Some Impacts of Atmospheric Aerosols Direct and Indirect Effects on Climate directly scattering solar radiation altering number and size distribution of cloud drops N2O5N2O5 HNO 3 Effects on Gas-Phase Composition Surfaces for heterogeneous chemistry Multiphase reaction volumes [S(IV)  S(VII)]

SO 2 +OH (+H 2 O)  H 2 SO 4 RH + OH  RCOOH Secondary Mass Growth Secondary Particles Primary particles Some Sources of Aerosol Particles

Aerosol Particle Size: Diameter vs. Effective Diameters For many particles, spherical geometry good assumption. “Diameter” has physical meaning Spherical?

Some Effective Diameters Aerodynamic Diameter rprp Same terminal falling speed in air as a particle with density 1g/cm 3 and radius r p rprp Electric Mobility Diameter Same trajectory in calibrated electric field as a spherical singly charged particle with radius r p Relation to aerodynamic diameter and other physical properties of particle not well understood for fractal like soot particles.

Number Concentration (cm -3 ) Diameter (  m) Problems 1.Information lost at small sizes due to large size range 2.Comparing particle concentrations in different bins marred by varying bin size 3.Area under curve is not proportional to total particle number concentration Bean Counting: Aerosol Size Distributions

Visual Representation of Particle Size Distributions N i /  D pi vs. D p N i /  log(D pi ) vs. Log(D p ) Area under both curves yields N total But dN/dlogD p vs. logD p is more informative

Questions The figure shows various representations of the same aerosol size distribution. Under which curve(s) is the area equal to the total particle number concentration? dN/dD p (blue) dN/dlogD p (green) dN/dlnD p (red) dN/dlogDp only. Why?

Area, Volume-Mass Distributions Heterogeneous and multiphase reaction rates depend on surface area or volume, respectively. Number Area Gravitational settling rates depend on mass and air quality standards are mass-based. --Assuming spherical geometry and dD p  0 dS(D p ) =  D p 2 n(Dp)dD p dV(D p ) = (  /6)D p 3 n(Dp)dD p

Questions 1.What are the units of S tot and V tot ? 2.How is the mass distribution function calculated? 3.What is the relevant property (area, volume, mass) for the following aerosol particle processes: Scavenging of HNO 3 by mineral dust Acidification of aerosols by gas-phase H 2 SO 4 Light scattering efficiency Amount of Fe deposited to ocean by dust

Smoothed Vertical Profiles of Aerosol Number Concentrations—(highly variable) Boundary layer: 10 – 10 5 cm -3 range in number concentration Free Troposphere: ~ cm -3 on average

Common Modes of Atmospheric Aerosol Distributions

Typical Number Distribution for Urban Aerosols Solid line: what would be observed, composed of 3 modes Dotted/Dashed lines: Two common parameterizations Junge Distribution (dashed line) is a power law. Has some useful properties but requires care. Log-Normal distribution (dotted line) is most often used

Continental and Marine Number Distributions Lower numbers in these regions relative to urban aerosols, especially in the nucleation mode. Giant aerosols over ocean dominated by sea salt. Dominant accumulation mode indicative of “aged” particles.

The Log-Normal Distribution The familiar normal (Gaussian) distribution Bell-curve shape in linear space 68% of variance about mean ( ) captured by 2  (width) 22 The Log-normal distribution Bell-curve shape in log space and  ?