Chemical characteristics of aerosols over Arabian Sea & Bay of Bengal: Impact of Anthropogenic Sources. Manmohan Sarin

Transcription

1 Chemical characteristics of aerosols over Arabian Sea & Bay of Bengal: Impact of Anthropogenic Sources Manmohan Sarin Physical Research Laboratory Ahmedabad , India 29th April 09 GESAMP 36 Annual Meeting: WMO, Geneva

2 Rationale: Climate Change: One of the most important societal concerns for the 21 st century Atmospheric Chemistry: Plays a critical role -Abundances & distributions of natural and anthropogenic constituents Greenhouse gases, aerosols and clouds -Influence incoming and/or outgoing solar radiation, temperature and precipitation

3 Climate Forcing by Anthropogenic Aerosols Key issue : Role of atmospheric chemistry in amplifying/damping climate change Questions : What are the effects of aerosols on clouds, their optical properties, precipitation and regional hydrologic cycle?

4 African drought Rosenfeld et al [2001]: Correlation between increasing dust frequency & Rainfall in Sahelian region (African Monsoon) (from 1950s through 1980s) Conclusion: Dust aerosols act as cloud condensation nuclei (CCN) Increased dust frequency Cause of decreased precipitation (cloud life-time effect)

5 Atmospheric Dust, Chemistry and Transport Variety of individual minerals (quartz, calcite, gypsum, clays, etc.) (different physical/chemical properties) Requires quantification of its global impacts Information on dust emission sources, transport, particle size and compositional data

6 Mineral Dust Atmospheric chemistry is over-simplified in current models (Global transport/chemistry model & dust model) Considered as single entity aerosol with single kinetic parameter for chemical reactions

7 Mineral dust & Atmospheric Chemistry O3 NO NO2 DUST HNO3 SO2 OH NO3 Alkaline nature favours uptake of SO 2 and NO x Changes surface properties: Hydrophobic Hydrophilic Ozone decomposition on mineral surface (2O 3 3O 2 ) ( mediated via POC, Organic complexes, Fe & Mn)

8 Mineral dust : Recent work * Provide stoichiometric/catalytic reaction centers - to perturb important gas-phase cycles * Enhances kinetics of SO 2 /NO 2 oxidation * Perturbation of photo-oxidation cycles of: O 3, HNO 3 & HO 2* -- radicals * Heterogeneous loss of organic compounds: Acetaldehyde, acetone. Comparable to loss by direct photolysis

9 Influence of Dust on Sulphate Asian regions: High SO 2 Present estimates of SO 4 cooling ignore reactions with mineral aerosols CaCO 3 + H 2 SO 4 CaSO 4 + H 2 O +CO 2 (fine) (coarse) Overestimate the forcing during high dust season

10 25 ORV Sagar Kanya Cruise Tracks Feb 2001 Latitude ( o N) A r a b i a n S e a I N D I A Goa Chennai B a y of B e n g a l 22 Mar Sept- 12 Oct Feb Maldives 0 I N D I A N O C E A N Longitude ( o E)

11 BOB Mean Wind Streamlines (March 2001)

12 EC (BC), OC, IC, K +, NO 3- and SO 4 and trace elements

13 MODIS AOD for Dec. month

14 Objectives 1. Characterize the temporal and spatial variability in chemical and isotopic composition of aerosols. 2. How this variability is related to atmospheric transport processes (physical and chemical). 3. Deposition fluxes of natural and anthropogenic constituents across the air-sea interface. IMPLICATIONS: Long-range transport of air pollutants from the south & south-east Asia towards the Indian Ocean.

15 Experiment : Sampling : Aerosols samples collected on quartz filters (using high-volume air samplers) Analysis : Water soluble constituents : NH 4 +, Na +,K +,Mg 2+,Ca 2+,SO 4,Cl -,NO 3 - Acid soluble constituents : Al, Fe, Ca, Mg, Mn, Cu, Zn, Pb Atmospheric nuclides : 7 Be (53.3 d) & 210 Pb (22.3 yr)

16 Total Ca 2+ (μg m -3 ) Dust & Biogenic Sources 4 March :1 nss-ca 2+ (μg m -3 ) Arabian Sea Indian Ocean Total Ca 2+ (μg m -3 ) nss-ca 2+ (μg m -3 ) Bay of Bengal Feb-March 2001 Feb :1

17 Dominance of Anthropogenic Sources nss-so 4 (μg m -3 ) nss-so 4 (μg m -3 ) March 2001 Arabian Sea Indian Ocean Total SO 4 (μg m -3 ) Bay of Bengal :1 Total SO 4 (μg m -3 ) 1:1 Feb-March 2001 Feb 2003

