O3, CO, Hydrocarbons and dimethyl sulfide over the Western Atlantic Ocean

m

Armspheric Enprronmmr Vd. 22. Irio. 11. pp. 2401 2809.1988. Printed in Great llritam.

0,,

CO, HYDROCARBONS AND D~~ET~YL SULFIDE THE WESTERN ATLANTIC OCEAN

6981/88 S3.M+O.w Pergamtwl Press plc

OVER

CHARLESC. VAN VALIN and MENACHEM LURIA* Air Quality Group, GMCC, Air Resources Laboratory, National Oceanic and Atmospheric Administration, Boulder. CO 80303, U.S.A. (First received 2 Ocroher 1986 and in ftnuili)rm I3 July 1987) con~ntrations of 0,. CO, dimethyi sulfide (DMS) and light hydrocarbons (C,-C,) were measured from an instrumented aircraft during February-April 1985, near the U.S. East Coast and in the vicinity of Bermuda as part of the Western Atlantic Ocean Experiment (WATOX). Sampling Rights were performed within the boundary layer (BL) and in the free troposphere(FT) at both locations. Photochemical generation of 0, in polluted air parcels transported from the continent within the BL was identi~ed as the probable source of excess 0, (up to 50 ppbv above background). Convective lifting of boundary layer air carried pollutants into the free troposphere. The concentrations of HC compounds in air sampled near Bermuda had a significant inverse relation to air mass transport time from the continent. The BL concentrations of the more reactive HCs (ethylene, propane, propylene, normal- and isobutane) declined faster than the less reactive HCs (acetylene and ethane). and were found to be proportional to air mass transport time over the ocean. DMS was detected, with few exceptions, only within the BL at both sampling locations. The average concentrations in the BL samples collected near the U.S. East Coast and in the vicinity of Bermuda were 27 and 54 pptv. In all samples taken in the BL the DMS con~ntration decreased sharply as a function of altitude. Abetract-The

Key word index: Non-methane hydrocarbons, ozone, carbon monoxide, dimethyl sulfide, boundary layer transport.

INTRODtJCTlON

During the 1985 Western Atlantic Ocean Experiment (WATOX), an instrumented aircraft was used to measure the transport of gases and aerosols eastward from the North American ~ntinent. Samples were taken with the aircraft between February 27 and March 26 over the Atlantic Ocean some 100 km east of Newport News, Virginia, and during the period 2-l 1 April near Bermuda. In concurrent papers Galloway et al. (1988) describe the objectives of WATOX, Pueschel et al. (1988) report on the aerosol measurements, Luria et al. (1987) discuss the SO2 data and Wellman et al. (1988) describe the aircraft systems. In this report we present the results of measurements of 0,. CO, hydrocarbons Cr-C, (HC) and dimethyl sulfide (DMS). We focus on two issues: a comparison between pollutant levels at the two locations in the boundary layer (BL) and in the free troposphere (FT), and the relationship between O,, HC and DMS, and other measured parameters.

EXPERIMENTAL The parameters measured, as included in this report, were wind direction, wind velocity and aircraft position by

* Permanent address: Environmental Sciences Division, The Hebrew University, Jerusalem 91904, Israel.

LORAN C, pressure, temperature, 0, concentration and aerosol size distributions (Particle Measuring Systems, Inc. ASASP-100X and FSSP-100 probes) for particle diameters from 0.1 to 47 pm (Wellman et al., 1988). Fifty-eight flask air samples were collected during this study; 22 near the U.S. East Coast and 36 near Bermuda. The flask samples were analyzed for Cz-C, HC, CO and DMS, usually within 72 h, by contract with R. A. Rasmussen, Biospherics Research Corp., Hillsboro, Oregon. The flask collection system was developed and reported by Rasmussen er cf. (19&b); the HC and CO analysis methods were described in the same reports. Validation of the DMS analysis method was described in Van Valin et al. (1987).The principal elements of the flask system are as follows: internally electropolished 3.2 t stainless sleel cans, previously filled with pure air, were Rushed with ambient air for 2-3 min and then pumped to an overpressure of 2-3 standard atmospheres with a stainless steel bellows pump. Analysis was by gas chromatog~phy with a flame photometric detector for the DMS and a Rame ionization detector for the HC and CO: CO was reduced to CH, prior to analysis. The 0, concentration was monitored using a Dasibi 1003AH photometric continuous monitor. The instrument was calibrated before and after the study by comparison with an NBS traceable 0, photometer. The differences between the two calibrations were < 1 ppbv. During the experiment, zero point calibrations were performed by scrubbing the 0, from an ambient air stream with activated charcoal and molecular seive. Flights from 7 February to 26 March 1985 were conducted over the Atlantic Ocean 50-150 km offshore from Newport News, Virginia. During 2-11 April 1985, Rights were conducted near Bermuda. The times, altitudes, winds and general observations are summarized in Table 1. Air mass back trajectories were calculated for each sampling day using National Meteorological Center’s griddcd

