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A major haze event near point barrow, Alaska: Analysis of probable source regions and transport pathways
w34-6981/89 $3.~+U.oo Pcrgamon Press plc
Atmospheic Environment Vol. 23, No. 1I, pp. 2537-2549.1989. Printed in Great Britain.
A MAJOR HAZE EVENT NEAR POINT BARROW, ALASKA: ANALYSIS OF PROBABLE SOURCE REGIONS AND TRANSPORT PATHWAYS l-3.A.
~~i~~~~,*~
R.C. SCWNELL,~J.D.KAWL,*~C.
A. HERBERT*
and E. JORANGEW
*Geophysical Monitoring for Climatic Change, National Oceanic and Atmospheric Administration, 325 Broadway, Boulder, CO 80303, USA, $Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309 and I/Norwegian Institute for Air Research, 2001, Lillestrsm, Norway
Abstract-On 2-3 April 1986, an unusually dense Arctic haze band with distinct horizontal and vertical boundaries was intercepted by a NOAA WP-3D aircraft north of Barrow, Alaska during the second Arctic Gas and Aerosol Sampling Program (AGASP-II). Within this haze event, condensation nuclei (CN) taunts exceeded 70,000cm-3, and aerosol scattering extinction coefficients (b,,) were > 70x lob6 m-r. SO, con~n~ations in the haze were I5 ppbv. Synoptic met~rolo~~l data coBected in Arctic and subArctic regions were supplemented with trajectory analyses to determine the probable source regions and transport pathways of the haze. Anatyses of these data, along with surface air quality measurements in Norway, suggest that the haze originated in central Europe 10 days earlier. The transport pathway followed a typical pattern characteristic of spring haze transport in the Arctic. Key word index: Arctic haze, polar meteorology, Arctic pollution, aerosols, long-range transport, Arctic trajectories.
types of trajectory analysis (Barrie et al., 1981; Harris,
lNTRODUCTION The occurrence of layered haze in the Arctic tropo-
sphere, originally documented by Mitchell (1957), has been the subject of considerable study in recent years (for example, Barrie et al., 1981; Schnell and Raatz, 1984; Radke et al., 1984; Raatz et al., 1985a,b; Lowenthal and Rahn, 1985). Haze layers are usually 2OOm-7 km thick with a horizontal extent of 20-200 km, and are often associated with distinct air mass boundaries (Raatz, 1985). Haze is most common between pressure altitudes of 700 and 900 mb. Radke et al. (1984) related the layering of Arctic haze to the strengthened thermal stability in the Arctic, the combined influences of different source regions and their locations relative to the polar front, reduced scavenging, phot~hemistry over the ice pack, and slow meridional transport in a weak atmospheric flow. Reconstruction of haze transport is complicated by the considerable distances between observed haze layers near Alaska and their potential sources (4000-10,000 km), and the sparseness of meteorological data in the Arctic basin. Nevertheless, it is possible to obtain estimates of source region and transport pathways through the use of chemical tracers (Pacyna, 1984; Lowenthal and Rahn, 1985), improved synoptic maps of the polar hemisphere, and various tPresent athfiation: Department of geography, University of Newcastle, NSW, 2308, Australia. fiTo whom correspondence should be addressed.
1984, for example). Because the Arctic atmosphere is so stable, the chemical and physical signatures in haze associated with source regions show little change over distances of the order of thousands of km (Lowenthal and Rahn, 1985). Many Arctic haze layers are thin, with pollutant concentrations elevated slightly above background levels (Herbert et al., 1987). On occasion, a haze layer can be quite dense, have condensation nuclei (CN) concentrations > 10,000 cme3, aerosol scattering extinction (b,& greater than 70 x 10m6 m-‘, accumulation-mode aerosols (0.1-1.0 pm diameter) an order of magnitude higher than background, and major increases in sulfate aerosol, sulfur dioxide, soot carbon, gaseous and solid organ&, and heavy metal components. Such haze events have been described for the 1983 Arctic Gas and Aerosol Sampling Program (AGASP-I) by Schnell and Raatz, 1984; Raatz et al., 1985a,b; Raatz, 1985; and others. The purpose of this paper is to describe the aerosol characteristics, meteorological conditions, and transport of a major haze event observed during AGASP-II and encountered by a NOAA WP-3D aircraft near Point Barrow, Alaska, on 2-3 April 1986. The instrumentation of the aircraft has been described by Schnell et al. (1989). Through the use of synoptic meteorological data and trajectory analyses, it is possible to track the movement of this haze layer from the general area of origin and the time when it was injected into the
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H. A.
BRIDGMAN er al. APRIL 2, lQ86
Arctic to the point of encounter with the aircraft. The trajectory model used in this study is the Atmospheric Environment Service (Canadian) Long Range Transport of Air Pollution (AES LRTAP) model (Olson et al., 1978; Barrie et al., 1981). Supporting meteorological data were obtained from the West German Meteorological Service (surface) and the U.S. National Climatic Data Center through the National Center for Atmospheric Research (NCAR) (850 mb and above). CN concentrations and h,, are used to characterize the haze.
HAZE LAYER CHARACTERISTICS, 2 3 APRIL
20000 _10000%
5000 tooo:h;-
J
2000 -
ii
500 2oofl
1986
On 2 April 1986 the NOAA WP-3D aircraft encountered an unusually dense haze layer while flying 300 km north of Point Barrow, Alaska (see Herbert et al., 1989, for full flight details). Initial contact with the haze occurred at 2331 GMT at 72.28”N, 158.27”W at a pressure altitude of 794 mb. After flying level for 10 min in the haze, the aircraft reversed track from 188” to 022” and exited the side of the haze flow at a point 30 km upwind of the entry point. The haze parameters are shown in Fig. 1, where it may be observed that an abrupt increase in CN concentrations from a few hundred to 70,000 cm- 3 occurred at the haze boundary coincident with a sharp increase in relative humidity (r.h.) from 50% to 75%. Aerosol b,, increased from 35 x 10m6 m-l at the haze edge to - 60 x 10m6 m-’ at the point where the aircraft reversed direction. Data from related studies show that, in the haze layer, black carbon reached 900 ng m-3 (Hansen and Novakov, 1989); aerosol optical depths, 0.75 (Dutton et al., 1989); and SO, 15 ppb (Thornton et al., 1989). Herbert et al. (1989) suggested that active SO* gas-to-particle conversion was occurring in this layer to produce the high CN concentrations. A vertical profile through the layer, shown in Fig. 2, established that the haze was
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Fig. 1. Time series of CN, dry aerosol bsp, ozone (OZ), water vapor mixing ratio (MR),and ambient temperature (TA) between 2330 and 2400 GMT on 2 April 1986when the aircraft first encountered the major haze layer 300 km off the north coast of Alaska. mainly concentrated between 900 and 750 mb (750-2500 m). Wind direction throughout the profile was steady at 270”-280”, and wind speed increased near the surface to 15 m s-l at 600 mb. from4ms-’
126 GMT 3 APRIL 1986
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Fig. 2. Vertical profiles of ozone (OZ), h,,, CN, ambient temperature (TA) relative humidity (RH), wind speed (WS), and wind direction (WD), through the major haze layer, 72”N, 158”W.
2539
Major haze event in Alaska
surface flow (Carlson, 1981; Iversen, 1984). Thus, air quality helow the inversion is largely determined by local sources rather than long-range transport except when the inversion decreases in intensity and pollutants mix down to the surface (Raatz et al., 1985b). Nerbert et al. (1987) found a strong positive correlation between CN and moisture in the haze layer. Figure 4 shows l-min averages of CN plotted against r.h. and temperature through the haze layer in the vertical profile shown in Fig. 2. The vast majority of CN concentrations > 6000cmi3 occur in air with r.h. 70-80% and temperatures of -25” to -27.5”C. On the basis of earlier research, the source of such a
Latitude-altitude cross-sections through the haze layer depict more completely its structure and extent, as shown in Fig. 3. In this figure, CN concentrations and bSP show that the base of the haze layer was coincident with the top of the surface temperature inversion. In the zone bounded by 800-900 mb, there is an undulation between maximum and minimum CN values with Iatitude. Between 750 and 6OOmb, there is a pronounced north to south CN gradient. Below the surface inversion, an abrupt decrease in CN concentration occurs. This is best explained by the fact that the surface inversion in the Arctic decouples the general tropospheric circulation from the 2 153-030 2-3
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APRIL
1986
74” N LAT 69
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Fig. 3. ~titud~aititude cross-sections of b,V and CN through the major haze layer, 72”N, 158”W. The thin~ash~ lines show the flight path; the thick-dashed lines indicate the boundaries of stable layers.
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Fig. 4. CN concentrations vs relative humidity (RH) and temperature for the period the aircraft was investigating the haze layer, showing that higher CN concentrations were associated with greater RH and warmer temperatures. Data are I-min averages.
heavy haze would most likely be either western Europe, eastern Europe or the U.S.S.R. (Rahn, 1981: Lowenthal and Rahn, 1985; Rahn and Lowenthal, 1986). Haze originating in the central U.S.S.R. has been associated with cold, dry air and that from western Europe with warmer, relatively moist air (Raatz, 1985). This haze often enters the eastern Arctic with a surge of air from the south. Surface air quality measurements obtained from the Norwegian Institute for Air Research (NILU) indicated that a large northward-flowing pollution mass passed over Scandinavia and entered the Arctic on 22--24 March, 9--l 1 days prior to the haze interception off of Alaska. This provided a starting point for a study with the goal of determining where this pollution over northern Europe had moved to, where the pollution north of Alaska had originated, and what pathway the pollution followed across the Arctic. Following is an analysis of the meteorology and transport over Eurasia and the Arctic leading up to the haze event observed north of Point flarrow, Alaska. The accumulation and northward movement of pollutants in the European sector is presented. Analyses of synoptic maps, restricted to the surface and 850 mb where the bulk of the haze existed, show that atmospheric stagnation over Europe in late March was followed by a surge of pollutants injected into the
0000 (4
Arctic with subsequent transport across the Arctic basin. Finally. results from the AES LRTAP trajectory model are used to establish movements and possible source regions of the haze layer. The presentation of the data generally follows the pattern and nomenclature introduced by Raatz et cd. (1985a).
