Improved simulation of isoprene oxidation chemistry with the ECHAM5/MESSy chemistry-climate model: lessons from the GABRIEL airborne field campaign T. M. Butler, D. Taraborrelli, C. Brühl, H. Fischer, H. Harder, M. Martinez, J. Williams, M. G. Lawrence, and J. Lelieveld Max Planck Institute for Chemistry, Mainz, Germany
Abstract. The GABRIEL airborne field measurement
campaign, conducted over the Guyanas in October
2005, produced measurements of hydroxyl radical (OH) concentration which
are significantly higher than can be simulated using current generation
models of atmospheric chemistry.
Based on the hypothesis that this "missing OH" is due to an
as-yet undiscovered mechanism for recycling OH during the oxidation
chain of isoprene, we determine that an OH recycling of about 40–50%
(compared with 5–10% in current generation isoprene oxidation
is necessary in order for our modelled OH to approach the lower error
the OH observed during GABRIEL.
Such a large amount of OH in our model leads to unrealistically low
mixing ratios of isoprene.
In order for our modelled isoprene mixing ratios to match those observed
during the campaign, we also require that the effective rate constant for the
reaction of isoprene with OH be reduced by about 50% compared with
the lower bound of the range recommended by IUPAC.
We show that a reasonable explanation
for this lower effective rate constant could
be the segregation of isoprene and OH in the mixed layer.
Our modelling results are consistent with a global, annual isoprene
source of about 500 Tg(C) yr−1, allowing experimentally derived
and established isoprene flux rates to be reconciled with global models.
Citation: Butler, T. M., Taraborrelli, D., Brühl, C., Fischer, H., Harder, H., Martinez, M., Williams, J., Lawrence, M. G., and Lelieveld, J.: Improved simulation of isoprene oxidation chemistry with the ECHAM5/MESSy chemistry-climate model: lessons from the GABRIEL airborne field campaign, Atmos. Chem. Phys., 8, 4529-4546, doi:10.5194/acp-8-4529-2008, 2008.