2401

2402

CHARI.FS c’. VAT VALIN and MFNACHEM LI~RI.~

Table 1. Research

flight summary ---I_

Flight

Date

Time (LST)

Altitude

begin-end

Near Newport

I

Wind [degrees(m s ‘)I

Cm) -_

News, Virginia

2 3 4 5 6 I 8 9 10

27/2/85 27/2/85 02/3/85 0213185 05/3/85 14,‘3,‘85 14/3/85 I8/3/85 I S/3/85 2513185

1040-1400 1710-195s 0845-l 110 1427-1655 1633-1713 1340-1710 213&2300 0826-0945 141@1720 1213-1435

3054570 Y&1525 91-305 1980-2740 760-910 91-1525 2135 152 2135-3660 760-3660

250-290( 14-35) 28&330(11-14) 30&320(10-11) 280-330(8-12) 25@260(17-19) 40-290(6-15) 280(20) 340(12) 310-340(9-14) 6&320(5-19)

II

2613185

124@-1255

1520-3660

320-360( l5--21)

132G1550 1620-1630 1138-1400 1427-1516 0945-l 209 1224-l 330 1254-1410 1420-1524 1412-1627 1636-1739 0437-0613 0627-0822

2130-3050 90-305 1520-3050 91-610 1520-3050 91-460 1520-4570 91-460 244@-3050 180-910 91-305 2440-3050

250(1619) 240(17) 28&310(12-14) 26&270(9-l 1) 140-170(4-8) 1 lo-130(5-8) 10&120(12-15) 150-160(19-25) 240-300( 19-29) 4&l 10(3-10) IOO-240(3-10) 26&300(1&18)

Near Bermuda _____.___ 12 0214185 13

04/4/85

14

05/4/85

15

06/4/g 5

16

10/4/85

17

1 l/4/85

Table 2. Flask sample analyses,

Flight

3 4 6 7 9 10 12 15

Clouds

Wind predominantly

north

BL, East Coast Trace gas concentrations C,H, CBHs C,H, (ppt) (ppt) (ppt)

2712185 2712185 0213185 0213185 05/3/85 05/3/85 14/3/85 18/3/85

1326 1722 0953 1031 1700 1746 1600 0853

305 91 305 91 780 91 1067 152

222 261 253 357 135 165 193 197

4440 4694 4165 3595 2059 1944 4170 2752

269 372 400 840 152 144 133 240

995 1233 1289 1659 373 502 251 888

2206 2424 1756 1977 573 695 1721 1215

223 +69

3477 *lo89

319 +234

898 k494

1571 k681

HC analyses of all flask samples are Tables 2-5, where they are separated into categories of “East Coast” or “Bermuda”, and BL,

up to 1550 m

Cloud top at 2300m

Date

and

in

clouds and light ppt.

S and NO, (a 213’5-2300 m Heavy haze above 3lOOm, cloud top (a: 1830m Scattered cloud ($ 1460 m, S and NO, @ 1520m, haze (4 2100-3100 m

C,H, (ppt)

Carbon monoxide and hydrocarbons

plume (a’ 910 m, cloud deck above

Occasional

C,H, (ppt)

RESULTSAND DISCUSSION

CO

Pollutant

CO (ppb)

analyzed wind field (Harris, 1982). Ten-day back trajectories were calculated for 700,850 and 1fIOOmb. During the time of the research effort at Newport News the air transport direction was from the west to north at velocities such that transport time over the ocean to the study area was from 2-3 h to less than a day. The transport time to Bermuda varied from as little as 3/4 day at the 700 mb level (3050 m) to as much as 7 days at the 1000 mb level (surface).