STAGNATION
ACCl;MLlLATION
OF POLLL’TANTS
OVER EUROPE
Synoptic analysis, 0000 GMT, 26 March 1986
The surface chart of the Arctic shown in Fig. 5a depicts a low-pressure system centered over central Scandinavia at 62’N latitude. An occluded front extended southward from the eastern section of the low across eastern Europe. A warm front extended northeast from the western section of the low to the Siberian coast. Surface winds associated with the system were Iight. Relative humidities between the two fronts were about 90%, but behind the occluded front, a zone of humidities (60% extended over central and western Europe. Skies were partly cloudy to cloudy over most of northern Europe. An extensive anticyclone centered on the Ural Mountain region covered most of the continent between the northern tip of Norway and the Soviet
0000
GMT 26 MARCH 1988 SURFACE
AND
(b)
OMT 26 MARCH 1986 850 mb
I
Fig. 5. (a) Surface (pressure, mb) and (b) 850 mb (geopotential height, m) synoptic analyses for the Arctic basin, 0000 GMT, 26 March 1986. The broad arrow indicates a probable path ofgreaterhaze flow. Wind arrows show the direction of air movement (one barb = 5 m s- ‘1.
Major haze event in Aiaska
island of Novaya Zemlya. Surface winds were light to calm throughout this region. Humidities near the center of the anticyclone dropped below 50%. East of the ridge, a low-pressure area previously over the Siberian coast had moved to the south and inland to central Siberia. Farther east, a high-pressure area dominated eastern Siberia and Alaska. Weak airfiow from eastern Siberia into the Arctic occurred between this high and the central Siberian low. At 850mb, as shown in Fig. Sb, airflow across Europe was south and southeast flow over Scandinavia associated with the low-pressure area located there. The southwest circulation pattern persisted along the northern edge of the Siberian ridge. Circulation in eastern Siberia was from the south and stronger, associated with the ridge interacting with the Siberian low. Relative humidities >80% were restricted to the areas dominated by low pressure, primarily over Scandinavia and eastern Europe. Wind speeds over northern Europe averaged 20 m s- ’ near the low, dropping to S-IOmsfarther south. A small low was developing near the North Pole. Air Rowed auross the Pole from the region of Novaya Zemlya, then around the high-pressure system near Point Barrow to create easterlies along the Alaskan coast. These conditions were the culmination of a stagnant meteorological situation that had existed over Europe since 3 March resulting in a strong build-up of pollutants. Any northward transport of haze associated with conditions on 26 March would most likely have occurred in the moist zone along the western side of the ridge located over the Urals ahead of the occluded front as indicated by the arrow in Fig. 5b in a manner described by Raatz and Shaw (1984) and Rahn (1981). Haze analysis Air quality data for Norway, representative of the polluted air mass over Europe, were obtained from the NILU monitoring network. The locations of the surface stations are shown in Fig. 6. As a first step in determining the flow and source of the haze, we examined sulfate concentrations from the mainland stations for March 1986. Figure 7a shows that the stagnating meteorological conditions over western Europe and Scandinavia during March led to a buildup of air pollution beginning 9 March progressing from south to north into the Arctic. At Birkenes and Skreildalen, in the southernmost section of the network (58”-59”N), con~ntrations peaked three times between i0 and 19 March. The multiple nature of the peaks suggests fluctuations in the transport of the polluted air in the relatively weak south to south southwest airflow during this period. At Hummelfjell (62.3”N) the peak of SOi- occurred between 18 and 20 March, several days after higher concentrations were measured farther south. Farther north at RBrvatn (62S”N, Fig. 7b) the peak in SO:- con~ntrations also occurred during this
2541
Norwegian Air Poltution Network
Fig. 6. Surface monitoring stations in the Norwegian Air Quality Network.
same period, but was about i/3 the concentration of that at Hummelfjell. Since Kgrvatn is about 2” (- 125 km) west of Hummelfjell (see Fig. 6) and was in prevailing south southwest flow, it may have been at the western edge of the polluted air mass. SOiconcentration peaks at Tustervatn (658’N) were similar in timing, but of lower concentration than those observed at Hummel~ell. Still farther north at Jergul(69.3”N), the bulk of the polluted air mass arrived between 22 and 24 March; light pollution was detected the previous week. It is worthy of note that the levels of pollutant SOi- at Jergul (Fig. 7b) are greater than at Klirvatn and Tustervatn, the next two stations south the Jergul in central Norway. This suggests that the pollution stream was located in northeast Norway at that time. Extending the gradient, one could suggest that the pollution was even stronger east of Jergul. At Bjern@ya (Fig. 7c), the western portion of the haze peaked on 25 and 26 March at values twice as large as the low values 2 weeks previously (note the difference in scale from Figs 7a and 7b. Sulfate concentrations at this station then diminished to below the monthly average during 27-28 March, and increased again to a secondary ak between 29 and 31 March. At Ny &4? lesund (78.9”N), about 500 km north of Bjerrnerya, the peak in SO:- concentrations occurred
H. A. BRIDGMAN
2542
et al.
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1986)
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’ 4 ’ 6 ’ 8 ‘10’12’14’16’18i20’22’24’26’28’30’ March
Cc) Fig. 7. Daily-indicator pollution from stations in the Norwegian
1
1986
Am
3 1986
Day
concentrations for March 1986 Air Quality Network shown in
Fig. 6: (a) southern mainland stations, (b) central and northern mainland stations, (c) northern island stations.
2 days later, on 28 March. Thus, the initial pulse of pollution traveled at an average speed of about 3 m s- ‘, as deduced from surface wind analyses. SOiconcentrations at Ny Alesund remained relatively high until 30 March as a portion of the polluted air mass remained over Svalbard. From Fig. 7, it may be further observed that SOiconcentrations over mainland Scandinavia fell to background levels after 25 March, as the haze was advected northward ahead of the front (discussed below). The movement of the low-pressure center toward Ny Alesund, plus a wind shift to the south on 27-28 March, terminated the pollution injection into the Arctic.
METEOROLOGY POLLUTANTS
OF THE MOVEMENT
OF THE
AWAY FROM THE EUROPEAN CONTINENT:
SYNOPTIC ANALYSIS, QOOO GMT, 27 MARCH 1986
Synoptic
nnalysis,
0000
GMT,
27 March
1986
Now that we have shown that pollution moved into the Arctic, we present data on the transport of that pollution across the Arctic basin. During 26 March the low over Scandinavia weakened considerably leaving a combined and moved north, occluded-stationary front extending from the northern tip of Norway, southeast, and then south across eastern Europe, as shown in Fig. 8a. The Siberian ridge ahead of the front, with a central pressure of
Major haze event in Alaska
0000 (a)
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ONIT 27 MARCH SURFACE
1986
GMT 28 MARCH
1986
SURFACE
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GMT 27 MARCH 1986 850 mb
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GMT 28 MARCH 1986
(b)
(4
2543
850
mb
Fig. 8. (a,~)Surface (pressure, mbf and {b,d) 850 mb(geopotential heigbt, m) synoptic anaiysesfor the Arctic basin, ooo0 GMT, 27 and 28 March 1986. The hatched arrows on the 850 mb mam indicate the probable movements of the return-flaw and Arctic basin components of the haze. Wind arrows show the-general direction of air movement (one barb= 5 ms-‘).
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H. A. BRILIGMAN et al.
1035 mb, had assumed a blocking position (Lejenas and Okland, 1983) and extended its influence northward across Novaya Zemlya. Circulation associated with this pattern comprised a gateway to the polar regions across northern U.S.S.R. and Novaya Zemlya. Surface wind speeds at this time were between 5 and lOms_‘, with a well-defined fetch extending back across western and eastern Europe. The air in this pathway was moist, having r.h. > 80%-probably the result of the air cooling radiatively as it crossed the Polar region. This northward surge of polluted air through the northern Scandinavia-northern U.S.S.R. region was most apparent at 850mb, as seen in Fig. 8b. The pathway is consistent with previous research, which suggests that Arctic haze injections occur closer to western Europe with the advance of spring conditions (Raatz and Shaw, 1984). There is a strong possibility that the haze followed two llow paths once in the Arctic, one portion taking the classic return flow route around the Scandinavian low (Rahn et al., 1980) and a second portion following the common pattern IIb described in detail by Raatz and Shaw (1984). We focus our study on the portion of haze over Scandinavia that had the higher potential to travel across the Arctic basin to Alaska.
