School of something FACULTY OF OTHER 1 Lecture 2: Aerosol sources and sinks Ken Carslaw

2 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?

3 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.)

4 Primary, Secondary and Aged Primary Primary particles Secondary particles Emitted gases gases coagulation condensation Aged primary particles Can contain primary and secondary matter chemistry Source ACPD Discussion by U. Poeschl: Amusing article on definitions: Schwartz, Henry’s law and sheep’s tails, Atmospheric environment, 22, , Reply: Clegg and Brimblecombe, p

5 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) SO 2, Dimethyl sulfide  oxidation  sulfate (H 2 SO 4 ) Nitrogen oxides  oxidation  nitrate (HNO 3 ) Volatile organic compounds (VOCs) -> oxidation -> low vapor pressure organic products (secondary organic aerosol, SOA) From natural and anthropogenic sources

6 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 NO x, SO 2, black carbon

7 Sulfur dioxide Domestic 9 Tg/a Power plants 48 Tg/a Industry 39 Tg/a Volcanic SO 2 = Tg/a Biogenic equiv SO 2 = 36Tg/a SO 2 (m=64)  H 2 SO 4 (m=98) Dimethyl sulfide

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

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

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

11 Biomass burning size distribution dN/dlogD from lognormal fitting

12 Traffic emissions size distribution nm nm ~80 nm Putaud et al, Aerosol Phenomenology, 2003 Rural UrbanKerbside

13 Aerosol production / emission rates

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

15 Sea spray production rates Integrated flux in a size range At nm: Mean concentration after one day N nm ~ 350 cm -3 N 1  m ~ 3.5 cm -3 F 1 km 1 m 2 Assume steady flux into 1 km deep well mixed boundary layer with u = 8 ms -1

16 Secondary aerosol production rates SO 2 + OH + M  H 2 SO 4 k OH ~ cm 3 molec -1 s -1 OH ~ 10 6 molec cm -3 SO 2 gas phase chemical lifetime ~ 10 6 s ~ 10 days Pham et al., JGR, 1995 * see Donahue! NO 2 + OH  HNO 3 NO 2 lifetime ~ 1 day OH+  -pinene  organic aerosol * k OH = 1.2×10 −11 exp(444/T) Monoterpene lifetime ~ 0.4 days

17 Sulfate aerosol production in clouds SO 2 evaporation cloud Involatile H 2 SO 4 remains in particles SO 2 H 2 SO 4 (gas) H 2 SO 4 (particles) deposition 42 ~4 times as much SO 4 from clouds as from gas phase oxidation: SO 2 lifetime ~ 2.5 days + OH

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

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

20 Wet scavenging In-cloud scavenging Below-cloud scavenging

21 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

22 Wet scavenging Below-cloud scavenging 3nm: h 10  m: h 100nm: 10days-weeks

23 Global aerosol production and loss timescales

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

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

26 Importance of aerosol lifetime It is short compared to most greenhouse gases CO 2 – 100 y CH 4 – 11y Aerosols do not accumulate in the atmosphere In the long term can expect GHGs to dominate forcing

27 Pb 210 tracer to quantify deposition lifetimes Huge model diversity in remote regions due to differences in deposition Rasch et al., Tellus, 2000