The

Hazy at 305 m

Altitude (m)

Average concentration Standard deviation

presented

Haze layer at 4570 m

Time EST

Flask

1 2 2 3 5 5 6 7

C‘omments

I’-C,,Hlo

DMS (PPt)

(PPt)

“-C,H,, (PPt)

55 60 55 204 99 35 27 80

591 695 503 700 141 224 419 359

I367 1634 1257 1977 249 497 874 637

24 37 22 35 15 29 18 35

77 k56

454 + 207

1061 k596

27 F8

“pseudo FT”, or FT. The BL is defined as the atmospheric layer of nearly uniform potential temperature (0) extending from the earth’s surface to the first temperature inversion. The FT is the atmosphere above the BL. The term “pseudo FT” is used to distinguish a zone, found above the temperature inversion, where the air mass had stronger similarities to BL air, rather than to other FT Bir at the same altitude during the same flight. The parameters used for comparison were those that were measured continuously, namely, 0, aerosol concentration*and OJ concentration. In the East Coast BL samples (Table 2) the average CO and HC concentrations are 2-3 times the corre-

2403

O,, CO, HCs and DMS concentrations over the Western Atlantic Ocean Table 3. Flask sample analyses, PT and pseudo FT, East Coast Trace gas concentrations Time Plight

PIask

FT samples 71 1 2 4 8 6 11 ;: 10 10 11 11 11 11

14 13 17 18 19 20 ::

Date

EST

Altitude (m)

27/2/85 27/2/85 02/3/85 14/3/8S

1109 1157 1451 1520

4570 3050 2740 1520

119 107 102 107

1248 1177 1332 1362

44 29 31 53

396 300 304 209

260 232 348 343

50 25 15 38

47 47 79 86

115 115


14/3/8S 14,‘3/‘85 25/3/85 25/3/U 26/3/U 26/3/8S 26/3/8S 26/3,J8S

2135 2247 1343 1402 1327 1420 1540 1609

2440 2130 3050 2130 is20 2130 3050 3660

128 t30 121 153 141 148 121 130

1693 1687 1695 2168 2280 2189 2282 2133

67 43 60 59

78 97

461 422 447 618 557 510 371 401

478 516 sss 790 708 669 624 510

I05 23 51 62 71 97

104 112 118 184 153 132 110 91

176 177 191 301 2% 261 185 146


126 116

1710 5425

57 &19

416 1114

503 +i79

51 529

104 &41

171 +75

<5

218 192

6079 2983

158 198

919 874

2837 1319

49 89

638 374

1398 794

IO
205

4531

178

896

2078

69

506

1096

Average Standard deviation Pseudo FT samples -9’?72/85 1550 1856 18/3/8S Average

2290 1520

CO (ppb)

C,D, gppt)

C2H4

(ppt)

fppt)

(Ppt)

‘(ppt)

(PPQ

(PPtI t:

C2W2

C3H,

C3H,

=,Hm

Pig. 1. Air mass back trajectories to Bermuda for the BL (1OOOmb)applying to flights on the six sampling days (2,4, 5,6, 10 and 11April). The numbers represent transport time in days from Bermuda.

~-C&I

Pp;

CHARLES C. VAN VALIN and MENACHEM LIJRIA

2404

sponding FT value (Table 3). Near Bermuda the BL concentrations (Table 4) were 15-2 times greater than in the FT (Table 5). The lowest FT concentrations at both locations are similar to previously reported northern hemisphere background values (Seiler and Fishman, 1981; Rudolph and Ehhalt, 1981; Blake and Rowland, 1986). Since the ocean is not a source of CO and the natural concentration of CO is not particularly altitude-dependent (Seiler and Fishman, 1981) one can assume that the high concentrations of CO in the Berumda BL are due to transport in the BL from the continent. Likewise, the Bermuda BL concentrations of C,H,, which has no known biological sources (Rudolph and Ehhalt, 1981), must be from transport from the continent. With C,H, one can make no assumption, since the ocean may be a source (Rudolph and Ehhalt. 1981).

