The aerosol module of HadGEM2 is called the Coupled Large-scale Aerosol Simulator for Studies In Climate (CLASSIC). It contains numerical representations for up to eight tropospheric aerosol species: ammonium sulphate, mineral dust, sea salt, fossil fuel black carbon, fossil fuel organic carbon, biomass burning aerosols, secondary organic (also called biogenic), and ammonium nitrate aerosols. Although each species is associated with a dedicated scheme, some aspects are shared. Emissions or chemical production of mineral dust, sea salt, and nitrate aerosols are interactive, as described below. Emissions for the other species or their precursors are from CMIP5 data sets. Transported species experience boundary layer and convective mixing and are removed by dry and wet deposition. Wet deposition by large-scale precipitation is corrected for reevaporation of precipitation: tracer mass is transferred from a dissolved mode to an accumulation mode in proportion of reevaporated precipitation. For convective precipitation, accumulation mode aerosols are removed in proportion to the simulated convective mass flux.
 Ammonium sulphate aerosols are part of the interactive sulphur cycle of HadGEM2-ES, as described by Jones et al.  and Roberts and Jones . The cycle starts with emissions of sulphur dioxide (SO2) and dimethyl sulphide (DMS), the latter being provided by the ocean biogeochemistry module which simulates plankton concentrations interactively. SO2 and DMS are prognostic tracers of the atmosphere model and are oxidized into sulphate (SO4=) by the hydroxyl radical (OH), hydrogen peroxide (H2O2), the peroxide radical (HO2) and ozone (O3). The UKCA tropospheric chemistry model (O'Connor et al., manuscript in preparation, 2011), called every time step of the atmosphere model, provides the interactive concentrations of these oxidants. A short description of the chemistry model is given below. Wet oxidation by ozone is the dominant pathway, representing 50% of total oxidation, with dry oxidation by OH and wet oxidation by H2O2 sharing equally the other half. SO4= is represented by three prognostic tracers, representing the Aitken and accumulation modes, and a mode for sulphate dissolved in cloud droplets. Mass is exchanged between those modes by nucleation (accumulation to dissolved mode), evaporation and reevaporation (dissolved to accumulation mode), coagulation and mode merging (Aitken to accumulation mode), and diffusion (Aitken to dissolved mode). Nucleation and diffusion are described by Jones et al.  and are applied to all hygroscopic HadGEM2 aerosol species. Nucleation of accumulation mode aerosols into the dissolved mode is assumed to happen quickly compared to the model time step, so accumulation mode aerosols in the cloudy part of the grid box instantaneously enter the dissolved mode and, conversely, dissolved mode aerosols are instantaneously converted into accumulation mode aerosols in the cloud-free part of the grid box. Cloud fraction is therefore the controlling parameter of that process. Diffusion of Aitken mode aerosols is slower and is computed from the cloud liquid water content, cloud droplet number concentration, and a fixed diffusion coefficient [Jones et al., 2001]. The model also includes a parameterization of the condensation of sulphuric acid (H2SO4) onto Aitken and accumulation modes. Both mode merging and condensation of H2SO4 are recent additions to the sulphur cycle, described by Bellouin et al. . Finally, the scheme does not partition sulphate into sulphuric acid and ammonium sulphate: sulphate mass is assumed to be fully in the form of ammonium sulphate.
 Mineral dust aerosol modeling is described by Woodward , with revisions described by Bellouin et al. . Emissions are computed interactively and depend on vegetation fraction, soil roughness length and moisture, and near-surface wind speeds. The horizontal flux is calculated for nine size bins covering particle radii from 0.0316 to 1000 μm. The vertical flux into the atmosphere is then obtained from the horizontal flux using equation (1) of Woodward  and partitioned across six size bins, covering radii from 0.0316 to 31.6 μm. These six size bins are then transported and experience deposition through gravitational settling, turbulent mixing, and below-cloud scavenging.
 The sea-salt aerosol scheme is described by Jones et al.  and is a diagnostic scheme: its number concentration is computed over open ocean grid boxes at each time step depending on the instantaneous near-surface wind speed. Sea-salt aerosols are not transported nor deposited.
 The fossil fuel black carbon (FFBC) aerosol scheme is described by Roberts and Jones . It involves three prognostic tracers representing fresh, aged, and in-cloud modes. Ageing is represented as an exponential decay with an e-folding rate of 1 day. FFBC aerosols are assumed to be hydrophobic and do not exert indirect effects. They are assumed to be interstitial within clouds and as such experience wet deposition after diffusional capture by cloud droplets as described in the appendix of Roberts and Jones .
