Canopy and leaf level 2-methyl-3-buten-2-ol fluxes from a ponderosa pine plantation

Canopy and leaf level 2-methyl-3-butene-2-ol (methylbutenol, MBO) emissions were measured from a ponderosa pine plantation in the Sierra Nevada mounta ins from July to October 1998. Canopy scale fluxes were measured using a gr adient approach, leaf level fluxes used a flow-through chamber. Emissions w ere dependent on ambient light and temperature levels and showed a diurnal cycle very similar to isoprene in deciduous forests. Maximum fluxes occurre d between 1000 and 1700 h with an average of 2 mg C m(-2) h(-1) in July and August, which equaled approximately 0.3-0.5% of the simultaneously measure d gross carbon uptake. MBO mixing ratios and fluxes over the pine plantatio n were also measured with a relaxed eddy accumulation (REA) system operated during part of our measurement campaign (Baker et al., 1999 Journal of Geo physical Research, in press). Mixing ratios measured by both systems were i n good agreement but the gradient approach estimated MBO fluxes twice as hi gh than the REA approach. Leaf level investigations revealed a seasonal cyc le in basal emission rate (emissions at 1000 mu mol m(-2) s(-1) PAR and 30 degrees C) with a maximum in August, decreasing towards the end of the seas on. We developed an emission model to scale MBO fluxes from the leaf level to the ecosystem level based on the well-known isoprene algorithm. The mode l estimates are substantially lower than our gradient flux measurements, an d match better with the REA flux measurements, and we conclude that the gra dient approach overestimated MBO fluxes at this site. Comparing seasonal ch anges of measured with modeled fluxes showed a pattern of basal emission ra tes similar to those observed at the leaf level, and the basal emission rat e was related to daytime air temperatures. While MBO contributes substantia lly to local photochemistry and its oxidation contributes significantly to the regional acetone budget, the latter probably does not represent a signi ficant global source of atmospheric acetone. (C) 2000 Elsevier Science Ltd. All rights reserved.