MODEL SIMULATION OF BrO CHEMISTRY AT THE DEAD SEA, ISRAEL

Transcription

1 MODEL SIMULATION OF BrO CHEMISTRY AT THE DEAD SEA, ISRAEL Eran Tas, Valeri Matveev, Menachem Luria and Mordechai Peleg The Hebrew University of Jerusalem Institute of Earth Science Air Quality Research Laboratory

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3 Distance (km) Haifa ISRAEL N Hadera 32 sea Mediterranean Tel-Aviv Nablus JORDAN Ashdod Jerusalem Ashkelon Be'er-Sheva Hebron Dead Sea 31 EGYPT

4 Ozone Levels (ppbv) over Central Israel and the Dead Sea 22 2 Netanya 31/7/97 14:4-17: km N Netanya 3/8/97 14:-16: Tel-Aviv 16 Tel-Aviv 14 Ashdod Jerusalem 14 Ashdod Jerusalem Dead Sea Dead Sea Beer-Sheva Beer-Sheva /8/97 14:-16: /8/97 15:15-17:45 2 Netanya 2 Netanya Tel-Aviv 16 Tel-Aviv 14 Ashdod Jerusalem 14 Ashdod Jerusalem Dead Sea Dead Sea Beer-Sheva Beer-Sheva

5 Ozone Destruction 2Br + 2O 3 2BrO + 2O 2 BrO + BrO Br + Br + O 2 Net 2O 3 3O 2

6 Qalya N Distance (km) sea Mediterranean Tel-Aviv Ashdod Ashkelon Be'er-Sheva Haifa Hadera ISRAEL Nablus Jerusalem Hebron JORDAN Metzoke Dragot (elevated site) Massada Ein Bokek Ovnat Mitzpeh Shalem Water channel Ein Gedi Neveh Zohar Dead Sea Works Peninsula 31EGYPT Dead Sea DOAS site DS South Ein Tamar Evaporation ponds

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8 Halides in Dead Sea Water and Enrichment Factors Site ph Cl - (g/l) Br - (g/l) Br - /Cl - x 1-3 Br - EF DS/OW Ocean Water North DS Mitzpeh Shalem Mid DS Evaporation ponds I Evaporation ponds II South DS

9 DOAS: Differential Optical Absorbance Spectrometer spectrometer mirror Xe lamp reflectors

10 DOAS: Gases absorb light at specific wavelength Optical absorbance, up to 8km path length Xe lamp; transmit/receive telescope spectrometer: 124 channels, 6nm window Computerized Spectra fit to standards Detection Limit order of subppb-ppt Important for BrO radical Only way to measure NO 2 directly

11 BrO measurements August 5, 21, 17:25.98 sp.# 364 BrO Ref λ [nm]

12 Reflector

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16 16 14 Ovnat -North Dead Sea 22 2 Ein Bokek - Mid Dead Sea BrO Levels (ppt) BrO Levels (ppt) Hour (UT+2) Hour (UT+2) 18 Evaporation Ponds - South Dead Sea BrO Levels (ppt) Hour (UT+2)

17 BrO Ozone August 3 21 North DS BrO (ppt) O3 (ppb) Mid DS South DS Hour (UT + 2)

18 16 Evaporation Ponds - December, Evaporation Ponds - South Dead Sea BrO Levels (ppt) BrO Levels (ppt) Hour UT Hour (UT+2)

19 Ein Tamar South of Dead Sea BrO (ppt) BrO (ppt) Hour (UT+2)

20 1 Metzoke Dragot - June, 22 8 BrO Levels (ppt) Hour UT+2

21 NO2 (ppb) August 5, 21 Dead Sea South BrO (ppt) O3 (ppb) Time (hr)

22 BrO (ppt) 5 Dead Sea South August 21 5 Aug. 3 8 Metzoke Dragot June 22 8 June O3 (ppb) O3 (ppb) 4 O3 (ppb) BrO (ppt) O3 (ppb) BrO (ppt) Y = -.33X R 2 =.87 Y = -1.28X R 2 = BrO (ppt) 6 Ein Tamar Aug BrO (ppt) Y = -.71 * X + 56 R 2 = Aug. 25 O3 (ppb) 3 2 O3 (ppb) BrO (ppt)

23 5 Dead Sea South August 21 Y = -.8*X R 2 =.77 5 Ein Tamar August 22 Y = -.9*X R 2 = NO2 (ppb) 3 2 NO2 (ppb) BrO (ppt) BrO (ppt) 5 4 Metzoke Dragot June 22 Y = -.91 * X R 2 =.96 NO2 (ppb) BrO (ppt)