 The biomass burning aerosol scheme is described by Davison et al.  with improvements described by Bellouin et al. . In contrast to most existing aerosol modules, which simulate biomass burning black carbon independently from biomass burning organic carbon, CLASSIC uses biomass burning tracers representing the sum of these two components. Biomass burning aerosol mass is distributed over three modes: fresh, aged, and dissolved in cloud droplets. Mass is emitted into the fresh mode and later converted into a hygroscopic aged mode with a short e-folding time scale of 6 h [Abel et al., 2003]. Upon ageing, biomass burning mass is increased by a factor of 1.62 to represent condensation of volatile organic compounds. This factor is chosen to decrease the mass fraction of black carbon from 8.75% in the fresh mode to 5.4% in the aged mode, following aircraft observations by Abel et al. .
 The fossil fuel organic carbon scheme also uses three tracers, representing the fresh, aged, and dissolved modes. Ageing from fresh to aged mode is represented as an exponential decay with a e-folding time scale of 1 day.
 Secondary organic aerosols represent biogenic aerosols from terpene emissions by vegetation. This is a climatology of monthly three-dimensional mass mixing ratios, taken from the chemistry-transport model STOCHEM [Derwent et al., 2003]. The climatology remains the same for all simulated years.
 The nitrate aerosol scheme is a recent addition to CLASSIC and is described here. Ammonium nitrate, NH4NO3, derives from the equilibrium reaction
where HNO3 and NH3 are nitric acid and ammonia gases, respectively. HNO3 concentrations are provided by the UKCA tropospheric chemistry scheme (O'Connor et al., manuscript in preparation, 2011), which is summarized below. NH3 is a variable of the atmosphere model and is depleted upon formation of ammonium sulphate. Formation of ammonium sulphate therefore takes priority over that of ammonium nitrate. The reaction between HNO3 and NH3, and the phase of the resulting nitrate aerosol, depend strongly on relative humidity and temperature. In CLASSIC, this dependence is parameterized by computing the dissociation constant, denoted kp in (molecules per cm3)2, of reaction (A1). The calculation of kp is different for relative humidities above and below the temperature-dependent deliquescence relative humidity (DRH). First, DRH is computed as
where T is the temperature (K). For relative humidities lower than DRH, kp depends only on temperature following
For relative humidities higher than DRH, kp depends on both temperature and relative humidity, following
where kp1 is computed using equation (A3), and p1, p2, and p3 are temperature-dependent parameters defined as
This formulation is by Mozurkewich  and is currently used in the European Monitoring and Evaluation Programme (EMEP) Unified Model (http://www.emep.int/UniDoc). This parameterization is less expensive than solving explicitly the thermodynamic equilibrium involved in nitrate formation and as such fits well the requirements for long integrations of complex and high-resolution Earth system models like HadGEM2-ES.
 Denoting concentrations (molecules per cm3) with brackets and defining [NH3] and [HNO3] as total concentrations of ammonia and nitric acid (both as gas and combined into ammonium nitrate), respectively, the equilibrium concentration of ammonium nitrate is computed by solving the following quadratic equation:
if [NH3][HNO3] > kp,
otherwise, which would correspond to dissociation of ammonium nitrate if [NH4NO3] was nonzero at the previous model time step. Finally, concentrations of [NH3] and [HNO3] are depleted or replenished to account for ammonium nitrate formation or dissociation, respectively. Chemical production of nitrate, as given in equation (A8), goes into an accumulation mode. Cloud formation transfers some of the accumulation mode mass into a dissolved mode, and evaporation and reevaporation transfer mass from the dissolved back to the accumulation mode.
