ARCTIC HAZE

Seasonal and Geographic Variations of the Arctic Haze and Meteorological Transport

In winter, the arctic air mass extends throughout the high Arctic and extends down over Eurasia and North America. Arctic haze has been described as having a

‘dome’ 7–8 km deep over the pole with shallow tongues of air 0–5 km deep spilling southward over the land masses. This air mass is statically stable because of the strong temperature inversions and has relatively small liquid cloud water or ice content.

During summer, cloud cover is extensive throughout the arctic basin. The uniform cloud deck forms when warm air flows over cold icepack; it is often associated with drizzle. The summer arctic stratus is one of the most pervasive and persistent cloud systems on the

planet. The turnover and almost constant drizzle is instrumental in cleansing the arctic atmosphere, reducing concentrations of arctic haze to insignificant levels. As a result, the concentration of arctic haze undergoes a strong seasonal variation with maximum in winter and minimum in summer. This ‘arctic air mass’ picture provides a rough understanding of the more persistent and larger features of arctic air pollution.

By correlating synoptic meteorological patterns with episodes of arctic haze, it was concluded that there is often a close connection between outbreaks of arctic haze and transport along or within anticyclonic pathways in the atmosphere. North American sources evidently contribute only in a minor way to the arctic pollution burden. Polluted air masses originating from the east coastal region of North America usually travel east across the Atlantic Ocean and experience rapid removal by storms within the Icelandic cyclone system.

Air trajectories associated with flow across the arctic basin are frequently associated with pollution episodes. During such instances, transport is often controlled by the mid-northern Eurasian high, extend-ing over Kamchatka and another cell over the Alaska area, or steered by cyclonic systems in the Barents Sea.

Studies of the meteorological patterns indicate that transport of arctic haze is associated with a quasi-stationary ‘blocking’ pattern in the atmosphere. This has been proposed to be a fundamental mechanism that provides conditions for poleward transport of

Figure 1 Annual emissions of sulfur dioxide (in units of millions of metric tons) in the high latitude Northern Hemisphere that influence the Arctic. (Map courtesy of Leonard Barrie.)

Output tap nearly closed off

Tap opened wide Slow drizzle in Large input

Arctic haze Normal mid latitude air pollution

Pollution level

Figure 2 Schematic diagram showing why arctic haze builds up in the high latitude regions. Removal for scrubbing out the atmosphere operates more slowly in the polar regions.

156 ARCTIC HAZE

midlatitudinal air pollution, particularly in the Eur-opean sector. The seasonal variation of such blocking seems to be an important factor, along with scavenging and other removal processes, for understanding the annual cycle of the arctic air pollution.

The isentropic dome of cold air centered roughly over the north pole helps explain the many elevated layers seen in arctic haze. The large static stability inhibits vertical mixing of the material throughout the troposphere. Also, the ‘dome’ causes midlatitude ground-based pollutants to ascend adiabatically as winds carry them poleward into the arctic basin.

The map of annual emissions of sulfur dioxide with superimposed arctic air mass (Figure 1) helps identify the major sources and currents of pollution-derived material affecting the Arctic. Thus, on the basis of the relatively strong source region in the central and western Eurasia sector, the occurrence of a deep lobe of the arctic air mass over much of this source, the occurrence of a poleward flowing circula-tion over this source area, and the absence of precipitation, clouds and turbulence along the path-way, one can conclude that Eurasia is of greater importance than North America as a source region for the Arctic haze.

In the past decade or so, the occurrence of arctic haze has become weaker and less frequent, perhaps due to breakup of the Soviet Union and associated decrease in heavy industrial emissions, though sys-tematic changes in meteorological patterns may also be involved.

Chemistry of the Arctic Haze

Samples of aerosol were collected in various arctic air masses in the 1970s and analyzed for their chemical composition in the hope that the origin of the hazy layers might become clear. Although sulfate dominates aerosol chemistry, the haze contains trace amounts of heavy metals, some of which suggest industrial pollution sources.

The chemical sampling also uncovered strong seasonal variation (discussed in the last section), with maximum occurrence of the arctic haze in the late winter and early spring.Figure 3shows the strong seasonal variation of sulfate aerosol concentration as sampled in the Canadian Arctic.

The largest fraction of the arctic haze is sulfate. Note from Figure 1 that the Eurasian sulfur dioxide emissions in areas liable to influence the Arctic are about a factor of 2–4 times larger than for North America. Note also that the major Eurasian sulfur dioxide emission sources are 5–101higher in latitude in comparison to those in North America. This, along with the fact that the arctic air mass lobes down

strongly over Eurasia, suggests major contribution to arctic haze from Eurasian sources.

