Acetone was one of the most abundant volatile organic compounds (VOCs) observed in the Sierra Nevada Mountains (California, USA). Mixing ratios were measured hourly above a ponderosa pine plantation using an automated in situ dual-channel GC-FID system throughout July 1997, from July through October 1998, and fluxes were measured with a relaxed eddy accumulator from July through September 1999. Acetone mixing ratios ranged from 1.4 to 7.8 ppb in July 1997, 1.1 to 7.8 ppb in July 1998, and 1.0 to 8.0 ppb in July 1999, and were correlated with compounds of biogenic and anthropogenic origin. During sunny periods in 1997 (PAR>1300 μmol cm−2 s−1) acetone was correlated with methylbutenol (biogenic emission from ponderosa pine, r2=0.48). Under the same conditions, acetone was also correlated with acetylene (anthropogenic emission, r2=0.52), yet acetylene and methylbutenol were not correlated with each other (r2=0.06). A linear combination of 1.34×methylbutenol+9.64×acetylene was highly correlated with acetone mixing ratios (r2=0.80), suggesting that the quantity of biogenic and anthropogenic contributions to the observed acetone could be determined using this correlation. Based on this method, 45% of the observed acetone could be attributed to biogenic sources, 14% to anthropogenic sources, and 41% to the regional background level. Comparison with direct emission ratios from tailpipe exhaust showed that the anthropogenic contribution to acetone mixing ratios could be attributed almost completely to secondary photochemical production (99%), with only a minor contribution from direct fuel combustion emissions (1%). Based on direct measurements of ecosystem scale fluxes, the biogenic contribution to acetone mixing ratios could be attributed to direct emissions (35%), methylbutenol oxidation (63%), and monoterpene oxidation (2%).