SULFATE & THE CLAW HYPOTHESIS

SULFATE & THE CLAW HYPOTHESIS

TOPICS FOR TODAY • The Sulfur Cycle & Sources of Sulfate • The CLAW hypothesis • How might sulfate concentrations be affected by climate change?

SULFATE: THE DOMINANT AEROSOL COUNTER-ACTING GHG WARMING [Hansen et al., 2005]

SULFUR CYCLE Most sulfur is tied up in sediments and soils. There are large fluxes to the atmosphere, but with short atmospheric lifetimes, the atmospheric S burden is small. SO2: Anthropogenic (fossil fuel combustion) source comparable to natural sources (soils, sediments, volcanoes, DMS oxidation) Sulfur is oxidized in the atmosphere: SO2 ---- > H2SO4 S(+IV) S(+VI) Sulfate is an important contributor to acidity of precipitation. Sulfuric acid has a low Pvap and thus partitions primarily to aerosol/aqueous phase Strongly perturbed by human activities!

GLOBAL SULFUR BUDGET [Chin et al., 1996](flux terms in Tg S yr-1) SO42- t = 3.9d SO2 t = 1.3d cloud 42 OH H2SO4(g) 18 8 4 NO3 OH (CH3)2S 64 dep 6 dry 44 wet DMS t = 1.0d 10 dep 27 dry 20 wet 22 Phytoplankton Volcanoes Combustion Smelters

GLOBAL SULFUR EMISSION TO THE ATMOSPHERE 2001 estimates (Tg S yr-1): Industrial 57 Volcanoes 5 Ocean 15 Biomass burning 1 Chin et al. [2000]

Thermodynamic rules: Sulfate always forms an aqueous aerosol Ammonia dissolves in the sulfate aerosol totally or until titration of acidity, whichever happens first Nitrate is taken up by aerosol if (and only if) excess NH3 is available after sulfate titration HNO3 and excess NH3 can also form a solid aerosol if RH is low Highest concentrations in industrial Midwest (coal-fired power plants) FORMATION OF SULFATE-NITRATE-AMMONIUM AEROSOLS SO42- (coal combustion) NO3- (fossil fuel) NH4+ (agriculture)

THE CLAW HYPOTHESIS Ncloud droplets (fixed LWP) Scattering of solar radiation by droplets + Cloud Albedo Cloud Nucleation + Loss of solar radiation to space - CCN NSS-sulfate Oxidation + Solar irradiance below clouds Tsurf of Earth DMS(g) Sea-to-air transport + Production of DMS by marine phytoplankton +/- ? DMS(aq)

THE GAIA HYPOTHESIS “Gaia: a complex entity involving the Earth's biosphere, atmosphere, oceans, and soil; the totality constituting a feedback or cybernetic system which seeks an optimal physical and chemical environment for life on this planet.” ~ James Lovelock CO2 Control Surface Temperature? DMS weathering …conversion to chalk/granite

DMS EMISSIONS DMS emitted by planktonic algae (living), but concentration in sea water not clearly connected with productivity. Note: phytoplankton contain chlorophyll so often use satellite products of chl-a as proxy  But phytoplankton speciation important! A=ocean surface area k=transfer velocity c = gradient across air/sea interface Flux (F)=Akc More DMS in ocean? More ice-free ocean? Increase winds? In Future:

ESTIMATING DMS EMISSIONS: A CHALLENGE! “The results were compared to published fields of geophysical and biological parameters. No significant correlation was found between DMS and these parameters, and no simple algorithm could be found to create monthly fields of sea surface DMS concentration based on these parameters.” [Kettle et al., 1999] Broken stick regression [Anderson et al., 2000]: DMS = a log(CJQ) < s DMS = a + b[log(CJQ) – s]log(CJQ) > s C = chlorophyll J = short-wave light Q = nutrient term All data Binned Data + Fit

ESTIMATES OF ANNUAL MEAN SURFACE [DMS] DMS fluxes are not known to within a factor of 2 – different regional success for different models [Belviso et al., 2004]

SULFATE SOURCE FROM DMS OXIDATION 18-27% of DMS is converted to sulfate [IPCC, 2007] DMS Sulfate % Sulfate from DMS DJF JJA [Gondwe et al., 2003]

THE IMPORTANT LINK: NSS-SULFATE (FROM DMS) AS CCN CLAW (1987) postulates: Marine CCN vary from 30-200 cm-3 Sea salt particles are not important CCN: concentrations at cloud height are ~1 cm-3 Sulfate is the only important water soluble aerosol in marine air (~100 cm-3) Increasing N, leads to enhanced A Average liquid water content of clouds (L) is constant: r = radius  = density of water N = number density of droplets ↑N implies compensating ↓r and thus an increase in total surface area of droplets and increase in cloud albedo Proxy for LWP (or thickness of cloud) Example: N +30% would lead to a globally averaged increase in solar albedo of +0.005 (and associated cooling of 1.3K)

IMPORTANCE OF CLAW? • Special issue of Environmental Chemistry in 2007 reviewed CLAW: • after 20 years of research the theory remains unproven • primarily because too many complex processes involved • theory was critical to start thinking of connections between biology, chemistry and cloud physics • Challenges: • literature explorations mostly statistical: CCN/sulfate correlations • the process as a whole would occur over 1000’s of km (a couple of days to oxidize DMS to SO2…transport away from any phytoplankton bloom) • on the whole, is the ocean light or temperature limited? Regional behaviour? • how important is MBL nucleation vs. FT detrainment? • how do sea salt and organic particles emitted from the ocean complicate CLAW? • what about the role of increasing ocean stratification (reduction in nutrient availability and plankton growth)? • the potential for enhanced convection to loft DMS higher into the FT • the effect of ocean acidification on ocean productivity? • floristic shifts due to warming oceans?

PREDICTED CHANGES IN (ANTHROPOGENIC) SO2 EMISSIONS ~50%+ Drop Globally SO2 emissions expected to decline, largely due to reductions expected in NA, EU, AS(?). Spread largely due to assumptions about timing of emissions controls in Asia/India. DMS source will become relatively more important. [IPCC 2007 (WG3)]

HOW WILL THE SULFUR CYCLE BE AFFECTED BY CLIMATE CHANGE? Transport? Lofting? SO42- SO2 cloud OH Oxidation processes? Lifetimes? H2SO4(g) NO3 OH (CH3)2S dep DMS dep Phytoplankton Volcanoes Combustion Smelters Species? Response? Ocean stratification? Flux out of ocean?

SULFATE & THE CLAW HYPOTHESIS