PhD Abstract
The terrestrial biosphere is a major source of volatile organic compounds (VOC), emitted by the vegetation, and significantly contributes to nitrogen oxides emissions (NOx) in the atmosphere, through soils nitrification and denitrification processes. Those compounds are highly reactive and thus play a key role in ozone cycle as well as in the chemical composition of the troposphere.Moreover, VOC involvement in tropospheric chemistry does not only concern gas phase reactions but also particulate formation, and several studies demonstrated that monoterpenes, but also isoprene, oxidation contributes to secondary aerosols formation in the troposphere.
The goal of this thesis is to better take into account biogenic VOC and NOx emissions at the global scale, implementing an emission scheme in the global dynamic vegetation model ORCHIDEE, and to better understand the role of those emissions in surface-atmosphere interactions.
Our scheme does not only calculate isoprene and monoterpenes emissions but also consider other compounds, such as methanol, acetone, formaldehyde, acetaldehyde, formic and acetic acids, which are not generally taken into account individually in emissions models. Isoprene emissions integrate the effect of radiation extinction into the canopy and the impact of leaf age is taken into account in isoprene and methanol emissions calculation. Estimate of NOx emissions is based on soil temperature and humidity, precipitations regime, industrial and natural fertilizers and also on the fertilizing effect of biomass burning in tropical regions.
An analysis of the impact of climate interannual variability on biogenic emissions, performed from 1983 to 1995, underlines a 8.4% increase between extreme years (1986 and 1995) of total VOC biogenic emissions over this period, with a strong correlation between emissions variation and El Niño events in tropical regions. A study of the evolution of VOC biogenic emission over longer time scale is completed from the Last Glacial Maximum (-21 000 years) to 2100, and shows that emissions could reach 1251 TgC/yr in the future, with an emission increase ranging from 59% for isoprene to more than 100% for other compounds, compared to present-day. VOC biogenic emissions decrease significantly in the Amazon region, in relation with a strong reduction in tropical forests coverage, but increase strongly in mid- and high-latitude regions of the northern hemisphere. In order to study the impact of biogenic emissions and their evolution on tropospheric chemical composition, off-line simulations are performed with the LMDz-INCA chemistry-transport model, and with the TM3 model, concerning the secondary aerosols formation.