The thermodynamics of pollutant removal as an indicator of possible source areas for arctic haze

Atmospheric Environment Vol. 26A, No. 16, pp. 2953 2961, 1992. 0004-6981/92 $5.00 + 0.00 Printed in Great Britain. 6) 1992 Pergamon Press Ltd

THE THERMODYNAMICS OF POLLUTANT REMOVAL AS AN INDICATOR OF POSSIBLE SOURCE AREAS

FOR ARCTIC HAZE

SUE ANN BOWLING and GLENN E. SHAW

Alaska Climate Research Center, Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 99775-0800, U.S.A.

(First received 8 September 1991 and in final form 22 April 1992)

Abstract--The majority of papers on arctic haze have concentrated on chemistry, with some consideration of source areas, synoptic structures associated with transport, trajectory analysis and transport modeling. Very little has appeared on the basic thermodynamics of air with normal humidity, with or without pollutants, traveling into and through the Arctic. When physically reasonable constraints are placed on sources and sinks of heat and water mass and on the relative humidity of near-surface air, it becomes apparent that the assumption of isentropic (adiabatic) flow without precipitation is incompatible with observed water mixing ratios in arctic haze layers. Arctic haze at elevations below 3 km can be explained if precipitation has occurred during transport from the pollution source or if the pollutants were injected from tall stacks into a layer of dry, subsiding air. Haze layers at higher elevations require extreme dryness and relatively high temperatures at their sources, and may be suspected of being of desert origin.

Key word index: Precipitation, transport, haze layer, desert origin.

1. I N T R O D U C T I O N

Haze layers in the Arctic were first reported 35 years ago (Mitchell, 1956), but the subject attracted little interest until the haze was rediscovered in the early 1970s (e.g. Shaw and Wendler, 1972; Shaw, 1975; Rahn et al., 1977). Although arctic haze has been the subject of considerable interest since the rediscovery, very little attention has been paid to the thermodynamics of its transport. The purpose of this contri- bution is to examine the temperature and water vapor mixing ratios of haze layers in the Arctic and utilize thermodynamics to deduce the temperatures and relative humidities the air in these layers must have had when it was last at the surface. This will be shown to have some important implications which seem not to have been widely recognized.

Air moving into and through the Arctic is in a constant process of change. In order to become arctic air, it must lose both heat and moisture. Heat can be and is lost by thermal infrared flux divergence. Al- though this is a slow process (on the order of 1 °C d - 1 : Sellers, 1965) it becomes significant over periods of weeks to months, and is ultimately the primary process in the transformation of temperate air into arctic air. This heat loss involves lowering the potential temperature of the air and usually the temperature as well. However, adiabatic processes are also very im-

* So far as this paper is concerned, the term "mixing ratio" will henceforth refer only to the mixing ratio of water vapor in the atmosphere, usually expressed in grams per kilogram (g kg-t).

portant in lowering the temperature of air moving into the Arctic, as temperate air will normally move upward as it moves into the Arctic (Carlson, 1981). The result in either case is to reduce the saturation mixing ratio (grams of water vapor per kilogram of air necessary to have the air in equilibrium with a plane surface of pure water at the observed atmospheric pressure and temperature) as air moves northward.* The order of magnitude of this change is to halve the saturation mixing ratio for each 10°C of radiative cooling or, equivalently, for each kilometer the air moves upward without condensation.

The actual mixing ratio, however, as opposed to the saturation mixing ratio, can change in stable air away from the surface only if water is added to the air by evaporation of precipitation from higher layers or subtracted by precipitation from the air which is in the process of transformation. This means that unless the relative humidity (approximately the ratio of the observed mixing ratio to the saturation mixing ratio) in the region where pollution is added to the air is very low, air moving poleward and cooling will eventually reach saturation. Once this occurs, further cooling must be accompanied by precipitation. (Cloud formation may of course occur without precipitation, but this is a transient situation as re-evaporation of the condensed water can only occur with warming of the air.) The precipitation, in turn, will remove some pollutants. This leads to a dilemma: the areas usually identified as source areas for arctic haze (northern Eurasia in early spring) are snow covered and near- surface air has relative humidities of over 50%. Yet arctic haze is common up to 800 mb, which is roughly

2953

2954 S.A. BOWLING and G. E. SHAW

1600 m (16°C cooling without condensation) above the surface, and has been observed with some fre- quency up to 500 mb (Barrie, 1986). There is no way that surface air with 50% or more relative humidity can reach the observed elevations without some of the original water being removed by precipitation.