18 Average Composition: Water-soluble Constituents SO 4 66% SO 4 60% Bay of Bengal Na + Cl- Feb, 2001 Ca 2+ Σ = 7.6 (μg m -3 ) Na K + NO - 3 Mg 2+ NH 4 + Σ = 9.2 (μg m -3 ) Feb, Na + NH + 4 Cl - K + Ca 2+ NO 3 - Mg 2+ 30% 16% SO 4 45% NO K + Ca 2+ Mg 2+ Cl - Σ = 12 (μg m -3 ) Sep, 2002

19 SO 4 in marine boundary layer (MBL) Sea-salts Biogenic: DMS SO 2 SO 4 Anthropogenic : SO 2 SO 4 Dust, CaSO 4 (gypsum) (cement industries) Biomass Burning? Volcanic (transient) oxi nss-so 4 = ΣSO Na + (non-sea-salt) SO 4 / Na + in sea water = 0.25 oxi oxi

20 E.F. (w.r.t. Fe) Bay of Bengal 2001(Feb) 2001(Mar) 2003(Feb) Cd Pb Zn E. F. (w.r.t. Fe) Bay of Bengal (Feb) Bay of Bengal (Mar) Indian Ocean Arabian Sea 1 Cd Pb Zn

21 (SEAREX data) Arabian Sea Pacific Ocean Indian Ocean Bay of Bengal Fe Al Av. Conc. (ng m -3 )

22 The cruise transects carried out in the Bay of Bengal and Arabian Sea for collection of bulk-aerosol samples during spring intermonsoon (March-May)

23 Average water-soluble composition of aerosols over Bay of Bengal and Arabian Sea, reflecting dominance of SO 4 Arabian Sea Bay of Bengal SO 4 Σ = 11 μg m -3 Σ = 9 μg m -3 47% NO - 3 5% 2% Mg2+ 1.5% K + SO 4 68% 1% 2% NO 3 - Mg 2+ + NH 10% 4 Na + 18% 15% 11 % Ca 2+ 9% 3% K + 7.5% Ca 2+ Cl - Na + Kumar, AKS, MMS (JESS 2008)

24 nss-ca 2+ (μg m -3 ) r 2 = 0.91, m = 0.87, ARS r 2 = 0.97, m = 0.97, BOB 1:1 line Ca (μg m -3 ) Enhanced solubility of Calcium as dust (CaCO 3 ) undergoes neutralization process with acidic species

25 Unequivocal evidence: Acid uptake by mineral dust HCO 3 - /(nss-ca 2+ + nss-mg 2+ ) Ahmedabad Excess Acid/(nss-Ca 2+ + nss-mg 2+ ) Nov-Feb Mar-Apr CaCO 3 + H 2 SO 4 CaSO 4 + CO 2 + H 2 O CaCO 3 + 2HNO 3 (neutralization reactions) Ca(NO 3 ) 2 + CO 2 + H 2 O Atmos. Environ, 2005a

26 Rainwater 40 Number of samples n = ph Rainwater ph Avg. ph = 6.7 ± 0.5 indicating Alkaline nature of rainwater Atmos. Environ, 2005b

27 HCO 3 - /(nss-ca 2+ + nss-mg 2+ ) Ahmedabad Excess Acid/(nss-Ca 2+ + nss-mg 2+ ) Nov-Feb Mar-Apr IMPLICATIONS: Change in size distribution of SO 4 and NO 3 - aerosols from fine to coarse mode Decrease in Cloud Condensation Nuclei Present-day models need to account for such chemical reactions to infer climate cooling due to Sulphate aerosols Atmos. Environ, 2005a

28 Chloride depletion in MBL Complete degassing of HCl from NaCl (Deliquescent sea-salt aerosols) % RH, buffered ph ~ oxi nss-so 4 SO 2 (S +4) SO 4 (S +6) NaCl + H 2 SO 4 2HCl + Na 2 SO 4 NaCl + HNO 3 HCl + NaNO 3 Consequence of decreasing aerosol ph as droplets evaporate HCl(aq.) reaches saturation HCl (g) Aqueous phase gas phase

29 SO 4 -rich aerosols: Implications to Cl - loss Cl - deficit (%) nss-so 4 (%) Feb 2001 Sept 2002 Feb 2003

30 Implications : (SO 2 ) SO 4 Enhanced anthropogenic (SO 2 ) SO 4 Sea-salt Sink limit CCN Not adequate Increase Ozone loss : oxi. 1. O 3 + S iv (SO 2 ) SO 4 + O 2 [major O 3 sink] (at ph : ~6-8, Rate constant 4 x ) 2. O 3 + Cl - oxi. ClO - + O 2 [slow rate] HCl degassing from sea-salt (Unaccounted O 3 sink!)

