| Popis: |
A laboratory and field study was conducted to evaluate artifact nitrate formation on filter media. The filter media evaluated included a cellulose acetate, a Teflon, a cellulose and five types of glass fiber filters covering a pH range from 4.6 to 9.4. Field studies employed simultaneous sampling using five high volume samplers and a low volume sampler system providing both a respirable (i.e. < 3.5 μm) fraction and a sample without size segregation. Laboratory studies evaluated interaction of clean filters with nitrogen dioxide and nitric acid as possible sources of artifact nitrate. The influences of temperature, relative humidity, sampling time, NH3, ozone and SO2 were determined. In 24-h field sampling in San Jose and Los Alamitos, California, mean nitrate values obtained with the filters studied differed by as much as a factor of 2.4. Of the filters studied only the Teflon filter (Fluoropore) provided collection of nitrate with negligible error due to interaction of the filter with the gaseous pollutants studied. A pH9.3 high volume glass fiber filter collected, on average, about 10 μg m−3 artifact nitrate compared with 8.5 μg m−3 of true particulate nitrate in 24h sampling. Laboratory studies suggest that collection of gaseous nitric acid is the dominant source of artifact nitrate. Nitrogen dioxide only became a substantial source of artifact nitrate in the presence of relatively high concentrations of ozone. |