Several solutions to this dilemma are possible, including precipitation with partial removal of pollutants, injection of pollutants into dry air by large effective stack heights or in mountain regions, downward transport of pollution from the original air mass in ice crystals, or a desert origin for high-elevation pollutant layers. Application of these solutions, however, invalidates several assumptions often taken for granted in the analysis of arctic haze.

2. PUTTING LIMITS O N TEMPERATURE AND HUMIDITY

O F THE P O L L U T I O N SOURCE AREAS FOR ARCTIC HAZE

One way of looking at the processes which remove material from the atmosphere is to consider the state (temperature, pressure and relative humidity) of mid- latitude air into which pollutants are being released, and the state of the same air in the Arctic. Carlson (1981) used this approach but without calculating actual humidities necessary at source areas. He also treated the polar front and positions of isentropic (constant potential temperature) surfaces as if they remained at their statistical positions.

We consider solely the air mass characteristics necessary at the source region if polluted air is to contribute to arctic haze, together with changes in those characteristics which occur during transport. Since we are concerned with temperature and humidity at the time and place pollutants are injected into an air mass, climatological characteristics of the polar or arctic fronts are significant only in that they indicate probabilities that an area will have the temperature and humidity necessary to carry pollutants into the Arctic without significant precipitation. To take an example familiar to American meteorologists, the type of arctic outbreak referred to by the media as the "Siberian Express" can carry arctic air well into the southernmost tier of states. When this happens, parts of Texas or Florida may be effectively north of the arctic front. If these areas are emitting pollutants under these conditions and these pollutants are not contributing to arctic haze, it is because the pollutants are being removed before they get back to high latitudes, not because they are well south of the climatological position of the front. (In this case, any trajectory of already cold air back into the Arctic would generally involve flow of cold arctic air over the Gulf of Mexico or the subtropical North Atlantic, with resultant strong moist convection and rainout of pollutants.)

For the purposes of the following illustration we assume the following:

(1) Both moisture and pollutants are added only at the surface, which we initially assume to be at a pressure of 1000 mb.

(2) Once the air has left the surface, the removal of moisture occurs only through precipitation from a cloud formed in the polluted air. (Moisture can still be added by moist convective processes or evaporation from precipitation falling through the air, but this turns out to be irrelevant for the cases examined, as the problem lies in explaining why the observed mixing ratios are so low.)

(3) Precipitation originating in the polluted air removes pollution. As a rough approximation, assume that for a small amount of precipitation formed in the air mass, the fraction of the pollution removed is proportional to the amount of water vapor removed from the air as precipitation. Then if H o is the concentration of pollution with the only removal process being dilution as initial plumes spread and merge, H is the observed pollutant concentration, Ar is the decrease in the mixing ratio due to precipitation, and k is a constant that will vary with the details of the precipitation process, the amount of pollutant remain- ing in the air is given by H = H o e -kA,. If r is in its usual units of g k g - 1, k will be of the order of 10- 3 times the scavenging ratio, Win, expressed in kg kg -1 (e.g. Barrie, 1985; values given later).

(4) In the absence of precipitation or evaporation, the only process which can change the potential temperature of the atmosphere is radiative cooling. Sellers (1965) calculated 1.3°C d -1 as an average radiative cooling rate for the atmosphere as a whole. Direct calculations of radiative cooling rates for soundings typical of ice crystal displays and ice fogs (surface temperatures below - 4 0 ° C ) gave values in clear air varying from 0.5 to 3°C d -1 (Bowling, 1970). As an order of magnitude estimate, we assume about 1 °C d - 1 radiative cooling for the Arctic.

