Abstract View
Imaginary Refractive Index Comparison of Water- and Methanol-soluble Brown Carbon Aerosol from western US Wildfires
Pai Liu, NISHIT SHETTY, Benjamin Sumlin, Yutong Liang, Allen Goldstein, Rajan K. Chakrabarty, Washington University in St. Louis
Abstract Number: 41
Working Group: Carbonaceous Aerosol
Abstract
Brown Carbon (BrC) aerosols absorb light at near-ultraviolet wavelengths and therefore contribute positively to Earth’s radiation balance. An accurate characterization of the complex refractive index (????=????+????????) for BrC aerosols is fundamental to a better estimation of their radiative forcing in climate models. The wavelength-dependent imaginary part of the refractive index ????, albeit being scrutinized via many laboratory studies over the past decade, remains highly uncertain when it comes to the real-world BrC emissions from large-scale biomass combustions, such as wildfires. Here we report the full absorption spectra of BrC aerosol samples taken from the Nethker fire during the FIREX-AQ campaign that took place between Aug 9th and Aug 16th, 2019. The filter collected BrC samples were dissolved into methanol and ultrapure water extractants via sonication with a 45 minutes duration. The solution-phase absorption spectra of the methanol and water extracts were measured with a spectrophotometer operated with incident wavelength (????) ranging between 300 and 800 nm. Subsequently, values of ????(????) within the complete visible range were determined with estimated BrC concentration in both extractants per the extraction efficiency values reported in previous publications. Our results show average ????BrC, Water≈ 0.076 and ????BrC, MeOH≈ 0.088 at 400 nm (respectively for the BrC extracted into water and methanol), beyond which the values of ????BrC, Water and ????BrC, MeOH are seen to converge on an identical curve, until ???? exceeds ca. 475 nm. These experimentally determined ????(????) spectra will next be integrated into radiative transfer models, towards an evaluation of their broader implications.