Forests aerosol emission

Echalar, F., A. Gaudichet, H. Cachier, and P. Artaxo. 1995. Aerosol emissions by tropical forest and savanna biomass burning characteristic trace elements and fluxes. Geophysical Research Letters 22 3039-3042. 

Aerosols may also play an important role in cHmate change. Natural aerosol emissions, similar to those caused by volcano eruptions and forest fires, can affect the radiation balance around the planet and, therefore, affect global temperatures quite distinctly from the heat directly released in such phenomena. Atmospheric aerosol emissions resulting from human industrial and deforestation activities can have the same effect, distinct from the associated greenhouse gas emissions. In both cases, these aerosols influence climate through the scattering of solar radiation, the absorption of terrestrial radiation, and through their effects on the properties of clouds [128, 129]. 

During biomass and fuel burning, a complex mixture of ill-characterized volatile organic matter are released into the atmosphere (Andreae and Merlet, 2001). It contributes to the formation of aerosols and fine particles of sizes up to 100 pm. After an estimated lifetime of 7.9 days (Cook and Wilson, 1996), they are either degraded or are removed from the atmosphere by precipitation. However, they can be transported a considerable distance. For example, boreal forest fires contribute substantially to atmospheric BC in the Arctic (Cook and Wilson, 1996), and Antarctica receives BC from biomass burning in the tropics (Wolff and Cachier, 1998). On a global scale the amount of atmospheric emission is estimated with 5-6Tg BC yr 1,... 

Barr et al. (2003) performed an analysis of the impact of phytogenic aerosol (PhA) which is defined as forming mainly due to monoterpene oxidation (primarily, a- and /3-pinenes), on the radiative regime of the ABL over the forest in the eastern part of Canada. In the forest ecosystem the level of emissions to the atmosphere of biogenic hydrocarbons is moderate, with the concentration of a- and /3-pinenes constituting about 1.6 ppb. NMHC oxidation resulted in the formation of PhA at a number density of particles of about 5 108 cm 3. For a given concentration and size distribution of aerosol, its impact on the short-wave radiation transfer in the ABL was assessed. 

The natural mechanisms of atmospheric aerosol generation are as follows soil-wind erosion, ejections to the atmosphere of salt particles from sea and ocean surfaces, emission of gases and vapors by photo-synthesizing plants and by decay products, ejections of the products (soot aerosol, first of all) of natural fires of forests, steppes, peat bogs, and also volcanic eruptions. 

The biosphere is a major contributor to the atmosphere of heavier hydrocarbons. Fritz Went (8, 9), who first recognized the global extent of smog, pointed out the general importance of natural emissions from vegetation. He estimated that sources in the biosphere annually emit between 170 X 10 and 10 tons of hydrocarbon material to the atmosphere. Went also observed that these materials are mainly in the terpene class and that, because they are photochemically reactive, these materials are polymerized in atmospheric photochemical reactions to form an organic aerosol. He attributes the blue haze found in many forested areas to the optical effects of this aerosol. 

As noted in Chapter 4, the dimensionless product x = beM z is called the optical depth of the layer, and (15.26) is called the Beer-Lambert law. In the visible portion of the spectrum, optical depth of tropospheric aerosols can range from less than 0.05 in remote, pristine environments to close to 1.0 near the source of intense particulate emissions such as in the plume of a forest fire. 

In addition to the important role biogenic terpenes play in gas-hase chemistry, their impact also extends to heterogeneous air chemistry. Although Went (1960) linked the formation of the blue haze over coniferous forests to the biogenic emission of 20 monoterpenes over 40 years ago, it was not until recently that terpenes received their due attention with respect to their role in secondary organic aerosol (SOA) formation. O Dowd et al. (2002) reported that nucleation events over a boreal forest were driven by the condensation of terpene oxidation products. Formaldehyde (HCHO) is a high-yield product of isoprene oxidation. The short photochemical lifetime of HCHO allows the observation of this trace gas to help constrain isoprene emissions (Shim et al. 2005). 

Andreae, M. O. and P. Merlot (2001) Emissions of trace gases and aerosols from biomass burning. Global Biogeochemical Cycles 15, 955-966 Andreae, M. O. (2004) Assessment of global emissions from vegetation fires. International Forest Fire News 31, 112-121... 

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