Typically have a higher organic content than coarse particles Also contain soluble inorganics: NH 4 +, NO 3 -, SO 4 2- A bimodal peak is often observed in this range Accumulation range: (0.08 to 2 μm) Contribute up to 50% by mass but 5% by number Sources: Condensation of low volatility vapours Self-coagulation of small particles within the nuclei range Coagulation of nuclei range particles with particles in the accumulation range

Particles are too small for much sedimentation Dominant removal processes are therefore: Rainout: incorporation into cloud droplets Washout: removal during precipitation Dry deposition: losses on surfaces The long lifetime of particles in this mode makes them important in atmospheric chemistry

Aitken nuclei particles grow larger either by: Acting as nuclei for condensation of low vapour pressure gaseous species Coagulation to form larger particles The lifetime of particles in this range is short owing to rapid coagulation Aitken nuclei: (0.01 to 0.08 μm) Contribute the largest number of particles, but the overall mass of particles in this range is small Sources: Gas-to-particle conversion at ambient temperatures May be formed in combustion – hot, supersaturated vapours form and condense

Ultrafine particles: (< 0.01 μm) The 4 th mode of the size distribution – sometimes referred to as the nucleation mode Particles in this size range are difficult to measure and are not well understood Can occur in significant numbers (e.g., 10 4 particles cm -3 ) but have a very small mass Formed via gas-to-particle conversion:

Characterising size distributions A log-normal distribution provides a good empirical fit to observed size distributions: where: σ g is the geometric standard deviation N T is the total number of particles D gN is the geometric number mean diameter This is analogous to a normal distribution: y = A exp[-(x – x 0 ) 2 / 2σ 2 ]

σ g is a measure of the spread of a distribution ln σ g = ln D – ln D gN →σ g = D / D gN 68% of particles fall between σ g = D gN / σ g and D gN σ g The value of σ g is the same for different distributions and mean diameters: geometric mass mean diameter, D gM geometric surface mean diameter, D gS geometric volume mean diameter, D gV

Other contributions? Moving air → turbulent motion 2.2 PARTICLE MOTION Particle motion driven by: Gravitational settling Brownian motion Approximately equal contributions at D ~ 0.2 μm important for: Gravitational settling  large particles D > 0.2 μm Brownian motion  smaller particles D < 0.2 μm

2.3 INTERACTION WITH RADIATION Extinction in the atmosphere is the sum of absorption and scattering of both particles and molecules: where andσ ag = absorption of gas σ sg = scattering of gas (Rayleigh) σ ap = absorption of particles σ sp = scattering of particles (Mie) Scattering: D << λ→ Rayleigh scattering D ~ λ→ Mie scattering D >> λ→ Mie scattering Mie scattering is dominant

Impact of aerosols on radiative balance: From the IPCC report on climate change: