Knowing where air pollutants come from helps assess their impact on human health and climate. One proposed tool is the ratio of two trace gases, glyoxal and formaldehyde, measured from the ground at four sites. This ratio was found to be lower at natural and higher at human-influenced sites, and to vary systematically over the day, season, and with temperature. These patterns suggest that our current understanding of this ratio remains incomplete, limiting its reliable use as a source indicator.