The origin of the arctic haze was determined to be Eurasian using a chemical fingerprint based on the ratio of vanadium to manganese concentrations in the collected filtered samples of the arctic haze. This simple tracer system immediately suggested that the greatest fraction of arctic haze aerosol derived from Eurasian industrial sources, especially in the eastern sectors. The reason is that the former Soviet Union and eastern European nations are coal burning societies, while the western nations are heavy users of petroleum products, laced with vanadium used as catalyst in the cracking process. The arctic haze value of V/Mn was very low, consistent with a coal burning source.

Concentrations of black carbon were also elevated, consistent with the dirtier combustion that takes place in inefficient coal power plants of the type used in the former Soviet Union. It is quite interesting to see that the essential picture about the sources and transport mechanisms for the arctic haze was recognized quite clearly during the 1970s. During later aircraft missions conducted by NOAA (the AGASP missions, Arctic Gas and Aerosol Sampling Program), the arctic haze was sampled directly and the concentrations and compositions of a number of minor constituents were determined. Again, sulfur in the form of sulfate was the predominant chemical compound in the arctic haze, even at the higher altitudes.

More elaborate multivariate analysis of heavy metal composition of the arctic haze aerosol confirmed that the greatest fraction of the arctic haze indeed originates in Eurasia. These deductions were in large part made from wide and generous sharing of data among the circumpolar nations in an informal ‘network’. By the mid-1980s, chemical fingerprinting methods had determined that Eurasia and North America were the first and second primary source regions for the general pollution phenomenon in the Arctic.

1980 0 500 1000 1500 2000 2500 3000 3500 4000

1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Year

SO4 = (cgm−3)

Aerosol sulfate at alert, NWT

Figure 3 Concentration of sulfate (in nanograms of sulfate per cubic meter) in the Canadian Arctic measured at Alert, in the Northwest Territories. (Courtesy of Leonard Barrie.)

ARCTIC HAZE 157

More recent investigations combined back wind meteorological trajectory models with geographic information and inventories of chemical emissions.

With the use of such models one can deduce partitioning of the source regions for the differing chemical species, although it has to be kept in mind that the accuracy suffers since the network of synoptic stations in the Arctic is sparse. Such investigations have indicated that a number of geographic locations in Eurasia have high potential as major emission sources. It was found that a Ni–Cu smelting complex at Norislk is probably one of the major contributors to the haze.

Arctic haze in mid-winter mainly consists of sulfur dioxide. At that time the polar atmosphere is a large chemical reactor. The gaseous form converts to particulate sulfates when the sun rises in spring.

More than sulfur-containing particles and heavy metals reach the Arctic. Organic compounds such as polychlorinated aromatics and pesticides have been measured in arctic air pollution.

Residence Time for Arctic Air Pollution

During the early investigations of the arctic haze phenomenon, in the 1970s, scientists were hampered by the erroneous, but pervasive, belief that aerosol pollution is confined to regional scales up to a few hundred kilometers at most. It was then clear that under conditions of great atmospheric stability and low water content, the residence time for aerosols and the corresponding air parcel travel distance increase dramatically. The majority of air pollution studies in the 1960s and 1970s had concluded that the lifetime of atmospheric aerosols is short, being only a few days.

This assumption breaks down in the polar regions since particles may remain airborne for weeks in the stable arctic air.

Two notable exceptions to the rule of ‘a few days residence time’ for aerosols had, however, been recognized for a long time. One was the phenomenon of stratospheric haze from explosive volcanic erup-tions, which is known to have lifetimes of several years. The other was the long-range transport of windblown dust from deserts. We now believe arctic haze to have an average lifetime shorter than strato-spheric dust veils, but comparable to long-distance transport of desert dust outbreaks, which also occur in dry air masses.

The connection between arctic haze and anti-cyclonic conditions, which was mentioned earlier, suggests that one reason why haze lasts so long is partly due to lowered exposure to rain and snow cleansing mechanisms.

Optical Transparency and Climatic Effects

In the early 1970s, unexpectedly high values for atmospheric turbidity were reported at the McCall Glacier in the Brooks Range in Alaska. In trying to understand the physical cause of the high turbidity, additional measurements were made of the wave-length dependence of optical extinction caused by the haze and the angular distribution of sky brightness.