31 High-altitude Stn: Temporal variability of aerosol iron (Fe A ) and water-soluble iron (WS-Fe) Jan 04 Jan 07 Jan 10 Jan 11 Jan 12 Jan 16 Jan 20 Jan 29 Jan 01 Feb 08 Feb 11 Feb 16 Feb 17 Feb 26 Feb 27 Feb 05 Mar 10 Mar 15 Mar 20 Mar 25 Mar 01 Apr 05 Apr 09 Apr 12 Apr 17 Apr 21 Apr 23 Apr 25 Apr 29 Apr 03 May 07 May 11 May 15 May 17 May 19 May 24 May 28 May 02 Nov 07 Nov 10 Nov 20 Nov 27 Nov 03 Dec 08 Dec 13 Dec 18 Dec 23 Dec 28 Dec WS-Fe (%) Winter Summer Winter Year Jan 04 Jan 07 Jan 10 Jan 11 Jan 12 Jan 16 Jan 20 Jan 29 Jan 01 Feb 08 Feb 11 Feb 16 Feb 17 Feb 26 Feb 27 Feb 05 Mar 10 Mar 15 Mar 20 Mar 25 Mar 01 Apr 05 Apr 09 Apr 12 Apr 17 Apr 21 Apr 23 Apr 25 Apr 29 Apr 03 May 07 May 11 May 15 May 17 May 19 May 24 May 28 May 02 Nov 07 Nov 10 Nov 20 Nov 27 Nov 03 Dec 08 Dec 13 Dec 18 Dec 23 Dec 28 Dec Fe A (ng m -3 ) 0 Year-2007 Bio-available Fe & surface ocean bio-geochemistry

32 Study sites SW-winds (May-Aug) Arabian Sea NE-Winds (Nov-Feb) Mt Abu, ~1700 m asl, clean site Ahmedabad, 49 m asl, a megacity Rainfall ~650 mm: Jun-Aug Bay of Bengal

33 m = 1.1 r = 0.95 NH 4 + (neq m -3 ) Summer Winter WS-Fe (%) nss-so 4 (neq m -3 ) nss-so 4 (μg m -3 )

34 Solubility of aerosol iron and Direct emission (fossil fuel combustion) 10 8 Summer Winter Cd/Al (x1000) WS-Fe (%) Fe A (ng m -3 )

35 WS-Fe (%) WS-Fe = 0.06 % and Fe A = 915 ng m -3 during summer season (as end member); WS-Fe of 16.1 % and Fe A = 50 ng m -3 (2nd end member) during winter season Fe A (ng m -3 ) Kumar and Sarin Tellus B, 2009 (Under Review) Fe solubility in semi-arid western India is controlled by composition and chemical nature of aerosols rather than chemical processing during long-range transport.

36 Nanoparticles: Photochemical mechanism + e - Hematite Fe 3+ Generate electronhole pairs Mineral Aerosols C 2 O 4 HSO 3 - Oxidation CO 2 SO 4 By product: Fe 3+ Fe 2+ Dissolution Reduction (cloud droplet)

37 Bay of Bengal (March-April 06)

38 Latitude( 0 N) Spatial distributions of Water SolubleFe(%) onboard ORV sagarkanya Cochin ICARB- Campaign Chennai March Bay of Bengal Longitude( 0 E)

39 Latitude( 0 N) Spatial distribution of WSFe(%) during April Goa Cochin Arabian Sea 0.02 onboard ORV Sagara Kanya ICARB- Campaign Longitude( 0 E)

40 25 Enrichment.Factors. of Cu from Mar-Apr06 ICARB-Campaign Latitude( 0 N) Visakhapatnam Bay of Bengal Longitude( 0 E)

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42 Conclusions : 1. Long-range transport of dust, biogenic and pollution-derived components dominate the aerosol composition in the MABL of Bay of Bengal & Arabian Sea. 2. On average, non-sea-salt-so 4 accounts for nearly 60% of the water soluble components indicating highly acidic environment (MABL of BOB). 3. Enhanced Fe deposition fluxes over Bay of Bengal and Arabian Sea could serve as dominant source for surface waters.