Aerosols (sources and processes)
Soot is composed of aggregates of small particles
(asbestos)
rainout (wet deposition)
Number Aerosol number is dominated by fine and ultrafine particles; aerosol mass by coarse particles Surface area Ultrafine Fine Coarse Volume
Fine particles are derived from gases turning to particles (e.g. combustion). Coarse particles are derived from mechanical processes breaking down larger substances (e.g. mineral dust). In between, the accumulation mode experiences a minimum in loss and therefore fine particles accumulate in this size range.
The removal of fine particles is dominated by wet deposition (~90%). The removal of coarse particles is more evenly divided between wet and dry deposition.
Atmospheric particles are made of inorganic and organic compounds Urban example Free troposphere example
Primary aerosol are particles emitted directly to the atmosphere
Secondary aerosol is aerosol mass formed by reactions in the atmosphere. Gases react and some of the products have low volatility; these can either (a) condense onto existing particles or (b) nucleate the formation of new particles.
Atmospheric particles take up water in amounts proportional to the ambient RH. Water uptake exhibits a hysteresis for pure compounds. Effluorescenc e RH Deliquescenc e RH
Present day aerosol optical depth
Annually-averaged particulate SO 4 formation from anthropogenic SO 2 (g) (units of kg/km 2 /hr)
Aerosol sulfate is primarily anthropogenic, but natural sources are also significant.
Annually-averaged SO4 source strength (units of kg/km 2 /hr)
Annually-averaged organic particulate matter source strength (units of kg/km 2 /hr)
Annually-averaged black carbon particle matter source strength (units of kg/km 2 /hr)
Annually-averaged dust source strength (units of kg/km 2 /hr)
Annually-averaged sea salt source strength (units of kg/km 2 /hr)
Aerosol and Radiation
Incredible pollution over eastern China. Much of the material in the NE is NOT from dust!
Models of aerosol-radiation interactions Mie scattering is a generalized, analytical theory for homogeneous spheres (there exist some extensions to slightly more complicated scenarios). Most models assume Mie scattering applies, even though it often doesn't. More complex models exist for arbitrary shapes and inhomogeneous compositions, but are computationally very expensive.
End-member models If d << λ, this is termed the Rayleigh scattering regime and simple analytical solutions exist If d >> λ, this is termed the geometric optics regime and simple analytical solutions exist
Incident light direction
Alpha is a particle size normalized by wavelength of light
Data from Aeronet
Total anthropogenc aerosol optical depth (model ensemble)
Net anthropogenic aerosol forcing of atmosphere only (model)
Net anthropogenic aerosol forcing of surface only (model)
Net TOA anthropogenic aerosol forcing (model)
Standard deviation of TOA anthropogenic aerosol forcing (model)
Radiative forcing (W/m 2 ) by: (a) Well-mixed greenhouse gases (b) stratospheric ozone depletion (c) tropospheric ozone (d) sulfate aerosols
(e) Biomass burning aerosols (f) Fossil fuel combustion aerosol s (g) Mineral dust aerosol (h) Indirect effect of sulfate aerosol
(i) aircraft contrails (j) surface albedo changes (k) solar variability
Aerosol indirect effects Twomey Effect (1 st Indirect Effect) Anthropogenic aerosol more CCN more cloud drops smaller cloud drops (for fixed liquid water amount) more surface area higher albedo
Aerosol indirect effects A B Precipitation effects (2 nd Indirect Effect) Anthropogenic aerosol smaller cloud drops harder to form precipitation (A) cloud stores more liquid water, increasing albedo (B) cloud lives for longer, increasing total cloudiness and albedo
Aerosol indirect effects Semi-direct effect : Absorbing aerosol more heating of the atmosphere increasing stability less convection less cloudiness lower albedo