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

00-199

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

0-14

5-19

>=20

Nitrogen Dioxide (ppb, hourly average)

<150

50-299

300-399

>=400

PM10 Particles (µg/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.

* Non seasonal episodes, for example particulate pollution episodes due to transport of particulate matter from industrial areas of central Europe, plume grounding episodes where specific weather conditions push emissions from large industrial chimney stacks down to ground level and episodes caused by release of a concentrated source (spill).

Winter Episodes

During cold calm periods of weather in the winter months, pollution emissions are trapped close to their sources and cannot disperse leading to a build up of pollution. This commonly happens due to an inversion layer in the atmosphere and, as a result, elevated concentrations of a range of pollutants can build up over several days. The biggest source of air pollution currently in most UK cities is road traffic and high concentrations of nitrogen oxides, particulates and hydrocarbons are usually observed during mid-winter episodes
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The adiabatic lapse rate is the temperature profile with respect to height which an air 'parcel' follows as it rises and expands [4]. The temperature of the atmosphere usually decreases with height and as a parcel of air rises, the pressure acting on it falls because the surrounding air is less dense, This causes it to expand (adiabatically and reversibly) and the temperature of the parcel to decrease [4]. If the air temperature then starts to increase with height (inversion, see figure 2) the air parcel cannot continue to rise as it becomes more dense than the surrounding ...

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