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Learning about Air Quality: 1 (Past)


Introduction:

This self-learning tutorial is intended to help you learn about the determinants of air quality, using UK examples of actual air quality data, and to give some guidance as to the understanding of these data. It also provides some indication of why an understanding of air quality is important for Environmental Health, i.e. the implications for human health. 
This tutorial is meant to complement other forms of learning about this subject area, and there are three parts to this: 
  The first part should easily be understood but it needs to be gone over carefully a couple of times. You will also need to engage in background reading to understand the physics and the chemistry of air quality. You might need to come back to it from time to time, since more examples may be added to supplement or to replace the current ones. Indeed, while doing the next tutorial in this series, you might come across some worthwhile data - if so please save it for possible inclusion in this tutorial. 

If you find that you have accessed archived data in the following tutorial at the first attempt but not at subsequent attempts, it may be because the window with the data has been hidden by the main display as a result of your having clicked the scrollbar or elsewhere on the main display in the meantime. To avoid this, it is recommended that you 'close' the special data windows after you have viewed the data, and before going back to the main text.


1. Learning from Past examples.

Several factors determine the concentration of air pollutants at any given time. However, in simple terms, these factors fall in two categories: 

[1] those causing the pollutant to be emitted into, or formed in the atmosphere, and 

[2] those causing the pollutant to be dispersed or otherwise removed from the atmosphere. 

Thus, as a guide: 

     
  • In a city, at a time when traffic is busy one would expect much higher emission of nitrogen dioxide, carbon monoxide, volatile hydrocarbons, and particulate matter than at quieter times in the same city, or than would be the case in the countryside.
  • If sulphur containing fossil fuel is being burnt, then sulphur dioxide as well as particulate matter will be emitted.
  • If a great deal of sunlight containing ultra-violet radiation, interacts with oxygen in the air, in the presence of certain pollutants acting as catalysts, ozone (a secondary pollutant) will be formed.
  • In windy conditions pollutants in the air will tend to be dispersed. Conversely if the air is still, pollutants will not be readily dispersed, (especially in so called 'inversion' conditions when air is trapped close to the ground).
  • In interpreting the data you may need to be aware of the Air Quality bands, as employed by the Department of the Environment.

Example 1. In Belfast, domestic heating still relies considerably on coal. Naturally on cold days, more coal is burnt and more pollution is emitted from this source. If the weather conditions are such that the atmosphere is very still with air trapped close to the ground, high concentrations of particulate air pollution can result. Click here for the actual data (then close the data window). You will note the level in Belfast exceeded 150 micrograms of particulate matter (PM10) per cubic metre of air.

Example 2. The background is the same as for the previous example. However it should be remembered that the coal in question contains significant amounts of sulphur, and hence sulphur dioxide is produced when it is burnt, thus explaining the 'poor' air quality concentration of 216 ppb in Belfast East. Note also the lower concentrations in the rural sites. Click here for the actual data (then close the data window). 

The air quality conditions in these two examples, used to be much commoner and affect many other cities seriously until clean air legislation was enacted following the severe air pollution of the early 1950s. 


Example 3. One would expect lower concentrations of nitrogen dioxide pollution in the countryside than in city centres, since this pollutant is produced largely by traffic. Click here for the actual data, and compare the differences in concentrations (then close the data window). Could the very low concentration in Glasgow on that day, at that time (a mere 1ppb - practically unheard of), perhaps be related to the fact that Glasgow was enjoying a bank holiday Monday, while Edinburgh and the other British cities were at work?

Example 4. Now consider a warmer time of year - with some sunshine setting in. 

ET Weather ForecastOn such occasions, an important concern is that of ozone pollution. However you should remember that this is a secondary pollutant which builds up as more and more of the sun's ultraviolet light gets through. 

The accompanying image from the archives of the Electronic Telegraph shows the forecast for the 6th of June 1996. 

Click here for one set of data, from noon on that day, and look carefully at the ozone concentrations (then close the data window). 

Were any values remarkably high -consistent with poor air quality? Your answer will probably be negative.

Now, click here to see what has happened to the concentrations, measured at the same sites, five hours later (then close the data window). 

Why are the levels so high in the rural sites? 

Remember the following chemical reaction: 

NO + O3 => NO2 + O2 

(Less nitric oxide in the rural environment to destroy the ozone perhaps?) 

Why are some of the urban sites in the second set of data (5 pm) showing higher ozone concentrations than others? 


Example 5.

This example relates to hydrocarbon emissions, specifically benzene. 

In the middle of one week in November, a spell of cold foggy weather with little air movement was experienced in Edinburgh, and perhaps to a lesser extent further south. 

The images show the benzene concentrations in Edinburgh and at a London site. What do you notice? 

 

Benzene - Edinburgh (archive)

Benzene - London (archive)
There is clearly a sharp rise in benzene concentration, on this occasion slightly earlier and slightly higher in Edinburgh, as a result of the very poor clearance of polluted urban air. 
 

You may also wish to make comparisons between these two sites over the last week, by accessing the latest weekly summary for Benzene concentrations in Edinburgh, shown below:- 
Benzene - Edinburgh (dynamic - recent)
Now compare the above Edinburgh benzene concentrations with the corresponding graph over the same few days for Benzene at a London site:- 
Benzene - London (dynamic - recent)

Please note that the scale of the two may be different, especially since concentrations at this latter site tend to be higher. A diurnal variation in concentration may also be evident. The London results tend to be higher than those in Edinburgh, but this is not invariably the case, for example if the windspeed in Edinburgh has been significantly lower than in London.


Other questions...

Some of your questions may have been answered, but probably not all. What exactly is 'very good' in terms of air quality, especially when a mixture of pollutants is concerned? Is it good enough?... for most people?... for everybody? 

Work is in progress to adapt teaching material that answers these questions, or to provide links to other sites which do. 


Acknowledgements etc