AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
Scaling Laws for Light Absorption Enhancement Due to Nonrefractory Coating of Atmospheric Black Carbon Aerosol
RAJAN K. CHAKRABARTY, William Heinson, Washington University in St. Louis
Abstract Number: 792 Working Group: Carbonaceous Aerosol
Abstract Recent field observations have shown atmospheric black carbon (BC) particles to exist in majority populations as internally mixed with non-refractory materials, including sulfate, nitrate, and organic carbon. These materials condense on BC particle surfaces as layers of external coating, which are typically non-absorbing in the visible solar spectrum but acts as “focusing lens” for the incoming light and results in an enhanced mass absorption cross-section (MACBC) compared to that for an equivalent external mixture. A broad range of enhancement factors for MACBC (E-MACBC ), from 1.05 to 3.5, has been observed during laboratory and field studies. This large spread in E-MACBC values accompanied by lack of any established scaling relationship makes it a cumbersome and challenging parameter to incorporate in climate models. Toward addressing this knowledge gap, we applied scaling theory to meta-analyzed observational data on BC light absorption in conjunction with numerically-exact electromagnetic calculations of simulated internally-mixed BC particles. Our results show that MACBC and E-MACBC evolve with increasing internal mixing ratios in simple power-law exponents of 1/3 in the shortwave solar wavelengths (λ = 400 - 900 nm). Remarkably, MACBC remains inversely proportional to wavelength 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 under prediction of MACBC. 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. Violations of these scaling laws are seen to occur only under scenarios where moderate-to-low amount of condensable coating materials are co-emitted with BC (e.g. vehicular emission).