SURGE CUT-OFF: SYNOPTIC ANALYSIS, WOO GMT, 28 MARCH 1986
By 0000 GMT on 28 March, the circulation pattern creating the surge from Europe was breaking down, as shown in Fig. 8c. The fronts over northeast U.S.S.R. and the European Arctic had combined into one stationary front, and overriding air from the south brought snow to the area. The low-pressure cancer over the North Pole had become slightly stronger (loo0 mb), forming a weak, closed circulation. This flow pattern acted as a draw for the haze then in the Norwegian Arctic. The weak low previously west of Scandinavia had moved north to 75”N, and a secondarly low (995 mb) had formed over Svalbard. The occluded front previously over eastern Europe had begun to move east and had crossed the Urals in northern Russia; a portion trailed back across Europe toward Italy as a warm front. Airflow behind the front was southerly at 5-10 ms- ‘. The high-pressure ridge over western Siberia was beginning to break down and had weakened by 10 mb in the Novaya Zemlya area. The high previously dominating the western Arctic had weakened and had begun to move southward into eastern Siberia. As at the surface, the 850 mb airflow into the polar regions was considerably weaker than before, averaging less than 7 m s-i (Fig. 8d). The zonal flow over northern Europe and northern Siberia had strengthened considerably, with southeast winds averaging 15-20 m s- *. The reestablishment of zonal flow over
Europe probably began moving haze material from the source regions quite rapidly to the northeast, toward Novaya Zemlya. Research on previous similar situations by Ottar et al. (1986) indicated that the haze would most likely have initially traveled at an altitude of lOO3OO m, but would have risen with the warm air over the front. HAZE TRANSPORT
In this section, we present trajectory analyses in the belief that objective analyses by computer models that take into account all the data available in the datasparse region should provide a complementary product to the more subjective synoptic weather map analyses discussed above. No surface or aircraft measurements of the anthropogenic haze layer over the Arctic basin were made until 2-3 April 1986, during the WP-3D flight near Point Barrow, Alaska. Therefore the transport of the haze must be based upon meteorological parameters that suggest the movement of the haze-containing air mass. In Europe and the northern U.S.S.R., there are over 200 meteorological stations from which synoptic analyses are produced. On the coastal areas and islands within the Arctic, there are 20 surface weather and rawinsonde stations. But on the ice cap itself, there is only one rawinsonde station, at approximately 85”N, 145”W. The shortage of data over the ice is somewhat mitigated by the fact that the Arctic basin is composed of relatively homogeneous terrain (ice) and has a relative lack of local circulations compared to lower latitude locations. The complete European, Asian, and Arctic basin data set was used in the following studies. Back-trajectories in Europe indicating possible haze source regions
Upstream trajectory analyses using the AES LRTAP trajectory model were performed to determine the probable transport pathways for haze reaching points in northern Europe. This model was used because it includes the effects of synoptic-scale vertical motions. These motions are of particular importance for determining source regions for Arctic pollution (Carlson, 1981; Iverson, 1984). Although trajectory analyses have proved useful in identifying the longrange transport of atmospheric pollutants, particularly in the Arctic, large uncertainties exist due to interpolation of sparse meteorological data (Kahl and Samson, 1986, 1988). Thus, trajectories should be interpreted as a general indication of air movement, recognizing that errors of up to several hundred kilometers may accrue over several days of transit. Upwind (backward) trajectories beginning at 1200 GMT on 26 March 1986 were calculated from three locations. Trajectories starting at 65”N, 14”E (point A, Fig. 9) in central Norway represent the movement of haze associated with stations in the Norwegian Institute for Air Research network. Trajectories starting
2545
Major haze event in Alaska
1200 QWr 26 MAR 1986
Fig. 9. Five-day 1000 mb backward trajectories calculated by the AES LRTAP map from three points in Europe: A (65”N, 14”E), B@O”N, 32”E), and C (7O”N, 50”E). Tick marks indicate 24-h intervals. at 6O”N, 32”E (point B) represent the potential
corridors of haze movement along the backside of the high (Raatz and Shaw, 1984), and back-trajectories from 70”N, 50”E (point C) represent potential haze injection into the Arctic across the Soviet Arctic coast. The results shown in Fig. 9 illustrate that the probable source region for air at 925 mb reaching all three locations was eastern Europe and the western U.S.S.R., areas with large anthropogenic emission rates (Barrie, 1986). Airflow to central Norway (point A) originated over the Austriaregions. Airflow to Hungary-Czechoslovakia point B originated over France, passing into eastern Europe before moving along the backside on the high. Airflow to the Soviet Arctic coast (point C) originated in the Black Sea region and flowed up the western side of the Ural Mountains. These results suggest that the main source of pollutants measured in the Norwegion network was eastern Europe and that some western Europe precursors were probably included. Trajectories
3 APR
1986
1200 OMf 26 MAR 1986
indicating polar basin transport
Figure 10 presents downwind (forward) trajectories originating at the same points A, B and C shown in Fig 9, beginning at 1200 GMT on 26 March 1986. Also shown are backward trajectories arriving at point D (72”N, 158”W), the point of aircraft contact with the haze off Alaska, terminating at 0000 GMT on 3 April 1986. When trajectories originating at two levels were simiIar, they are shown for clarity as one.
Fig.10. Five-day forward trajectories at 900 and 925 mb calculated by the AES LRTAP model originating at points A, B and C (as in Fig. 9) on 1200 GMT, 26 March 1986, and a five-day backward trajectory originating at 0000 GMT, 3 April 1986 from point D (72”N, 158”W). the point of aircraft contact with the hare layer. Tick marks are printed at 24-h intervals. When trajectories were similar at two levels, only one is shown for clarity (a) 925 and 900 mb; (b) 850, 800 and 700 mb.
2546
H. A.
BRIDGMAN
Trajectories starting at point A (Fig. 10a) in central Norway moved westward into the Atlantic after remaining stagnant over Norway for several days, These results support the suggestion that some haze moved in a return flow pattern around the low (Raatz and Shaw, 1984). At 850mb and above (Fig. lob), airflow for the 5-day period was northward to the Norwegian Arctic coast, and then east toward the Soviet coast. Trajectories starting at point B generally represent airflow and potential haze movement away from eastern Europe. Air at all levels in Figs lOa,b between 900 and 700 mb, moved north-northeast along the Soviet Arctic coast, finishing just north of the Taymyr Peninsula after 5 days. These trajectories suggest that a transport pathway across the Murmansk U.S.S.R. area into the Soviet Arctic was a likely pathway for further movement across the Arctic basin. Trajectories from point C represent Arctic haze that would have originated in the western U.S.S.R., mainly from sources along the western flank of the Ural Mountains. Trajectories beginning at levels up to 850 mb parallel the equivalent trajectories from point B, indicating that the airflow at these levels was probably part of the injecting corridor along the Soviet Arctic coast. Airflow from point C at the higher 700 mb level, however, moved southeast into central Siberia. The forward trajectory calculations shown in Figs lOa,b suggest that haze probably originated in eastern Europe and the western U.S.S.R., and most likely followed a pathway into the Arctic basin east of Novaya Zemlya, skirting the Taymyr Peninsula before moving toward Point Barrow. Haze transport out of Europe was most likely between 900 and 800 mb, and probably was not strictly isentropic because radiative cooling of the haze layer would allow gradual sinking as it transected the Arctic basin. Haze measured by the air quality stations in Norway was not injected directly into the Arctic, but was instead eventually carried southwest over the Atlantic Ocean. Back-trajectories
from the Alaskan Arctic
Backward trajectories from point D, where the aircraft measured the haze, are also shown in Figs 10a and b. At levels from 925 to 700 mb the trajectories track back across the Arctic basin to essentially intersect the forward trajectories coming out of Europe. This overlap of 5-day trajectories suggests’a reasonable path for the haze transport, and the 9-10 day transit time from Europe to Alaska agrees with ground measurements of the haze departure from the Norwegian network and arrival in Alaska as determined with aircraft measurements (Herbert et al., 1989). Meteorology
in the Arctic Basin
In a final effort to further diagnose the meteorology of the haze transport across the Arctic basin, data
et al.
from a number of sources were merged and interpreted as follows. Surface synoptic maps of the Arctic from the West German Weather Service were analyzed to obtain an initial indication of meteorological conditions across the basin. These maps were supplemented at 850 and 700 mb by computer depictions of the spatial variations of temperature, humidity, winds and geopotential height. These latter data were originally compiled for use in the NCAR Global Climate Model. Using these data, along with the modeled trajectory paths, we developed a scenario to approximate the movement of the haze across the Arctic basin from the time of the surge over the Soviet Arctic coast to the arrival of the haze off Point Barrow, Alaska. The aircraft data established and trajectory data suggested that the haze flowed between 900 and 750 mb. The movement of the haze is shown on a series of 850 mb synoptic maps (Fig. 11) for the period 30 March-2 April. In Fig. 11, the suggested location for a portion of the haze layer is depicted by an oval shaded area. The shaded region is not meant to specify exact boundaries of the haze, but to indicate a probable general location of the haze region subsequently sampled by the aircraft. From Fig. 11, it may be observed that variations in three major synoptic features determined the estimated flow of the haze across the basin. The first was the movement of the low from Svalbard to the east-northeast. Winds were weak during this period ( < 5 m se I). In the return-flow pattern, the haze over Svalbard was probably caught in the northerly flow and moved south into the Atlantic. This southward movement of the western part of the haze layer would explain why SO: concentrations at Bjornoya showed a secondary peak between 29 and 31 March (Fig. 7) when no direct surge occurred from any known source region. Meanwhile, the eastern portion of the haze probably moved northward across the basin. The second major synoptic feature of importance was the low that developed over the North Pole on 27-28 March, and moved southward toward Alaska between 28 and 30 March. This low become stationary about halfway between the pole and the Alaskan coast by 31 March. It controlled the circulation in the western Arctic through 2 April, creating northerly and northeasterly winds of 1O--l 5 m s ’ on its polar side. By 31 March, the movement and strengthening of this low created westerly winds of 15 ms- ’ along the Alaskan coast. On 31 March, the haze layer (Fig. 1lb), previously off the Taymyr Peninsula, was transported in the northerly flow associated with the west Arctic low and began to move more rapidly toward the Alaskan coast. Winds behind the haze layer were now northeasterly toward the Siberian coast, and the haze became completely divorced from the eastern Arctic. The third major synoptic feature influencing the movement of the haze layer was the high-pressure system over eastern Siberia, which began strengthening on 30 March. Over the next 3 days, the high
2541
Major haze event in Alaska
0000 (4
0000 (c)
0000
GMT 30 MARCH 1988 850 mb
@I
GMT 1 APRIL 1986 850 mb
(4
0000
GMT 31 MARCH 1988 850 mb
GMT 2 APRIL 1986 850
mb
Fig. 11. The 850 mb synoptic analyses (geopotential height, m) for the Arctic basin at 0000 GMT: (a) 30 March,(b) 31 March, (c) 1 April, (d) 2 April 1986. The shaded areas indicate the probable locations of a portion of the haze layer, and the hatched arrows the layer’s general direction of movement. Wind arrows show the direction of air movement (one barb = 5 m s- ‘).
