Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE)

E-PEACE, an experiment which while not identifying as SRM resulted in clear implications for marine cloud brightening (MCB) technology post hoc;

E-PEACE combined a targeted aircraft campaign off the coast of Monterey in July and August 2011 with embedded ship and satellite observations (Fig. 2) and modeling studies. Atmospheric conditions in the northeastern Pacific during July are ideal for the formation of homogeneous layers of persistent stratocumulus clouds. The layers observed have consistent diurnal characteristics, cloud thicknesses of 100–300 m, and cloud-top heights typically below 500 m. The susceptibility of cloud albedo to particle perturbations is well documented for the eastern Pacific near 36°N (Coakley et al. 1987, 2000; Platnick et al. 2000).

We employed the research vessel (R/V) Point Sur to measure the aerosol below cloud and as a platform for well-characterized smoke emissions to produce a uniquely identifiable cloud signature. The Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft was used with a full payload of instruments (Table 3) to measure particle and cloud droplet number, mass, and composition. E-PEACE combined 1) controlled release of smoke from the deck of the Point Sur, salt aerosol from the Twin Otter, and exhaust from container ships transiting across the study region; 2) flight plans designed to investigate results from large-eddy simulations (LES) and to provide constraints for aerosol–cloud parcel (ACP) modeling studies, to test our ability to quantitatively predict the cloud dynamical response to increases in particle concentrations in the natural atmosphere; and 3) satellite analyses of marine stratocumulus to constrain the radiative properties of the natural, perturbed, and regional cloud systems.

Table 3.
Instruments on CIRPAS Twin Otter and R/V Point Sur. PM10 = particulate matter ≤10 μm.

With 12 days of ship time on the R/V Point Sur and 30 flights (each ~4.5 h long) on the CIRPAS Twin Otter (Tables 4 and 5), we could take full advantage of the persistence of stratocumulus clouds to probe the effect of particle sources on marine stratocumulus properties. Since the particles would be emitted in high concentrations over small areas in crosswind directions, their effects on clouds could be separated from those of meteorology. And in terms of number concentration and duration, the impacts of these particle emissions would be large enough to be distinguished from natural cloud variability.

Table 4.
Summary of CIRPAS Twin Otter flights during E-PEACE 2011.

Table 5.
Summary of R/V Point Sur cruise during E-PEACE 2011..

As noted above, three types of particles were involved in E-PEACE:

1) combustion exhaust particles from cargo ships of opportunity, which are the emissions responsible for ship tracks;
2) shipboard smoke-generated particles; and
3) aircraft-based milled salt particles (Fig. 3).

Type 1 is the exhaust that consists of 50–100-nm dry-diameter particlesemitted at rates of 1016–1018 s–1 from the engines of large (2,000 ton) cargo ships, in this instance on trans-Pacific, Los Angeles to San Francisco, or other commercial routes. Such emissions were responsible for the first observed ship tracks (Conover 1969). At a fuel cost of about $100,000 (U.S. dollars) per day, operations of such vessels dedicated solely to research are not feasible. However, real-time tracking of commercial vessels (www.marinetraffic.com) was used to identify fast-moving (>30 km h–1) cargo or container ships in the region within the aircraft operating area (as illustrated in Fig. 4).

Type 2 involves smoke particles produced at an estimated rate of 1011–1013 s–1 on the stern deck of the R/V Point Sur (described in “Tailor-made particles with a battlefield smoke generator”), with dry diameters that ranged from 50 nm up to 1 μm and very low hygroscopicity.

Type 3 particles were dispersed from the Twin Otter aircraft in cloud. An adjustable auger fed a fluidized bed that dispensed NaCl particles, which had been milled to diameters of 3–5μm and mixed with SiO2 to prevent particles from sticking together (Drofa et al. 2010). [1]

FIGURE 1: Photographs of the R/V Point Sur from the CIRPAS Twin Otter, showing (a) the persistence of the plume of smoke from the ship in the atmosphere and some of the aircraft instruments for measuring particles and clouds, (b) the production of smoke, © one of the two smoke generators used for producing smoke, (d) the operation of the smoke generators on the stern of the R/V Point Sur, and (e) the aerosol instrumentation on the bow of the R/V Point Sur.

FIGURE 2: Illustration of E-PEACE design and observations of emitted particles in marine stratocumulus in Jul and Aug 2011 west of central California. The diagram shows the three platforms used in making observations of particle and cloud chemical and physical properties, namely, the R/V Point Sur, the CIRPAS Twin Otter, and the A-Train satellites and GOES. The design included using smoke generated on board the R/V Point Sur that was measured after emission by the CIRPAS Twin Otter in clouds. The satellite was used to measure the changes in reflectance of sunlight due to the effects of the emitted particles on the clouds. The CVI was used as an inlet for evaporating droplets as they were brought into the aircraft, allowing sampling of droplet chemical composition. [1]