Aerosol measurements have been made continuously at Mauna Loa Observatory (MLO) from 1974 to the present. Condensation nucleus (CN) concentration has been measured using automatic CN counters, and aerosol scattering extinction (&sgr;sp) has been measured using a four-wavelength nephelometer. The Mauna Loa Observatory Photochemistry Experiment (MLOPEX) was conducted in 1991--1992 to study intensively many important variables in the field of atmospheric chemistry. Because of a strong diurnal cycle in nearly everything measured at MLO, caused by an upslope-downslope wind system, it is important to develop data-editing criteria that can safely identify background conditions as opposed to other conditions when the site may be contaminated by local sources. Ordinarily, background conditions occur during nighttime downslope wind conditions, and contaminated conditions occur during daytime upslope wind conditions. However, occasionally unusual weather conditions or contamination caused by a local source such as the Mauna Loa caldera can confuse the issue. It is recommended that background aerosol data be chosen during 0000--0800 Hawaiian standard time (HST) to generally avoid upslope wind conditions, and that wind direction and speed, CN, and SO2 data be used if available to further eliminate local pollution episodes. In addition, all data should be examined by a human editor, if possible, in order to recognize certain episodes that may not fit automated criteria. ¿ American Geophysical Union 1996