2548
H. A.
BRIDCMAN
extended its influence northward into the Arctic basin, enhancing the northwest airflow along the Alaskan coast toward the west Arctic low. A ridge from this high developed toward the pole beginning on 1 April (Fig. 1lc), eventually creating high pressure along the entire Siberian side of the Arctic. By 0000 GMT, 2 April (Fig. lid), the haze was in the westerly airflow along the Arctic coast where it was subsequently
intercepted by the aircraft.
SUMMARY
AND FINAL REMARKS
Analyses of the meteorology involved in the transport of a dense haze layer encountered by an aircraft north of Point Barrow, Alaska (72”N, 158”W), on 2-3 April 1986 have revealed a coherent picture pointing to potential sources and probable transport path ways. From the AES LRTAP trajectory model and relevent synoptic analyses, the source region of the haze was suggested as being mainly central Europe. The primary transport of haze began as a surge across the Soviet Arctic coast on 27 March 1986 and followed a pathway east of Novaya Zemlya to just north of the Taymyr Peninsula. Haze movement at first was relatively rapid, but by 29 March, it had slowed in the light and variable circulation just east of Novaya Zemlya. The haze layer was then drawn into the circulation pattern around the Iow located near the North Pole on 31 March, and from that time moved more rapidly to its position off Point Barrow on 2 April. This transport appeared to be confined to a layer between 900 and 800 mb. A westerly portion of the haze was probably transported in the return flow pattern associated with a weak low near Svalbard (Raatz and Shaw, 1984; Rahn, 1981), between 26 and 29 March. This part of the haze would eventually move southward into the Atlantic. Raatz and Shaw (1984) established that the transport pathway taken by the major haze layer in 1986 was typical of that associated with spring conditions in the European Arctic. Surge events in spring occur almost exclusively west of the Ural Mountains and are often associated with a return-flow pattern in which some of the haze moves toward Svalbard then flows west then south in circulation around a lowpressure system north of Scandinavia. On the basis of the strength of evidence presented here, we conclude that central Europe is the most likely source of the haze layer observed off Alaska on 2 April, and that the haze travelled from Europe to Alaska in IO+1 days. Because of the lack of haze measurements between northern Scandinavia and Point Barrow, however, as well as the sparsity of meteorological data in the Arctic basin, contributions from sources in western U.S.S.R. cannot be totally excluded. Ultimately, chem+caI analyses of the haze aerosol constituents will have to be relied upon as a final determinant of the source regions.
ef ul.
Acknow&gements--We thank L. Barrie and M. Olson for the AES LRTAP trajectory results, and J. Harris, D. Lowenthai, and K. Rahn for valuable comments and advice. An early version of this paper presented at the 4th International Conference on Arctic Haze in Hurdal, Norway (28 September-2 October 1987), elicited many valuable comments from the audience. This research was performed while J. Kahl worked as a NRC Research Associate at the Geophysical Monitoring for Climatic Change (GMCC) division of the National Oceanic and Atmospheric Administration (NOAA). This research was financially supported in the field by GMCC and the NOAA Ofice of Aircraft Operations; Office of Naval Research (ONR); National Aeronautics and Space Administration; Naval Air Systems Command; and grants to individual investigators from the National Science Foundation. Data analysis was supported by the Defense Nuclear Agency, and by ONR through the University Research Initiative Program, contract N~l4-86-0695. A special thanks to the staff and air crew from the Miamibased Office of Aircraft Operations, who equipped the aircraft and flew in the Arctic under trying conditions. Manuscript typing was provided by L. Prendergast, R. Rossen, and M. Van Asche of NOAAiGMCC. Drafting was by J. Howe. REFERENCES
Barrie L. A. (1986) Arctic air pollution: an overview of current knowledge. Atmospheric Environment 20,643-663. Barrie L. A.. Hoff R.M. and Danaunatv S.M. (1981) The influence of midlatitudinal pollution sources on haze in the Canadian Arctic. Atomospheric Enoironment 15, ““.
<
1407-1419.
Carlson T. N. (1981) Speculations on the movement of polluted air to the Arctic. Afmospherir Environment 15, 1473-1477. Dutton E. G., DeLuisi J. J. and Herbert G. H. (1989) Spectral aerosol optical depth of Arctic haze as measured on board the NOAA WP-3D during AGASP-II, April 1986. J. atmos. Chem. (in press). Hansen A. D. A. and Novakov T. (1989) Aerosol black carbon measurements in the Arctic troposphere. J. ntmos. Chem. (in press). Harris J. M. (1984) Trajectories during AGASP. Geophys. Res. Lett. 11, 453-456. Herbert G. A., Bridgman H. A., Schnell R. C., Bodhaine B. A. and Oltmans S. J. (1987) The analysis of meteorological conditions and haze distribution for the second Arctic Gas and Aerosol Sampting Project (AGASP-II). NOAA Technical Memorandum, ERL ARL-158, Boulder, Colorado. Herbert G. A., Schnell R. C., Bridgman H. A., Bodhaine B. A., Oltmans S. J. and Shaw G. E. (1989) Meteorology and haze structure during AGASP-II, Part I: Alaskan Arctic Flights, 2-10 April 1986. J. atmos. Chem. (in press). iverson T. (1984) On the atmospheric transport of pollution to the A&tic. Geophys. Res. iett. 11, 45%460. Kahl J. D. and Samson P. J. (1986) Uncertainty in traiectory calculations due to low resolutibn meteorolbgical data. j. dim. appl. Met. 25, 18161831. Kahl J. D. and Samson P. I. (1988) Trajectory sensitivity to rawinsonde data resolution. Atmospheric Environment 22, 1291-1299.
Leienas H. and Okland H. (1983) Characteristics of northern Hemisphere blocking as determined from a long time series of observational data. T&s %A. 350-362. Lowenthai D. H. and Rahn K. A. (1%) Regional sources of pollution aerosol at Barrow, Alaska, during winter 1979-1980 as deduced from elemental tracers. Atmospheric Environment.
19, 201 l-2024.
Mitchell J. M. Jr. (1957) Visual range in the polar regions _ with particular reference to the Alaskan A&c. J. &OS. terres. phys. Spec. Suppf. Pt. 1, 195-211. Olson M. P.. Oikawa K. K. and MacAfee A. W. (1978) A I
Major haze event in Alaska trajectory model applied to the long-range transport of air pollutants: a technical description and some model intercomparisons. LRTAP 78-4, Atmospheric Environment Service, Ontario, Canada. Ottar B. and staff (1986) Air pollutants in the Arctic. Final report, Norwegian Institute for Air Research, Lillestrom, Norway. Pacyna J. (1984) Estimation of the atmospheric emissions of trace elements from anthropogenic sources in Europe. Atmospheric. Environment l&41-50. Raatz W. E. (1985) Meteorological conditions over Eurasia and the Arctic contributing to the March 1983 Arctic haze episode. Atmospheric Environment 19, 2121-2126. Raatz W. E., Schnell R. C. and Bodhaine B. A. (1985a) The distribution and transport of pollution aerosol over the Norwegian Arctic on 31 March and 4 April 1983. Atmospheric Environment 19, 213552142. Raatz W. R., Schnell R. C., Bodhaine B. A., Oltmans S. J. and Gammon R. H. (1985b) Air mass characteristics in the Vicinity of Barrow, Alaska, 9-19 March 1983. Atmospheric Environment 19, 2127-2134. Raatz W. E. and Shaw G. E. (1984) Long-range troposperic transport of pollution aerosols into the Alaskan Arctic. J. dim. appl. Met. 23, 1052-1064.
2549
Radke L. F., Lyons J. H., Hegg G. A., Hobbs P. V. and Bailey I. H. (1984) Airborne observations of Arctic aerosol-I. Characteristics of Arctic haze. Geophys. Res. Lett. 11, 393-396. Rahn K. A. (1981) Relative importances of North America and Eurasia as sources of Arctic aerosols. Atmospheric Environment 15, 1447-1455. Rahn K. A., Joranger E., Semb A. and Conway T. J. (1980) High winter concentrations of SO, in the Norwegian Arctic and transport fro-n Eurasia. Nature u17, 824-826. Rahn K. A. and Lowenthal D. H. (1986) Who’s polluting the Arctic? Why is it so important to know? An American perspective. In Arctic Air Pollution (edited by B. Stonehouse), pp. 85-95. University Press, Cambridge, London. Schnell R. C. and Raatz W. E. (1984) Vertical and horizontal characteristics of Arctic haze during AGASP: Alaskan Arctic. Geophys. Res. Lett. 11, 309-372. Schnell R. C., Watson T. B. and Bodhaine B. A. (1989) NOAA WP-3D instrumentation and flight operations on AGASP-II. J. ntmos. Chem. (in press). Thornton D. C., Bandy A. R. and Driedger A. R. III (1989) Sulfur dioxide in the North American Arctic. J. atmos. Chem. (in press).
Atmospheic Environment Vol. 23, No. 1I, pp. 2537-2549.1989. Printed in Great Britain.
A MAJOR HAZE EVENT NEAR POINT BARROW, ALASKA: ANALYSIS OF PROBABLE SOURCE REGIONS AND TRANSPORT PATHWAYS l-3.A.
~~i~~~~,*~
R.C. SCWNELL,~J.D.KAWL,*~C.
