Feature

Something in the Air Tonight

A new method of monitoring the contents of our city air may reveal a few unpleasant surprises.

Andy Reisinger

As yet another winter prepares to lift its grip on New Zealand, the discussion about air pollution is likely to evaporate just like the layers of fog and smoke that make cities like Christchurch a rather unpleasant place to be in during the cold months.

Much of New Zealand's research into air pollution to date has focused on smoke emissions from open fires and the associated health risks; it is now well known that wood and coal fires used for domestic heating contribute at least 90% of the total smoke emissions in Christchurch, regularly pushing the concentrations of fine particulate matter (PM10) well above the levels considered safe by the World Health Organisation and the Canterbury Regional Council alike. Despite the lively discussion about potential solutions to the Christchurch smoke pollution, it is generally accepted that problems mainly lie in the area of politics -- there is no question that smoke levels are too high, and that they must come down.

It is only beginning to dawn on many people, however, that air pollution usually consists of many more species than just smoke, with sulfur dioxide (SO₂), carbon monoxide (CO), nitrogen dioxide (NO₂), ozone (O₃), and benzene (C₆H₆) being just a few examples. These gases come from a variety of sources (domestic heating, traffic emissions, industry) and all affect human health in some direct or indirect way, ranging from temporary reduction of physical fitness to inducing cancer.

Relatively little, however, is known about the typical concentrations of some of these gases, their detailed sources, and effects on human health in the New Zealand context. This is mainly due to the fact that with traditional measurement methods, one requires almost a separate miniature chemical laboratory for each individual compound to be detected, and even then the sensitivity of conventional techniques is often not high enough to study the behaviour of these gases in the atmosphere in detail.

Almost two decades ago, however, a technique was developed in the United States and Europe which measures the concentration of pollutant trace gases through their ability to absorb light, allowing the measurement of a large number of gases in the atmosphere with a single instrument, often with higher precision than conventional techniques and shorter averaging times. This technique has now found its way into New Zealand through development work undertaken by the University of Canterbury in conjunction with the National Institute of Water and Atmospheric Research (NIWA).

Light Absorption

The technique of differential absorption spectroscopy utilises the fact that all gases absorb light at certain wavelengths. If a beam of light is shone through the atmosphere, some wavelengths of the light will therefore be filtered out by pollutant gases, while other wavelengths pass through without loss of intensity. The presence of air pollutants thus creates a unique absorption pattern in a beam of light that allows one to both identify individual species and quantify their concentrations.

A good example is the typical absorption pattern of carbon monoxide, an air pollutant which stems mainly from traffic emissions and domestic heating. The range of concentrations observed for this trace gas can be very large. In clean air, the concentration of carbon monoxide (CO) is about 60 parts per billion (ie 60 molecules of CO per billion molecules of air). During polluted conditions in the Christchurch winter, however, carbon monoxide levels often climb above 6,000 parts-per-billion, resulting in a drastic change of the absorption characteristics of the atmosphere.

Many other gases, including important air pollutants such as SO₂, NO₂, C₆H₆, and O₃, can be measured using the same principle with very good detection limits and high time resolution (typically less than five minutes). The main challenge in this technique exists in the measurement of atmospheric absorption spectra which have sufficiently low noise to allow the detection of even very weak absorbers, increasing the range of gases which can be measured successfully.

The only component which cannot be measured with the differential absorption technique is smoke, one of the prime components of the Christchurch air pollution. Smoke absorbs light at almost every wavelength and does not create an absorption pattern that would allow one to quantify its concentration in the light path.

The great advantage of this new technique, compared with conventional chemical detection methods, is that a single instrument allows the measurement of a great number of gases -- basically all gases which absorb light in the region for which the spectrometer is tuned. This includes some gases which have never or rarely been measured in New Zealand cities before, for example benzene, nitrous acid, and ozone.

Three such instruments have come recently into operation in New Zealand. One is a commercial instrument which has been purchased by NIWA in Auckland, the other two are original developments from a cooperation between the University of Canterbury and NIWA in Lauder, Central Otago. While all instruments are still in their testing and validation phase, first results from Christchurch already show their potential in investigating the concentrations and possible sources for some trace gases whose presence has hitherto been underemphasised in regional and national air quality planning.

Some results

One of the prime goals of the new measurement equipment is the measurement of pollutants whose concentrations are currently only poorly known, and the building of an air quality database which allows assessment of their long-term average concentrations and contribution to public health. While current equipment is well able to detect peak pollution episodes and act as smog warning sensors, detailed studies of the sources and distributions of many pollutant gases requires a degree of precision which only the new equipment can deliver.

The instrumentation in Christchurch allows the simultaneous measurement of carbon monoxide, nitrogen monoxide, and benzene. While the first two gases come from both traffic and domestic heating, the latter has traffic as its only source. First measurements of these gases in Christchurch this winter show a very high correlation between all three, which in the long-term could be used to estimate the relative contributions of traffic and domestic heating to carbon monoxide pollution, and to estimate benzene concentrations in other locations where only carbon monoxide and nitrogen monoxide are monitored.

Benzene is mainly emitted from the combustion of high-octane rated petrol. Its importance in air quality issues stems from the fact that according to current medical theory, there is no safe exposure limit for benzene. The attributed health damage potential is simply proportional to a person's total lifetime exposure; therefore a person living in an area which suffers constantly from mild traffic pollution might be at the same risk level as a worker who is exposed to very high benzene concentrations at a roadside, but only for a limited number of days per month. While there is currently insufficient data to allow any detailed conclusions or recommendations to be made, it is an area where more attention will have to paid in future planning strategies.

Another issue that has been tackled with the new instrumentation is the possibility of summer smog in the Canterbury area. Air pollution is generally associated with cold, wintry conditions which are responsible for producing clear, still days with strong inversion layers which trap pollutants underneath. Under certain conditions, however, strong summer sunshine and a mix of trace gases like nitrogen oxides and hydrocarbons could produce a "second generation" of air pollutants through chemical reactions within the atmosphere. Such pollution episodes are well known in the northern hemisphere (particularly the United States and many European areas), but appear to be virtually non-existent in New Zealand. Detailed investigations into the possible occurrence and composition of such episodes in this country are now possible. The past summer of 1996/97 was not exactly conducive to producing such still, clear, hot days, but monitoring will continue in the coming seasons to further investigate this potential problem area.

Conclusions

We can hope that the availability of this new air pollution monitoring technique will allow city and regional councils to incorporate air quality considerations more readily into their medium- and long-term planning strategies than has previously been the case. While the balance between potential health hazards of pollutant trace gases and the cost of equipment to monitor them often seems to have put air quality to the bottom of the priorities list, there will be only few such excuses left in the coming years. At the same time, it would certainly be worth keeping in mind that it is not the measurement of air pollution, but political action and personal responsibility of the citizens that will ultimately clean up the air in our cities.

Andy Reisinger is with NIWA in Wellington