1. Aerosol Properties    

 

    In spite of the complexity associated with compositions of aerosols from various sources, we still need to gain as much knowledge as possible on their chemical and physical properties. AR5 focuses on four key aerosol properties – the chemical composition, the mixing state, the morphology and the particle number size distribution. These properties are important because they can directly affect aerosol particles’ hygroscopicity, optical properties, and their ability to act as either cloud condensation nuclei (CCN) or ice nuclei (IN). Furthermore, these understandings will affect parameterization of aerosol radiation forcing (RFari) and aerosol-radiation and aerosol-cloud interactions in GCMs.

      Since AR4, advanced instrumentation has fostered improved measurements of key aerosol properties in laboratory and field experiments, which ultimately enables better representation of the chemical, physical and optical properties of aerosols in models [Ghan & Schwartz, 2007].

 

• characterizing aerosol chemical composition

    It is commonly known that sulfate aerosols (from coaling burning, volcanic eruption, etc.) are good at scattering off solar radiation and therefore, helping with climate cooling. While black carbon (BC, from fossil fuel combustion and biomass burning, etc.) is strongly light-absorbing. BC enhances climate warming, especially when existing in stratosphere.

    In AR5, characterizing black carbon (BC) and secondary organic aerosols (SOA) have received increasing attention, because the former’s strong warming effect and the latter’s ubiquitous nature and ability to mix with other aerosols.

   Great progress has been made thanks to advances in analyzing instruments. For instance, measuring individual BC-containing particles becomes possible with laser-induced incandescence (single particle soot photometer, a.k.a. SP2). This achievement also makes it possible to measure the size of BC cores, the total BC mass concentration, condensation of gas-phase compounds on BC and then coagulation with other aerosols. Insights have also been gained in the compositions of organic aerosol (OA) and SOA from aerosol mass spectrometer studies, which concludes that the majority of SOA is likely to be oxygenated OA and that most of the time, SOA consists of the majority of the total OA mass.

These advances have enables people to better estimate the lifetime and atmospheric loading of those aerosol species.

 

• characterizing the mixing state of important aerosol types

    Mixing state of aerosols affects their optical properties, by altering their morphology and thus their abilities of scattering and absorbing light. Commonly, BC and SOA are internally mixed with other aerosols, for example, BC with urban polluted aerosols, and SOA with all kinds including mineral dusts. This internal mixing can easily change the sizes and shapes of the resulting aerosols than there is no such mixing. Often, their abilities to act as CCN or IN may also be altered.

    Sometimes, changes in humidity might also affect aerosols’ mixing state and consequently, the above-mentioned radiative and physical properties [Freney et al.,2010].

    In AR5, some global aerosol models have been able to approximate the aerosol mixing state using size-resolving bin or model schemes [Kim et al., 2008; Mann et al., 2010].

 

• Simulating aerosol number size distribution (n(log D), number of particles cm^-3)

   Prior to AR5, aerosol volume size distribution (nv(log D), μm^3 cm^-3) were commonly simulated by CMIP models rather than the number size distribution. However, there are important reasons for simulating the latter [Asmi et al. 2011].

   For reason one, it is important for estimating aerosol-clouds interactions. Aerosols can affect climate via increasing cloud cover and then increasing cloud albedo (Twomey effect). In this process, cloud condensation nuclei (CCN), defined as the subset of aerosols that provide the initial sites for condensation of liquid water from surrounding supersaturated water vapor, will be activated to form cloud droplets. The ability of particles to be activated is affected by particle size, water vapor supersaturation, and particle hygroscopicity (i.e. the ability of a substance to attract and hold water molecules from the surrounding environment, through absorption or adsorption) [Lohmann & Feichter, 2005; McFiggans et al., 2006]. Among them, the particle size is the most important factor in order to obtain reasonable activation fraction of particles [McFiggans et al., 2006]. For this reason, one needs to know the number of particles associated with a specific size (or size range), i.e. the number size distribution.

    Another important reason is due to the requirement of studying new particle formation process and sources of small particles (e.g. combustion particles). These studies request small size aerosols being better resolved. In this case, using the number size distribution is superior to the volume size distribution, as the latter is insensitive to the size of small particles when it is too small.

 

• cloud condensation nuclei

    Good CCN candidates are usually water-soluble, e.g. sea salt, sulfates and sulfuric acid, nitrates and nitric acid, and some organics.

    As discussed above, it has been recognized that the most important factor of CCN activation is the particle size. Other than this, particle hygroscopicity is another important factor. In AR5, some new CCN activation models have introduced the bulk hygroscopicity parameter (κ-theory) for water-soluble particles [Rissler et al., 2004, 2010; Petters & Kreidenweis, 2007] and adsorption theory [Kumar et al., 2011] for insoluble particles.

 

• ice nucleation

    Good IN candidates are usually insoluble, e.g. mineral dust, volcanic ashes and primary bioaerosols (e.g. bacteria, fungal spores, pollen).

    In AR5, four heterogeneous ice-nucleation (freezing) mechanisms have been distinguished. They are immersion freezing (initiated from within a cloud droplet), condensation freezing (freezing during droplet formation), contact freezing (due to collision with an existing IN) and deposition nucleation (direct deposition of water vapor onto IN). Which mechanism(s) is the major one in mixed-phase clouds remains to be a question with great uncertainties.

 

 

2. Aerosol Processes

 

     The aerosol process studies are categorized into two major types – processes related with aerosol production in (source process) and removal from (sink process) the atmosphere. Obviously, this categorization is adopted for convenience of parameterization of aerosols in climate models. In AR5 cycle, there is significantly improved understanding of some source processes. However, much less progress has been made on our knowledge of sink processes.

 

Source Processes

 

• New Particle Formation (nucleation)

      This is the process by which low-volatility gas molecules nucleate into non-volatile molecule clusters. In AR5, nucleation involving sulfuric acid and nucleation of charged/uncharged particles are extensively examined.

 

• Condensation & Coagulation

      This is the process by which the nucleated non-volatile molecule clusters formed from new particle formation, when conditions favorable, rapidly grow into nanometer-sized (10^-9 m) aerosol particles. In AR5, the condensation of organic vapors turns out to be important. Therefore, condensation process can be very closely related to SOA formation process.

      Other than the initial condensation process, in AR5, the coagulation process is also studied. This refers to the process by which small aerosol particles collide into each other and stick into each other after the collision. Therefore, it is an important mechanism for aerosol internal mixing. Coagulation usually occurs near source areas when aerosol concentration is relatively high. In contrast, if the lifetime of aerosol is long enough, coagulation can also occur under low concentration circumstance.

 

• Release of Aerosol from cloud evaporation

This process is evaluated in AR5 since it can alter the number concentration, composition, size and mixing state of ambient aerosols.

 

Sink Processes

 

• Dry deposition

      This is the process by which large aerosol particles are removed from the atmosphere due to gravitational sedimentation. In climate models, this process is relatively well parameterized.

 

• Wet deposition

      This is the process by which aerosol particles are scavenged from the air by precipitation (e.g. rain, snow). Parameterization of this term remains to be a key source of uncertainty in aerosol models.