Exhausts adding to smog

Press, 24 September 1988, Page 9

Exhausts adding to smog

By

NIGEL MALTHUS

Sophisticated new equipment installed by the Health Department in Christchurch could show for the first time that the city’s winter smog is at least partly because of motor vehicle exhausts.

Smog of the type which troubles Christchurch on cold nights occurs when water vapour in the air condenses around a seed of pollutant matter into droplets.

dioxide levels: one which was specific for sulphur dioxide, but was accurate only at high levels, and one which measured total acid gases to World Health Organisation standard.

Of the three main air pollutants in Christchurch — smoke, nitrates, and sulphur dioxide — smoke particles are usually thought to be the seed.

Acid gases include sulphur dioxide and the oxides of nitrogen — nitric oxide and nitrogen dioxide — but a public perception of sulphur dioxide as a more serious problem meant that the nitrogen component was largely overlooked, when pollution figures were published. Sulphur dioxide came from burning any fuel containing sulphur, primarily coal, although diesel fuel oils also contain small amounts.

There were now indications from places such as Denver and Phoenix, however, that “a substantial portion” of the loss of visibility experienced in winter smog was because of nitrates, produced from car exhausts, said Dr Terry Brady, the Health Department’s district air pollution control officer. Christchurch people would frequently say “it was a bad night last night,” when the department’s smoke measurements did not show that it had been particularly smoky, he said. The new equipment installed in Christchurch this year will allow the various pollutants to be more accurately measured and related to smog.

Sales of high-sulphur coals were now restricted in Christchurch, however, and sulphur levels in the city were low, said Dr Brady. Oxides of nitrogen come from high-tempera-ture combustion — hotter than domestic fires. Motor vehicles are the only significant source in Christchurch.

Dr Brady has begun research aimed at identifying the primary cause of loss of visibility. The department had installed “a whole rack of new instruments,” including devices to measure the “visibility index” and relevant weather factors including wind speed, he said.

Smoke, the third main pollutant, comes from any combustion which is inefficient or incomplete. Domestic fires are the main source.

The oxides of nitrogen further react to form the nitrates which are now thought to play a part in seeding winter smog.

Nitrates are already known to cause Los Angeles-style photochemical smogs, but these are a different phenomenon, requiring sunlight and high temperatures. Photochemical smogs have been recorded in Christchurch in summer.

These also included devices to accurately distinguish between sulphur dioxide and the oxides of nitrogen. The department formerly had used two methods to assess sulphur