(5) If the mixing ratio does change due to precipitation from the air mass, each g kg - 1 decrease in mixing ratio leads to 2.82 K increase in potential temperature from the release of latent heat. This is calculated directly from the latent heat of vapor/ice conversion (2835 J g - 1 of water) and the specific heat of dry air (1005 J K -1 kg -~ of air). The variation in the latent heat with temperature is neglected. Evapor- ation of precipitation falling through the arctic air would cool the air while increasing the mixing ratio, but the results of the calculations below indicate that this would only make finding a source area even more difficult.

Transport without precipitation

As an extreme case, assume that k is very large, so that any precipitation at all will effectively cleanse the air, and that dry deposition is negligible. Assume further that transport to the Arctic is rapid, so that radiative cooling can be neglected. In this extreme case, the potential temperature, mixing ratio and

Source areas for arctic haze 2955

pollutant content of the polluted air are conserved. Air which retains its pollutants into the Arctic has traveled along surfaces of constant potential temperature (isentropic surfaces), which generally slope upward toward the poles. If the pressure, temperature and relative humidity of the air in an arctic haze layer are known, its temperature and relative humidity at any other pressure height (specifically, the pressure when and where pollution was added) can readily be calculated. Table 1 shows the initial temperatures and decimal relative humidities calculated for pollution sources at 1000 rob. The temperatures across the top and pressures along the left edge of the table are those observed in the haze layer, where the air is assumed to be just saturated over water. To obtain the source relative humidities for haze layers with relative humidities less than 100%, multiply the relative humidity in the haze layer by the number given in Table 1. This rather simple approach is based on the same assumptions used in isentropic trajectory analyses.

Barrie (1986) shows vertical profiles of arctic haze which suggest that arctic haze is most common and/or intense at elevations below 2 km (roughly 800 mb) , but that layers of haze occur with reasonable fre- quency up to around 500 mb. It seems clear that one of the reasons arctic haze is normally confined to the region below 800 mb in the atmosphere is that humidities in pollution source areas are rarely low enouyh to allow transport to higher elevations without precipitation. Eurasia may be the primary source area for arctic haze not only because it has extensive develop- ment at high latitudes, but because it is the only land area open to the Arctic at low elevations which is barricaded by mountains from subtropical water to the south (Fig. 1).

If the requirement for rapid transport is relaxed, the required source temperature must be raised by rough-

ly 1 ° C for each day of transport in order to allow for radiative cooling. If we allow this cooling rate, 10 days of travel (which according to Raatz and Shaw, 1984, is a typical transport time from Eurasia to Alaska) would require that the source area for the pollution be 10°C warmer than and have roughly half of the relative humidity indicated by the table.

Sample calculation

Values of pressure, temperature and mixing ratio were scaled from regions of maximum bsp (backscatter, indicating maximum aerosol concentration and generally correlating with high black carbon and pollutant concentrations) observed during the 1986 AGASP-II flights 201,202 and 203 in arctic haze north of Barrow (Herbert et al., 1989; Bridgeman et al., 1989a). Table 2 shows the characteristics of the haze layers and the temperature and humidity in the same air assuming travel times which allowed 0, 5, 10 and 15°C of radiative cooling from a source at 1000 mb.

Herbert et al. (1989) pointed out that the characteristics of the pollution aerosols in the three cases suggested that flight 201 encountered a relatively young aerosol, flight 202 was in a middle-aged pollution layer, while the aerosol in flight 203 was well- aged. If the aerosol in flight 202 is assumed to be 5 days older than that of flight 201 5 days earlier, there is a very close match in source temperatures. Given the considerable variability of the mixing ratios in the hazy air mass along the airplane flight tracks, the mismatch in the mixing ratios between the two flights does not exclude the possibility that the two flights represent the same source. Considering that three out of four of the trajectories calculated for the event sampled by flight 202 approached being closed loops, it seems very likely that the first two flights were

Table 1. Temperature/decimal relative humidity which air must have had at' 1000 mb (assumed origin of pollution) in order to reach the temperature and pressure shown

Temperature (°C)

