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Air Pollution Episodes

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Episodes Figure 1: Air pollution episodes can often be a problem in cities such as Middlesbrough [Picture from 1]. Summa Walker EERE UG 4: Air and Water Pollution Modelling UK Air Pollution Episodes An air pollution episode is the term used for a period of poor air quality, lasting up to several days, often extending over a large geographical area [2]. Concentrations of all the measured species may increase at the same time, or only one species may be affected. 'Air Pollution Bands' classify pollution levels into bands to enable air quality levels to be identified, see table 1. Description... Low S Moderate I High A Very High Sulphur Dioxide (ppb, 15 minute averages) <100 100-199 200-399 >=400 Ozone (ppb) <50 (8hr running average) 50-89 (hourly average) 90-179 (hourly average) >=180 (hourly average) Carbon Monoxide (ppm, 8 hour running average) <10 10-14 15-19 >=20 Nitrogen Dioxide (ppb, hourly average) <150 150-299 300-399 >=400 PM10 Particles (�g/m3, 24 hour running average) <50 50-74 75-99 >=100 S = Standard Threshold, I = Information Threshold, A = Alert Threshold Table 1: Air Pollution bands for some major pollutants [from 5] There are different types of episode, caused in different ways: * Winter episodes occur during periods of cold calm weather when pollution emissions are trapped close to their sources and cannot disperse. * Summer episodes are characterised by high ozone levels and occur during warm sunny weather in the summer months. ...read more.


Figure 2: Variation of temperature in the atmosphere with height, showing inversion caused by ozone layer [From 6] A temperature inversion at the earth's surface can occur on clear still nights as the ground is a better emitter of infrared radiation than the air. Consequently the ground cools much more rapidly than the air above it. Throughout the night this process creates a layer of air where the coolest temperatures are found closest to the surface and the temperature increases with height up to several feet [3]. Temperature inversions also usually form near the boundaries of warm and cold air that define weather fronts (frontal inversions) and are most pronounced in warm fronts. They form near the air boundaries as warm air overruns colder air at the surface creating a thin layer of air near the cold air in which temperatures increase with height. This inversion layer is parallel to the slope of the cold air-warm air boundary above the earth's surface [3]. Winter Episode Example The most severe winter episode in recent years occurred in London on 12-15 December 1991. The meteorological conditions were cold and foggy for several days, and the resulting build-up of vehicle emissions led to record breaking levels of nitrogen dioxide (NO2). The maximum hourly concentration was over 400ppb, the highest seen in the UK since automatic monitoring began in the early 1970s [2]. ...read more.


Plume grounding episodes occur when winds blow emissions from industrial chimneys down to ground level. The area affected depends on wind direction and distance from the industrial source. The power stations and heavy industry around the Thames corridor can produce such episodes and to a lesser extend plume grounding can be measured from industrial sources in north London [5]. Non-seasonal related episodes also can occur due to the release of a concentrated source. For example a rapid rise in 1,3-butadiene levels was recorded in Middlesbrough on 31 July 1995, see figure 6 [2]. Concentrations rose from normal background levels (0.3ppb) to 83.1ppb within one hour, taking concentrations into the "very high" air quality band for the first time in the networks history while levels of other pollutants remained at normal background levels [2]. Local investigation revealed the most likely source to be a shipping tanker purging its tanks of a previous cargo of 1,3-butadiene. It is estimated that some seven tonnes of the chemical were released, and a prevailing Easterly wind transported it across the town, giving elevated concentrations at the monitoring site. Figure 6: Levels of Butadiene over Middlesbrough, Summer 1995 [From 2]. Summary There are 3 main types of air pollution episode where levels of pollutants exceed that of normal levels. These are Winter episodes caused by inversion layers and stable weather conditions. Summer episodes where transportation of pollutants combined with high levels of sunlight cause photochemical smogs. Non seasonal episodes caused by local or transported releases of pollutants. ...read more.

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