Interannual variation in biogenic emissions on a regional scale

Abstract

Interannual variation in biogenic emissions is not well quantified, especially on regional scales. We use a land surface model augmented with a short‐term dynamic phenology scheme to estimate the interannual variation in the emission of biogenic volatile organic compounds (BVOCs) between 1982 and 2004. We use North American Regional Reanalysis data to drive two versions of the National Center for Atmospheric Research Community Land Model (CLM) on a 0.1° grid over eastern Texas. The first version is the standard CLM with prescribed leaf area index (LAI) (i.e., LAI varies seasonally but not interannually); the second version is the standard CLM augmented with a dynamic phenology scheme (CLM‐DP) that allows LAI to respond to environmental variation. We calibrate CLM‐DP using satellite‐derived LAI as our visual constraint. When phenology is prescribed, the domain‐mean (domain‐maximum) average absolute departure from the monthly mean BVOC flux is 11.7% (70.6%); when phenology is allowed to vary with environmental conditions, it is 22.4% (137.7%). The domain‐mean (domain‐maximum) average absolute departure from the monthly mean flux is lower during summer: using CLM‐DP, it is 15.7% (35.3%); using the standard CLM, it is 7.0% (23.0%). The domain‐average, mean‐normalized standard deviation of the June‐July‐August mean BVOC flux is 0.0619 when LAI is prescribed and 0.183 when LAI varies with environmental conditions. Our results imply that interannual variation of leaf biomass density, which is primarily driven by interannual variability of precipitation, is a significant contributor to year‐to‐year differences in BVOC flux on a regional scale, of at least equal importance to interannual variation of temperature and shortwave radiation. Phenology‐driven biogenic emission variability is most pronounced in regions with relatively low emissions: as a grid cell's mean BVOC flux decreases, the mean‐normalized standard deviation of BVOC flux tends to increase. BVOC flux is most variable between years in subhumid, sparsely wooded regions where interannual variability of precipitation is relatively large.