The biogeochemical impacts of forests and the implications for climate change mitigation

Vegetation emits biogenic volatile organic compounds (BVOCs) into the atmosphere which, once oxidised, may partition into the particle-phase, forming secondary organic
aerosol (SOA). In this thesis, the climatic impacts of biogenic SOA are quantified, using a detailed global aerosol microphysics model, and the sensitivity of these radiative effects to the representation of various atmospheric processes is examined.
By altering the size, composition and number of particles in the atmosphere, the presence of biogenic SOA very likely has a negative radiative effect on the climate (i.e. a cooling), via both the direct radiative effect (DRE) and first aerosol indirect effect (AIE). The DRE from biogenic SOA is sensitive to the large uncertainty in the amount of biogenic SOA being produced in the atmosphere (estimated to be between 12 and 1870 Tg(SOA) a-1). The presence of biogenic SOA increases the global annual mean concentration of particles with the potential to form cloud droplets (i.e. cloud condensation nuclei; CCN). Consequently, biogenic SOA exerts a global annual mean first AIE of between +0.03 W m-2 and -0.77 W m-2. Most of the range in the first AIE due to biogenic SOA can be attributed to uncertainty regarding the role of biogenic oxidation products in the very early stages of atmospheric new particle formation (i.e. nucleation). The most negative
first AIEs (up to -0.77 W m-2) are simulated when BVOC oxidation products do participate in the very early stages of new particle formation; an approach which best captures the observed seasonal cycle in particle concentrations across the continental northern hemisphere. At the outside of the uncertainty range examined here, the DRE and
first AIE due to biogenic SOA are almost half the strength (in terms of absolute magnitude) of the estimated net anthropogenic radiative forcing from 1750 to 2005 of
+1.6 W m-2.
The sign of the first AIE due to biogenic SOA is also sensitive to assumptions regarding the volatility of biogenic oxidation products and the manner in which their addition to the existing aerosol distribution is modelled. Taking a kinetic approach, in which SOA is partitioned according to particle surface area, gives a negative first AIE due to the role of secondary organics in the growth of newly formed particles. However, taking a thermodynamic equilibrium approach, in which SOA is added in proportion to existing organic mass, gives a positive first AIE because the growth of newly formed particles is suppressed in the presence of larger particles (i.e. due to the enhanced condensation sink). As a result, the thermodynamic approach is not able to capture the observed growth of
new particles in the atmosphere and may not be suitable when examining processes that depend strongly on changes to ultrafine particle number, such as the first AIE.
The negative radiative effects of biogenic SOA have implications for the climatic impact of forests and any changes to their distribution. The contribution of the first AIE, due to changes in the production of biogenic SOA, is quantified here using simplified deforestation scenarios. Globally, the replacement of forests with grass results in a first AIE of +0.26 W m-2 due to a 91% reduction in biogenic SOA production. This increases
the total warming effect of deforestation, from the combined changes to carbon dioxide concentration and surface albedo, by 21%.
Regionally, the strongest first AIE (+0.12 W m-2) comes from tropical (20°N – 20°S) deforestation which reduces global SOA production by 73%. The largest AIE per change
in SOA comes from simulated temperate (20°N – 50°N and 20°S – 50°S) deforestation, which reduces global SOA production by only 15%, but leads to strong warming over
remote ocean regions with high cloud fraction. This work suggests that present-day tropical deforestation is warming the climate more than previously thought, confirming
that a reduction in deforestation should be a priority for climate change mitigation. These results also suggest that present-day afforestation in temperate regions may be exerting more of a cooling than would be attributed to CO2 sequestration alone, warranting further investigation.