7.5.1.5 Aerosols from Iodine Compounds

Intense new aerosol particle formation has been frequently observed in the coastal environment (O’Dowd et al., 2002a). Simultaneous coastal observations of reactive iodine species (Saiz-Lopez et al., 2005), chamber studies using iodocarbon precursors and laboratory characterisation of iodine oxide particles formed from exposure of Laminaria macroalgae to ozone (McFiggans et al., 2004) have demonstrated that coastal particle formation is linked to iodine compound precursor released from abundant infralittoral beds of macroalgae. The particle bursts overwhelmingly occur during daytime low tides (O’Dowd et al., 2002b; Saiz-Lopez et al., 2005). Tidal exposure of kelp leads to the well-documented release of significant fluxes of iodocarbons (Carpenter et al., 2003), the most photolabile of which, di-iodomethane (CH₂I₂), may yield a high iodine atom flux. However, the iodine monoxide (IO) and iodine dioxide (OIO) radicals, and new particles are thought more likely to result from emissions of molecular iodine (McFiggans et al., 2004), which will yield a much greater iodine atom flux (Saiz-Lopez and Plane, 2004). It is unclear whether such particles grow sufficiently to act as CCN (O’Dowd, 2002; Saiz-Lopez et al., 2005). Thus, a hitherto undiscovered remote ocean source of iodine atoms (such as molecular iodine) must be present if iodine-mediated particle formation is to be important in the remote marine boundary layer (McFiggans, 2005).

7.5.1.6 Climatic Factors Controlling Aerosol Burdens and Cycling

As discussed above, near-surface wind speed determines the source strength for primary aerosols (sea salt, dust, primary organic particles) and precursors of secondary aerosols (mainly DMS). Progress has been made in the development of source functions (in terms of wind speed) for sea salt and desert dust (e.g., Tegen et al., 2002; Gong, 2003; Balkanski et al., 2004). Wind speed also affects dry deposition velocities and hence the lifetime of aerosols. In addition, biogenic emissions are strongly dependent on temperature (together with humidity/moisture; e.g., Guenther et al., 1995). Temperature also is a key factor in the gas-aerosol partitioning of semi-volatile secondary organics (Kanakidou et al., 2005).

Precipitation directly affects the wet removal and hence the lifetime of atmospheric aerosols. More aerosols decrease the precipitation formation rate, which in turn increases the lifetime of aerosols and results in more long-range aerosol transport to remote regions where wet removal is less efficient. At the same time, precipitating boundary layer clouds maintain themselves by keeping aerosol concentrations low (e.g., Baker and Charlson, 1990; Stevens et al., 2005; Sharon et al., 2006). Precipitation also affects soil moisture, with impacts on dust source strength and on stomatal opening/closure of plant leaves, hence affecting biogenic emissions. Cloud processing is an important pathway in the gas-to-particle conversion. It is the most important oxidation pathway for sulphate aerosols and shifts the aerosol size distribution to larger sizes, such that aerosols are more easily activated in subsequent cloud events (e.g., Hoppel et al., 1990; Kerkweg et al., 2003; Yin et al., 2005). It is also important in the conversion of hydrophobic to hydrophilic carbon.

Aerosol burden and lifetime are also affected by microphysical interactions among the different aerosol compounds as well as by changes in the spatial and seasonal distribution of the emissions. Sea salt aerosols, for example, provide surfaces for conversion of SO₂ into sulphate aerosols (Sievering et al., 1992) with consequences for cloud formation (Gong and Barrie, 2003; Section 7.5.2.1). A future reduction in SO₂ emissions and the associated reduced conversion of hydrophobic to hydrophilic soot could lead to a prolonged residence time of soot (Cooke et al., 2002; Stier et al., 2006b) and increased ammonium nitrate (Liao and Seinfeld, 2005). However, in a transient AOGCM climate simulation with an em-bedded microphysical aerosol module, Stier et al. (2006a) show that the effect on the hydrophobic to hydrophilic conversion can be outweighed by a general shift to low-latitude dry-season soot emissions. Consequently, soot lifetime increases in a future climate despite an enhanced conversion of hydrophobic to hydrophilic soot.