Elisabeth Andrews — University of Colorado
Paul Zieger — Stockholm University
Gloria Titos — University of Granada
Category
General topics – Aerosols
Description
Temporal and spatial variability of hygroscopicity and its influence on particle light scattering (parameterized by γ, defined as f(RH)=a(1-RH)-) at Graciosa Island. Top plot shows observed seasonality, while the bottom four panels show trajectory footprints from 7-day back trajectories indicating regions associated with observed γ-values. Larger values of gamma indicate increased influence of particle hygroscopicity on particle light scattering. Note the slightly different ranges for γ on the color scales.
Aerosol optical properties strongly depend on ambient relative humidity (RH). Depending on their size, composition, and the ambient humidity, atmospheric particles will take up varying amounts of water, thereby altering their optical properties and impacting their contribution to aerosol radiative forcing. In this project, the ultimate goal is to assess how well global models simulate the aerosol/water interaction using in situ measurements of aerosol hygroscopicity. In the initial phase of the research, it is vital to compile and assess the data quality of the available tandem nephelometer humidograph measurements and to harmonize the data sets in terms of data treatment (instrument corrections, hygroscopic fitting assumptions, etc.). We will be working with observations from DOE ARM and AMF1 sites as well as other, non-DOE observations (e.g., NOAA and ACTRIS network data).
Here we summarize the availability and status of humidograph data sets including the availability of ancillary data such as size distribution and aerosol chemistry. For several sites with well-constrained observations, initial findings on the temporal variability of hygroscopicity are presented. Additionally, for these sites, trajectory footprint analysis is used to explore the influence of air mass source region on hygroscopicity and to explain observed seasonal patterns in hygroscopicity. The figure provides an example of this type of analysis for the ARM AMF1 deployment at Graciosa Island, Azores. At this site there are both seasonal and spatial dependencies of scattering hygroscopicity. The aerosol hygroscopicity effect in the scattering coefficient is higher in winter and tends to be associated with north-westerly air masses (i.e., open ocean). Model simulations will be evaluated against these types of findings.
Finally, preliminary results will be shown to demonstrate the importance of characterizing the presence of aerosol water at low and intermediate relative humidity (RH<40-50%). This is especially important for marine sites where the influence of sea salt results in atmospheric water uptake at low and intermediate RH, which can contribute significantly to the observed light scattering and may also influence the characterization of hygroscopicity. This highlights the importance of sufficiently drying the aerosol particles to lower than 40% in order determine their actual dry light scattering properties.
