In situ polarimetry measurements of smoke from wildfires during FIREX-AQ

The open burning of biomass fuels is an important source of aerosols because they contribute significantly to the pre-industrial radiative forcing budget and they are a large source of aerosol in the modern era that is anticipated to increase due to climate change. Wildfires are a global phenomenon which are most practically monitored by remote sensors. If we increase confidence in the optical properties of smoke, then we can improve the ability of remote sensing techniques to measure emissions from wildfires and quantify the radiative impact of those emissions. Two extrinsic aerosol optical quantities frequently measured by passive remote instruments, and therefore critical to constrain with in situ measurements for wildfire smoke, are the aerosol scattering phase function and polarized phase function.

We will present in situ measurements of the scattering phase function and polarized phase function from the NOAA Laser Imaging Nephelometer (LiNeph) during the FIREX-AQ campaign. These measurements will be compared with those derived from particle size distributions measured directly by the Langley Aerosol Research Group (e.g. laser aerosol spectrometer.) We will also present some parameters that are retrieved from the polarimetry measurements by an open-source inversion algorithm, GRASP. The algorithm is based on Mie theory and allows the retrieval of particle size distributions and optical properties from the measured phase functions. We will also show that using measured size distributions and retrieved refractive indices (versus literature values) allows us to better reproduce the total measured scattering observed by integrating nephelometers during FIREX-AQ.