SEASALT AEROSOL PARTICLES COUNTING ANGELS ON THE HEAD OF A PIN?

Stephen E. Schwartz

Environmental Sciences Department

Workshop on Aerosol Production

Yorkshire, U. K.

Ron Giddings(http://graphicexchange.com/6Portfolio.html) May 11-13, 2004 http:// www. ecd. bnl. gov/ steve/ schwartz. html IMPORTANCE OF SEA SALT AEROSOL TO CHANGE

Aerosols — the key uncertainty in of over the industrial period. Role of sea salt aerosol? Understanding/quantifying climate influences of other aerosol species Attribution of aerosol radiative influences to SSA and non-SSA species. Evaluating CTM calculations of aerosol optical depth and other properties Decreasing the “wiggle room” in modeling aerosol forcing. Influencing forcing of climate change by other aerosol species Direct forcing. Indirect forcing. Forcing of climate change by SSA Increased SSA production in a greenhouse warmed world? IMPORTANCE OF SSA IN UNDERSTANDING AND QUANTIFYING CLIMATE INFLUENCES OF AEROSOL SPECIES OTHER THAN SSA MONTHLY AVERAGE AEROSOL JUNE 1997 Polder radiometer on Adeos satellite Optical Thickness τ λ = 865 nm

0 0.5

Ångström Exponent α ατ=−ddln / lnλ

-0.2 1.2 SOURCE STRENGTHS OF KEY AEROSOL TYPES AND AEROSOL OPTICAL DEPTH (IPCC, 2001)

Anthropogenic production rate Natural sulfate production rate

Anthropogenic organic matter Natural organic matter

Anthropogenic black carbon Dust (D < 2 µm)

Sea salt (D < 2 µm) Total optical depth

Source strength, kg km-2 hr -1 Aerosol optical depth SEASALT AEROSOL MASS CONCENTRATION Modeled and observed annual concentrations vs. location From IPCC (2001) intercomparison

- Prospero et al. -3 Concentration, g m Concentration, g

Cheju IslandHokkaido Midway Oahu Fanning Island American SamoaNorfolk IslandCape GrimWellington (corrected) SEASALT AEROSOL MASS CONCENTRATION Modeled vs. observed annual concentrations From IPCC (2001) intercomparison

30 GISS GSFC 25 LLNL

) ULAQ -3 ECHAM 20 PNNL

15

10 Modeled [SSA]/(µg m 5

0 0 5 10 15 20 25 30 Observed [SSA]/(µg m-3) SENSITIVITY OF MODELED TOA FLUX TO SSA Difference between modeled and measured annual TOA outgoing shortwave radiation -- Haywood, Ramaswamy & Soden, Science, 1999 No aerosol Pronounced deficit in modeled flux in areas of high surface winds.

All aerosols Difference reduced by factor of 2 except sea salt indicates importance of non-sea-salt aerosols. All aerosols; Low sea-salt aerosol burden leads low sea salt to further improvement.

All aerosols; High sea-salt burden eliminates high sea salt difference in SH but overestimates mid-to-high latitude NH oceans. All aerosols Ignoring sulfate yields good except sulfate; agreement almost everywhere. high sea salt Anthropogenic sulfate aerosol over oceans may be overestimated. SEASALT AEROSOL MASS SCATTERING EFFICIENCY At 80% RH; dependence on particle radius at 80% RH

λ = 0.532 µm

The mass scattering efficiency depends strongly on particle radius. IMPORTANCE OF SSA IN INFLUENCING FORCING OF CLIMATE CHANGE BY AEROSOL SPECIES OTHER THAN SSA UPTAKE OF SULFATE AND NITRATE BY SEA SALT

'

A, nitrate, partial conversion B, B' sulfate C, nitrate

Buseck and Pósfai, PNAS, 1999 SUPPRESSION OF NUCLEATION BY SSA

By providing a sink for H2SO4, SSA particles can keep concentrations sufficiently low to suppress new particle formation, changing the size distribution of the concentration of the resultant marine aerosol.

If [H2SO4] is below the If [H2SO4] exceeds the nucleation nucleation threshold, then threshold, nucleation occurs nucleation is suppressed. forming new particles.