A. HERBERT*
and E. JORANGEW
*Geophysical Monitoring for Climatic Change, National Oceanic and Atmospheric Administration, 325 Broadway, Boulder, CO 80303, USA, $Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309 and I/Norwegian Institute for Air Research, 2001, Lillestrsm, Norway
Abstract-On 2-3 April 1986, an unusually dense Arctic haze band with distinct horizontal and vertical boundaries was intercepted by a NOAA WP-3D aircraft north of Barrow, Alaska during the second Arctic Gas and Aerosol Sampling Program (AGASP-II). Within this haze event, condensation nuclei (CN) taunts exceeded 70,000cm-3, and aerosol scattering extinction coefficients (b,,) were > 70x lob6 m-r. SO, con~n~ations in the haze were I5 ppbv. Synoptic met~rolo~~l data coBected in Arctic and subArctic regions were supplemented with trajectory analyses to determine the probable source regions and transport pathways of the haze. Anatyses of these data, along with surface air quality measurements in Norway, suggest that the haze originated in central Europe 10 days earlier. The transport pathway followed a typical pattern characteristic of spring haze transport in the Arctic. Key word index: Arctic haze, polar meteorology, Arctic pollution, aerosols, long-range transport, Arctic trajectories.
types of trajectory analysis (Barrie et al., 1981; Harris,
lNTRODUCTION The occurrence of layered haze in the Arctic tropo-
sphere, originally documented by Mitchell (1957), has been the subject of considerable study in recent years (for example, Barrie et al., 1981; Schnell and Raatz, 1984; Radke et al., 1984; Raatz et al., 1985a,b; Lowenthal and Rahn, 1985). Haze layers are usually 2OOm-7 km thick with a horizontal extent of 20-200 km, and are often associated with distinct air mass boundaries (Raatz, 1985). Haze is most common between pressure altitudes of 700 and 900 mb. Radke et al. (1984) related the layering of Arctic haze to the strengthened thermal stability in the Arctic, the combined influences of different source regions and their locations relative to the polar front, reduced scavenging, phot~hemistry over the ice pack, and slow meridional transport in a weak atmospheric flow. Reconstruction of haze transport is complicated by the considerable distances between observed haze layers near Alaska and their potential sources (4000-10,000 km), and the sparseness of meteorological data in the Arctic basin. Nevertheless, it is possible to obtain estimates of source region and transport pathways through the use of chemical tracers (Pacyna, 1984; Lowenthal and Rahn, 1985), improved synoptic maps of the polar hemisphere, and various tPresent athfiation: Department of geography, University of Newcastle, NSW, 2308, Australia. fiTo whom correspondence should be addressed.
1984, for example). Because the Arctic atmosphere is so stable, the chemical and physical signatures in haze associated with source regions show little change over distances of the order of thousands of km (Lowenthal and Rahn, 1985). Many Arctic haze layers are thin, with pollutant concentrations elevated slightly above background levels (Herbert et al., 1987). On occasion, a haze layer can be quite dense, have condensation nuclei (CN) concentrations > 10,000 cme3, aerosol scattering extinction (b,& greater than 70 x 10m6 m-‘, accumulation-mode aerosols (0.1-1.0 pm diameter) an order of magnitude higher than background, and major increases in sulfate aerosol, sulfur dioxide, soot carbon, gaseous and solid organ&, and heavy metal components. Such haze events have been described for the 1983 Arctic Gas and Aerosol Sampling Program (AGASP-I) by Schnell and Raatz, 1984; Raatz et al., 1985a,b; Raatz, 1985; and others. The purpose of this paper is to describe the aerosol characteristics, meteorological conditions, and transport of a major haze event observed during AGASP-II and encountered by a NOAA WP-3D aircraft near Point Barrow, Alaska, on 2-3 April 1986. The instrumentation of the aircraft has been described by Schnell et al. (1989). Through the use of synoptic meteorological data and trajectory analyses, it is possible to track the movement of this haze layer from the general area of origin and the time when it was injected into the
2537
2538
H. A.
BRIDGMAN er al. APRIL 2, lQ86
Arctic to the point of encounter with the aircraft. The trajectory model used in this study is the Atmospheric Environment Service (Canadian) Long Range Transport of Air Pollution (AES LRTAP) model (Olson et al., 1978; Barrie et al., 1981). Supporting meteorological data were obtained from the West German Meteorological Service (surface) and the U.S. National Climatic Data Center through the National Center for Atmospheric Research (NCAR) (850 mb and above). CN concentrations and h,, are used to characterize the haze.
HAZE LAYER CHARACTERISTICS, 2 3 APRIL
20000 _10000%
5000 tooo:h;-
J
2000 -
ii
500 2oofl
1986
On 2 April 1986 the NOAA WP-3D aircraft encountered an unusually dense haze layer while flying 300 km north of Point Barrow, Alaska (see Herbert et al., 1989, for full flight details). Initial contact with the haze occurred at 2331 GMT at 72.28”N, 158.27”W at a pressure altitude of 794 mb. After flying level for 10 min in the haze, the aircraft reversed track from 188” to 022” and exited the side of the haze flow at a point 30 km upwind of the entry point. The haze parameters are shown in Fig. 1, where it may be observed that an abrupt increase in CN concentrations from a few hundred to 70,000 cm- 3 occurred at the haze boundary coincident with a sharp increase in relative humidity (r.h.) from 50% to 75%. Aerosol b,, increased from 35 x 10m6 m-l at the haze edge to - 60 x 10m6 m-’ at the point where the aircraft reversed direction. Data from related studies show that, in the haze layer, black carbon reached 900 ng m-3 (Hansen and Novakov, 1989); aerosol optical depths, 0.75 (Dutton et al., 1989); and SO, 15 ppb (Thornton et al., 1989). Herbert et al. (1989) suggested that active SO* gas-to-particle conversion was occurring in this layer to produce the high CN concentrations. A vertical profile through the layer, shown in Fig. 2, established that the haze was
0056-O
50000 -
,
-2o-
’
,I ”
I
I
I
’
’
’
-
I TlME
w%Im;30te
LAT LON
72.34 I 158.24
:
71.76 : 158.51
2400 72.19 157.70
72.35” N i 158.11’ W
Fig. 1. Time series of CN, dry aerosol bsp, ozone (OZ), water vapor mixing ratio (MR),and ambient temperature (TA) between 2330 and 2400 GMT on 2 April 1986when the aircraft first encountered the major haze layer 300 km off the north coast of Alaska. mainly concentrated between 900 and 750 mb (750-2500 m). Wind direction throughout the profile was steady at 270”-280”, and wind speed increased near the surface to 15 m s-l at 600 mb. from4ms-’
126 GMT 3 APRIL 1986
500 AGASP
0 1.
10 , 0
20 40 so b., -- (x 16’ni’) -1.
*I*
II
80 TA (“C) *I
C*“.**B*‘l
100 1000 10000 0 CN hi’) . . * * ‘. ‘. . I 20 40 00 80 100 02 (ppb)
20
40 60 RH (%)
80
100
8
0
100
200 WD (9
300
Fig. 2. Vertical profiles of ozone (OZ), h,,, CN, ambient temperature (TA) relative humidity (RH), wind speed (WS), and wind direction (WD), through the major haze layer, 72”N, 158”W.
2539
Major haze event in Alaska
surface flow (Carlson, 1981; Iversen, 1984). Thus, air quality helow the inversion is largely determined by local sources rather than long-range transport except when the inversion decreases in intensity and pollutants mix down to the surface (Raatz et al., 1985b). Nerbert et al. (1987) found a strong positive correlation between CN and moisture in the haze layer. Figure 4 shows l-min averages of CN plotted against r.h. and temperature through the haze layer in the vertical profile shown in Fig. 2. The vast majority of CN concentrations > 6000cmi3 occur in air with r.h. 70-80% and temperatures of -25” to -27.5”C. On the basis of earlier research, the source of such a
Latitude-altitude cross-sections through the haze layer depict more completely its structure and extent, as shown in Fig. 3. In this figure, CN concentrations and bSP show that the base of the haze layer was coincident with the top of the surface temperature inversion. In the zone bounded by 800-900 mb, there is an undulation between maximum and minimum CN values with Iatitude. Between 750 and 6OOmb, there is a pronounced north to south CN gradient. Below the surface inversion, an abrupt decrease in CN concentration occurs. This is best explained by the fact that the surface inversion in the Arctic decouples the general tropospheric circulation from the 2 153-030 2-3
LA1
69
70
71
72
73
1 GMT
APRIL
1986
74” N LAT 69
70
71
72
73
74’ N
Fig. 3. ~titud~aititude cross-sections of b,V and CN through the major haze layer, 72”N, 158”W. The thin~ash~ lines show the flight path; the thick-dashed lines indicate the boundaries of stable layers.
,
60000~
,
,
AGASP
46000
_
APRIL
,
,
,
,
,
,
-
‘,‘,,,‘,” -
- II
_
.
2, 1986
CN vs RH
-$
36000
-
24000
-
AGASP-II APRIL
2, 1986
.
CN “o TEMP
l
.
l
l
s 5
.
l
l l
l;.
S l
i 20
40
60
RH (%)
80
100
-45
-40
-35
-30
-25
-20
TEMP (“C)
Fig. 4. CN concentrations vs relative humidity (RH) and temperature for the period the aircraft was investigating the haze layer, showing that higher CN concentrations were associated with greater RH and warmer temperatures. Data are I-min averages.
heavy haze would most likely be either western Europe, eastern Europe or the U.S.S.R. (Rahn, 1981: Lowenthal and Rahn, 1985; Rahn and Lowenthal, 1986). Haze originating in the central U.S.S.R. has been associated with cold, dry air and that from western Europe with warmer, relatively moist air (Raatz, 1985). This haze often enters the eastern Arctic with a surge of air from the south. Surface air quality measurements obtained from the Norwegian Institute for Air Research (NILU) indicated that a large northward-flowing pollution mass passed over Scandinavia and entered the Arctic on 22--24 March, 9--l 1 days prior to the haze interception off of Alaska. This provided a starting point for a study with the goal of determining where this pollution over northern Europe had moved to, where the pollution north of Alaska had originated, and what pathway the pollution followed across the Arctic. Following is an analysis of the meteorology and transport over Eurasia and the Arctic leading up to the haze event observed north of Point flarrow, Alaska. The accumulation and northward movement of pollutants in the European sector is presented. Analyses of synoptic maps, restricted to the surface and 850 mb where the bulk of the haze existed, show that atmospheric stagnation over Europe in late March was followed by a surge of pollutants injected into the
0000 (4
Arctic with subsequent transport across the Arctic basin. Finally. results from the AES LRTAP trajectory model are used to establish movements and possible source regions of the haze layer. The presentation of the data generally follows the pattern and nomenclature introduced by Raatz et cd. (1985a).
