10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Changes in Direct Radiative Forcing due to Differences in Marine Aerosol Size Distributions during NAAMES Field Campaigns
GEORGES SALIBA, Raghu Betha, Savannah Lewis, Chia-Li Chen, Lynn Russell, Timothy Bates, Patricia Quinn, Scripps Institution of Oceanography
Abstract Number: 1432 Working Group: Aerosol Chemistry
Abstract Oceans are the largest source of biogenic particles to the atmosphere. Marine aerosols are mostly comprised of sea salt, sulfate, and organics which are efficient cloud condensation nuclei. The shape of the size distribution of marine aerosols has direct radiative effects on the climate due to light-scattering from these particles. Furthermore, changes in marine aerosols affect the climate indirectly by perturbing cloud formation, lifetime, and precipitation patterns. However, size distribution measurements from pristine marine environments are scarce. The reported data show variability and seasonal and geographical differences.
We present measurements of aerosol size distributions and chemical composition of marine aerosols measured during four consecutive North Atlantic Aerosol and Marine Ecosystem Studies (NAAMES) field campaigns. Measurements were made onboard the R/V Atlantis and cover the period from November 2015 till May 2018. These time periods were selected because they coincide with opposite extremes of the Phytoplankton life cycle, which have been shown to influence marine aerosol number and composition. We retrieved total marine aerosol size distributions by merging independent size distributions from a scanning electron mobility spectrometer (SEMS), an optical particle counter (OPC), and an aerodynamic particle sizer (APS). Our data cover the range of particle sizes from 0.01 µm to 20 µm. Single-particle chemical composition was measured using the Aerodyne high-resolution aerosol mass spectrometer (HR-AMS) light-scattering (LS) and Event Trigger (ET) modes.
The measured number and mass distributions are multi-modal with identifiable modes around 0.02 µm (Aiken mode), 0.1 µm (accumulation mode), and greater than 0.2 µm (PMA mode), in agreement with published results. Interestingly, we measured a shift to larger diameters of the size distributions measured during periods of high phytoplankton activity (2016 NAAMES) compared to size distributions measured during periods of low phytoplankton activity (2015 NAAMES). The difference was about a factor of two increase in geometric mean diameter. We are investigating the source of this shift and its implication on the aerosol direct radiative forcing. To achieve that goal, we are: (1) fitting the different modes to lognormal distributions, and (2) quantifying chemical composition from each mode using single-particle chemical information. Chemical composition constrains the effect of condensation (from oxidation of ocean-emitted gas-phase organic and inorganic precursors) on the measured size shift. (3) We are finally performing Mie theory calculations to quantify the change in aerosol scattering (and therefore direct radiative forcing) from shifting size distributions. Refractive indices, critical inputs to Mie calculations, are calculated for each particle mode using the volume-mixing rule (obtained from chemical composition information from AMS). Our results will help elucidate the aerosol direct radiative forcing in an environment that is sensitive to changes in particle number and mass.