The seven non-methane HC that are shown in Tables 2-S have been divided into two groups designated “Reactive” (R) and “Less Reactive” (I.). with C2H, and C,H, forming the L group, and C’,H,, C,H,, C,H,, I‘-C,H,, and rt-C,H,, forming the R group; this grouping is based on the reactivity with the HO radical. The rate constant k(H0) is nearly 20 times greater for C,H, than for CH,, and C,H, is I .6 times more reactive than C,H,, but the next least reactive material is C,H,, which is almost an order of magnitude more reactive than C,H,. In Table 6 we show the sums of the average concentrations within the eight groups, converted to C. and indicate the ratios of LI’R at the four locations. In the East Coast BL, which contains the highest concentrations and freshest material, L/R = 0.75. In the East Coast FT the ratio is I .5. whereas at Bermuda the values are 1.5 and 2.9. respectively.

Table 4. Flask sample analyses,

Fligbt 12 12

. Flask 25 26

Date

Altitude (m)

CO (PPb)

C,H, (PPt)

C,H, (PPt)

0214185 0214185

1620 1630

305 91

123 119

1320 1360

86 51

239 251

121

1340

68

157 155 157

2188 2174 2188

36 44 40

156

2183

155 153 151

2043 2030 2085

153 139 139

i-C,H,O (PPt)

n-C,H,, (PPt)

27 21

34 19

DMS (PPt)

148 135

122 50

245

142

86

24

26

59

531 541 522

627 615 620

30 22 51

117 115 117

211 216 213

45 85 32

40

531

621

34

116

213

54

81 46 50

472 495 478

521 527 532

71 45 41

96 90 97

152 151 151

30 42 58

2053

59

482

527

52

94

151

43

1969 1879

51 49

403 371

403 395

38 67

69 65

100 93

38 50

139

1924

50

387

399

52

67

96

44

186 192 189 186 183 183 182 180

2489 2499 2516 2487 2516 2516 2559 2239

93 110 88 96 98 111 93 66

733 825 729 755 705 692 740 732

808 891 860 892 873 883 890 767

23 25 30 27 54 43 28 18

214 255 240 240 232 232 233 210

377 442 397 407 381 381 391 340

25 34 56 66 70 72 83 91

185

2478

94

739

858

31

232

390

62

174 173 176 178 176 178 178 175

2235 2239 2303 2332 2253 2358 2287 2284

64 66 70 72 83 70 72 76

738 732 704 713 709 730 717 754

775 767 781 794 778 799 790 786

14 18 34 33 27 21 30 23

206 210 222 221 218 215 217 211

344 340 356 370 356 366 361 350

26 38 52 58 66 74 74 84

176

2286

72

725

784

25

215

355

59

Average concentration Standard deviation

167 +20

2159 &488

72 +22

616 &166

679 +217

38 i23

169 *75

280 +128

56 *20

Average of Averages Standard deviation

155 k23

2044 +394

64 _+19

518 &192

555 +262

47 +22

125 283

205 +144

54 i8

13 13 13

28 29 30

0414185 04/4/85 0414185

1453 1501 1510

14 14 14

32 33 34

05/4/85 05/4/85 05/4/85

1248 1304 1310

15 15

37 38

0614185 0614185

1452 1502

305 91 610

Average 460 305 91

Average 305 91

Average 41 42 43 44 45 46 47 48

10/4/85 10/4/85 10/4/85 10/4/85 10/4/85 1014185 lOjij85 10/4/85

1658 1710 1718 1724 1730 1736 1745 1754

910 610 460 370 270 180 91 30

Average 17 17 17 17 17 17 17 17

Trace gas concentrations C,H, C,Hs C,H, (PPt) (PPt) (PPt)

Time AST

Average

16 16 16 16 16 16 16 16

BL, Bermuda

49 50 51 52 53 54 55 56

1 l/4/85 1 l/4/85 11/4/85 1 l/4/85 1 lj4j85 1 l/4/85 11;4/85 11/4/85

0508 0514 0522 0526 0532 0538 0546 0552

915 610 460 370 270 180 91 30

Average

48 70

;; 58

31 35

I:

flask

;;

40

::

16

Pseudo FT samplezi

12 12 14 IS 15 17 17

FT samples

Flight

1337 1456 1020 1309 1332 064.5 0744

213a 3050 1520 4570 3050 2440 3050

AItitude (m)

2448 1453 1726 1876 is14

Ill 149 t24

E

1520

1330 f 262

116 it2

1520

1368 1012 1461 tt29 t800 1166 1367

C,H, (Ppt)

120 loo 119 to9 138 113 t12

W4

CO

1235 1450

Average Standard deviation

04/4185 to14/85 to/4/8s

Average Standard deviation

02/4/85 02J4J8S 05[4/as 06/4/B w4Pt5 t t/4/85 1l/4/85

Date

Time AST

39

524 310 426 420 f 107

&Z

35 fl8 E!