STAGNATION
ACCl;MLlLATION
OF POLLL’TANTS
OVER EUROPE
Synoptic analysis, 0000 GMT, 26 March 1986
The surface chart of the Arctic shown in Fig. 5a depicts a low-pressure system centered over central Scandinavia at 62’N latitude. An occluded front extended southward from the eastern section of the low across eastern Europe. A warm front extended northeast from the western section of the low to the Siberian coast. Surface winds associated with the system were Iight. Relative humidities between the two fronts were about 90%, but behind the occluded front, a zone of humidities (60% extended over central and western Europe. Skies were partly cloudy to cloudy over most of northern Europe. An extensive anticyclone centered on the Ural Mountain region covered most of the continent between the northern tip of Norway and the Soviet
0000
GMT 26 MARCH 1988 SURFACE
AND
(b)
OMT 26 MARCH 1986 850 mb
I
Fig. 5. (a) Surface (pressure, mb) and (b) 850 mb (geopotential height, m) synoptic analyses for the Arctic basin, 0000 GMT, 26 March 1986. The broad arrow indicates a probable path ofgreaterhaze flow. Wind arrows show the direction of air movement (one barb = 5 m s- ‘1.
Major haze event in Aiaska
island of Novaya Zemlya. Surface winds were light to calm throughout this region. Humidities near the center of the anticyclone dropped below 50%. East of the ridge, a low-pressure area previously over the Siberian coast had moved to the south and inland to central Siberia. Farther east, a high-pressure area dominated eastern Siberia and Alaska. Weak airfiow from eastern Siberia into the Arctic occurred between this high and the central Siberian low. At 850mb, as shown in Fig. Sb, airflow across Europe was south and southeast flow over Scandinavia associated with the low-pressure area located there. The southwest circulation pattern persisted along the northern edge of the Siberian ridge. Circulation in eastern Siberia was from the south and stronger, associated with the ridge interacting with the Siberian low. Relative humidities >80% were restricted to the areas dominated by low pressure, primarily over Scandinavia and eastern Europe. Wind speeds over northern Europe averaged 20 m s- ’ near the low, dropping to S-IOmsfarther south. A small low was developing near the North Pole. Air Rowed auross the Pole from the region of Novaya Zemlya, then around the high-pressure system near Point Barrow to create easterlies along the Alaskan coast. These conditions were the culmination of a stagnant meteorological situation that had existed over Europe since 3 March resulting in a strong build-up of pollutants. Any northward transport of haze associated with conditions on 26 March would most likely have occurred in the moist zone along the western side of the ridge located over the Urals ahead of the occluded front as indicated by the arrow in Fig. 5b in a manner described by Raatz and Shaw (1984) and Rahn (1981). Haze analysis Air quality data for Norway, representative of the polluted air mass over Europe, were obtained from the NILU monitoring network. The locations of the surface stations are shown in Fig. 6. As a first step in determining the flow and source of the haze, we examined sulfate concentrations from the mainland stations for March 1986. Figure 7a shows that the stagnating meteorological conditions over western Europe and Scandinavia during March led to a buildup of air pollution beginning 9 March progressing from south to north into the Arctic. At Birkenes and Skreildalen, in the southernmost section of the network (58”-59”N), con~ntrations peaked three times between i0 and 19 March. The multiple nature of the peaks suggests fluctuations in the transport of the polluted air in the relatively weak south to south southwest airflow during this period. At Hummelfjell (62.3”N) the peak of SOi- occurred between 18 and 20 March, several days after higher concentrations were measured farther south. Farther north at RBrvatn (62S”N, Fig. 7b) the peak in SO:- con~ntrations also occurred during this
2541
Norwegian Air Poltution Network
Fig. 6. Surface monitoring stations in the Norwegian Air Quality Network.
same period, but was about i/3 the concentration of that at Hummelfjell. Since Kgrvatn is about 2” (- 125 km) west of Hummelfjell (see Fig. 6) and was in prevailing south southwest flow, it may have been at the western edge of the polluted air mass. SOiconcentration peaks at Tustervatn (658’N) were similar in timing, but of lower concentration than those observed at Hummel~ell. Still farther north at Jergul(69.3”N), the bulk of the polluted air mass arrived between 22 and 24 March; light pollution was detected the previous week. It is worthy of note that the levels of pollutant SOi- at Jergul (Fig. 7b) are greater than at Klirvatn and Tustervatn, the next two stations south the Jergul in central Norway. This suggests that the pollution stream was located in northeast Norway at that time. Extending the gradient, one could suggest that the pollution was even stronger east of Jergul. At Bjern@ya (Fig. 7c), the western portion of the haze peaked on 25 and 26 March at values twice as large as the low values 2 weeks previously (note the difference in scale from Figs 7a and 7b. Sulfate concentrations at this station then diminished to below the monthly average during 27-28 March, and increased again to a secondary ak between 29 and 31 March. At Ny &4? lesund (78.9”N), about 500 km north of Bjerrnerya, the peak in SO:- concentrations occurred
H. A. BRIDGMAN
2542
et al.
27.0
270
24.0
210 18 0 o^ k s IIe
180
c^ 15.0
i
15.0
12.0
2 II-z
12.0
is
s: 9.0
9.0
6.0
6.0
3.0
0 !
(a)
/
4
1
6
I
a
,
I I I I , I I I 10'12 14 16 la 20 22 24 26'28'30
Day (March
2'4' (b)
1986)
7.8 72-
-
SOT Concentrations Bj0m0ya 74O31'N, 1V'Oi'E
6 6 ~-
---
Ny besund
7P55’N
llO55’E
Day (March
54
;
4.8 4.2 -
5:
3.0 --
1986)
: II ::
--
9 II-f
30'
7
6.0
67
6'8'10'12'14'16'18'20'22'24'26'28
3.6 -~
2.4 1.8
1.2 0.6 0 ‘2
’ 4 ’ 6 ’ 8 ‘10’12’14’16’18i20’22’24’26’28’30’ March
Cc) Fig. 7. Daily-indicator pollution from stations in the Norwegian
1
1986
Am
3 1986
Day
concentrations for March 1986 Air Quality Network shown in
Fig. 6: (a) southern mainland stations, (b) central and northern mainland stations, (c) northern island stations.
2 days later, on 28 March. Thus, the initial pulse of pollution traveled at an average speed of about 3 m s- ‘, as deduced from surface wind analyses. SOiconcentrations at Ny Alesund remained relatively high until 30 March as a portion of the polluted air mass remained over Svalbard. From Fig. 7, it may be further observed that SOiconcentrations over mainland Scandinavia fell to background levels after 25 March, as the haze was advected northward ahead of the front (discussed below). The movement of the low-pressure center toward Ny Alesund, plus a wind shift to the south on 27-28 March, terminated the pollution injection into the Arctic.
METEOROLOGY POLLUTANTS
OF THE MOVEMENT
OF THE
AWAY FROM THE EUROPEAN CONTINENT:
SYNOPTIC ANALYSIS, QOOO GMT, 27 MARCH 1986
Synoptic
nnalysis,
0000
GMT,
27 March
1986
Now that we have shown that pollution moved into the Arctic, we present data on the transport of that pollution across the Arctic basin. During 26 March the low over Scandinavia weakened considerably leaving a combined and moved north, occluded-stationary front extending from the northern tip of Norway, southeast, and then south across eastern Europe, as shown in Fig. 8a. The Siberian ridge ahead of the front, with a central pressure of
Major haze event in Alaska
0000 (a)
0000 w
ONIT 27 MARCH SURFACE
1986
GMT 28 MARCH
1986
SURFACE
0000
GMT 27 MARCH 1986 850 mb
0000
GMT 28 MARCH 1986
(b)
(4
2543
850
mb
Fig. 8. (a,~)Surface (pressure, mbf and {b,d) 850 mb(geopotential heigbt, m) synoptic anaiysesfor the Arctic basin, ooo0 GMT, 27 and 28 March 1986. The hatched arrows on the 850 mb mam indicate the probable movements of the return-flaw and Arctic basin components of the haze. Wind arrows show the-general direction of air movement (one barb= 5 ms-‘).
2544
H. A. BRILIGMAN et al.
1035 mb, had assumed a blocking position (Lejenas and Okland, 1983) and extended its influence northward across Novaya Zemlya. Circulation associated with this pattern comprised a gateway to the polar regions across northern U.S.S.R. and Novaya Zemlya. Surface wind speeds at this time were between 5 and lOms_‘, with a well-defined fetch extending back across western and eastern Europe. The air in this pathway was moist, having r.h. > 80%-probably the result of the air cooling radiatively as it crossed the Polar region. This northward surge of polluted air through the northern Scandinavia-northern U.S.S.R. region was most apparent at 850mb, as seen in Fig. 8b. The pathway is consistent with previous research, which suggests that Arctic haze injections occur closer to western Europe with the advance of spring conditions (Raatz and Shaw, 1984). There is a strong possibility that the haze followed two llow paths once in the Arctic, one portion taking the classic return flow route around the Scandinavian low (Rahn et al., 1980) and a second portion following the common pattern IIb described in detail by Raatz and Shaw (1984). We focus our study on the portion of haze over Scandinavia that had the higher potential to travel across the Arctic basin to Alaska.
