Statement of problem – why are combustion aerosols important?

Combustion aerosols have direct, semi-direct and indirect impacts on the Earth’s energy budget. This involves the energy balancing of the incoming solar radiation taking into accounts the incoming, outgoing, absorbed and part of the radiation that have interacted the other components of the Earth (Forster, 2007). Types of indirect aerosol effects which can contribute to the global energy budget include the following: (1) the cloud albedo or Twomey effect which explains why smaller cloud particles reflect more radiation (Warner and Twomey, 1967). (2) the cloud life time effect that describes the efficiency of smaller cloud particles in inhibiting precipitation, hence, increasing the cloud lifetime. (3) semi-direct effect explains the evaporation of cloud particles by the absorptive abilities of aerosol particles e.g. soot. (4) the thermodynamic effect which explains that smaller cloud droplets delay the onset freezing. (5) glaciation indirect effect which suggests that more ice nuclei increase the precipitation efficiency. (6) riming effect which has to do with the decrease in the riming efficiency by smaller cloud droplets, and finally, (7) the surface energy budget effect, which predicts that the increase in the optical thickness of clouds and aerosol can decrease the net surface solar radiation (Lohmann and Feichter, 2005; Forster, 2007). Of these indirect effects, thermodynamic, glaciation and riming are the most dominant effects in mixed-phase clouds. In addition, of the seven effects listed here, only glaciation effect is very crucial to soot INPs components of the cloud/aerosols (Lohmann and Diehl, 2006; Lohmann, 2002).

For an overall assessment of the Earth’s radiative budget, the total contributions by direct, semi-direct and all the indirect effect must be considered. Some scientific studies and reports have focused on the warming effect of soot in the atmosphere (direct effect)

because of its high absorbing properties e.g. Jacobson (2001); Ramanathan and Carmichael (2008); Yu et al. (2006); Saleh et al. (2013); Jacobson (2012). In addition to the direct aerosol effect, there is also the semi-direct effect (Hansen et al., 1997). Black carbon has been reported to cause a warming in the boundary layer which leads to overall reduction in the liquid-water path (LWP), hence, causing a positive semi-direct radiative forcing (Hill and Dobbie, 2008). However, less attention has been given to its indirect contributions. Hence, the understanding of indirect effects caused by combustion aerosols on clouds is limited due to ageing of these materials in the atmosphere. This aging process can involve their interactions with other atmospheric species such as ozone, SO2, nitrates, organics and other particles, which can alter their initial chemistry and behaviour. This forms part of the indirect aerosol chemistry effect.

This has resulted in significant uncertainties associated with the assessment of total radiative properties of clouds (Bond et al., 2013; Ervens and Feingold, 2013; Forster, 2007; Boucher, 2013; Graf, 2004; Tao et al., 2012). More so, attempts by aerosol-climate models to represent the role of combustion aerosols are handicapped by limited parameterizations (Phillips et al., 2008; Phillips et al., 2012).

Recently, data from analyses of ice crystals residues from in-situ observations showed that there is a very strong correlation (R² = 0.996) between the number of ice particles and the concentration of black carbon (BC) measured in the same region as depicted in in Figure 1.4 (Twohy et al., 2010). This study was conducted over the Rocky Mountains, USA, to investigate the ice concentrations in orographic wave clouds at temperatures -24 to -29 °C (see Table 1.1). A variety of techniques was used to measure the ice nuclei compositions as listed on Table 1.1. Electron microscopy images of the various particles collected during the flights showed soot, sulphates and other biomass components. Although a strong correlation is established with black carbon, the work also suggested internal mixing of soot with soluble materials like sulphate or other salts (Twohy et al., 2010). It should be noted in Figure 1.4 that the data is not forced through the origin hence; the data should not be extrapolated beyond what is reported.

Other field measurements also indicate that soot, combustion ashes, and other combustion particles are seen in ice crystal residues to varying degrees (Pratt et al., 2009; Pratt et al., 2010; Twohy et al., 2010; Kamphus et al., 2010; Stith et al., 2011;

Cozic et al., 2008).

