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Aerosols and Climate Peter J. Adams and Neil Donahue Center for Atmospheric Particle Studies (CAPS) Carnegie Mellon University CEDM Annual Meeting May 25, 2017 Outline • • • • • • Aerosols 101 Their Climate Forcings Climate Sensitivity and Aerosols Why Aerosol Forcings are Uncertain Health Effects and Hidden Warming Black carbon (soot) as short-term climate mitigation Recall Definitions • Aerosol = collection of particles (solid or liquid), suspended in a gas (=atmosphere for us) • Particulate matter (PM) = aerosol • Two National Ambient Air Quality Standards (NAAQS) • • • PM2.5: the mass concentration of particles whose aerodynamic diameter is <= 2.5 microns PM10: … same but <= 10 microns PM2.5 is more likely to penetrate deeper in the lungs than particles larger than 2.5 microns Composition Is Complex • Inorganic ions: sulfate, nitrate, ammonium, others Biggest contributors to PM2.5 (fine) mass • Organic carbon conc. • Elemental carbon (“soot”) Health effects? • Trace metals: Pb, V, Fe, Ni, Cu, etc. • Crustal/mineral dust species: Si, Ca, Al, Fe, Ti, etc Coarse PM, key natural sources, • Sea spray: NaCl (and some organics) obvious in satellite • Biological particles: pollen, spores, etc. imagery • Fly ash Size Ranges • Atmospheric particles vary greatly in size • • 1 nm • • < 1 nm, you are probably a gas molecule > 10 mm, you won’t be in atmosphere very long (gravitational settling) 10 nm 100 nm 1 mm 10 mm factor of 104 in diameter, 1012 in volume from a cluster of a few molecules … to a cloud droplet Dp (diameter) Terminology • *for regulatory purposes, 2.5 mm is cutoff between “fine” and “coarse” Anthropogenic aerosol forcing Nucleation mode 1 nm 10 nm “Aitken” mode / ultrafine mode “Fine” mode / accumulation mode 100 nm Submicron 1 mm* Natural, windblown sea spray, mineral dust “Coarse” mode 10 mm Supermicron Dp (diameter) Key Processes: Particle Formation Making big particles • 1) wind-blown emission (primary particles) out of bigger • high winds lift dust and sea spray into atmosphere “particles” (the Earth) • 2) mechanically generated (primary particles) • • construction, abrasion of tires on roadway etc. 3) regional nucleation (secondary particles) • • high (super-saturated) levels of H2SO4(g) … plus “other stuff” (ammonia, amines, oxidized organics … a research question) Makingstill small • gas molecules cluster until a stable particle is formed particles out of •gas 4) near-source nucleation (“primary” particles) molecules • • • at point of emission (e.g. tailpipe of car), emissions are gases but rapid cooling causes nucleation and growth of particles near source sometimes in source: e.g. combustion soot Particle Evolution • Condensation (and evaporation) • • recall gas-phase reactions that produce: H2SO4, HNO3, semi-volatile organic compounds these tend to condense onto particles, causing them to grow (1-20 nm/h is typical during day) • Coagulation • • particles collide and stick due to Brownian (“random walk”) diffusive motion mostly a mechanism for ultrafine and nucleation mode particles to add to larger particles • It should be clear that particles are mixtures of lots of stuff, not all from same source Particle Removal • Dry deposition • mostly for super-micron particles (tD is ~1 day) • Wet deposition • mostly for Dp > 100 nm (tD is 4-8 days) • *In some sense, coagulation is also “removal” • (decreases N conc., M conc. remains the same) Super-micron vs Sub-micron Nucleation mode “Ultrafine” mode “Fine” mode / accumulation mode Freshly nucleated particles are smallest (by definition) 1 nm 10 nm Near-source nucleation (from combustion) are a little bigger “Coarse” mode Windblown and mechanically generated particles are bigger 100 nm Submicron 1 mm* 10 mm Supermicron Dp (diameter) Super-micron vs Sub-micron • This drives a big difference in composition between sub-micron / super-micron • • Super-micron: wind-blown soil, sea spray, mechanically generated dust, tire wear, etc. Sub-micron: primary combustion particles and all secondary material produced by gas-phase chemistry • • • inorganics: sulfate, nitrate, ammonium organics: POA, SOA (primary/secondary organic aerosol) black carbon (elemental carbon / soot) Life Cycle of Super-micron (Boring) Nucleation mode “Ultrafine” mode “Fine” mode / accumulation mode • Particle emitted • Negligible evolution due to condensation/coagulation •1 nm Removed10usually by100dry nm nm deposition Submicron “Coarse” mode Windblown and mechanically generated particles are bigger 1 mm* 10 mm Supermicron Dp (diameter) Life Cycle of Sub-Micron Particle (Interesting) Nucleation mode “Ultrafine” mode “Fine” mode / accumulation mode Freshly nucleated particles are smallest (by definition) condensation 1 nm 10 nm Near-source nucleation (from combustion) are a little bigger 100 nm Submicron “Coarse” mode • Particles grow by condensation (1-20 nm/h) • Ultrafine/nucleation mode particles lost by Dp coagulation on timescale (diameter) of hours 1 mm* 10 mm • Both processes drive mass to “accumulation” mode (slow dry deposition) Super• Eventual micronremoval by precipitation Aerosols Scattering Sunlight Dust and smoke over Australia (Terra) Aerosols Absorbing Sunlight (Direct) photo courtesy of Jay Apt Kuwaiti oil fires Clouds and Climate •fluxes are W m-2 •Width of arrow proportional to flux Atmosphere 342 77 67 (reflected) (absorbed) 30 168 Earth • • 23% of incoming sunlight reflected by atmosphere (mostly by clouds) Without cloud reflection, the Earth would be ~15º C warmer Aerosol Cloud Reflectivity Effect Clean air mass •Lower CCN concentration •Higher Transmittance •Lower reflectivity (albedo) •Better chance of precipitation? •Shorter cloud lifetime? Polluted air mass •Higher CCN concentration •Lower Transmittance •Higher reflectivity •Less precipitation? •Longer cloud lifetime? Cloud Optics: Surface Area For a given amount of liquid water (or ice): • More pollution/CCN → More cloud droplets → More surface area • → More scattering → Brighter cloud → Cooler Earth Darker clean cloud Brighter polluted cloud (Few CCN) (More CCN) photo courtesy of Amy Sage Cloud Condensation Nuclei (CCN) • • In a particle-free atmosphere, a strong supersaturation (~400% relative humidity) is required to nucleate new liquid droplets Instead, cloud water condenses onto pre-existing particles: cloud condensation nuclei (CCN) Clear Sky (RH < 100%) Activation: Cloudy Sky (RH > 100%) water condenses on CCN to form cloud droplets Other particles (aerosols) CCN (~100 nm) Cloud droplets (~10 mm) Aerosol Activation “Activation” = formation of cloud droplet involves a competition between solute and surface tension effects Number Depends on number concentration above “critical diameter” Diameter Aerosols and Climate: Semi-Direct Effect High RH Atmospheric Heating Low RH • Atmospheric heating from absorptive aerosol • Warmer air lower RH less cloud formation How direct is direct? • Direct effect: scattering/absorbing sunlight • Semi-direct effect: • • • • aerosol absorption heats atmospheric layer warmer air → lower relative humidity → less/no cloud forcing or feedback? Indirect effect: modifying cloud properties • “brightness (first) effect” • “lifetime (second) effect” Other Forcings • BC on snow/ice surfaces • • • Makes surface darker and more absorbing Relatively small global forcing May be regionally important • BC as ice nuclei • • • Ice nuclei are important triggers of precipitation May change cloud cover, affect hydrological cycles Almost nothing very concrete known about this effect Long-lived GHGs: +3 W/m2 (+/- 20%) Aerosol effects: mostly cooling, highly uncertain Radiative Forcing (W m-2) Climate Change Uncertainty • “Climate sensitivity” is a key parameter global average temperature change • • • • global average radiative forcing Key parameter is l,“climate sensitivity” • • • T lF 0.3 to 1 °C per W/m2 1.5 - 4.5 °C for doubling of CO2 Implicit assumption that l is same for all kinds of forcings (absorbing aerosols are known exception) Boundary between “forcing” and “feedback” is fuzzy sometimes In climate models, representation of cloud feedback is largest source of uncertainty In retrospective studies, knowledge of aerosol forcing is lacking Aerosols and Climate Uncertainty 3.5 GHG forcing High sensitivity Temperature Change (K) 3.0 2.5 Aerosol (haze) + GHG forcing 2.0 1.5 Low sensitivity 1.0 20th century T increase 0.5 0.0 0.0 0.5 1.0 1.5 2.0 Forcing (W m -2) 2.5 3.0 