E 2ao

280 197 365 237

E 454 524 f249

$s

153 to7 300 122 375 133 232

23 32 14 ;;

*:

E 30

z 28 18

Trace gas ~n~ntratioo~ C&H, C,H, C,H, (PPt) (PPt) @pt)

:;: 39 24 7f 17 22

C,H, f.PPl)

Tabte 5. Flask sample analyses, FT and pseudo FT. Bermuda

15-f 58 92 102 iSO

319 79 153 184 f123

20 C5 <5

<5 -

2 45 &34

::

: 66

36 +20

C5

DMS (Pit)

<5
tPPt)

~-C&3

;: 95 18 93

24 17

i-C,H , o (PPl)

2406

CHARLESC. VAN VALIN and MENACHEM LIJRIA

Table 6. Summary of reactive and less reactive hydrocarbon concentrations R* (PpbCf

Location Average values -~-.-Boundary layer, East Coast Free troposphere, East Coast $ Boundary layer, Bermuda Free troposphere, Bermuda $

Il.6 2.9 3.3 1.1

8.8 4.4 5.1 3.2

*R-Reactive hydrocarbons (C,H,, C,H,, C&i,, &.,H,,, t L--Less reactive hydrocarbons (C,H, and C,H,). $ Less the ‘“pseudo” FT samples.

1raw-iTime(hewn)

Fig. 2. Pseudo first-order decay piots for R (reactive HC), L (less-reactive HC) and CO.

The transport times of the air masses from the continent to Bermuda can be estimated from trajectory analysis (Harris, 1982), and these are shown in Fig. 1. As can be seen, the transport time varied from 38 to 168 h. Combining’this information with the average measured CO, L and R concentrations, the pseudo first-order decay rates were estimated as shown in Fig. 2. The descriptioti of the decay in terms of a first-order rate process does not necessarily mean that there is a single removal process. It is, however, a convenient means of quantitatively describing the overall process responsible for the removal of HC and CO over the ocean. The vahdity of such treatment is demonstrated by the excellent fit; in ali three cases the correlation coefficient is greater than 0.98. The calculated decay rates are 0.003, 0.005 and 0.013 h - ’ for CO, L and R. Due to the long atmospheric lifetime of CO and L, the first two decay rates can be taken as indicative of dilution during transport. The difference between the average of these values and the value for R can be assumed to be due to chemical loss processes at an approximate average rate of 0_009h- ’ that can be easily explained by the reaction of R with the HO radical. Ozone A summary of the 0, me~u~rnen~

is presented in Table 7 as a listing of the averages and standard

(P$C)

Total HC (PPbC1

20.4 7.3 8.4 4.3

Ratio L/R

0.75 1.5 l.5 2.9

K,H,,)