Figure 1.4: Correlation of ice number concentration and refractory black carbon as observed from analyses of ice crystal residues. The plot is from Twohy et al. (2010).

A summary of these data are shown in Table 1.1. High concentrations of ice particles have been measured in both primary and secondary wakes of aircrafts and this observation is attributed to ice nucleation induced by soot particles (Schumann et al., 2013). Again, this provides another piece of evidence that soot may be playing an important role in modifying cloud properties.

However, the data from these observations seems to contradict some experimental data which considers soot (black carbon) as relatively poor ice nuclei. Although, some studies observed soot to nucleate ice, the overall picture of soot’s ice nucleation activity is mixed (Dymarska et al., 2006; Petters et al., 2009; Crawford et al., 2011; DeMott et al., 1999; Gorbunov et al., 2001; Sakaeda et al., 2011; Murray et al., 2012; Hoose and Möhler, 2012). Also, aged soot particles by organics and sulphates are reported to only improve its hygroscopic property and not necessarily its ice nucleation efficiency (Mohler et al., 2005a; Pant et al., 2006; Friedman et al., 2011). This is because the materials such as sulphates and organics are coated on the surface of the soot and do not

necessarily alter the surface chemical structure of the soot. For example, in the immersion mode where these coated soot particles are fully immersed, the coatings may dissolve into the bulk solution leaving behind bare soot surface. However, a strong oxidizing substance such as ozone may modify the chemical surface of soot (Disselkamp et al., 2000) which may change its ice nucleation behaviour.

Aside from soot, investigations on the interactions of combustion ashes particles with clouds are rare. Only three studies, that the author is aware of, reported the potential of combustion ash as IN and it was shown that no significant ice-forming abilities were observed (Schnell et al., 1976; Parungo et al., 1978; Langer et al., 1967); although ice residue analyses of cirrus clouds have suggested mineral/fly ash to be one of the dominant components (DeMott et al., 2003). In addition, many reports usually grouped combustion ashes with mineral dust particles (DeMott et al., 2003; Kamphus et al., 2010; Chen et al., 1998). DeMott et al. (2003) observed that about 33 % of the ice residues from cirrus clouds were from mineral dust/fly ash and Chen et al. (1998), 65

%. These data sets give a clue that combustion ashes may contribute significantly to the primary ice formation in other cloud types such as mixed-phase clouds; however, data is lacking.

Paucity of data and limited understanding of the behaviour of combustion aerosols as IN in mixed-phase clouds, can limit the robustness of ice parameterization schemes e.g.

Phillips et al. (2008). Clearly, there is need to improve the current understanding of the behaviour of soot and combustion ashes as IN which will help in reducing the level of uncertainty (IPCC, 2013) associated with cloud adjustments by aerosols, especially, in ice clouds.

Table 1.1: Proportion of soot (black carbon) in ice crystal residues sampled in a temperature regime of mixed-phase clouds. The percentage of ice composition due

2-Dimensional-Cloud Probe (2D-C probe), Counterflow Virtual Impactor (CVI), Continuous Flow Diffusion Chamber (CFDC), Aircraft-Aerosol Time-of-Flight Mass Spectrometer (A-ATOFMS), Single Particle Soot Photometer (SP2), Optical Particle Counter (OPC), Aerodyne Compact Time-of-Flight Aerosol Mass Spectrometer (C-ToF-AMS), Scanning Transmission Electron Microscopes-Energy-Dispersive X-ray (STEM-EDX), Cloud Droplet Probe (CDP), Forward Scattering Spectrometer Probe – 100 (FSSP-100), Tunable Diode Laser (TDL), Cloud Condensation Nuclei spectrometer (CCN spectrometer), Ultra-High Sensitivity Aerosol Spectrometer (UHSAS), Condensation Particle Counter (CPC), Single Particle Laser Ablation Time-of-flight mass spectrometer (SPLAT).

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