Atmospheric Chemistry and Physics Discussions
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2009
10.5194/acpd-9-17265-2009
http://www.atmos-chem-phys-discuss.net/9/17265/2009/
http://www.atmos-chem-phys-discuss.net/9/17265/2009/acpd-9-17265-2009.html
http://www.atmos-chem-phys-discuss.net/9/17265/2009/acpd-9-17265-2009.pdf
17265
17296
2009-08-14
Closing the Dimethyl Sulfide Budget in the Tropical Marine Boundary Layer during the Pacific Atmospheric Sulfur Experiment
S. A. Conley
saconley@ucdavis.edu
I. Faloona
G. H. Miller
B. Blomquist
D. Lenschow
A. Bandy
Department of Land, Air and Water Resources, University of California, Davis, CA, USA
Department of Applied Science, University of California, Davis, CA, USA
National Center for Atmospheric Research, Boulder, CO, USA
Department of Chemistry, Drexel University, Philadelphia, PA, USA
Department of Oceanography, University of Hawaii, Honolulu, HI, USA
Fourteen research flights were conducted with the National Center for
Atmospheric Research (NCAR) C-130 near Christmas Island (2° N, 157° W)
during the summer of 2007 as part of the Pacific Atmospheric Sulfur
Experiment (PASE). In order to tightly constrain the scalar budget of DMS,
fluxes were measured at various levels in the marine boundary layer (MBL)
from near the surface (30 m) to the top of the mixed layer (500 m) providing greater accuracy of the flux divergence calculation in the
DMS budget. The observed mean mole fraction of DMS in the MBL exhibited the
well known diurnal cycle, ranging from 50 pptv in the daytime to 110 pptv at night. Contributions from horizontal advection are included using a
multivariate regression of all DMS flight data from within the MBL to
estimate the mean gradients and trends. With this technique we consider the
residual term in the DMS budget as an estimate of overall photochemical
oxidation. Error analysis of the various terms in the DMS budget indicate
that chemical losses acting on time scales of up to 110 h can be
inferred with this technique. On average, photochemistry accounted for 7.3 ppt hr<sup>−1</sup>
loss rate for the seven daytime flights, with an estimated
error of 0.6 ppt/hr. The loss rate due to expected OH oxidation is
sufficient to explain the net DMS destruction without invoking the action of
additional oxidants (e.g. reactive halogens.) The observed ocean flux of
DMS averaged 3.1 (±1.5)μmol m<sup>−2</sup> d<sup>−1</sup>, and generally
decreased throughout the sunlit hours. The average entrainment flux at the
top of the MBL was 2.5 μmol m<sup>−2</sup> d<sup>−1</sup>; therefore the flux
divergence term in the budget equation only contributed an average increase
of 1.3 ppt hr<sup>−1</sup> to the mean MBL mole fraction. Over the entire mission,
the horizontal advection contribution to the overall budget was 0.2 ppt hr<sup>−1</sup>, indicating a mean atmospheric DMS gradient nearly perpendicular
to the east-southeasterly trade winds and the chlorophyll gradient in the
equatorial upwelling ocean. Nonetheless, horizontal advection was a
significant term in the budget of any given flight, ranging from −1.5 to
2.3 ppt hr<sup>−1</sup>, indicating a patchy and random seawater DMS distribution,
and thus needs to be accounted for in budget studies.
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