Impacts of aerosol particles on the microphysical and radiative properties of stratocumulus clouds over the southeast Pacific Ocean 1College of Earth, Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, USA
05 Mar 2013
2School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona, USA
3Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, Colorado, USA
4School of Earth and Environment, University of Leeds, Leeds, UK
5Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA
6Center for Global and Regional Environmental Research, University of Iowa, Iowa City, Iowa, USA
7Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, USA
8Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming, USA
*currently at: Atmospheric, Oceanic and Planetary Physics, Department of Physics, University of Oxford, Oxford, UK
Received: 06 Jul 2012 – Published in Atmos. Chem. Phys. Discuss.: 09 Aug 2012 Abstract. The southeast Pacific Ocean is covered by the world's
largest stratocumulus cloud layer, which has a strong impact on ocean
temperatures and climate in the region. The effect of anthropogenic sources
of aerosol particles on the stratocumulus deck was investigated during the
VOCALS field experiment. Aerosol measurements below and above cloud were
made with a ultra-high sensitivity aerosol spectrometer and analytical
electron microscopy. In addition to more standard in-cloud measurements,
droplets were collected and evaporated using a counterflow virtual impactor
(CVI), and the non-volatile residual particles were analyzed.
Revised: 05 Feb 2013 – Accepted: 08 Feb 2013 – Published: 05 Mar 2013
Many flights focused on the gradient in cloud properties on an E-W track
along 20° S from near the Chilean coast to remote areas offshore.
Mean statistics, including their significance, from eight flights and many
individual legs were compiled. Consistent with a continental source of cloud
condensation nuclei, below-cloud accumulation-mode aerosol and droplet
number concentration generally decreased from near shore to offshore. Single
particle analysis was used to reveal types and sources of the enhanced
particle number that influence droplet concentration. While a variety of
particle types were found throughout the region, the dominant particles near
shore were partially neutralized sulfates. Modeling and chemical analysis
indicated that the predominant source of these particles in the marine
boundary layer along 20° S was anthropogenic pollution from
central Chilean sources, with copper smelters a relatively small
Cloud droplets were smaller in regions of enhanced particles near shore.
However, physically thinner clouds, and not just higher droplet number
concentrations from pollution, both contributed to the smaller droplets.
Satellite measurements were used to show that cloud albedo was highest
500–1000 km offshore, and actually slightly lower closer to shore due to the
generally thinner clouds and lower liquid water paths there. Thus, larger
scale forcings that impact cloud macrophysical properties, as well as
enhanced aerosol particles, are important in determining cloud droplet size
and cloud albedo.
Differences in the size distribution of droplet residual particles and
ambient aerosol particles were observed. By progressively excluding small
droplets from the CVI sample, we were able to show that the larger drops,
some of which may initiate drizzle, contain the largest aerosol particles.
Geometric mean diameters of droplet residual particles were larger than
those of the below-cloud and above cloud distributions. However, a wide
range of particle sizes can act as droplet nuclei in these stratocumulus
clouds. A detailed LES microphysical model was used to show that this can
occur without invoking differences in chemical composition of
Citation: Twohy, C. H., Anderson, J. R., Toohey, D. W., Andrejczuk, M., Adams, A., Lytle, M., George, R. C., Wood, R., Saide, P., Spak, S., Zuidema, P., and Leon, D.: Impacts of aerosol particles on the microphysical and radiative properties of stratocumulus clouds over the southeast Pacific Ocean, Atmos. Chem. Phys., 13, 2541-2562, doi:10.5194/acp-13-2541-2013, 2013.