ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-5237-2011 South East Pacific atmospheric composition and variability sampled along 20° S during VOCALS-REx Allen G. 1 Coe H. 1 Clarke A. 2 Bretherton C. 3 Wood R. 3 Abel S. J. 4 Barrett P. 4 Brown P. 4 George R. 3 Freitag S. 2 McNaughton C. 2 Howell S. 2 Shank L. 2 Kapustin V. 2 Brekhovskikh V. 2 Kleinman L. 5 Lee Y.-N. 5 Springston S. 5 Toniazzo T. 6 Krejci R. 7 Fochesatto J. 8 Shaw G. 8 Krecl P. 9 Brooks B. 9 McMeeking G. 1 Bower K. N. 1 Williams P. I. 1 Crosier J. 1 Crawford I. 1 Connolly P. 1 Allan J. D. 1 Covert D. 3 Bandy A. R. 10 Russell L. M. 11 Trembath J. 12 Bart M. 9 McQuaid J. B. 9 Wang J. 5 Chand D. 3 13 Centre for Atmospheric Science, University of Manchester, Manchester, M13 9PL, UK Department of Oceanography, University of Hawaii at Manoa, Honolulu, USA Department of Atmospheric Science, University of Washington, Seattle, Washington, USA Met Office, Exeter, UK Brookhaven National Laboratory, USA Department of Meteorology, University of Reading, Reading, UK Department of Applied Environmental Science (ITM), Stockholm University, Sweden Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK, USA Institute for Climate and Atmospheric Science, University of Leeds, UK Drexel University, Philadelphia, USA Scripps Institution of Oceanography, University of Califronia-San Diego, USA Facility for Airborne Atmospheric Measurements, Cranfield University, UK now at: Atmospheric Sciences and Global Change Division, PNNL, Richland, WA, USA 01 06 2011 11 11 5237 5262 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/11/5237/2011/acp-11-5237-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/5237/2011/acp-11-5237-2011.pdf

The VAMOS Ocean-Cloud-Atmosphere-Land Regional Experiment (VOCALS-REx) was conducted from 15 October to 15 November 2008 in the South East Pacific (SEP) region to investigate interactions between land, sea and atmosphere in this unique tropical eastern ocean environment and to improve the skill of global and regional models in representing the region. This study synthesises selected aircraft, ship and surface site observations from VOCALS-REx to statistically summarise and characterise the atmospheric composition and variability of the Marine Boundary Layer (MBL) and Free Troposphere (FT) along the 20° S parallel between 70° W and 85° W. Significant zonal gradients in mean MBL sub-micron aerosol particle size and composition, carbon monoxide, sulphur dioxide and ozone were seen over the campaign, with a generally more variable and polluted coastal environment and a less variable, more pristine remote maritime regime. Gradients in aerosol and trace gas concentrations were observed to be associated with strong gradients in cloud droplet number. The FT was often more polluted in terms of trace gases than the MBL in the mean; however increased variability in the FT composition suggests an episodic nature to elevated concentrations. This is consistent with a complex vertical interleaving of airmasses with diverse sources and hence pollutant concentrations as seen by generalised back trajectory analysis, which suggests contributions from both local and long-range sources. Furthermore, back trajectory analysis demonstrates that the observed zonal gradients both in the boundary layer and the free troposphere are characteristic of marked changes in airmass history with distance offshore – coastal boundary layer airmasses having been in recent contact with the local land surface and remote maritime airmasses having resided over ocean for in excess of ten days. Boundary layer composition to the east of 75° W was observed to be dominated by coastal emissions from sources to the west of the Andes, with evidence for diurnal pumping of the Andean boundary layer above the height of the marine capping inversion. Analysis of intra-campaign variability in atmospheric composition was not found to be significantly correlated with observed low-frequency variability in the large scale flow pattern; campaign-average interquartile ranges of CO, SO<sub>2</sub> and O<sub>3</sub> concentrations at all longitudes were observed to dominate over much smaller differences in median concentrations calculated between periods of different flow regimes. The campaign climatology presented here aims to provide a valuable dataset to inform model simulation and future process studies, particularly in the context of aerosol-cloud interaction and further evaluation of dynamical processes in the SEP region for conditions analogous to those during VOCALS-REx. To this end, our results are discussed in terms of coastal, transitional and remote spatial regimes in the MBL and FT and a gridded dataset are provided as a resource.

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