These measurements confirmed Mitchell’s earlier suspicions that the winter arctic haze consisted mainly of small aerosols. During the early 1970s, arctic haze over Alaska was found to be layered by flying a sun photometer aboard a light (Cessna) aircraft. The AGASP experiments of the 1980s demonstrated that the layers consist of aerosol particles that can have climatic effects by interacting with the solar radiation field all across the Arctic. It was later determined that in addition to the submicrometer haze, there is often another source of haziness in the Arctic from precipitations of ice crystals, which sometimes form in clear air. Such ‘diamond dust’, so called because of its sparkling appearance, can also reduce visibility and interact with the radiation field.

Scattering and absorption of sunlight by arctic haze was shown to have a slight warming effect on the Earth–atmosphere column, of magnitude about 0.11C per day, because carbonaceous material lowers albedo. There is estimated to be a slight cooling at the surface caused by scattering of light. In the winter months there may be a slight warming of the arctic atmosphere caused by the interaction of the aerosol with outgoing infrared radiation.

However, in addition to slight climate effects caused by absorption and scattering of light by the arctic haze aerosols, there may be subtle and so far not well evaluated influences on climate from indirect radiative effects. These may result from the modification of cloud parameters, since the arctic haze introduces new sources of cloud condensation nuclei and, possibly, new sources of freezing nuclei into the arctic basin.

Ecological Implications

Compounds such as pesticides, PCBs, persistent organics, as well as trace metals are detected through-out the arctic basin in the atmosphere and also in the surface land and sea domains and scattered through-out the region’s biome. Most of the atmospheric pollution and some of the surface and biological pollution is undoubtedly caused from the arctic haze phenomena. But it should also be recognized that in places in the Arctic the surface concentrations of pollutants can be extremely small, even during times 158 ARCTIC HAZE

when the air is quite contaminated. This of course is due to the long residence time of the haze.

In addition to simple transport of material through the atmosphere, another depositional mechanism was suggested that involves a fractional distillation. The surprisingly high concentration of organic material is determined in part by the temperature-dependent partitioning of the low volatility compounds. A process of global fractionation may be occurring, during which time organic compounds become latitudinally fractionated, condensing at different ambient temperatures depending on their volatility.

Substances with low vapor pressure preferentially accumulate in the polar regions, much like deposition of vapor products on cold regions in vacuum systems.

Aerosols and gases are scrubbed out of the atmo-sphere in precipitating air masses. Many of these are over oceanic regions in the North Atlantic, the

Norwegian Sea and possibly in the Bering Sea. These are important world fisheries and the consequences of dumping industrial pollutants that had congregated in the arctic atmosphere are unknown.

ATMOSPHERIC TIDES

M Hagan, National Center for Atmospheric Research, Boulder, CO, USA

J Forbes, University of Colorado, Boulder, CO, USA A Richmond, National Center for Atmospheric Research, Boulder, CO, USA

Introduction

Atmospheric tides are ubiquitous features of the Earth’s atmosphere. They are the persistent global oscillations that are observed in all types of atmos-pheric fields, including wind, temperature, pressure, density, and geopotential height. Tidal oscillations have periods that are some integer fraction of a solar or lunar day. The solar diurnal and semidiurnal tides have

24 h and 12 h periods, respectively. The lunar diurnal tidal period is about 24.8 h, while the lunar semidiur-nal period is 12.4 h. Scientists often use a shorthand notation to represent solar and lunar tides. S1 and S2 refer respectively to the solar diurnal and semidiurnal tides. Their lunar counterparts are M1 and M2.

Atmospheric tides have been studied for many years, since they are evident in both surface pressure and magnetic observations that date back to the early part of the twentieth century.Figure 1illustrates a time series of surface pressure measurements made at Batavia (now known as Jakarta, Indonesia) during the first 5 days of January in 1925. The dominant feature of this time series provides evidence of the solar semidiurnal atmospheric tide. Specifically, there is a 1–2 hPa deviation from the average pressure of about 1011 hPa that occurs regularly at 12 h intervals. This

1017

1013

1009

1005

1 2 3 4 5

Day

Atmospheric pressure (hPa)

Figure 1 Surface pressure (hPa) at Batavia (Jakarta, Indonesia) against time during the first 5 days of January 1925.

ATMOSPHERIC TIDES 159

semidiurnal variation is modulated by other varia-tions, but the former is such a persistent oscillation that the semidiurnal tide is also the dominant oscilla-tion in monthly, yearly, and even multiyear averages of daily surface pressure measurements made at Batavia.

