Artifact Formation of Sulfate, Nitrate, and Hydrogen Ion on Backup Filters: Allegheny Mountain Experiment

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Abstract
Artifact sulfate formation in ambient conditions on filters of several types was evaluated by comparison between upstream and downstream tandem filters and also by comparison between filters operated in parallel. Some evaluation of artifact H+ and NO3 − was also performed. The experimental site was a tower on the top of Allegheny Mountain in southwestern Pennsylvania. Artifact SO4 = on backup filters was observed to be directly related to filter alkalinity, relative humidity, and cumulative exposure to SO2 with the more alkaline types converting ≥ 10% of the SO2 to which they were exposed, even after very long exposure. On the quartz-fiber backup filters, which gave relatively little artifact, the artifact SO4 = and NO3 − precursors seem to have the same origin as the atmospheric SO2. Artifact H+ was also related to atmospheric SO2, though not clearly to artifact SO4 =; on a mole basis, H+ was a leading, if not the major, artifact species on quartz-fiber backup filters. However, the backup filter method grossly overestimates the amount of artifact SO4 = and NO3 − on front filters under the conditions of this experiment which was dominated by an acidic SO4 − aerosol. The problem observed with NO3 − on nonalkaline front filters was not one of artifact but rather of metathetical loss of real and/or artifact aerosol NO3 −, resulting from attack by the less-volatile acidic aerosol.

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