This section will discuss primary aerosol emission sources and aerosol processes pertaining to the formation, growth and the removal of atmospheric aerosols.
3.2.3.4.1 Nucleation of new sulfate aerosols
The formation of new nanometre-sized particles has been widely observed at numerous global sites (Kulmala et al., 2004; Kulmala et al., 2007); from the boundary layer (Clarke et al., 1998) to the free troposphere (Clarke et al., 1999). The majority of aerosol processes driving new aerosol formation occur at particle diameters of 3 nm or less, while observations are only able to cover larger particles (Kulmala et al., 2007).
Binary homogeneous nucleation (BHN) within GLOMAP-mode is parameterised using the classical nucleation theory. Critical nucleated particles within GLOMAP-mode contain usually less than 100 molecules of H2SO4 (Kulmala et al., 1998), and due to their size (<3 nm) (Kulmala et al., 2007) they are inserted in to the nucleation mode within GLOMAP-mode (Mann et al., 2010; Scott, 2013).
89 3.2.3.4.2 Condensation
GLOMAP-mode includes the condensation of gas-phase H2SO4 and low-volatility secondary organic material (SEC-ORG) on to all aerosol modes. The sulfate and particulate organic matter (POM) component masses are updated in the next timestep as per the mass of H2SO4 and SEC-ORG condensing on each mode (Mann et al., 2010). Within GLOMAP-mode particles grow due to condensation to a new particle diameter (D̅), with the mass of H2SO4 and SEC-ORG condensate being stored and passed on to the ageing routine (Mann et al., 2010).
3.2.3.4.3 Ageing
Ageing is a process, through which the condensation of soluble gas-phase species or coagulation occurs with smaller particles, whereby previously water-insoluble particles can become partly soluble (Mann et al., 2010; Scott, 2013). Particles are transferred to the corresponding hydrophilic mode once adequate soluble material has been accumulated, assumed to be 10 monolayers in GLOMAP-mode in order to make the particle soluble; a process known as physical ageing (Mann et al., 2010; Scott, 2013).
In GLOMAP-mode the flux of soluble material to the insoluble modes is passed on to the corresponding soluble mode. This ensures that the ageing process only changes the number concentration of the insoluble modes, leaving their composition and size unperturbed (Mann et al., 2010).
3.2.3.4.4 Hygroscopic growth
Using parameters for calculating molalities of binary electrolytes as a function of relative humidity from Jacobson (2005) water uptake by each component within each aerosol mode is calculated using the Zdanovskii-Stokes-Robinson (ZSR) method (Zdanovskii, 1948; Stokes and Robinson, 1966; Mann et al., 2010). This method assumes spherical particles.
Any organic material present in the insoluble modes is assumed to be primary emitted material and non-hygroscopic. In the soluble modes organic material is considered as either primary or secondary organic material that has been aged. Moderate hygroscopicity is assigned to organic material in the soluble modes consistent with a water uptake per mole at 65% of sulfate assuming a molar mass of 0.15 kg mol-1 for the aged organic molecule (Mann et al., 2010).
90 3.2.3.4.5 Coagulation
GLOMAP-mode includes representations for both intra-modal and inter-modal coagulation, i.e.
the collision of particles of the same and different modes respectively. Particles within soluble modes can coagulate with particles within the large soluble and insoluble modes. While insoluble mode particles can only coagulate with larger insoluble mode particles (Mann et al., 2010; Scott, 2013). Within the nucleation mode the source of nucleated particles is also included (Mann et al., 2010).
3.2.3.4.6 Aerosol dry deposition
Dry deposition is the removal of particles and gases from the atmosphere in the absence of precipitation. GLOMAP-mode represents dry deposition using the same approach as in Spracklen et al. (2005a), using the same methodology as Zhang et al. (2001) and Slinn (1982).
3.2.3.4.7 Aerosol scavenging
Aerosol removal via nucleation scavenging from both large-scale and convective scale precipitation is calculated using rain-rates diagnosed from ECMWF analysis fields (Mann et al., 2010). Akin to Spracklen et al., (2005) large-scale rain removes particles at a constant rate equivalent to 99.9% conversion of cloud water to rain over a 6 hour period (Spracklen et al., 2005a). Tiedtke (1989) is used to calculate the conversion rate for convective precipitations;
assuming a rainfall fraction of 30%.
Nucleation scavenging only occurs where precipitation is formed in that model level; evaluated by comparing calculated rain-rates to those in the level above. In GLOMAP-mode, nucleation scavenging removes only soluble particles with a dry radius greater then rscav, which here is taken as 103 nm (Mann et al., 2010; Scott, 2013).
Impact scavenging represents the removal of aerosols by falling raindrops, simulated in GLOMAP-mode in a way analogous to the method used in GLOMAP-bin; described in Pringle (2006) (Mann et al., 2010). Raindrop particle collection efficiencies are determined from a look-up table using the Marshall-Palmer raindrop size distribution modified by Sekhon and Srivastava (1971) and geometric mean diameter (Dg) for each mode (Mann et al., 2010).
Following GLOMAP-bin, an empirical relationship from Easter and Hales (1983) is used to calculate raindrop terminal velocity.
91 3.2.3.4.8 Mode-merging
To prevent modes from continuing to grow indefinitely due the processes of coagulation and condensation, and grow outside their specified size ranges, a mode-merging approach is applied (Mann et al., 2010). The mode-merging routine used checks whether the geometric mean diameter (Dg) is outside the range given in Table 3.1. If this is the case, then the fractions of the mode number and mass concentrations outside the given ranges are transferred to the next largest mode as described Mann et al. (2010).
3.2.3.4.9 Cloud processing
The growth of aerosol particles through the uptake and chemical reaction of gases while the growing particle exists as a water droplet in non-precipitating clouds is known as cloud processing (Mann et al., 2010).
GLOMAP-mode simulates the activation of soluble particles to cloud droplets, and their subsequent growth (Mann et al., 2012; Mann et al., 2010). Following Spracklen et al. (2005a) the smallest particles that can be activated to cloud droplets are determined to have an activation dry radius (ract) of 37.5 nm, corresponding to a cloud supersaturation of 0.2% which is typical of marine stratocumulus clouds (Mann et al., 2010). This defines the Aitken and accumulation modes frequently seen in size distribution observations within the marine boundary layer (Hoppel et al., 1994; Mann et al., 2010).
In GLOMAP-mode, cloud processing is treated as a two stage process. Firstly, the fractions of particle mass and number in the soluble Aitken mode from larger particles are calculated, and are transferred to the soluble accumulation mode. Secondly sulfate mass produced by the oxidation of SO2 is portioned between the soluble accumulation and coarse modes. Through treating cloud processing in this way, particles at the larger end of the Aitken size range can be activated and cloud-processed, with the minimum between the soluble Aitken and accumulation modes created (Mann et al., 2010).