SURGE CUT-OFF: SYNOPTIC ANALYSIS, WOO GMT, 28 MARCH 1986
By 0000 GMT on 28 March, the circulation pattern creating the surge from Europe was breaking down, as shown in Fig. 8c. The fronts over northeast U.S.S.R. and the European Arctic had combined into one stationary front, and overriding air from the south brought snow to the area. The low-pressure cancer over the North Pole had become slightly stronger (loo0 mb), forming a weak, closed circulation. This flow pattern acted as a draw for the haze then in the Norwegian Arctic. The weak low previously west of Scandinavia had moved north to 75”N, and a secondarly low (995 mb) had formed over Svalbard. The occluded front previously over eastern Europe had begun to move east and had crossed the Urals in northern Russia; a portion trailed back across Europe toward Italy as a warm front. Airflow behind the front was southerly at 5-10 ms- ‘. The high-pressure ridge over western Siberia was beginning to break down and had weakened by 10 mb in the Novaya Zemlya area. The high previously dominating the western Arctic had weakened and had begun to move southward into eastern Siberia. As at the surface, the 850 mb airflow into the polar regions was considerably weaker than before, averaging less than 7 m s-i (Fig. 8d). The zonal flow over northern Europe and northern Siberia had strengthened considerably, with southeast winds averaging 15-20 m s- *. The reestablishment of zonal flow over
Europe probably began moving haze material from the source regions quite rapidly to the northeast, toward Novaya Zemlya. Research on previous similar situations by Ottar et al. (1986) indicated that the haze would most likely have initially traveled at an altitude of lOO3OO m, but would have risen with the warm air over the front. HAZE TRANSPORT
In this section, we present trajectory analyses in the belief that objective analyses by computer models that take into account all the data available in the datasparse region should provide a complementary product to the more subjective synoptic weather map analyses discussed above. No surface or aircraft measurements of the anthropogenic haze layer over the Arctic basin were made until 2-3 April 1986, during the WP-3D flight near Point Barrow, Alaska. Therefore the transport of the haze must be based upon meteorological parameters that suggest the movement of the haze-containing air mass. In Europe and the northern U.S.S.R., there are over 200 meteorological stations from which synoptic analyses are produced. On the coastal areas and islands within the Arctic, there are 20 surface weather and rawinsonde stations. But on the ice cap itself, there is only one rawinsonde station, at approximately 85”N, 145”W. The shortage of data over the ice is somewhat mitigated by the fact that the Arctic basin is composed of relatively homogeneous terrain (ice) and has a relative lack of local circulations compared to lower latitude locations. The complete European, Asian, and Arctic basin data set was used in the following studies. Back-trajectories in Europe indicating possible haze source regions
Upstream trajectory analyses using the AES LRTAP trajectory model were performed to determine the probable transport pathways for haze reaching points in northern Europe. This model was used because it includes the effects of synoptic-scale vertical motions. These motions are of particular importance for determining source regions for Arctic pollution (Carlson, 1981; Iverson, 1984). Although trajectory analyses have proved useful in identifying the longrange transport of atmospheric pollutants, particularly in the Arctic, large uncertainties exist due to interpolation of sparse meteorological data (Kahl and Samson, 1986, 1988). Thus, trajectories should be interpreted as a general indication of air movement, recognizing that errors of up to several hundred kilometers may accrue over several days of transit. Upwind (backward) trajectories beginning at 1200 GMT on 26 March 1986 were calculated from three locations. Trajectories starting at 65”N, 14”E (point A, Fig. 9) in central Norway represent the movement of haze associated with stations in the Norwegian Institute for Air Research network. Trajectories starting
2545
Major haze event in Alaska
1200 QWr 26 MAR 1986
Fig. 9. Five-day 1000 mb backward trajectories calculated by the AES LRTAP map from three points in Europe: A (65”N, 14”E), B@O”N, 32”E), and C (7O”N, 50”E). Tick marks indicate 24-h intervals. at 6O”N, 32”E (point B) represent the potential
corridors of haze movement along the backside of the high (Raatz and Shaw, 1984), and back-trajectories from 70”N, 50”E (point C) represent potential haze injection into the Arctic across the Soviet Arctic coast. The results shown in Fig. 9 illustrate that the probable source region for air at 925 mb reaching all three locations was eastern Europe and the western U.S.S.R., areas with large anthropogenic emission rates (Barrie, 1986). Airflow to central Norway (point A) originated over the Austriaregions. Airflow to Hungary-Czechoslovakia point B originated over France, passing into eastern Europe before moving along the backside on the high. Airflow to the Soviet Arctic coast (point C) originated in the Black Sea region and flowed up the western side of the Ural Mountains. These results suggest that the main source of pollutants measured in the Norwegion network was eastern Europe and that some western Europe precursors were probably included. Trajectories
3 APR
1986
1200 OMf 26 MAR 1986
indicating polar basin transport
Figure 10 presents downwind (forward) trajectories originating at the same points A, B and C shown in Fig 9, beginning at 1200 GMT on 26 March 1986. Also shown are backward trajectories arriving at point D (72”N, 158”W), the point of aircraft contact with the haze off Alaska, terminating at 0000 GMT on 3 April 1986. When trajectories originating at two levels were simiIar, they are shown for clarity as one.
Fig.10. Five-day forward trajectories at 900 and 925 mb calculated by the AES LRTAP model originating at points A, B and C (as in Fig. 9) on 1200 GMT, 26 March 1986, and a five-day backward trajectory originating at 0000 GMT, 3 April 1986 from point D (72”N, 158”W). the point of aircraft contact with the hare layer. Tick marks are printed at 24-h intervals. When trajectories were similar at two levels, only one is shown for clarity (a) 925 and 900 mb; (b) 850, 800 and 700 mb.
2546
H. A.
BRIDGMAN
Trajectories starting at point A (Fig. 10a) in central Norway moved westward into the Atlantic after remaining stagnant over Norway for several days, These results support the suggestion that some haze moved in a return flow pattern around the low (Raatz and Shaw, 1984). At 850mb and above (Fig. lob), airflow for the 5-day period was northward to the Norwegian Arctic coast, and then east toward the Soviet coast. Trajectories starting at point B generally represent airflow and potential haze movement away from eastern Europe. Air at all levels in Figs lOa,b between 900 and 700 mb, moved north-northeast along the Soviet Arctic coast, finishing just north of the Taymyr Peninsula after 5 days. These trajectories suggest that a transport pathway across the Murmansk U.S.S.R. area into the Soviet Arctic was a likely pathway for further movement across the Arctic basin. Trajectories from point C represent Arctic haze that would have originated in the western U.S.S.R., mainly from sources along the western flank of the Ural Mountains. Trajectories beginning at levels up to 850 mb parallel the equivalent trajectories from point B, indicating that the airflow at these levels was probably part of the injecting corridor along the Soviet Arctic coast. Airflow from point C at the higher 700 mb level, however, moved southeast into central Siberia. The forward trajectory calculations shown in Figs lOa,b suggest that haze probably originated in eastern Europe and the western U.S.S.R., and most likely followed a pathway into the Arctic basin east of Novaya Zemlya, skirting the Taymyr Peninsula before moving toward Point Barrow. Haze transport out of Europe was most likely between 900 and 800 mb, and probably was not strictly isentropic because radiative cooling of the haze layer would allow gradual sinking as it transected the Arctic basin. Haze measured by the air quality stations in Norway was not injected directly into the Arctic, but was instead eventually carried southwest over the Atlantic Ocean. Back-trajectories
from the Alaskan Arctic
Backward trajectories from point D, where the aircraft measured the haze, are also shown in Figs 10a and b. At levels from 925 to 700 mb the trajectories track back across the Arctic basin to essentially intersect the forward trajectories coming out of Europe. This overlap of 5-day trajectories suggests’a reasonable path for the haze transport, and the 9-10 day transit time from Europe to Alaska agrees with ground measurements of the haze departure from the Norwegian network and arrival in Alaska as determined with aircraft measurements (Herbert et al., 1989). Meteorology
in the Arctic Basin
In a final effort to further diagnose the meteorology of the haze transport across the Arctic basin, data
et al.
from a number of sources were merged and interpreted as follows. Surface synoptic maps of the Arctic from the West German Weather Service were analyzed to obtain an initial indication of meteorological conditions across the basin. These maps were supplemented at 850 and 700 mb by computer depictions of the spatial variations of temperature, humidity, winds and geopotential height. These latter data were originally compiled for use in the NCAR Global Climate Model. Using these data, along with the modeled trajectory paths, we developed a scenario to approximate the movement of the haze across the Arctic basin from the time of the surge over the Soviet Arctic coast to the arrival of the haze off Point Barrow, Alaska. The aircraft data established and trajectory data suggested that the haze flowed between 900 and 750 mb. The movement of the haze is shown on a series of 850 mb synoptic maps (Fig. 11) for the period 30 March-2 April. In Fig. 11, the suggested location for a portion of the haze layer is depicted by an oval shaded area. The shaded region is not meant to specify exact boundaries of the haze, but to indicate a probable general location of the haze region subsequently sampled by the aircraft. From Fig. 11, it may be observed that variations in three major synoptic features determined the estimated flow of the haze across the basin. The first was the movement of the low from Svalbard to the east-northeast. Winds were weak during this period ( < 5 m se I). In the return-flow pattern, the haze over Svalbard was probably caught in the northerly flow and moved south into the Atlantic. This southward movement of the western part of the haze layer would explain why SO: concentrations at Bjornoya showed a secondary peak between 29 and 31 March (Fig. 7) when no direct surge occurred from any known source region. Meanwhile, the eastern portion of the haze probably moved northward across the basin. The second major synoptic feature of importance was the low that developed over the North Pole on 27-28 March, and moved southward toward Alaska between 28 and 30 March. This low become stationary about halfway between the pole and the Alaskan coast by 31 March. It controlled the circulation in the western Arctic through 2 April, creating northerly and northeasterly winds of 1O--l 5 m s ’ on its polar side. By 31 March, the movement and strengthening of this low created westerly winds of 15 ms- ’ along the Alaskan coast. On 31 March, the haze layer (Fig. 1lb), previously off the Taymyr Peninsula, was transported in the northerly flow associated with the west Arctic low and began to move more rapidly toward the Alaskan coast. Winds behind the haze layer were now northeasterly toward the Siberian coast, and the haze became completely divorced from the eastern Arctic. The third major synoptic feature influencing the movement of the haze layer was the high-pressure system over eastern Siberia, which began strengthening on 30 March. Over the next 3 days, the high
2541
Major haze event in Alaska
0000 (4
0000 (c)
0000
GMT 30 MARCH 1988 850 mb
@I
GMT 1 APRIL 1986 850 mb
(4
0000
GMT 31 MARCH 1988 850 mb
GMT 2 APRIL 1986 850
mb
Fig. 11. The 850 mb synoptic analyses (geopotential height, m) for the Arctic basin at 0000 GMT: (a) 30 March,(b) 31 March, (c) 1 April, (d) 2 April 1986. The shaded areas indicate the probable locations of a portion of the haze layer, and the hatched arrows the layer’s general direction of movement. Wind arrows show the direction of air movement (one barb = 5 m s- ‘).