SSA SSA OH OH

H2SO4 H2SO4

SO2 SO2 H2O NH3 ≈ × 6 -3 Nucleation is suppressed if [H2SO4] < [H2SO4]* 3 10 cm (Napari et al., JGR, 2002). SOURCES AND SINKS OF H2SO4 AND CRITICAL SSA CONCENTRATION kr OH + SO2 → H2SO4 kmt H2SO4 + SSA → H2SO4áSSA

dHSO At steady state 24==0kk [OH ][ SO ] − [ H SO ][ SSA ]. dt r2mt24

> kr2[][]OH SO Nucleation is suppressed if kmt[]SSA . []*HSO24 = π For diffusion limited uptake kDRmt[]SSA 4 g where Dg = diffusion coefficient and R = SSA “radius concentration,” Rrdndrdr= ∫ ( / log ) log .

× 6 -3 -1 For [OH] = 3 10 cm and SO2 = 0.02 nmol mol (ppb) the critical radius concentration R* ≈ 4 µm cm-3, typical of SSA (1-10 µm cm-3). UPTAKE OF CONDENSABLE GASES ON SEA SALT AEROSOL Dependence on particle radius for “canonical” size distribution; RH = 80%

10-2 n / 103 cm-3

10-3 (d τ-1 / d log r) / s-1

10-4

10-5 -3 -1 kmt / cm s

-6 10 2 3 4 5 6 7 8 9 2 3 4 5 6 7 8 9 2 0.1 1 10 r80/µm

Mass adds preferentially near r80 = 1 µm. UPTAKE OF CONDENSABLE AND REACTIVE GASES ON SEA SALT AEROSOL Dependence on particle radius for “canonical” size distribution; RH = 80%

10-2 n / 103 cm-3

10-3 (d τ-1 / d log r) / s-1

10-4

10-5 -3 -1 kmt / cm s

-6 10 2 3 4 5 6 7 8 9 2 3 4 5 6 7 8 9 2 0.1 1 r /µm 10 80

λ = 0.532 µm

In clear air mass adds preferentially near r80 = 1 µm.

In clouds mass adds preferentially near r80 = 0.3 µm. DEPENDENCE OF CLOUD ON CLOUD DEPTH Influence of Cloud Drop Radius and Concentration CLOUD LIQUID WATER PATH 1 g m-2 10 g m-2 100 g m-2 1000 g m-2 1.0

-3 0.8 LWC = 0.3 g m -3 g = 0.858 -3 -3 0.6

cd cd cd N = 1120N cm = 140 cmN = 17.5 cm 0.4 8 µ r = 4 16 m 0.2

0 1 m 10 m 100 m 1 km 10 km CLOUD DEPTH Twomey, Atmospheric Aerosols, 1977 For a given liquid water path, cloud albedo is highly sensitive to cloud drop number concentration or radius. SENSITIVITY OF ALBEDO AND FORCING TO CLOUD DROP CONCENTRATION (Global MeanShortwaveForcing,Wm

0.14 0.025 0.08 (Top-of-Atmosphere Albedo) 8

0.12 0.020 (Global MeanAlbedo) 0.10 0.06 6 0.015 0.08 0.04 4 0.06 0.010

(Cloud-Top Albedo) (Cloud-Top 0.04 0.02 0.005 2 0.02 0.00 0.00 0.000 0 1.3 2 2.7 3 4 5 6 -2

1 ) 1 Relative Number Density of Cloud Drops Schwartz and Slingo (1996) Indirect forcing is highly sensitive to small perturbations in cloud drop concentration. A 30% increase in cloud drop concentration results in a forcing of ~1 W m-2. INDIRECT (TWOMEY) FORCING Dependence on incremental cloud drop concentration ∆N and Sensitivity to initial cloud drop concentration N0  NN+ ∆∆  N F /(W m-2 )≈ 441 ln 0  =+ ln      N00N

-2 12

10 -3 N0 / cm 8 10

6 30 4

2 100

Global Average Forcing, W m 0 0 50 100 150 200 Change in Cloud Drop Concentration ∆N, cm-3 RESEARCH REQUIREMENTS FOR SSA PRODUCTION Accurate SSA mass production flux and its dependence on controlling parameters. Necessary to model SSA mass concentration (but insufficient). Accurate size-dependent SSA production flux and its dependence on controlling parameters. Necessary to model SSA size dependent concentration Necessary to determine optical properties and radiative influences. Necessary to model SSA influences on clouds and . Essential to model direct effects of anthropogenic aerosols. Essential to model indirect effects of anthropogenic aerosols. The sensitivity of aerosol direct and indirect forcing to size-dependent SSA concentration necessitates accuracies of ×÷ 2 or better.