Baselining the Indirect Effect by Improving Quantification of Sea Spray and Marine Sources at Ascension Island

Abstract

Oceans cover two-thirds of the Earth and understanding the interactions of aerosols with clouds in these regions requires quantifying the man made contributions to the budget of cloud-drop forming particles (known as cloud condensation nuclei, or CCN) relative to the non-man made “baseline” conditions, as well as understanding the meteorology of boundary layer clouds.  Modeling studies have shown substantial uncertainties and sensitivities to natural marine CCN sources, meaning that to reduce uncertainties in indirect effects we must be able to better quantify the CCN budget in ocean regions.  While models provide important constraints on these uncertainties, actually reducing uncertainties requires substantial observations in open-ocean and coastal regions in order to establish the baseline on which man made emissions are added.  The tropical South Atlantic Ocean is one of the least-sampled regions of the planet, making the comprehensive measurements of the Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign an important resource for quantifying marine and biomass burning aerosol-cloud interactions in this region.

This proposal will address the first research topic specified in the Funding Opportunity Announcement (FOA): (1) Aerosol-cloud interactions.  The proposal targets the Atmospheric Radiation Measurement (ARM) LASIC deployment because it provides an excellent location for studying the South Atlantic trade cumulus and transitions from stratocumulus clouds with both baseline marine and smoky aerosol. The aerosol size and chemical signatures will be compared with the light scattering and absorbing signatures of clouds to find overlapping properties.  In particular, we expect that the larger size (and higher hygroscopicity) of sea spray particles will produce cloud droplets that are on average larger in low-level clouds in well-mixed boundary layers, and that this size difference will be detectable by ARM sensors. Identifying this signature in marine areas provides an important constraint for climate models in a range of clean (baseline) and continent-influenced (smoky) conditions.

The proposed work is new relative to the existing literature on this topic and to the work completed to date from LASIC in that it provides a specific link between aerosol and cloud properties that complements the existing use of this data set.  In addition, it targets the specific role of sea spray particles to better understand the baseline aerosol. Prior work has tracked either CCN only (and not cloud properties) or it has focused on smoke aerosol types rather on sea spray. The comprehensive ARM sensor suite used during the LASIC campaign has the unique ability to answer the more specific and relevant question of which aerosol type (namely sea spray) changes how much of a particular cloud property (in addition to CCN).

Moreover, LASIC provides the longest record of comprehensive aerosol size distribution measurements in a cloud-influenced marine location in the ARM database. These measurements are from a differential mobility analyzer (DMA), which is the most accurate type of instrument for measuring particles smaller than 1 micrometer in diameter. Here we will make use of the ARM measurements as well as value-added fitting and parameterizations in a novel technique that was pioneered by the Russell group for quantifying sea salt. In addition, we will use clustering of meteorological conditions to categorize and retrieve column optical properties and effective cloud droplet size from Atmospheric Emitted Radiance Interferometer (AERI) data for thin clouds. The Russell and Lubin groups have demonstrated success in all of these types of analyses in past publications, and our analyses will build on techniques developed as part of the ARM West Antarctic Radiation Experiment (AWARE) and other marine-influenced campaigns.