Pressure (mb) -40 -30 -20 -10 0

500 11/0.04 23/0.04 35/0.04 48/0.05 60/0.05 600 - 3/0.07 8/0.08 20/0.09 31/0.10 43/0.11 700 - 15/0.14 -4/0.16 7/0.18 18/0.19 29/0.21 750 -20/0.20 -9/0.22 2/0.24 13/0.26 23/0.28 800 -25/0.28 -14/0.31 -3/0.33 7/0,35 18/0.37 850 - 29/0.40 - 18/0.42 - 8/0.44 3/0,46 13/0.48 900 -33/0.54 -23/0.56 -12/0.58 -2/0.60 8/0.62 950 -37/0.74 -26/0.75 -16/0.77 -6/0.78 4/0.79

This calculation assumes no radiative cooling and no precipitation. The calculations of relative humidity assume that the air is just saturated with respect to water at the temperature and pressure on the axes. If a layer is not saturated, the decimal relative humidity shown should be multiplied by the observed relative humidity to obtain the relative humidity at the 1000 mb origin. If radiative cooling is allowed, temperatures at 1000 nab will be higher and relative humidities will be lower than shown. As an example, a haze layer at 700 mb with a temperature of - 20 ° C and a relative humidity of 100% would have had a temperature of 7°C and a relative humidity of 18% at 1000 mb.

2956 S. A. BOWLING and G. E. SHAW

80°

Oo

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-%

° ¢

i 0 0 °

Fig. 1. Circum-Arctic topography. Note that the American plains and Eurasia between Scandinavia and the Lena River provide the only possible land routes to the Arctic with elevations below 500 m. Light shading: elevations above 500 m;

medium shading: elevations above 1000 m; heavy shading: elevations above 2000 m.

Table 2. Observed arctic haze conditions and inferred conditions at an assumed 1000 mb origin assuming no precipitation and travel times of 0, 5, 10 and 15 days with a radiative cooling rate of 1 °C d-1

Haze Source characteristics

0 days 5 days 10 days 15 days Flight Date P T r r.h. T r.h. T r.h. T r.h. T r.h.

201 4/03 850 - 2 3 0.35 49 --11 21 --6 14 - 1 10 4 07 202 4/08 900 --25 0.25 50 --17 25 - 1 2 16 - 7 11 - 2 08 203 4/10 925 --9 0.20 10 --3 6.5 2 4.5 7 03 12 02

Pressures (P) in mb, temperatures (T) in °C, mixing ratios (r) in g kg- 1, relative humidities (r.h,) in %. All data refer to the 1986 AGASP-II flights north of Barrow, Alaska, and the P, T and r values in the haze columns were scaled from figures in Herbert e t al. (1989). It should be noted that the same data for flight 201 were reported in Bridgeman e t al. (1989a) with water content expressed as relative humidity. That article gave the haze relative humidity as 75%, which would increase all flight 201 relative humidities to 150% of the values given. This still produces much lower relative humidities than were observed in central Europe 10 days earlier.


sampling slightly different portions of the same haze layer. The relative humidities at assumed 1000 mb origins, allowing for transit time, are unexpectedly low, however, especially for flight 203. In fact, when attempts were made to find stations on the northern hemispheric maps of the European Weather Bulletin (1986) with temperatures and dew points compatible with the values shown in Table 2 in the time period from 21 March to 10 April, 1986, no such stations could be found. It appears that the assumptions made were too rigid; they cannot explain the low mixing ratios found in arctic haze layers.

Modifications to the dry transport assumption

There are at least five possible ways of interpreting the mismatch between calculated source temperatures and humidities and those actually occurring:

(1) Pollutants were added at a lower pressure (higher elevation) than the assumed value of 1000 rob. This could be due either to sources in mountainous terrain or to high stacks. (In the latter case, humidities lower than those observed at the ground are also a possibility.)

(2) Heat was added to the air radiatively, perhaps by the absorption of solar radiation by suspended carbon.

(3) Water was removed from the polluted air by precipitation between the time the air moved out of the boundary layer and the time of measurement, which implies that a substantial amount of pollution survived the cloud precipitation process.

(4) The air mass in which the pollutants were observed was not the air mass to which they were originally added, i.e. the pollutants were somehow transferred from air with one potential temperature and mixing ratio to air with a much lower mixing ratio. (The most likely form of transport would be vertical settling with the aid of ice crystals.)

(5) The published temperatures and humidities are in error.

