Lecture 2: Aerosol sources and sinks

Lecture 2: Aerosol sources and sinks Ken Carslaw

Key issues • What are the relative source strengths and distributions? • How quickly is aerosol produced and removed? • How do these factors change with particle size? • Following lecture: how are aerosol properties altered between emission and removal?

Primary and secondary particles • Primary particles are emitted directly into the atmosphere • Secondary particles are formed in the atmosphere (by condensation or nucleation of gaseous precursors). • One person’s primary is another’s secondary • E.g., global model: urban particles may be treated as primary because they are formed below the grid scale. I.e., they can be primary if formed within a source (e.g., an engine, city, etc.)

Primary, Secondary and Aged Primary Secondary particles coagulation Aged primary particles Can contain primary and secondary matter condensation gases chemistry Primary particles Emitted gases Source Source ACPD Discussion by U. Poeschl: http://www.cosis.net/copernicus/EGU/acpd/5/S5095/acpd-5-S5095.pdf Amusing article on definitions: Schwartz, Henry’s law and sheep’s tails, Atmospheric environment, 22, 2331-2332, 1988. Reply: Clegg and Brimblecombe, p2332-2333.

Primary and secondary emissions • Primary • Dust (including re-suspended), combustion products of elemental and organic carbon (biomass burning, wildfires, vehicles), sea spray, primary biological particles (spores, etc) • Secondary • Ammonia ammonium (dissolution) • SO2, Dimethyl sulfide  oxidation sulfate (H2SO4) • Nitrogen oxides  oxidation nitrate (HNO3) • Volatile organic compounds (VOCs) -> oxidation -> low vapor pressure organic products (secondary organic aerosol, SOA) • From natural and anthropogenic sources

Quantifying emissions • Active emissions • (Depend on the environment) • Sea spray, dust – wind speed • DMS – wind speed and biological activity etc. • Biogenic VOCs – temperature, biological activity • Passive emissions • (Depend on emission factors, energy use, etc) • Anthropogenic NOx, SO2, black carbon

Sulfur dioxide SO2 (m=64)  H2SO4 (m=98) Domestic 9 Tg/a Power plants 48 Tg/a Dimethyl sulfide Volcanic SO2 = 25-35 Tg/a Biogenic equiv SO2 = 36Tg/a Industry 39 Tg/a

Organic matter Biogenic SOA = 10 – 100’s Tg/a (see Donahue) Biomass burning = 34Tg/a Fossil fuel 3Tg/a Biofuel = 9Tg/a

Sea spray • Global sea spray mass production rate Total = 8000 Tg/a

Sea spray size distribution Number, area, volume Wind speed = 8 m/s ~1.3% of sea spray is in the accumulation mode ~ 100 Tg/a Marine aerosol production: a review of the current knowledge, O'Dowd and De Leeuw, Phil Trans Roy Soc A, 365, 2007

dN/dlogD from lognormal fitting Biomass burning size distribution

Traffic emissions size distribution Rural Urban Kerbside 20-30 nm 30-50 nm ~80 nm Putaud et al, Aerosol Phenomenology, 2003

Aerosol production / emission rates

Sea spray flux versus wind speed • Flux is proportional to 10 m wind speed cubed

Sea spray production rates Assume steady flux into 1 km deep well mixed boundary layer with u = 8 ms-1 • Integrated flux in a size range • At 10-100 nm: • Mean concentration after one day • N10-100nm ~ 350 cm-3 • N1mm ~ 3.5 cm-3 1 km F 1 m2

Secondary aerosol production rates • SO2 + OH + M  H2SO4 • kOH ~ 10-12 cm3 molec-1 s-1 • OH ~ 106 molec cm-3 • SO2 gas phase chemical lifetime ~ 106 s ~ 10 days NO2 + OH  HNO3 NO2 lifetime ~ 1 day • OH+a-pinene  organic aerosol* • kOH = 1.2×10−11 exp(444/T) • Monoterpene lifetime ~ 0.4 days Pham et al., JGR, 1995 * see Donahue!

Sulfate aerosol production in clouds SO2 Involatile H2SO4 remains in particles evaporation cloud H2SO4 (gas) ~4 times as much SO4 from clouds as from gas phase oxidation: SO2 lifetime ~ 2.5 days + OH 12 H2SO4 (particles) SO2 42 45 deposition

Aerosol removal (scavenging) processes • Dry deposition – diffusion to and deposition on surface • Wet deposition • In-cloud or “nucleation” scavenging • Impaction

Dry deposition Deposition velocity over forest Brownian diffusion Gravitational settling 20 m/s 1 m/s Accumulation mode! 10 cm s-1 lifetime of 1 km deep well mixed boundary layer aerosol~ 3h 0.1 cm s-1 lifetime of ~12 days

Wet scavenging In-cloud scavenging Below-cloud scavenging

Wet scavenging • Characteristic time scale for the conversion of cloud droplets into raindrops in precipitating clouds ~ 3 hours • PROBLEMS: • Cloud-scale processes • Particle size dependence In-cloud scavenging

Wet scavenging 3nm: 0.5-20h 10 mm: 0.5-10h 100nm: 10days-weeks Below-cloud scavenging

Global aerosol production and loss timescales

Production of global aerosol mass ~1 month to reach steady state Based on Leeds GLOMAP global aerosol model

Decay of global aerosol mass and number Switch off all emission processes in a global model Number lifetime ~ 10 days Arctic Global models predict a ~factor 2 difference in aerosol lifetime between US, Asia & Europe 10% remains after 1 month Mass lifetime ~ 3 days Based on Leeds GLOMAP global aerosol model

Importance of aerosol lifetime • It is short compared to most greenhouse gases • CO2 – 100 y • CH4 – 11y • Aerosols do not accumulate in the atmosphere • In the long term can expect GHGs to dominate forcing

Pb210 tracer to quantify deposition lifetimes • Huge model diversity in remote regions due to differences in deposition Rasch et al., Tellus, 2000

Lecture 2: Aerosol sources and sinks