Vertical mixing and chemistry of isoprene in the atmospheric boundary layer: Aircraft-based measurements and numerical modeling

Vertical profiles of isoprene, methanol, and ozone (O-3) concentrations wer e measured between the middle and upper atmospheric boundary layer (ABL) fr om a research aircraft and were numerically simulated for the ABL and a dec iduous forest canopy with a one-dimensional model coupling turbulence diffu sion and atmospheric chemistry. Isoprene emissions from the deciduous fores t canopy were estimated by coupling an existing biogenic emission algorithm with estimates of canopy leaf density inferred from satellite remote sensi ng observations. Numerical simulations predicted low isoprene concentration s in the middle and upper ABL; however, the agreement between the simulatio ns and the measured values was poor for two of the three profiles, indicati ng that a three-dimensional transport model might be necessary in future si mulations. Chemical oxidation of isoprene by O-3 and hydroxyl radical (OH), particularly in the middle and upper ABL, tends to reduce the isoprene con centrations and influences the vertical fluxes in that layer; however, chem ical reactions have little effect on fluxes of isoprene near the emission s ource, where turbulent mixing is much faster than chemical reactions and wh ere the emission process controls the vertical flux. The isoprene flux decr eases rapidly with increasing height, with little isoprene escaping from th e ABL. Vertical profiles of methanol concentrations were simulated with the biogenic emission algorithm used for isoprene; these vertical profiles wer e similar to the measured values for the well-mixed ABL but were much lower than the measured concentrations in the lower layers of the growing ABL be cause of weaker calculated mixing in the upper ABL during the morning. The results of this investigation indicate that chemical oxidation of isoprene is rapid enough to allow O-3 and other oxidants to accumulate in the ABL on a regional scale if sufficient levels of nitrogen oxides are present; howe ver, methanol is much more stable, and biogenic emissions of this compound have the potential to form O-3 and other oxidants in areas distant from the emission source.