24 1 8 Dead Sea - Ein Gedi June 24 NO2 (ppb) BrO (ppt) Time (hr)

25 Measurement Conclusions BrO present all along the Dead Sea Valley The BrO levels observed at the Dead Sea (~15 ppt) are higher by a factor of ~1 as compared to measurements reported for other sites Elevated BrO levels were observed both during the summer and winter BrO was also observed at a site some 5 km south of the Dead Sea with a maximum measured level around1 ppt

26 Measurement Conclusions (cont) BrO was also observed at an elevated site some 4 meter above the Dead Sea water level. Maximum concentrations recorded 6 ppt Frequency & intensity of BrO formation more pronounced towards the south (over the evaporation ponds)

27 Why are BrO levels high at the Dead Sea? High Br - concentrations in the Dead Sea waters (up to.4 µg/m 3 Br - and 12 µg/m 3 Cl - measured in aerosols) Low ph values of Dead Sea waters (between 6 and 5) Reaction of BrO with NO 2 to form HNO 3 and further lower ph (NO 3 in aerosols - avg.6 µg/m 3 ; max. 12 µg/m 3) Elevated sulfate levels (SO 4 2 µg/m 3) large surface area low ph in aerosols - avg. 1 µg/m 3 ; max.

28 Model description One-Dimensional Chemical Transport Model The model takes into account: The vertical motion of the different species based on diffusion and advection calculations Deposition velocity values Explicit gas phase chemical mechanism Photochemical reactions Heterogeneous reactions parameterization Treatment of temperature according to Arrhenius equation Pressure dependent reactions

29 Model calculations for each species at each grid point and time interval (Concentration) time = t Boundary emissions Horizontal advection Vertical diffusion Vertical advection Chemical transition Dry deposition (Concentration) time = t+dt

30 C t i = w C z i + z K ( z) C z i + q i + p i C l i i Ci w - mixing ratio of species i - vertical wind speed component Z - vertical height K ( z) - exchange coefficient at height q i - sum of emissions fluxes or advection fluxes for species i P- gas phase chemical production for species i i Cli - gas phase chemical loss

31 Model Inputs Basic photochemical processes based on 166 gas-phase reactions. Bromine gas phase mechanisms described by 32 reactions Hydrocarbon species (13), NO and NO 2 fluxes, from ground level up to boundary layer based on measurements.

32 Model Inputs (cont) Br 2 flux of 1 molecules.cm -2.s -1 added as initiation species from ground level upwards. Meteorological parameters representative of actual real conditions for simulation days were obtained from a 1-d model. Meteorological conditions, fluxes, solar data and heterogeneous parameterizations were up-dated at 15 minute intervals.

33 Reaction # G1 G2 G3 G4 G5 G6 G7 G8 G9 G1 G11 G12 G13 Bromine gas phase reactions Br + O 3 BrO + O 2 BrO + BrO Br+Br+O 2 BrO + BrO Br 2 +O 2 Br + HO 2 HBr + O 2 BrO + HO 2 HBr + O 3 Br + HO 2 HOBr + O 3 BrO + OH Br 2 + HO 2 BrO + OH HBr + O 2 Br 2 + OH HOBr + Br HBr + OH H 2 O + Br Br + NO 3 BrO + NO 2 BrO + NO 3 BrOO + NO 2 OBrO + NO BrO + NO 2 Rate Constant (cm 3 molecules -1 s * * * * * * * * * * * * * 1-12

34 Reaction # G14 G15 G16 G17 G18 G19 G2 G21 G22 G23 G24 G25 G26 Bromine gas phase reactions BrO + NO 2 BrONO 2 Br + NO 2 BrNO 2 BrO + NO Br + NO 2 BrO + CH 2 O HBr + HCO Br + C 2 H 2 C 2 H 2 Br Br + C 2 H 4 BrC 2 H 4 Br + CH 3 CHO HBr + CH 3 CO Br + CH 3 O 2 BrO + CH 3 O Br + C 3 H 6 HBr + C 3 H 5 Br + C 3 H 6 BrC 3 H 6 BrO+CH 3 O 2 Br + CH 3 O + O 2 BrO+CH 3 O 2 OBrO + CH 3 O BrO+CH 3 O 2 HOBr + CH 3 O 2 Rate Constant (cm 3 molecules -1 s * * * * * * * * * * * * * 1-12

35 Reaction No. GP1 GP2 GP3 GP4 GP5 GP6 Gas phase photolytic reactions Br 2 Br BrO Br + O 2 HOBr Br + OH BrONO 2 Br + NO 3 BrNO 2 Br + NO 2 OBrO BrO + NO 2 Rate Constant (s-1) 3.12 * * * * * *1 -

