion of more realistic growing seasons has only small effects on annual average concentrations of pollutants such as ozone, 15 the metrics associated with vegetation-risk from ozone are significantly affected. This work demonstrates a strong need to include more realistic treatments of growing seasons in CTMs. The method used here could also be suitable for other types of models which require information on vegetation cover, such as meteorological and regional climate models. In future work, the " T5 " and other methods will also be further 20 evaluated for use with agricultural and grassland land-covers, which are important for emissions and deposition of reactive nitrogen compounds.