AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Characterization of Black Carbon Aging Processes with a Size-Dependent Timescale
LAURA FIERCE, Nicole Riemer, Tami Bond, University of Illinois at Urbana-Champaign
Abstract Number: 440 Working Group: Carbonaceous Aerosols in the Atmosphere
Abstract The microphysical properties of particles containing black carbon (BC) are altered soon after emission by condensation, coagulation, and photochemical reactions, collectively termed “aging”. The adequate representation of these processes in models is a key challenge in determining the optical properties, chemical reactivity, and cloud condensation nuclei (CCN) activity of aged particles. The simplest representation classifies fresh BC as hydrophobic and aged BC as hydrophilic, assuming that hydrophobic BC is converted to hydrophilic BC using a fixed first-order timescale. Estimates of BC’s lifetime are sensitive to the assumed aging timescale, so constraining this value is essential in determining BC’s burden and climate forcing.
In this work, we present a method for explicitly calculating a size-dependent aging timescale using the particle-resolved model PartMC-MOSAIC. We define aging as the rate at which particles transition from CCN-inactive to CCN-active at a specified supersaturation threshold. The aging timescale is a function of the wet diameter of BC-containing particles D$_(wet,BC), the supersaturation threshold S, and the time of day t.
We determined size-dependent aging timescales for combustion-generated particles under about 300 scenarios, exploring a range of environmental parameters and pollution characteristics. The value of the timescale varied from minutes to weeks, depending on the size of the fresh BC-containing particles and the local plume dynamics. The condensation aging timescale was shortest (~10 minutes at S=0.3%) for the largest (D$_(wet,BC)>100 nm) fresh particles and decreased with the secondary aerosol mass production rate per wet aerosol surface area (from ~1 week to ~1 hour for D$_(wet,BC)≈100 nm at S=0.3%). The coagulation aging timescale was shortest (~1 hour at S=0.3%) for the smallest (D$_(wet,BC)<20 nm) fresh particles and decreased with the total number concentration of large (D$_(wet)>200 nm) particles. Therefore, both condensation and coagulation play important roles in aging, and their relative impact depends on the particle size range.