7.5.1.2 Sea Salt

Sea salt aerosol is a key aerosol constituent of the marine atmosphere. Sea salt aerosol particles affect the formation of clouds and rain. They serve as sinks for reactive gases and small particles and possibly suppress new particle formation. Sea salt is also responsible for a large fraction of the non-sea salt sulphate formation (e.g., Sievering et al., 1992). The major meteorological and environmental factors that affect sea salt formation are wind speed, atmospheric stability and wind friction velocity, sea surface and air temperatures, present and prior rain or snow and the amount and nature of surface-active materials in the near-surface ocean waters (Lewis and Schwartz, 2005). The average annual global sea salt flux from 12 models is estimated to be 16,300 Tg ± 200% (Textor et al., 2005) of which 15% is emitted into the submicron mode.

7.5.1.3 Natural Organic Carbon

Biogenic organic material is both directly emitted into the atmosphere and produced by VOCs. Primary emissions from the continents have been thought to be a relatively minor source but some studies suggest that these emissions could be much higher than previously estimated (Folberth et al., 2005; Jaenicke, 2005). Kanakidou et al. (2005) estimate a global biogenic secondary organic aerosol production of about 30 Tg yr^–1 and recognise the potentially large, but uncertain, flux of primary biogenic particles. Annual global biogenic VOC emission estimates range from 500 to 1,200 Tg yr^–1 (Guenther et al., 1995). There is a large range (less than 5 to greater than 90%) of organic aerosol yield for individual compounds and atmospheric conditions resulting in estimates of global annual secondary organic aerosol production from biogenic VOCs that range from 2.5 to 44.5 Tg of organic matter per year (Tsigaridis and Kanakidou, 2003). All biogenic VOC emissions are highly sensitive to changes in temperature, and some emissions respond to changes in solar radiation and precipitation (Guenther et al., 1995). In addition to the direct response to climatic changes, biogenic VOC emissions are also highly sensitive to climate-induced changes in plant species composition and biomass distributions.

Global biogenic VOC emissions respond to climate change (e.g., Turner et al., 1991; Adams et al., 2001; Penner et al., 2001; Sanderson et al., 2003b). These model studies predict that solar radiation and climate-induced vegetation change can affect emissions, but they do not agree on the sign of the change. Emissions are predicted to increase by 10% per °C (Guenther et al., 1993). There is evidence of physiological adaptation to higher temperatures that would lead to a greater response for long-term temperature changes (Guenther et al., 1999). The response of biogenic secondary organic carbon aerosol production to a temperature change, however, could be considerably lower than the response of biogenic VOC emissions since aerosol yields can decrease with increasing temperature. A potentially important feedback among forest ecosystems, greenhouse gases, aerosols and climate exists through increased photosynthesis and forest growth due to increasing temperatures and CO₂ fertilization (Kulmala et al., 2004). Increased forest biomass would increase VOC emissions and thereby organic aerosol production. This couples the climate effect of CO₂ with that of aerosols.

New evidence shows that the ocean also acts as a source of organic matter from biogenic origin (O’Dowd et al., 2004; Leck and Bigg, 2005b). O’Dowd et al. (2004) show that during phytoplankton blooms (summer conditions), the organic aerosols can constitute up to 63% of the total aerosol. Surface-active organic matter of biogenic origin (such as lipidic and proteinaceous material and humic substances), enriched in the oceanic surface layer and transferred to the atmosphere by bubble-bursting processes, are the most likely candidates to contribute to the observed organic fraction in marine aerosol. Insoluble heat-resistant organic sub-micrometre particles (peaking at 40 to 50 nm in diameter), mostly combined into chains or aggregated balls of ‘marine microcolloids’ linked by an amorphous electron-transparent material with properties entirely consistent with exopolymer secretions (Decho, 1990; Verdugo et al., 2004), are found in near-surface water of lower-latitude oceans (Benner et al., 1992; Wells and Goldberg, 1994), in leads between ice floes (Bigg et al., 2004), above the arctic pack ice (Leck and Bigg, 2005a) and over lower-latitude oceans (Leck and Bigg, 2005b). This aerosol formation pathway may constitute an ice (microorganisms)-ocean-aerosol-cloud feedback.