Atmospheric pollution becomes problem over a large area because atmosphere transports relatively uniform concentrations of air pollutant(s) over considerable distances. Rhdhe et al. (1982) categorized the scale of atmospheric transport as follows:
- Local transport: This occurs from individual point or line source for a distance of few kilometres only. It is mainly associated with plumes and is affected by local meteorological conditions. As the plume disperses, most of the pollutant falls back to the ground as dry deposition.
- Regional transport: This occurs to distances less than a thousand kilometres. At this scale, individual plumes merge together and development of a relatively uniform profile of pollutant after about 1-2 hours becomes possible. Such transport causes spread of air pollution problem from urban/industrial sources to the surrounding downwind countryside. In such transport, pollutants come down both by dry and wet deposition depending upon meteorological conditions.
- Sub-continental and continental transport: Such transport occurs over several to a few thousand kilometres. In such transport, relatively uniform pollutant profile becomes well developed the pollution undergoes several diurnal cycles. At this scale, wet deposition of pollutants becomes more important than dry deposition and interchange of pollution between troposphere and stratosphere becomes possible.
- Global transport: Such transport extends from a few thousand kilometres to the entire global atmosphere. At this scale, definable pathways of pollutant transport disappear due to inevitable mixing in atmosphere. Continental and global transports of air pollution are termed long range transport (LRT). Such transport requires an organized meteorological system of atleast synoptic scale that will allow movement of pollution without much dispersion or loss.
Most unnfavourable conditions for LRT are windy-blustry conditions associated with heavy rain and strong turbulent mixing. Under such conditions, any organised mass of pollution will almost immediately be blown apart. Scavenging from within and below the clouds will remove virtually all the pollutant material quite close to source region. Further, there would be little time available for secondary chemistry to develop.
Most favourable conditions for LRT involve circulation associated with the back side of a high pressure system where two major features become crucial:
1. Persistence of a synoptic-scale inversion over a wide area reducing vertical distribution of pollution and restricting it to a fairly shalow depth of atmosphere.
2. Existence of conditions allowing slow horizontal advection in the layer containing air pollution.
Both these situations are assisted by flat terrain that allows uninturrupted horizontal movement of air mass. During development of LRT, pollutants collect initially under the influence of a shallow, stable and high-pressure system that has been stagnating over a source region for previous several days. During this period, the surface wind speeds have been less than 3 metres per second, mixing heights below the inversion have been restricted to 1500 metres or less, hours of sunshine have been close to maximum possible and there have been no weather fronts or precipitation. Over the period, which is most likely to persist for four days, the concentration of air pollutants increases significantly, extensive secondary reactions occur and poor visibility persists. Under such circumstances, LRT of air pollution can occur in three major ways with first one having strongest impact.
1. The high pressure slowly begins to move out of the area and polluted air is drawn towards the trailing edge (in the back side) where it starts moving northwards (in northern hemisphere) in the prevailing circulation. If a weak cold or warm front is in the vicinity, transport is improved and is often beteer directed by the enhanced pressure gradient. Inversion and stable atmosphere often restrict LRT to layers below 700 mb pressure altitude and the polluted air mass may move several thousand kilometres without much dispersion or dilution. Separation from the friction layer near the surface is also important if polluted air mass is to maintain its cohesion. Over continental areas, extreme stability of lowest atmospheric layers in winter suggests that at airflow at 850 mb, pressure level might be the best indicator of LRT. During summers, transport is usually not so well defined as the solar heating and vertical convection tend to dominate the stability situation.
2. Variants to LRT situation provide smaller but still important concentrations of pollutants to great distances downwind of sources. If edge of a high is associated with cool, cloudy weather and a stiff breeze, the transport will be quite rapid in a dynamic and unstable atmosphere. high humidity and cloud cover will allow oxidation of precurssors such as SO 2 or NOx without a great deal of dispersion away from the air mass core. This results in transfer of moderate concentrations of pollutants that are not removed by rainwater because of the absence of enough vertical diffusion for precipitation.
3. If circulation system on the edge of a high pressure draws moist air from ocean regions, which then moves over source areas of pollution, weak turbulent mixing will draw the polluted material into the air stream. This material will mix rapidly through the depth of the air flow creating quite uniform and mild pollution concentrations. This pollution will be transported for considerable distances. If it meets a mountain barrier in the way, the mildly polluted air can be drawn up orographically and forced over the top. Pollutants are then scavenged resulting in mildly acidic rainfall on the windward side of mountain, which may persist for several hours. In the tropics, LRT largely depends on the persistence and frequency of trade winds associated with emission source regions. Transport occurs at the altitude of trade wind inversion and in stable atmospheric conditions over long distances. This allows minimal dispersion of pollutant material in the air mass.
In southern hemisphere, there are very few air pollution sources and, therfore, no important LRT associated with air pollution. In the northern hemisphere, four major continental to hemispherical scale air pollution problems due to long range transport of air pollution. These are Arctic haze, Western Atlantic ocean air quality, Saharan dust and Asian dust.
Measurements of LRT are often difficult and expensive. Therefore, trajectory analysis is often used to establish back trajectories (general source area of pollution origin) and forward trajectories (general location to which pollutants are being transported). Trajectories are calculated from synoptic level upper wind data and are normallly based on isobaric or isentropic principles. Two-dimensional models can not describe the vertical movements of either the airflow or the polluted air mass. Three-dimensional models, using a series of grid points at various altitudes have been designed to establish the sources of polluted air masses that have moved several thousand kilometres. Trajectories are best used as a support method to establish general transport movements and not to estimate the changes in pollution concentrations over the time. Benefits of such use are:
- Support for chemical tracer experiments from different source regions;
- Simulation of dry deposition;
- Evaluation of acute air pollution problems;
- Establishment of the source of a chronic pollution problem. At best, trajectories show general accuracy for five days assuming that air flow is consistent with little vertical or horizontal changes.
Most often trajectories lose accuracy after 48 hours since spatial distribution of upper synoptic grid and the number of measurements from them are too thin to obtain accurate interpolations of air flow variations. Distortions occur with the presence of turbulent eddies, evolving synoptic patterns, increasing diffusion and inaccuracies in determination of mean wind. Trajectory analysis fails in presence of fronts or other rapidly changing atmospheric conditions.