This last possibility is outside our ability to evalu- ate. It will not be investigated further here.

(1) Pollutants added at elevations above sea level. If pollutants are being added at elevations above the 1000 mb level, the source temperatures can be lower and the source relative humidities higher than those calculated above. Table 3 shows calculations of temperature, relative humidity and dew point for origins at lower pressures, allowing 5°C of radiative cooling while in transit from the origin to the point sampled. The elevations are for a standard atmosphere; at the temperatures involved the actual elevations would be slightly lower.

The calculated source conditions for flights 201 and 202 suggest that sources around 2000 m elevation could be compatible with the greatest dew point depressions actually observed in the Caucasus area and along the northern rim of the central and eastern Asian highlands (both of these areas are considerably drier than stations in the snow-covered part of Siber- ia). However, the absolute values of the dew point temperatures in these regions are at least 10°C too high to match the calculations. Calculations for flight 203 predict unrealistically low relative humidities even for source elevations on the order of 3000 m. In any event, no sources are known at these elevations.

It appears that the only way in which heat, mixing ratio and pollutants can all be conserved in the same air mass to produce the observed arctic haze is if the relatively moist air near the ground in the source area is overlain by dry air (probably air with a recent history of subsidence) and if effective stack heights are sufficient to inject pollutants directly into the dry air mass. This mechanism needs further investigation. Here we continue by evaluating the effects of other deviations from our original conservation assumptions.

(2) Absorption of solar energy by suspended carbon. The presence of black carbon in the arctic haze aerosol leads to the possibility that the air could be heated radiatively by the absorption of sunlight, especially by early April, when the measurements used were made. Hansen and Novakov (1989) give peak aerosol black carbon concentrations of 750 ng m- 3 at 850 mb on flight 201, 500 ng m - a at 700 mb (300 ng m 1 at

Table 3. Calculated source characteristics for sources at various elevations, assuming 5°C radiative cooling during transport

Flight 201 Flight 202 Flight 203 Source Source elevation pressure (m) (mb) T T d r.h. T Td r.h. T Td r.h.

100 1000 - 6 -29 14 - 12 -33 16 2 -35 04 550 950 - 10 --29 18 - 16 -33 21 - 2 -35 06 1000 900 - 14 --30 24 -20 -34 29 --6 -36 07 1450 850 - 18 --31 32 -24 -34 39 -- 10 -36 09 1950 800 -22 -31 42 --28 --35 52 - 15 -37 12 2450 750 -27 -32 62 --32 --35 75 -19 -38 18 3000 700 -31 -33 89 --37 --36 sat -24 -38 25

T= temperature in °C, Td = dew point in °C, r.h. = relative humidity in %.


lower elevations) on flight 202 and 500 ng m - 3 just below the 900 m b level on flight 203. Atmospher ic density at the levels used for the calculat ions above was 1.2 kg m - 3 for all three cases, giving black ca rbon mass concent ra t ions ranging from 250 to 630 ng k g - 1 .

One gram of carbon, in aerosol form, has an absorp t ion cross-section of 10 m 2 (Roessler and Faxvog, 1980). The effective absorp t ion due to aerosol black ca rbon in the three cases considered is therefore 2.5 x 10 -6 to 6.3 × 10 -6 m 2 k g - 1. As an upper limit

to heating, assume tha t in early April the ent ire solar cons tan t is incident on the volume conta in ing the c a r b o n aerosol for 12 h a day. Using a solar cons tan t of 1350 W m -2 and a specific heat for air of 1005 J kg -1 , 5 .8x 10 7 J m -2 are incident on the aerosol each day, and between 145 J d - 1 k g - 1 (of air) and 370 J d - 1 k g - 1 will actually be absorbed, for a m a x i m u m heat ing rate between 0.14 and 0.37°C d - l . This will rarely be enough to offset the radiat ive cooling rate. M o r e realistically, the very low solar e levat ion angle (less than 30 ° at local solar noon) in the Arctic at this t ime of year means tha t a significant fract ion of the incoming solar rad ia t ion will be scat tered back to space or absorbed in the long a tmospher ic pa th before it ever reaches the low

elevations for which the heat ing rates were calculated. In practice, the effect of heat ing due to absorp t ion by black ca rbon would be to make the calculat ions for a 5-day travel t ime actually apply to travel times of 6 to 7 days, a change which is well within the uncer ta inty of the cooling rate. Solar heat ing therefore canno t be responsible for the low relative humidit ies calculated for the source areas, and the only remain ing possibility is tha t water and pol lu tants were separated dur ing transit .

