During the VOCALS (VAMOS Ocean-Cloud-Atmosphere-Land Study) experiment, a suite of instruments will be deployed on the NSF/NCAR C-130 aircraft to measure a wide variety of aerosol properties: number, volatility, size distributions from 10 nm to 10 um in diameter, size-resolved aerosol composition, mixing state, and optical properties at low and ambient humidities. The data will be used to determine the origin of aerosol particles (from the sea surface, the free troposphere, or formed in situ), their suitability as cloud condensation nuclei (CCN), the mechanisms that control particle and CCN concentrations in pockets of open cells (POCs) and the VOCALS region, and their related optical properties. These measurements will help answer several of the hypotheses proposed in the overall VOCALS plan, including the role of aerosol properties in cloud and drizzle formation, scavenging, POC formation and the relationship between aerosol sources and cloud properties. Close collaborations will be established with other aerosol scientists on the C-130 and other platforms during VOCALS, as well as with cloud physicists, meteorologists and remote sensing specialists.
The data should lead to improved understanding of the processes controlling aerosol effects on the radiative balance and climate in regions with extensive stratocumulus cloud cover. This will enable modelers to better parameterize those processes and will also help in understanding the impacts of large anthropogenic sulfur dioxide (SO2) and aerosol sources along the coast of Chile. The results could potentially be applied to the study of CCN evolution and the aerosol indirect effects on cloud properties. One graduate student will be involved in the experiment and data analysis.
Our HiGEAR (Hawaii Group for Environmental Aerosol Research) objectives on VOCALS have been to characterize the physical, chemical and optical properties of the aerosol in order to assess their impacts on and/or modification by the extensive stratus decks over the South Equatorial Pacific (SEP). Our work contributed significant aerosol data collection and analysis for the C-130 flights as included in the survey paper by Dr. Grant Allen et al. published in ACPD in 2011 and in the Wood et al paper. Southeast Pacific Atmospheric Composition and Variability Sampled Along 20±S During VOCALS-Rex, G. Allen, H. Coe, A. Clarke, C. Bretherton, R. Wood, et al. An aircraft case study of the spatial transition from closed to open mesoscale cellular convection over the Southeast Pacific, Wood, R., C. S. Bretherton, D. Leon, A. D. Clarke, P. Zuidema, G. Allen, and H. Coe., ACP 11, no. 5 (2011): 2341-2370. Our own HiGEAR papers examine the role of combustion aerosol and its influence over the SEP not only near the coast in the marine boundary layer (MBL) but in the free troposphere (FT) and near cloud top over the region, Fig.1. AMS organic aerosol concentrations (green) and SP2 black carbon were often highest above cloud (see Fig 2) and indicative of biomass burning sources. Our data implies entrainment of combustion aerosol can replenish MBL aerosol lost to drizzle over the SEP and could help stabilize MBL stratus clouds against dissipation. Aerosol aloft entrained into the MBL contributes to marine aerosol in many regions. Organic aerosol in the MBL may have marine or continental sources. In the SEP MBL we found organic aerosol was linearly related to combustion derived black carbon (Fig. 3) showing that even for the cleanest VOCALS conditions with CO<60 pptv and BC< 15ng m-3 a linear relation between AMS organics and BC reflects a combustion origin. Our paper argues that most organic aerosol in clean marine air was combustion related and linked to inputs from the FT and only a small fraction could be attributed to a natural marine source. Organic matter and non-refractory aerosol over the remote Southeast Pacific: oceanic and combustion sources; L. M .Shank, A.D. Clarke ,S. Howell, S. Freitag, V. Brekhovskikh, V. Kapustin, C. McNaughton, T. Campos, ACP 12.1 (2012): 557-576. Sea-spray aerosol (SSA) number at sizes effective as CCN is an important element of the SEP aerosol-POC-cloud system We demonstrated (Blot et al.) that VOCALS SSA data for very clean conditions was similar to coastal breaking waves (Clarke et al., 2006) allowing us to use our coastal source function in the VOCALS region (Fig. 4a). We show SSA production during VOCALS, expected to be associated with the increased winds measured offshore (Fig. 4b - heavy black line with circles), is not evident in measured data (red) or in data corrected for inversion height changes (grey) (Fig. 5). This is because precipitation also increases dramatically further offshore (Fig. 4c) (See Wood et al, 2012) and this removal sink dominates over production for precipitation rates >1mm day-1 (Fig. 4 d). This helps explain high variability in observed SSA vs. wind speed (Lewis and Schwartz, 2004) because most measurements cannot account for previous scavenging by precipitation . We show that SSA contributes about 20% to CCN over the SEP. Ultrafine Sea Spray Aerosol over the South Eastern Pacific : Open-Ocean contributions to Marine Boundary Layer CCN, Blot, R., A. D. Clarke, S. Freitag, V. Kapustin, S. G. Howell, J. B. Jensen, L. M. Shank, C. S. McNaughton, and V. Brekhovskikh. ACP 13, no. 14 (2013): 7263-7278. Our final year focused on understanding the multiple factors controlling CCN over the SEP. This is the PhD thesis topic, Interactions between Aerosol and Marine Stratocumuli Over the South East Pacific for Steffen Freitag, who defends in December 2014. Although Dr. Clarke is now Emeritus, he continues to explore the link between aerosol, clouds, CCN and both natural and pollution sources over the SEP. A preliminary invited paper was presented at the AGU fall meeting in Dec. 2011 [paper OS42A-05] Combustion Aerosol over Marine Stratus: Long Range Transport, Subsidence and Aerosol-Cloud Interactions over the SEP, A. Clarke, J. Snider, S. Freitag, J. Kazil, G. Feingold, T. Campos, V. Brekhovskikh, V. Kapustin, S. Howell, L. Shank References Clarke, A., S. Owens and J. Zhou, An ultrafine sea-salt flux from breaking waves: Implications for CCN in the remote marine atmosphere, J. Geophys. Res., 111, D06202, doi:10.1029/2005JD006565. Lewis, E. and Schwartz, S.: Sea salt aerosol production: mechanisms, methods, measurements and models: a critical review, vol. 152, Amer. Geophys. Union, 2004. Wood, R., D. Leon, M. Lebsock, J. Snider, A. Clarke, Precipitation driving of droplet concentration variability in marine low clouds, submitted JGR June 14, 2012.
