Chemistry, transport and dry deposition of trace gases in the boundary layer over the tropical Atlantic Ocean and the Guyanas during the GABRIEL field campaign A. Stickler1,**, H. Fischer1, H. Bozem1, C. Gurk1, C. Schiller2, M. Martinez-Harder1, D. Kubistin1, H. Harder1, J. Williams1, G. Eerdekens1, N. Yassaa1, L. Ganzeveld1,*, R. Sander1, and J. Lelieveld1 1Department of Atmospheric Chemistry, Max Planck Institute for Chemistry, Mainz, Germany 2Department of Chemistry, York University, Toronto, Canada *now at: Department of Earth System Science, University Wageningen, The Netherlands **now at: Institute for Atmospheric and Climate Science, ETH Zürich, Switzerland
Abstract. We present a comparison of different Lagrangian and chemical box
model calculations with measurement data obtained during the GABRIEL campaign
over the tropical Atlantic Ocean and the Amazon rainforest in the Guyanas, October 2005.
Lagrangian modelling of boundary layer (BL) air constrained by measurements is used to derive a horizontal gradient
(≈5.6 pmol/mol km−1) of CO from the ocean to the
rainforest (east to west). This is significantly smaller than that derived
from the measurements (16–48 pmol/mol km−1), indicating that photochemical
production from organic precursors alone cannot explain the observed strong gradient.
It appears that HCHO is overestimated by the Lagrangian and chemical box models,
which include dry deposition but not exchange with the free troposphere (FT).
The relatively short lifetime of HCHO implies substantial BL-FT
exchange. The mixing-in of FT air affected by African and South American
biomass burning at an estimated rate of 0.12 h−1 increases the CO
and decreases the HCHO mixing ratios, improving agreement with
measurements. A mean deposition velocity of 1.35 cm/s for H2O2
over the ocean as well as over the rainforest is deduced assuming BL-FT exchange
adequate to the results for CO. The measured increase of the organic peroxides
from the ocean to the rainforest (≈0.66 nmol/mol d−1)
is significantly overestimated by the Lagrangian model, even when
using high values for the deposition velocity and the entrainment rate.
Our results point at either heterogeneous loss of organic peroxides and/or
their radical precursors, underestimated photodissociation or missing reaction paths of peroxy radicals not
forming peroxides in isoprene chemistry. We calculate a mean integrated
daytime net ozone production (NOP) in the BL of (0.2±5.9) nmol/mol (ocean)
and (2.4±2.1) nmol/mol (rainforest). The NOP strongly correlates with NO
and has a positive tendency in the boundary layer over the rainforest.
Citation: Stickler, A., Fischer, H., Bozem, H., Gurk, C., Schiller, C., Martinez-Harder, M., Kubistin, D., Harder, H., Williams, J., Eerdekens, G., Yassaa, N., Ganzeveld, L., Sander, R., and Lelieveld, J.: Chemistry, transport and dry deposition of trace gases in the boundary layer over the tropical Atlantic Ocean and the Guyanas during the GABRIEL field campaign, Atmos. Chem. Phys., 7, 3933-3956, doi:10.5194/acp-7-3933-2007, 2007.