(3) Loss of water by precipitation. As stated above, ~educing the mixing rat io by precipi ta t ion loss adds heat to the a tmosphere , thus lowering the required tempera tures in the source region. Calculat ions of source tempera tures and relative humidit ies were carried out for all three flights for precipi ta t ion losses of 0.1, 0.25, 0.5, 1, 1.5 and 2 g kg -1. The results of these calculat ions are shown in Table 4. Precipi ta t ion loss on the order of 1 g k g - 1 is clearly capable of p roduc ing much more realistic relative humidit ies for flights 201 and 202, bu t is this loss of water while substant ia l amoun t s of pol lu tants remain in the air mass consis tent with what is k n o w n abou t pre- c ipi ta t ion scavenging?

Barrie (1985) gives sulfate mass scavenging ratios (Win) for Bergeron-process precipi ta t ion in C a n a d a

Table 4. Flights 201-203 with partial removal of initial water through precipitation

Haze Source characteristics Precipitation loss 0 days 5 days 10 days 15 days (g kg - 1 ) P T CB 1 CB 2 T r.h. T r.h. T r.h. T r.h.

Flight 201 0.1 850 - 2 3 790 780 -11.3 28 -6 .3 18 -1.3 13 3.7 09 0.25 850 - 2 3 830 780 - 11.7 38 -6 .7 25 - 1.7 18 3.3 12 0.5 850 - 2 3 900 780 - 12.4 57 -7 .4 38 -2 .4 26 2.6 18 1.0 850 - 23 990 780 - 13.8 98 - 8.8 68 - 3.8 46 1.2 32 1.5 850 - 2 3 - - 780 . . . . . 5.2 71 -0 .2 49 2.0 850 - 23 - - 780 . . . . . 6.6 100 - 1.6 69

Flight 202 0.1 900 - 2 5 830 820 -17.3 35 -12.3 23 -7 .3 15 -2 .3 11 0.25 900 - 2 5 885 820 -17.7 52 -12.7 35 -7 .7 23 -2 .7 11 0.50 900 - 25 960 820 - 18.4 83 - 13.4 54 - 8.4 37 - 3.4 25 1.0 900 - 2 5 - - 820 -19.8 - - -14.8 - - -9 .8 69 -4 .8 47 1.5 900 - 2 5 - - 820 -21.2 - - -16.2 - - -11.2 - - -6 .2 73 2.0 900 - 2 5 - - 820 -22.6 - - -17.6 - - -12.6 - - -7 .6 - -

Flight 203 0.1 925 - 9 650 640 -3 .5 10 1.5 7 6.5 5 11.5 3 0.25 925 - 9 700 640 - 4 16 1 11 6 8 11 5 0.5 925 - 9 770 640 -4 .6 26 0.4 17 5.4 12 10.4 9 1.0 925 - 9 870 640 - 6 49 - 1 34 4 23 9 17 1.5 925 - 9 950 640 -7 .4 77 -2 .4 52 2.6 37 7.6 26 2.0 925 - 9 - - 640 - 8.8 - 3.8 76 1.2 52 6.2 37

CB1 is the pressure height in mb at which the ascending air from the source reaches water saturation. CB2 is the pressure height in mb at which ice in air descending to the final haze layer would evaporate. Both calculations neglect radiative cooling. If such cooling occurred after the air descended from the final cloud base, CB2 must have been 8 mb higher in the atmosphere (lower pressure) for each °C of radiative cooling. In order for water loss by precipitation to have occurred, the air parcel must have reached at least a pressure height of CB 2 in the course of its trajectory. Changes in CB 1 due to varying transit times depend on cooling rates and times both inside and before entering the cloud layer, but in general initial cloud bases will be higher in the atmosphere with longer total transit times. Dashes indicate that there is no way of obtaining the given amount of precipitation from an air mass starting at 1000 mb without supersaturation and finishing at the given temperature, pressure and relative humidity.


