District Fine-Tunes Smog Readings

The South Coast Air Quality Management District on Monday changed the method by which it reports the hourly amount of nitrogen dioxide pollution, which will change the appearance of the air quality charts published daily in The Times.

The new method corrects an error that has caused nitrogen dioxide levels to be reported at higher than actual levels, according to Art Davidson, acting deputy executive officer for public affairs.

Hourly levels of three pollutants, ozone, carbon monoxide and nitrogen dioxide are displayed in charts according to a “pollutant standard index.” The index is a computed number based on atmospheric measurements of pollutant concentration compared with allowable limits at each of several standards established by state and federal government.

The old formula for computing the NO2 index compared concentrations only against the one-hour standard, while the new method also takes into consideration the 24-hour standard.

Davidson noted that the district is within 2% of meeting the federal annual standard for NO2 and that during 1987, exceeded the state one-hour standard on only four days and only at the downtown Los Angeles measurement station. But the former method of reporting the pollutant made it appear that there were many more violations.

November and December are traditionally the worst months for NO2 pollution, Davidson said. He estimated that during the course of a year, concentrations will range between zero and 150 on the PSI scale reported by The Times.