deviations found in the BL, the FT or pseudo FT for each flight. The average BL concentration near the East Coast was 44 ppbv, with 55 ppbv near Bermuda. However, the average FT values at the two locations were nearly identical at about 51 ppbv. LJsually the standard deviations of the OX concentrations measured within the BL and the FT were minimal (less than 10% of the measured value), but in some cases the standard deviations were much larger; these were associated with the concurrent presence of other pollutants, and were taken to be examples of photochemical generation of 0,. There are several examples of active photochemical generation or long-dis~nce transport of excess 0, during the research flights reported in this paper. Two of these were encountered near the East Coast and they apparently involved aged urban plumes; the first, on 5 March, is used in Fig. 3 as an example of 0, enhancement (maximum = 65 ppb), probably due to in-plume photochemi~i activity. The second example of Fig. 3 shows 0, depletion concurrent with SO, enhancement Elevated 0, concentrations were found during some of the flights near Bermuda, mostly at altitudes above the BL. The greatest enhancements occurred in air masses that had the shortest transport times from the continent (4, 10 and 11 April). The highest con~ntrations (Table 7) were found above the BL in the previously defined pseudo FT. The lowest 0, non-plume analysis of the entire project, 22 ppbv, is at the low end of the range of values reported by Seiler and Fishman (1981), who carried out measurements over the tropical Pacific Ocean. This low 0, con~ntration is consistent with the estimated 7-day over-ocean trajectory that includes the sub-tropical Atlantic Ocean. Furthermore, the CO and HC concentrations at the same time were also among the lowest found during our study (Table 41, and were in reasonabfe agreement with background values. We regard these 0, concentrations as being representative of the sub-tropical oceanic lowertroposphere. Case Study, 10 April 1985 The flight of 10 April provided the most notable example of probable vertical transport of a photochemically active BL parcel into the FT. Figure 4

O,, CO, HCa and DMS ~~~trat~ons

over the Western Atlantic Ocean

2407

Table 7. @%medata ~~a~

Flight 1

BL

Date

ozone (ppbv) ~m~~d~ S.$l

31.711.7 4S.S&l.i

31.5k3.3 38.2fS.I 4K6k7.2

2 3 4 5

27J2JRS 2?/2J85 02J3JSS 02J3J8S OSJ3,‘sS

6

14/3/85

8’ 9 10

~4J3J~S 18/3~5 18/3/8S 2SJ3J8S

If

26J3JW

12 13 54

02f4JsS W/4/85 OS/4J85

21.7~1.1 5x0* t..S* 53.411.6

:z 17

06/4/8S 1Of4J85 1lJ4J35

45.3 57,7f2.O.O* f 2.7 61.0~2.~*

lT

47.8 f4.5 28.1 f 2.4 37.2rt: 13.1* 45.5 k4.3 60.949.3+

42.9f LO 35.2f2.0

41*6f 1.0 47.8 4 2.0

64.04 1.6’

87.2 63.5 f4.6* + 11.2*

S6.0&:5.3 54.1 f3.1 69.4&L?* 54.8 f 2.9 w 49.2~9‘5 47.2cS.3 56.6+ 5.9* 48.0&8.5 53.6 f 7.6 52.0&5.7

* Enhan~

0, ~n~ntmtion. t Pqveer piant plume.

du&g ~~~ fraud ~11~~ air par&s. Fig. 3. 0, and SO2 ~~atmt~~ns measu lotion of 0, in an a~nt~y freshly poltutui parcel is shown in the left,and fo~ation of excess Cl3 in an aged parcel is d~#~sFr8t~ on the ~~t-~and side. The aircraft air speed was 65-70 m s-l.

shows the plots of pressure ~a~titude~,OS conce~tra~ tion, a~umu~ation-rn~e aerosol ~n~n~~o~ & wind dim&on and wind velocity vs time of day. F’igureS sfiows the flight track, with times, altitudes and 0s notes. Almost as soot as the altitude of 3050m was reached for Segment I of the flight the aircraft flew into a region of high 0, co~ntration and highly

variable 8. The s~~uati~~was similar during the 6rst half of Segment II when the Bight dire&on was ~ut~~s~~ at 244Qm, which was about 1SQm above a broken stratocumulus cloud deck, but during the southeastern half of §egment IXboth the O3 and 6 had little va~ab~~t~.This was also the case during the no~h~tw~ flight of nt III at 910m and the

C‘HAKI.FS <‘. Vnh VALIIU

2408

lxul

14

15

16

17 18

Time (ASI’)

13Ml 14

15

16 17

18

and MENACHEM LURM

Fig. 6. DMS as a function of altitude. The open circles represent the data obtained in the late afternoon on 10 April 1985; the filled circles represent observations from 11 April 1985, shortly before sunrise.