36 August 9, 21 Julian Day BrO (ppt) 6 3 O 3 (ppb) Hour (UT+2)

37 .5 BrO ppbv) O3 (ppbv) Time (hours) BrO O 3

38 .12 H2 (ppbv) H1 (ppbv) HOM (ppbv) FUL (ppbv) Time (hours) Diurnal profiles of BrO obtained in different simulations BrONO 2 + H 2 O HOBr + HNO 3 HOBr + H + + Br - Br 2 + H 2 O (H1) (H2)

39 BrONO2 NO 2 BrNO 2 NO 2 H 2 O Aerosol BrONO 2 ( g ) HOBr ( g) BrO NO O3 hν hν Br hν hν hν RH Br 2 HO 2 HOBr HBr HOBr (g) Br 2 ( g ) + HOBr H Br (aq) Br 2 ( aq ) Aerosol SEA SALT AEROSOL PRODUCTION

40 .12 Julian day Julian day Julian day Time (hours) BrO concentrations (ppbv) simulations vs measured BrO BrO obtained by simulations BrO obtained by measurements

41 Cycle 1 Br 2 + hν 2 Br Br + O 3 BrO + O 2 BrO + BrO 2BrO + O 2 (GP1) (G1) (G2) 2O 3 3O 2 Cycle 2 BrO + HO 2 HOBr + O 2 HOBr + hν OH + Br Br + O 3 BrO + O 2 (G6) (GP3) (G1) HO 2 + O 3 OH + 2O 2

42 pptv E-5 1E Time (hours) Diurnal profile of bromine species for 'FULL' simulation HBr BrONO 2 Br BrO HOBr BrNO 2 Br 2 OBrO

43 Cycle 3 BrO + NO 2 BrONO 2 BrONO 2 + H 2 O HOBr + HNO 3 (a) HOBr OH+ Br (b) HOBr + H + + Br - Br 2 + H 2 O Br 2 2 Br Br + O 3 BrO (G14) (H1) (GP3) (H2) (GP1) (G1)

44 5E-6 4E-6 [H2] t 3E-6 (ppbv/min) 2E-6 1E [H1].4 t (ppbv/min) BrOx (ppbv) Time (hours) The dependence of BrOx production on cycles 3.b and 3.a [H2] Line/Symbol Plot 12 t [H1] Line/Symbol Plot 13 t fghfghf BrOx

45 1 8 Dead Sea - Ein Gedi June 24 NO2 (ppb) BrO (ppt) Time (hr)

46 Metzoke Dragot June NO2 (ppb) NOz (ppb) BrO (ppt)

47 pptv E-5 1E Time (hours) Diurnal profile of bromine species for 'FULL' simulation HBr BrONO 2 Br BrO HOBr BrNO 2 Br 2 OBrO

48 1 (ppbv) -1-2 (b) -.2 (ppbv) (a) Time (hours) (a) - Only H2, without H1 (b) - FULL HNO 3 (FULL- Without Br Chemistry) NO 2 (FULL- Without Br Chemistry)

49 CONCLUSIONS: In order to obtain compatibility with measurements at the Dead Sea, the model needs to include two heterogeneous processes: The release of Br - into the gas phase via the Bromine Explosion mechanism which occurs in sea salt aerosols. The decomposition of BrONO 2 that occurs on the surface area of sulfates aerosols. These processes, only if included together as illustrated in cycle 3b, can explain the efficient BrOx production and O 3 destruction at the Dead Sea. The entrainment of O 3 fluxes from outside the area was found to be essential for continuation of RBS activity and O 3 destruction, especially after O 3 levels are beneath ~1-2ppb.

50 CONCLUSIONS (cont.) The significant contribution of the heterogeneous decomposition of BrONO 2 to BrOx production causes NO 2 to have an important influence in controlling and a limiting factor in BrOx production and recycling, O 3 destruction and the diurnal profile of Br and BrO. An essential link in these efficient heterogeneous processes is the heterogeneous decomposition of BrONO 2. The high contribution of the heterogeneous decomposition of BrONO 2 is due to the faster production of HOBr, by a factor of ~4. Model simulations indicate that the release of Br 2 by activation of HOBr may occur more directly from the sea water and not only from the sea salt aerosols.

51 CONCLUSIONS (cont.) The heterogeneous decomposition of BrONO 2 leads to an increase in HNO 3 and an anti-correlation with NO 2. The formation of HNO 3 explains the measured increase in NO z concentrations simultaneously with NO 2 decrease, during RBS activity. The contribution of BrONO 2 and BrNO 2 to NO z is small and not more than 25% of total NO z.