ranging from 800 to 1500 kg kg-1 , with standard deviations of factors of 2.5 to 3. Davidson et al. (1985) give values between 100 and 2 0 0 k g k g -~ for precipitation from polluted arctic air in Greenland. These values correspond roughly to values of k from 0.l to 1 . 5 k g g -1. Including values a standard deviation below the lower value and above the higher value, we can consider a range from 0.06 to 5 kg g - as possible values of k. If the decrease of the mixing ratio through precipitation is Ilk, pollution levels will be lowered to 1/e (37%) of their original concentrations. These values thus suggest that mixing ratio decreases ranging from 0.2 to 16 g kg-1 are consistent with survival of roughly a third of the initial pollution, though values above 5 g k g - I are supported only by the Greenland data. This is more than sufficient to account for the observed temperatures and mixing ratios during flights 201 and 202. Even flight 203 source parameters now look reasonable for sources in the Urals on a short trajectory, though they are not consistent with the long residence time ( > 5 days) suggested by the size distribution (Herbert et al., 1989).

The assumption that precipitation has occurred during transport does have serious implications for trajectory models, however, especially for flight 203. If any precipitation at all occurred, this air mass must have subsided over 300 mb since the last traces of its clouds evaporated simply to reach its observed low relative humidity. Isobaric trajectory analysis for this air mass would be credible only if 1000, 850, 700 and preferably 500 mb trajectories were in agreement with each other. Herbert et al. (1989) show only 1000- and 850-mb back trajectories, which diverged considerably. According to the authors, the 1000-mb trajectories, which are inconsistent with the extreme

dryness of the air, are the only ones even suggesting a source. Kahl et al. (1989) have calculated more detailed trajectories (including isentropic and 3D trajectories) for flight 203 as their case 16. Dr Kahl was kind enough to send us a copy of the unpublished 3D trajectories for flights 201, 202 and 203, which confirmed our deduction of subsidence in the trajectory for flight 203. The back trajectory of the air winding up at 850 mb shows approximately 150 mb subsidence, based on calculated vertical motions, in the 5 days preceding the haze measurement. (This also helps confirm the age of the haze, as the air mass appears not to have been in contact with the ground during this period.) The thermodynamics indicate that the air must have subsided from a cloud base at or above 640 mb. In this case it is clear that isentropic back trajectories are meaningless before the time that the air left the cloud base.

(4) Vertical redistribution of pollutants into the dry layer by ice crystal precipitation. Borys et al. (1988) observed that the vapor-phase growth of snowflakes through Bergeron distillation results in exclusion of pollutants from snow (especially unrimed snow) relative to rain. This would seem to argue against transport of pollution downward by ice crystals. Hoff (1988), nevertheless, observed what appeared to be such downward transport. If the crystals nucleate as supercooled water droplets on pollution aerosols, each ice crystal will initially have essentially the same absolute amount of pollutant as a cloud droplet (Ohtake et al., 1978). This will be diluted by further vapor-phase growth, but the dilution involved in growing a small (10-100/~m diameter) ice crystal is at least two to four orders of magnitude less than that involved in growing even a rather small (1 mm) snowflake. Examples of the kind of ice crystals being

o

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J ~lO0/~

d O

O

. @

b Fig. 2. Comparison of snowflakes and "diamond dust" type ice crystals shown on the same scale. Note that the snowflake

shown is a small one, only a little over a millimeter in diameter.


considered are shown in Fig. 2; they are relatively common in the Arctic (Curry et al., 1990). The fall speeds of these small ice crystals will not be high. Stokes law velocities for water droplets of this size are of the order of 10-1000 m h - 1. Allowing for the non- spherical shapes and lower density of ice crystals, fall speeds from 10 to 1 0 0 m h -1 are probably reasonable. For cloud droplets of this size falling through an tmdersaturated atmosphere, evaporation would be complete in a few tens of meters. Ice saturation and even supersaturation, however, are relatively common in the atmosphere, as evidenced by the occurrence of cirrus fibratus and persistent contrails. Given an atmosphere at or slightly above ice saturation and a day or two of time, vertical transport of several kilometers by this mechanism seems possible. This would provide a vertical downward transport mechanism for pollutants. When crystals fell into a dry layer, they would evaporate, leaving a concentrated layer of pollutants. Such a scenario could explain the high level of backscatter and the local maximum of condensation nuclei seen in very dry air (relative humidity 11% relative to ice saturation) on flight 203. Measurements of pollutant concentrations in arctic clear-sky ice crystal precipitation are clearly needed.