Time (AST)

Fig. 4. Time profiles of aerosol concentration, pressure (altitude), 0, concentration, wind direction, 6 and wind velocity, for the duration of Flight 16 on 10 April 1985.

ratio between the FT concentrations on these days was quite different. On 11 April the concentrations in the FT were the same as background measurements found on other days during this study, but the 10 April FT levels were much higher, being closer to those measured in the BL Dimethyl suljide

spiral down 30"N 67w

/ 65

63'W

Fig. 5. Illustration of the flight track of Flight 16.

stepwise descent of Segment IV. It is evident from the 0 and the stratocumulus deck that rising air motions were taking place in the northwestern area. Further support for this is seen in the accumulation mode aerosol concentrations during the northwestern part of Segment II. While the concentrations here are only a small fraction of those in the BL, they are greater than in the southeastern part of Segment II. Further evidence for the existence of convective activity on 10 April can be obtained from the CO and HC data. While the BL concentrations on the two days, 10 and 11 April, were practically identical, the

Measureable levels of DMS (> 5 pptv) were found in all samples taken in the BL. In only three out of 23 FT samples were detectable levels of DMS found. These findings are similar to our previous observations from the Gulf of Mexico (Van Valin et af., 1987) where no DMS was detected in the FT. The average BL concentration calculated for the East Coast samples is 27 4 8 ppv (Table 3); while the average for the Bermuda measurements is double that (54 + 8 pptv, Table 4). The higher value for the Bermuda samples is expected. An obvious reason for this is the greater length of transport time over the ocean for equihbration of the atmosphere to oceanic DMS input. It is also probable that the ocean near Bermuda was more productive of DMS. The reduced concentrations of R HC, as compared to the East Coast atmosphere, woutd lead to tower HO concentrations and consequently lower photochemical activity near Bermuda; offsetting this would have been the lower CO and NO,X and higher 0, and water vapor concentrations. Therefore, it is not possible to make a valid comparison between the photochemical removal rates of DMS from the atmospheres near Bermuda and closer to the East Coast. The average DMS ~~nt~ti~s measured in this study, 27 and 54 pptv for poButed and clean samples, respectively, are sign%cant#y greater than the corresponding averages of 7 and 27 pptv

0,. CO, HCs and DMS concentrations over the Western Atlantic Ocean obtained in the Gulf measurements. Nevertheless, these values are well within the range reported by Andreae and Raemdonck (1983), Andreae et al. (1985) and Ferek et al. (1986), which vary from < 5 pptv for Atlantic Ocean areas subject to airflow from continental regions, up to a few hundred pptv for remote oceanic areas not affected by continental air. The average concentrations of DMS are consistent with previous observations, but an expected diurnal cycle was not observed. Two altitude profiles of DMS concentration, taken on 10 and 11 April, were intended to test the existence of diurnal cycles of DMS concentration. The measurements were carried out shortly before sunset on 10 April and prior to sunrise on 11 April (Table 1). Model simulation of the marine atmospheric chemistry (Luria and Meagher, 1988; Andreae et al., 1985) predicted that lower DMS concentrations would be observed in the first set and higher in the second. The results of the two sets of measurements (Fig. 6), did not support this expectation. As can be seen from this figure, the two data sets are alike in both concentration and altitude dependency. The strong vertical gradient in DMS is consistent with our other measurements taken here and in our previous study (Van Valin et al., 1987). and with the report of Ferek et al. (1986). Both model simulations mentioned above include loss reactions with both HO during the day and NO, at night. The expected greater loss rate of DMS from the atmosphere on IO April may have been offset by a greater flux rate from the Ocean surface due to winds. It is known that the wind speed is a significant factor in determining the flux rate from the ocean to the atmosphere (Andreae et a/., 1985). On 10 April, the wind speeds at the lowest altitudes were 2-3 times the speeds on the following morning, and this may explain the similarities of the profiles.

CONCLUSIONS

(1) Greater 0, concentrations were measured in the vicinity of Bermuda at altitudes above the BL in air containing measureable quantities of anthropogenic pollutants than in similarly polluted air near the U.S. East Coast. (2) The concentrations of CO and HC in the BL at Bermuda were inversely related to travel time from the continent, while FT concentrations were near hemispheric background levels. The ratios of L/R HC had a high correlation to transport time. (3) DMS concentrations were always found to decrease with increasing altitude in the BL. It was not found in the FT.

2409

Acknowledgement-We acknowledge the expert conduct of the field phase of this experiment by Dr R. F. Pueschel, Mr D. L. Wellman and Mr T. Gates.

REFERENCES

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