Atmospheric tides are further characterized by their sources. The Moon’s gravity forces the lunar atmos-pheric tide, while solar atmosatmos-pheric tides can be excited in several ways, including the absorption of solar radiation, large-scale latent heat release associ-ated with deep convective clouds in the troposphere, and the gravitational pull of the Sun. The restoring force that acts on atmospheric tides is gravity, so tides are a special class of buoyancy or gravity waves.

Unlike other gravity waves, tides are affected by the Earth’s rotation and sphericity because of their com-paratively large periodicities and horizontal scales.

The general mathematical expression for a tidal oscillation is given by eqn [1], whereAis the magnitude of the variation in some atmospheric field, s is its frequency,tis universal time;lis longitude, andsis the zonal wavenumber. The magnitude ofsrepresents the number of wave crests that occur along a latitude circle, while the sign of s is indicative of the zonal direction of propagation.fis the so-called tidal phase.

A crest of the wave occurs whenfsatisfies eqn [2].

Acosðst!sl!fÞ ½1"

f¼st!sl ½2"

The horizontal phase speed of the tide,cph, is defined by differentiating eqn [2] and holdingfconstant (eqns [3] and [4]).

sdt!sdl¼0 ½3"

c_ph#dl dt¼s

s ½4"

s>0 corresponds to eastward tidal propagation, and so0 corresponds to westward tidal propagation.

For solar tides, the nth harmonic frequency is sn¼ns1, where n is a positive integer and s1¼ ð2p=24Þh^!1. Rewriting the mathematical expression for a tide in terms of local solar time (hours), tL ¼tþl=s1, results in a mathematical expression of the form of eqn [5].

AcosðsntL! ðsþnÞl!fÞ ½5"

For the subset of atmospheric tides known as migra-ting solar tides,s¼ !nand eqn [5] reduces to eqn [6].

AcosðsntL!fÞ ½6"

Thus, migrating solar tides have the same local time variation at all longitudes. Ifn¼1 ands¼ !1, the

tide is diurnal and moves or migrates westward in longitude with the apparent motion of the Sun from the perspective of a ground-based observer. Further, cph¼ !ð2p=24Þh^!1. Similarly, if n¼2 and s¼ !2, then the wave is a migrating semidiurnal tide. The remaining set of global scale waves with tidal periods that are not Sun-synchronous are known as non-migrating tides. Nonnon-migrating tides may be viewed as waves that propagate to the west more rapidly or slowly than the Sun, or that propagate eastward, or that are standing.

All tides contain components that propagate in the vertical direction. The effects of upward-propagating tidal components are particularly important because these waves grow in amplitude as they conserve energy in an atmosphere whose density decreases with increasing altitude. Thus, tides with insignificant amplitudes in their lower atmospheric regions of excitation often affect the upper atmosphere pro-foundly because they introduce large atmospheric variations with local time and because they may dissipate and deposit their energy and momentum therein.

Migrating Solar Tides

The absorption of radiation by a longitudinally invariant atmosphere is the primary source of migrat-ing solar tides. Owmigrat-ing to the rotation of the Earth, this absorption is periodic in time from the perspective of the ground-based observer. The resultant heating gives rise to migrating tidal oscillations. Solar radiation is absorbed throughout the Earth’s atmosphere, thereby exciting migrating solar tides at almost all altitudes.

Atomic oxygen, which is the most abundant atmos-pheric constituent at altitudes about 150 km above the Earth’s surface, absorbs the shortest-wavelength solar radiation, known as the extreme ultraviolet. Increas-ingly longer wavelengths are absorbed as the solar radiation approaches the Earth’s surface. Molecular oxygen, (O2) absorbs the far-ultraviolet radiation (100–200 nm) at altitudes near about 100–150 km, and ozone (O3) absorbs the 200–300 nm solar ultra-violet radiation at middle atmospheric altitudes be-tween about 30 and 70 km. Solar infrared radiation may be absorbed by water vapor, (H₂O) in the lowest part of the atmosphere.

Even though there is little, if any, tidal forcing due to solar heating in the upper mesosphere (B80–100 km), measurements of winds and temperatures exhibit strong tidal signatures in this region. Figure 2 illus-trates an example of the magnitude of the mean winds and the tidal oscillations over Adelaide, Australia at these altitudes. The data points represent the eastward winds that were measured with the Buckland Park 160 ATMOSPHERIC TIDES

In document The Speed of Sound in the Atmosphere (sider 155-166)

Further Reading