Growth Factors and Optical Hygroscopicity of Organic Aerosols Determined from Multi-Wavelength Humidity-Controlled Extinction Measurements

While greenhouse gases (CO₂, CH₄, N₂O, etc.) trap radiation in the atmosphere, aerosols reflect incoming radiation back towards the source - both contributing to observable climate change in opposing directions. The latter is associated with the largest uncertainty in scientific understanding of the terrestrial climate due to several inter-related direct and indirect aerosol effects. In this laboratory study, optical extinction coefficients were determined at multiple wavelengths using a humidity-controlled cavity ring-down spectroscopy (HC-CRDS) instrument, from which effective radii for the extant aerosol were extrapolated. The effective radii, coupled with the relative humidity (RH) control and measurement, allowed investigation of growth curves for organic laboratory-generated aerosol surrogates. The effective radii were calculated using the spectral deconvolution algorithm with fine-mode curvature (SDA-FMC) developed by O'Neill et al. (2005). The growth factors and hygroscopicity parameter (κ) are extracted from the particle growth curve (due to water uptake). The preliminary results were qualitatively correct for organic aerosol surrogates; effect radii increased in the presence of high RH. Further experimentation with a more expansive set of compounds would be necessary before utilizing these results in the field for compositional characterization.

O'Neill, N. T., Thulasiraman, S., Eck, T. F., & Reid, J. S. (2005). Robust optical features of fine mode size distributions: Application to the Québec smoke event of 2002. Journal of Geophysical Research, 110. https://doi.org/10.1029/2004JD005157