Journal article

Free troposphere as a major source of CCN for the equatorial pacific boundary layer: Long-range transport and teleconnections

Clarke A, Freitag S, Simpson R, Hudson J, Howell S, Brekhovskikh V, Campos T, Kapustin V, Zhou J ...see all

Atmospheric Chemistry and Physics, vol. 13, issue 15 (2013) pp. 7511-7529 Published by Copernicus GmbH

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Airborne aerosol measurements in the central equatorial Pacific during
PASE (Pacific Atmospheric Sulfur Experiment) revealed that cloud
condensation nuclei (CCN) activated in marine boundary layer (MBL)
clouds were strongly influenced by entrainment from the free troposphere
(FT). About 65% entered at sizes effective as CCN in MBL clouds, while
similar to 25% entered the MBL too small to activate but subsequently
grew via gas to particle conversion. The remaining similar to 10% were
inferred to be sea salt aerosol.
FT aerosols at low carbon monoxide (CO) mixing ratios (mostly volatile at 360 degrees C with a number mode peak of around 30-40
nm dry diameter and tended to be associated with cloud outflow from
distant (3000 km or more) deep convection. Higher CO concentrations were
commonly associated with trajectories from South America and the Amazon
region (ca. 10 000 km away) and occurred in layers indicative of
combustion sources (biomass burning season) partially scavenged by
precipitation. These had number modes near 60-80 nm dry diameter with a
large fraction of CCN.2 (those activated at 0.2% supersaturation and
representative of MBL clouds) prior to entrainment into the MBL. Flight
averaged concentrations of CCN.2 were similar for measurements near the
surface, below the inversion and in the FT just above the inversion,
confirming that subsidence and entrainment of FT aerosol strongly
influenced MBL CCN.2. Concurrent flight-to-flight variations of CCN.2 at
all altitudes below 3 km also imply MBL CCN.2 concentrations were in
quasi-equilibrium with the FT over a 2-3 day timescale.
The observed FT transport over thousands of kilometers indicates
teleconnections between MBL CCN and cloud-scavenged sources of both
natural and/or residual combustion origin. Nonetheless, in spite of its
importance, this source of CCN number is not well represented in most
current models and is generally not detectable by satellite because of
the low aerosol scattering in such layers as a result of cloud
scavenging. In addition, our measurements confirm nucleation in the MBL
was not evident during PASE and argue against a localized linear
relation in the MBL between dimethyl sulfide (DMS) and CCN suggested by
the CLAW hypothesis. However, when the FT is not impacted by long-range
transport, sulfate aerosol derived from DMS pumped aloft in the ITCZ
(Inter-Tropical Convergence Zone) can provide a source of CCN to the
boundary layer via FT teleconnections involving more complex non-linear

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  • A. D. Clarke

  • S. Freitag

  • R. M C Simpson

  • J. G. Hudson

  • S. G. Howell

  • V. L. Brekhovskikh

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