3.5 Climate Models: Sensitivity / Aerosols Figure from Kiehl et al., GRL v34 doi:10.1029/2007GL0313832007 Climate Models: Sensitivity / Aerosols Figure from Kiehl et al., GRL v34 doi:10.1029/2007GL0313832007 Challenges • Need to characterize particle • • • • • • mass/number concentration size distribution: ~10 nm to 10 mm chemical composition: >hundreds compounds mixing state interactions with clouds (sub-grid) separate anthropogenic from natural • Highly variable in space and time: intrahemispheric mixing Mean aerosol residence hourly daily monthly annual Mean CO2 residence NH/SH mixing decadal century Aerosol Variability Uncertainty in pre-industrial/natural baseline matters 20th Century Forcings Temperature Reconstruction Temperature Reconstruction Temperature Reconstruction Temperature Reconstruction Temperature Reconstruction PM and Life Expectancy: 1980 PM and Life Expectancy: 2000 Global Distribution of PM2.5 Global Mortality from PM2.5 Hidden Warming from China’s Aerosols • China is ~25% of world emissions for anthropogenic aerosol precursors • • Global aerosol forcing is • • • -0.8 W/m2 (best guess) … so China is -0.2 W/m2 -2 W/m2 (coolest) … so China is -. 5 W/m2 Climate sensitivity • • • IPCC AR5 Table 7.1 1.5 to 4.5 deg C for 2xCO2 (~4 W/m2) 0.3 to 1.0 K/(W/m2) Multiplying by climate sensitivity… • • Best guess: China is -0.2 W/m2 x 0.6 K/W/m2 = -0.12 deg C Worst case: so China is -. 5 W/m2 x 1.0 K/W/m2 = -0.5 deg C Won’t Climate Response Take a Long Time? … No Surface air T Instantaneous global desulfurization (red) Y2000 levels of sulfate and GHGs held constant (blue) Won’t Climate Response Take a Long Time? … No Surface air T Surface layer of ocean adjusts quickly (0.8 K warming in one decade) More gradual response as deep ocean equilibrates Black Carbon as Climate Mitigation? Warming Cooling Sunlight Absorption Cloud Brightening Cloud Burnoff Co-emitted Reflectors Snow/Ice Darkening • Black carbon sources • • contain cooling/scattering aerosol contribute to cloud condensation nuclei How much cooling goes with the warming? Kuwaiti oil fires (photo courtesy of Jay Apt) 45 Timing • BC mitigation would lead to fast benefits Source: UNEP report, 2011 Climate Effects BC 0.05 to 0.55 W/m2 (IPCC) but maybe ~1 W/m2? Not easily separable from cooling aerosols Source: IPCC AR4 Co-Emitted Species: Early Debate • Response from Joyce Penner (selected quotes) • • • • he obtains a warming from the combined direct and indirect effects of f.f. BC + OM that is inadequately documented warming may disagree with the results of published models by the aerosol-climate community. Penner et al. [2003] … obtain a forcing for f.f. BC + OM that is not significantly different from zero Jacobson [2002] has an inferred forcing for f.f. BC + OM that is 0.5 W/m2 Co-Emitted Species • Most recent comprehensive analysis (Bond et al., 2012) shows coemitted cooling could completely offset BC warming Source: Bond et al., 2012 Co-Emitted Species • Different BC sources have different warming potential A, H, RC all different models Warming more likely than not Source: Kopp and Mauzerall, 2010 Net Warming/Cooling By Source Diesel: + ~.15 W/m2 “high confidence in net positive total climate forcing is possible only for black-carbon source categories with low co-emitted species, such as diesel engines.” (Bond et al.Biofuel 2013) cooking: + but small Residential coal: + but small Biomass burning: net cooling ~.2 W/m2 from right sources → 0.06 to .2 K BC Controls Reduce CDNC In global annual average, 50% FF: CDNC reduced by 4.6% 50% CARB: CDNC reduced by 8.7% CDNC [cm-3 ] Smax [%] Reff [μm] 195.6 0.26 8.27 Ratio of cloud droplet number (CDNC) 50% FF Base case Black Carbon “Warming and Drying” Aerosol Forcing (W m-2) -0.5 W m-2 Governs global mean T +2.0 W m-2 Reduces evaporation -2.5 W m-2 Ramanathan et al., Science 294, 2119-2124, 2001. Summary • Estimating aerosol climate forcing is tough • Similar/related to cloud feedback problem • Estimates of aerosol forcing and climate sensitivity are correlated • Strong aerosol cooling is bad news • Implies high climate sensitivity, lots of hidden warming (e.g. China) • Mitigating black carbon is worthwhile • but expectations for climate benefits should be limited