Evolution of Multispectral Aerosol Absorption Properties in a Biogenically-Influenced Urban Environment during the CARES Campaign

Optical Characterization of Fresh and Photochemically Aged Aerosols Emitted from Laboratory Siberian Peat Burning

Carbonaceous aerosols emitted from biomass burning influence radiative forcing and climate change. Of particular interest are emissions from high-latitude peat burning because amplified climate change makes the large carbon mass stored in these peatlands more susceptible to wildfires and their emission can affect cryosphere albedo and air quality after undergoing transport. We combusted Siberian peat in a laboratory biomass-burning facility and characterized the optical properties of freshly emitted combustion aerosols and those photochemically aged in an oxidation flow reactor (OFR) with a three-wavelength photoacoustic instrument. Total particle count increased with aging by a factor of 6 to 11 while the total particle volume either changed little (<8%) for 19 and 44 days of equivalent aging and increased by 88% for 61 days of equivalent aging. The aerosol single-scattering albedo (SSA) of both fresh and aged aerosol increased with the increasing wavelength. The largest changes in SSA due to OFR aging were observed at the shortest of the three wavelengths (i.e., at 405 nm) where SSA increased by less than ~2.4% for 19 and 44 days of aging. These changes were due to a decrease in the absorption coefficients by ~45%, with the effect on SSA somewhat reduced by a concurrent decrease in the scattering coefficients by 20 to 25%. For 61 days of aging, we observed very little change in SSA, namely an increase of 0.31% that was caused a ~56% increase in the absorption coefficients that was more than balanced by a somewhat larger (~71%) increase in the scattering coefficients. These large increases in the absorption and scattering coefficients for aging at 7 V are at least qualitatively consistent with the large increase in the particle volume (~88%). Overall, aging shifted the absorption toward longer wavelengths and decreased the absorption Ångström exponents, which ranged from ~5 to 9. Complex refractive index retrieval yielded real and imaginary parts that increased and decreased, respectively, with the increasing wavelength. The 405 nm real parts first increased and then decreased and imaginary parts decreased during aging, with little change at other wavelengths.

Scaling Laws for Light Absorption Enhancement Due to Nonrefractory Coating of Atmospheric Black Carbon Aerosol

Black carbon (BC) aerosol, the strongest absorber of visible solar radiation in the atmosphere, contributes to a large uncertainty in direct radiative forcing estimates. A primary reason for this uncertainty is inaccurate parametrizations of the BC mass absorption cross section (MAC_{BC}) and its enhancement factor (E_{MAC_{BC}})-resulting from internal mixing with nonrefractory and nonlight absorbing materials-in climate models. Here, applying scaling theory to numerically exact electromagnetic calculations of simulated BC particles and observational data on BC light absorption, we show that MAC_{BC} and E_{MAC_{BC}} evolve with increasing internal mixing ratios in simple power-law exponents of 1/3. Remarkably, MAC_{BC} remains inversely proportional to the wavelength of light at any mixing ratio. When mixing states are represented using mass-equivalent core-shell spheres, as is done in current climate models, it results in significant underprediction of MAC_{BC}. We elucidate the responsible mechanism based on shielding of photons by a sphere's skin depth and establish a correction factor that scales with a ¾ power-law exponent.