 As noted above, sulphur cycle oxidants (OH, H2O2, HO2, and O3) and nitric acid (HNO3) are provided to the aerosol scheme by the UKCA tropospheric chemistry model. This model is described in detail by O'Connor et al. (manuscript in preparation, 2011) and a short summary is given here. The chemistry model represents the chemistry of 41 chemical species, of which 25 are transported tracers. The chemical solver is an explicit iterative backward Euler approach with a chemical time step of 5 min. The model describes odd oxygen (Ox), nitrogen (NOy), hydrogen (HOx), and carbon monoxide (CO) chemistry with near-explicit treatment of methane (CH4), ethane (C2H6), propane (C3H8), and acetone (Me2CO) degradation (including formaldehyde (HCHO), acetaldehyde (MeCHO), peroxy acetyl nitrate (PAN), and peroxy propionyl nitrate (PPAN)). This chemistry is similar to that described by Law et al. . The model accounts for 25 photolytic reactions and 96 molecular reactions. HNO3 is generated through the inorganic chemistry and is destroyed by dry and wet deposition, photolysis, and reaction with OH. Treatment of heterogeneous chemistry is not included, thus heterogeneous reactions between NO3 and N2O5 are not represented. The model is applied across all layers of the atmosphere models, reaching 39 km. However, to account for missing processes in stratospheric chemistry, O3, CH4, and NOy are relaxed to time-varying climatologies above the tropopause.
 The direct radiative effect due to scattering and absorption of radiation by all eight aerosol species represented in the model is included. The semidirect effect, whereby aerosol absorption tends to change cloud formation by warming the aerosol layer, is thereby included implicitly. Wavelength-dependent specific scattering and absorption coefficients are obtained using Mie calculations from prescribed size distributions and refractive indices, as given in Table A1. Sets of wavelength-dependent complex refractive indices are taken from Toon et al.  for ammonium sulphate and sea salt, World Climate Research Program  for fossil fuel black carbon, Haywood et al.  for biomass burning, Jarzembski et al. , Gosse et al. , and Weast  for ammonium nitrate, Balkanski et al.  for mineral dust, and Lund-Myhre and Nielsen  for biogenic aerosols. The real part of the refractive index of fossil fuel organic carbon is taken to be equal to that of biomass burning, with the imaginary part set to −0.006i for all wavelengths. In addition, hydrophilic species experience hygroscopic growth, whereby the modal radius increases with increasing relative humidity, leading to an increase in specific extinction compared to the dry aerosol. Hygroscopic growth is parameterized following Fitzgerald  for sulphate, sea-salt, and nitrate aerosols; Haywood et al.  for biomass burning aerosols, with fossil fuel organic carbon aerosols assumed to follow the same growth rate; and Varutbangkul et al.  for biogenic aerosols. Finally, all aerosol species except mineral dust and fossil fuel black carbon are considered to be hydrophilic, act as cloud condensation nuclei, and contribute to both the first and second indirect effects on clouds, treating the aerosols as an external mixture. Jones et al.  detail the parameterization of the indirect effects used in HadGEM2-ES. The cloud droplet number concentration (CDNC) is calculated from the number concentration of the accumulation and dissolved modes of hygroscopic aerosols. For the first indirect effect, the radiation scheme uses the CDNC to obtain the cloud droplet effective radius. For the second indirect effects, the large-scale precipitation scheme uses the CDNC to compute the autoconversion rate of cloud water to rainwater.
Table A1. Parameters Describing Aerosol Components in HadGEM2 and the Resulting Aerosol Optical Propertiesa
|Accumulation sulphate||0.095||1.4||1769||1.53–10−7i||3.1, 5.8, 182.6||1.00|
|Aitken sulphate||0.0065||1.3||1769||1.53–10−7i||0.001, 0.004, 1.1||1.00|
|FF black carbon||0.04||2.0||1900||1.75–0.44i||5.4||0.41|
|Sea-salt film||0.1||1.9||2165||1.55–10−7i||3.0, 6.9, 101.7||1.00|
|Sea-salt jet||1.0||2.0||2165||1.55–10−7i||0.2, 0.5, 9.7||1.00|
|Fresh biomass||0.10||1.30||1350||1.55–0.029i||4.2, 4.6, 8.8||0.84, 0.85, 0.92|
|Aged biomass||0.12||1.30||1350||1.54–0.018i||5.1, 6.9, 16.7||0.91, 0.93, 0.97|
|Biogenic||0.095||1.50||1300||1.43–0.0i||3.5, 3.8, 19.1||1.0|
|Fresh FF organic carbon||0.10||1.30||1350||1.54–0.006i||3.7, 4.1, 8.4||0.96, 0.97, 0.98|
|Aged FF organic carbon||0.12||1.30||1350||1.54–0.006i||5.0, 6.8, 16.6||0.97, 0.98, 0.99|
|Accumulation nitrate||0.095||1.40||1725||1.61–10−8i||4.2, 8.5, 211||1.00|
|Mineral Dust Division||Size Range||Density||m||kext||ϖ0|