Aerosols create large uncertainty in the planetary energy balance due to both direct and indirect radiative forcing. Understanding aerosol seasonal patterns is essential for accurate climate change prediction. Mountain regions are often difficult for climate models to resolve. Therefore, long term observations collected at high elevations are particularly useful. In-situ surface aerosol optical measurements were analyzed for the years 2011–2016 at a mountain site located in western Colorado and tied to potential sources based on relationships among the aerosol properties.
Peak scattering and absorption coefficients were observed during the summer months, suggesting greater aerosol loading (likely due to wildfires), and were lowest during the winter months indicating cleaner conditions (due to less boundary layer influence). The scattering Ångström exponent, a property that provides information about size distributions, revealed coarse-mode particles during the spring, which is consistent with dust aerosols. During the summer months the scattering Ångström exponent revealed size distributions composed of mostly fine-mode particles. This observed increase in fine particles points to the presence of combustion aerosols, likely attributable to wildfires during the dry season (Hallar, 2015). The absorption Ångström exponent was lowest (close to one) during the summer which is consistent with the presence of combustion aerosols and was slightly higher (~1.3) during the spring season. Schmeisser et al. (2017) suggests that, for in-situ aerosol, absorption Angstrom exponents larger than 1.5 may be indicative of dust if they are associated with low (< 1.3) scattering Angstrom exponents. The increase in combustion aerosols during the summer accompanied by high values for single scattering albedo suggests that these aerosols have undergone processing in the atmosphere before reaching Storm Peak Laboratory. These results are important for improving visibility and predicting future aerosol concentrations in the Western U.S.