Production mechanisms, number concentration, size distribution, chemical composition, and optical properties of sea spray aerosols

摘要

The impact of sea spray aerosols on global climate remains one of the most uncertain components of the aerosol–radiation–climate problem, but has received less attention than the impacts of terrestrial and anthropogenic aerosols. The last decade has produced a large body of information regarding the sources and composition of marine aerosols, resulting in a reassessment of the complex role that sea spray particles play in climate and various geophysical phenomena. As sea spray aerosol contributes substantially to the preindustrial, natural background which provides the baseline on top of which anthropogenic forcing should be quantified, and because the ocean covers over 70% of the Earth's surface, the representation of sea spray aerosol in climate models strongly influences the predicted impact on climate of anthropogenic aerosols via direct and indirect effects. In addition, climate change affects atmospheric parameters, such as wind speed which has controlling effect on the production of sea spray aerosol. Recent reviews on sea spray aerosol production and composition (de Leeuw et al., 2011) summarized the state of the art and remaining uncertainties. Over forty scientists from six countries convened in Raleigh, NC on June 4–6 2012 to review the status and prospects of sea spray aerosol research. Participants were researchers from the oceanography and atmospheric science communities, including academia, private industry, and government agencies. The workshop was held with the primary objectives of (1) identifying the most critical open questions regarding sea spray aerosol and developing a list of priorities for conducting and facilitating novel research and (2) ranking the most pressing science questions based on their feasibility impact on reducing the current uncertainty ranges for different processes. The four main focus groups followed by the three breakout sessions determined the most urgent questions that would improve quantification of sea spray aerosol-radiation-climate interactions, with special emphasis on the production flux, number concentrations, chemical composition, nucleation properties, and optical properties. Instead of relying on individual presentations, the meeting format was structured to emphasize consensus-building among participants who collectively discussed successes, weaknesses, and research goals and methods of achieving them in their specific expertise of sea spray aerosol research. Following a keynote presentation by Gerrit de Leeuw highlighting open questions from his perspective, the workshop participants were divided into four focus groups (Figure 1): in situ field measurements, laboratory experiments, remote sensing, and regional and global modeling. Following short presentations by the group leaders in which successes and difficulties of past measurements/modeling/remote sensing efforts were reviewed and important missing parameters were identified, participants deliberated in breakout sessions to discuss three major topics: sea spray source function, atmospheric aging and budget, and optical properties. In each session, participants elaborated on the current sea spray aerosol-related research status and identified future priorities. In the following we reiterate the main discussion points summarized in each of the four main sessions. The discussion in this session focused on methods to measure fundamental quantities of interest for determining the impact of sea spray aerosols on climate, specifically, size-resolved number concentrations and production fluxes with information on size-resolved chemical composition. Various participants described the need for measurements of additional parameters to better understand the sources and sinks of sea spray aerosol. Key among these were background meteorology, biogeochemistry and physics of the ocean mixed layer including surface roughness/wind stress, wave breaking and bubble plume dynamics, and whitecap fraction (defined as the fraction of the