Global Isoprene Fluxes inferred from OMI Formaldehyde Columns over 2005-2013

Abstract:

Among VOCs, isoprene is the most largely emitted hydrocarbon in the atmosphere, with global annual emissions estimated a about 500 Tg, but with large uncertainties (Arneth et al., 2011). Here we use the source inversion approach to derive top-down biogenic isoprene emission estimates for the period between 2005 and 2013 constrained by formaldehyde columns retrieved from the Ozone Monitoring Instrument (OMI). To this aim, the IMAGESv2 global chemistry-transport model and its adjoint code are adopted, and the MEGAN-MOHYCAN-v2 updated model is used as bottom-up inventory for isoprene emissions (Stavrakou et al., 2014). The inversions are performed for each year of the study period, and monthly emissions are derived for every model grid. The inversion results are compared to independent isoprene emissions from GUESS-ES (Arneth et al., 2007) and MEGAN-MACC (Sindelarova et al., 2014), to ground-based isoprene flux measurements in Amazonia and to top-down fluxes based on GOME-2 formaldehyde columns (Stavrakou et al., 2015).

The mean global annual OMI-based isoprene flux for the period 2005-2013 is estimated at 307 Tg, with small interannual variation between a minimum in 2008 and 2013 (294 Tg) and a maximum in 2010 (317 Tg). This estimate is by 12% lower with regard to the a priori inventory on average, and substantially lower than the MEGAN-MACC and the GUESS-ES inventory. The inferred reduction is well in line with the GOME-2 based inversion, and is found to be the strongest in tropical regions for both inversions. Flux measurements in the Amazon forest show large variability, and are generally higher than the bottom-up and top-down fluxes especially in the dry season. Top-down isoprene fluxes are available at a resolution of 0.5°x0.5° between 2005 and 2013 on the GlobEmission website (http://www.globemission.eu).