2548
H. A.
BRIDCMAN
extended its influence northward into the Arctic basin, enhancing the northwest airflow along the Alaskan coast toward the west Arctic low. A ridge from this high developed toward the pole beginning on 1 April (Fig. 1lc), eventually creating high pressure along the entire Siberian side of the Arctic. By 0000 GMT, 2 April (Fig. lid), the haze was in the westerly airflow along the Arctic coast where it was subsequently
intercepted by the aircraft.
SUMMARY
AND FINAL REMARKS
Analyses of the meteorology involved in the transport of a dense haze layer encountered by an aircraft north of Point Barrow, Alaska (72”N, 158”W), on 2-3 April 1986 have revealed a coherent picture pointing to potential sources and probable transport path ways. From the AES LRTAP trajectory model and relevent synoptic analyses, the source region of the haze was suggested as being mainly central Europe. The primary transport of haze began as a surge across the Soviet Arctic coast on 27 March 1986 and followed a pathway east of Novaya Zemlya to just north of the Taymyr Peninsula. Haze movement at first was relatively rapid, but by 29 March, it had slowed in the light and variable circulation just east of Novaya Zemlya. The haze layer was then drawn into the circulation pattern around the Iow located near the North Pole on 31 March, and from that time moved more rapidly to its position off Point Barrow on 2 April. This transport appeared to be confined to a layer between 900 and 800 mb. A westerly portion of the haze was probably transported in the return flow pattern associated with a weak low near Svalbard (Raatz and Shaw, 1984; Rahn, 1981), between 26 and 29 March. This part of the haze would eventually move southward into the Atlantic. Raatz and Shaw (1984) established that the transport pathway taken by the major haze layer in 1986 was typical of that associated with spring conditions in the European Arctic. Surge events in spring occur almost exclusively west of the Ural Mountains and are often associated with a return-flow pattern in which some of the haze moves toward Svalbard then flows west then south in circulation around a lowpressure system north of Scandinavia. On the basis of the strength of evidence presented here, we conclude that central Europe is the most likely source of the haze layer observed off Alaska on 2 April, and that the haze travelled from Europe to Alaska in IO+1 days. Because of the lack of haze measurements between northern Scandinavia and Point Barrow, however, as well as the sparsity of meteorological data in the Arctic basin, contributions from sources in western U.S.S.R. cannot be totally excluded. Ultimately, chem+caI analyses of the haze aerosol constituents will have to be relied upon as a final determinant of the source regions.
ef ul.
Acknow&gements--We thank L. Barrie and M. Olson for the AES LRTAP trajectory results, and J. Harris, D. Lowenthai, and K. Rahn for valuable comments and advice. An early version of this paper presented at the 4th International Conference on Arctic Haze in Hurdal, Norway (28 September-2 October 1987), elicited many valuable comments from the audience. This research was performed while J. Kahl worked as a NRC Research Associate at the Geophysical Monitoring for Climatic Change (GMCC) division of the National Oceanic and Atmospheric Administration (NOAA). This research was financially supported in the field by GMCC and the NOAA Ofice of Aircraft Operations; Office of Naval Research (ONR); National Aeronautics and Space Administration; Naval Air Systems Command; and grants to individual investigators from the National Science Foundation. Data analysis was supported by the Defense Nuclear Agency, and by ONR through the University Research Initiative Program, contract N~l4-86-0695. A special thanks to the staff and air crew from the Miamibased Office of Aircraft Operations, who equipped the aircraft and flew in the Arctic under trying conditions. Manuscript typing was provided by L. Prendergast, R. Rossen, and M. Van Asche of NOAAiGMCC. Drafting was by J. Howe. REFERENCES
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1407-1419.
Carlson T. N. (1981) Speculations on the movement of polluted air to the Arctic. Afmospherir Environment 15, 1473-1477. Dutton E. G., DeLuisi J. J. and Herbert G. H. (1989) Spectral aerosol optical depth of Arctic haze as measured on board the NOAA WP-3D during AGASP-II, April 1986. J. atmos. Chem. (in press). Hansen A. D. A. and Novakov T. (1989) Aerosol black carbon measurements in the Arctic troposphere. J. ntmos. Chem. (in press). Harris J. M. (1984) Trajectories during AGASP. Geophys. Res. Lett. 11, 453-456. Herbert G. A., Bridgman H. A., Schnell R. C., Bodhaine B. A. and Oltmans S. J. (1987) The analysis of meteorological conditions and haze distribution for the second Arctic Gas and Aerosol Sampting Project (AGASP-II). NOAA Technical Memorandum, ERL ARL-158, Boulder, Colorado. Herbert G. A., Schnell R. C., Bridgman H. A., Bodhaine B. A., Oltmans S. J. and Shaw G. E. (1989) Meteorology and haze structure during AGASP-II, Part I: Alaskan Arctic Flights, 2-10 April 1986. J. atmos. Chem. (in press). iverson T. (1984) On the atmospheric transport of pollution to the A&tic. Geophys. Res. iett. 11, 45%460. Kahl J. D. and Samson P. J. (1986) Uncertainty in traiectory calculations due to low resolutibn meteorolbgical data. j. dim. appl. Met. 25, 18161831. Kahl J. D. and Samson P. I. (1988) Trajectory sensitivity to rawinsonde data resolution. Atmospheric Environment 22, 1291-1299.
Leienas H. and Okland H. (1983) Characteristics of northern Hemisphere blocking as determined from a long time series of observational data. T&s %A. 350-362. Lowenthai D. H. and Rahn K. A. (1%) Regional sources of pollution aerosol at Barrow, Alaska, during winter 1979-1980 as deduced from elemental tracers. Atmospheric Environment.
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Mitchell J. M. Jr. (1957) Visual range in the polar regions _ with particular reference to the Alaskan A&c. J. &OS. terres. phys. Spec. Suppf. Pt. 1, 195-211. Olson M. P.. Oikawa K. K. and MacAfee A. W. (1978) A I
Major haze event in Alaska trajectory model applied to the long-range transport of air pollutants: a technical description and some model intercomparisons. LRTAP 78-4, Atmospheric Environment Service, Ontario, Canada. Ottar B. and staff (1986) Air pollutants in the Arctic. Final report, Norwegian Institute for Air Research, Lillestrom, Norway. Pacyna J. (1984) Estimation of the atmospheric emissions of trace elements from anthropogenic sources in Europe. Atmospheric. Environment l&41-50. Raatz W. E. (1985) Meteorological conditions over Eurasia and the Arctic contributing to the March 1983 Arctic haze episode. Atmospheric Environment 19, 2121-2126. Raatz W. E., Schnell R. C. and Bodhaine B. A. (1985a) The distribution and transport of pollution aerosol over the Norwegian Arctic on 31 March and 4 April 1983. Atmospheric Environment 19, 213552142. Raatz W. R., Schnell R. C., Bodhaine B. A., Oltmans S. J. and Gammon R. H. (1985b) Air mass characteristics in the Vicinity of Barrow, Alaska, 9-19 March 1983. Atmospheric Environment 19, 2127-2134. Raatz W. E. and Shaw G. E. (1984) Long-range troposperic transport of pollution aerosols into the Alaskan Arctic. J. dim. appl. Met. 23, 1052-1064.
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Radke L. F., Lyons J. H., Hegg G. A., Hobbs P. V. and Bailey I. H. (1984) Airborne observations of Arctic aerosol-I. Characteristics of Arctic haze. Geophys. Res. Lett. 11, 393-396. Rahn K. A. (1981) Relative importances of North America and Eurasia as sources of Arctic aerosols. Atmospheric Environment 15, 1447-1455. Rahn K. A., Joranger E., Semb A. and Conway T. J. (1980) High winter concentrations of SO, in the Norwegian Arctic and transport fro-n Eurasia. Nature u17, 824-826. Rahn K. A. and Lowenthal D. H. (1986) Who’s polluting the Arctic? Why is it so important to know? An American perspective. In Arctic Air Pollution (edited by B. Stonehouse), pp. 85-95. University Press, Cambridge, London. Schnell R. C. and Raatz W. E. (1984) Vertical and horizontal characteristics of Arctic haze during AGASP: Alaskan Arctic. Geophys. Res. Lett. 11, 309-372. Schnell R. C., Watson T. B. and Bodhaine B. A. (1989) NOAA WP-3D instrumentation and flight operations on AGASP-II. J. ntmos. Chem. (in press). Thornton D. C., Bandy A. R. and Driedger A. R. III (1989) Sulfur dioxide in the North American Arctic. J. atmos. Chem. (in press).