If this mechanism is occurring, the implications for trajectory analysis are even more severe than would be the case for the occurrence of substantial amounts of precipitation, as the air mass in which the pollutants are observed could be of totally different origin from the one into which they were injected.

Were the cases used representative of arctic haze? Unfortunately the AGASP-II flights north of Canada did not encounter significant elevated layers of arctic haze (Bridgeman et al., 1989b). The aged, diluted haze that they did encounter appeared to have t e m p e r - atures similar to those observed in flights 201 and 202, with somewhat higher mixing ratios. The original AGASP flights (Raatz et al., 1985) showed haze layers below the 800-mb height to have relative humidities generally between 50 and 100%, again consistent with flights 201 and 202, though some very low relative humidities were observed in higher layers with less haze. One of the authors (GS) made several airplane ascents from Barrow in 1974 and compared the airplane data with the Barrow soundings. These flights all suggested that relatively clear layers below elevated haze layers were often near saturation, while the elevated haze layers had dew point depressions of 5-15°C, corresponding to relative humidities of 25-75% for haze layers generally between 800 and 900 mb. Flights 201 and 202, then, appear on the basis of this limited data set to be typical, while flight 203 may have observed an abnormally dry layer of arctic haze. It is unfortunate that humidity has been measured so infrequently in arctic haze layers.

Summary of calculations

Observed humidities in at least some arctic haze layers appear to be too low to be consistent with

isobaric or isentropic transport, and the problem is even more acute if radiative cooling of the air during transport is taken into account. Heating by solar radiation absorbed by carbon in the haze does not appear sufficient to do more than partly offset longwave radiative cooling. For average arctic hazes with relative humidities of the order of 50% and heights not much over a kilometer, the observations can be explained by some combination of injection into dry layers aloft by high effective stack heights, precipitation along the path into the Arctic, and downward transport by ice crystals. It should be noted that the last two mechanisms are to some extent in opposition to each other, the one implying that removal of pollutants by precipitation is ineffective while the other (ice crystal deposition) suggests that the incorporation of pollutants into ice crystals is very efficient. Very dry haze layers or those at elevations above the 700-mb level are difficult to explain as pollution without the operation of more than one of these mechanisms, and the possibility that such elevated arctic haze layers consist of desert dust (as in Rahn et al., 1977) should be considered. All the proposed mechanisms need further investigation.

3. IMPLICATIONS AND CONCLUSIONS

According to our analysis, the transport of pollutants that wind up as arctic haze layers at high elevations or low relative humidities clearly presents problems. In particular, the simplest explanation for the low mixing ratios observed in many arctic haze events is that some precipitation has originated in the pollutant layer during transport. In order for precipitation to have occurred previously in air with low relative humidity, the air must have descended at least from its lifting condensation level--on the order of a kilometer for air with 50% relative humidity. This means that isobaric and even isentropic trajectory analyses may have little relationship to the true trajectories. In the case of rare haze layers at elevations above 600 rob, even saturated air requires source temperatures so high and source relative humidities so low that, in the absence of data on chemical composition, a desert source may be suspected.

Further research is badly needed both in the process of pollutant injection (what is the relative humidity of the air into which pollutants are actually being added?) and in removal mechanisms expected to operate in arctic air. In particular, the role of relatively small (10-100/~m) ice crystals needs to be clarified (Curry, 1990).

Acknowledgements--Particular thanks are due to J. D. Kahl and J. M. Harris, who provided back trajectories for the haze layers of the three AGASP-II flights. This research was supported by State of Alaska funds.


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Documents

The thermodynamics of pollutant removal as an indicator of possible source areas for arctic haze