Measurements of background atmospheric constituents at Mauna Loa Observatory, Hawaii, are occasionally subject to contamination by local effects such as upslope wind flow or volcanic emissions from Mauna Loa volcano. Volcanic episodes have been identified during 1978 with a sulfur dioxide meter operated at the observatory. In combination with surface wind data these SO2 measurements have been used to examine aerosol data, taken at the observatory, for volcanic episodes and to develop criteria to be used for screening other data taken at the observatory when SO2 measurements are not available. It is found that a requirement that Aitken nuclei concentration be less than about 840 cm-3 will eliminate only about 2% of th data during southerly wind conditions and still approximate the background conditiond determined by independently removing known volcanic episodes.