TY - JOUR
T1 - Rapid evolution of aerosol particles and their optical properties downwind of wildfires in the western US
AU - Kleinman, Lawrence I.
AU - Sedlacek, Arthur J.
AU - Adachi, Kouji
AU - Buseck, Peter R.
AU - Collier, Sonya
AU - Dubey, Manvendra K.
AU - Hodshire, Anna L.
AU - Lewis, Ernie
AU - Onasch, Timothy B.
AU - Pierce, Jeffery R.
AU - Shilling, John
AU - Springston, Stephen R.
AU - Wang, Jian
AU - Zhang, Qi
AU - Zhou, Shan
AU - Yokelson, Robert J.
N1 - Publisher Copyright:
© Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License.
PY - 2020/11/11
Y1 - 2020/11/11
N2 - During the first phase of the Biomass Burn Operational Project (BBOP) field campaign, conducted in the Pacific Northwest, the DOE G-1 aircraft was used to follow the time evolution of wildfire smoke from near the point of emission to locations 2-3.5 h downwind. In nine flights we made repeated transects of wildfire plumes at varying downwind distances and could thereby follow the plume's time evolution. On average there was little change in dilution-normalized aerosol mass concentration as a function of downwind distance. This consistency hides a dynamic system in which primary aerosol particles are evaporating and secondary ones condensing. Organic aerosol is oxidized as a result. On all transects more than 90 % of aerosol is organic. In freshly emitted smoke aerosol, NH+4 is approximately equivalent to NO3. After 2 h of daytime aging, NH+4 increased and is approximately equivalent to the sum of Cl, SO24, and NO3. Particle size increased with downwind distance, causing particles to be more efficient scatters. Averaged over nine flights, mass scattering efficiency (MSE) increased in ∼ 2 h by 56 % and doubled in one flight. Mechanisms for redistributing mass from small to large particles are discussed. Coagulation is effective at moving aerosol from the Aitken to accumulation modes but yields only a minor increase in MSE. As absorption remained nearly constant with age, the time evolution of single scatter albedo was controlled by age-dependent scattering. Near-fire aerosol had a single scatter albedo (SSA) of 0.8-0.9. After 1 to 2 h of aging SSAs were typically 0.9 and greater. Assuming global-average surface and atmospheric conditions, the observed age dependence in SSA would change the direct radiative effect of a wildfire plume from near zero near the fire to a cooling effect downwind.
AB - During the first phase of the Biomass Burn Operational Project (BBOP) field campaign, conducted in the Pacific Northwest, the DOE G-1 aircraft was used to follow the time evolution of wildfire smoke from near the point of emission to locations 2-3.5 h downwind. In nine flights we made repeated transects of wildfire plumes at varying downwind distances and could thereby follow the plume's time evolution. On average there was little change in dilution-normalized aerosol mass concentration as a function of downwind distance. This consistency hides a dynamic system in which primary aerosol particles are evaporating and secondary ones condensing. Organic aerosol is oxidized as a result. On all transects more than 90 % of aerosol is organic. In freshly emitted smoke aerosol, NH+4 is approximately equivalent to NO3. After 2 h of daytime aging, NH+4 increased and is approximately equivalent to the sum of Cl, SO24, and NO3. Particle size increased with downwind distance, causing particles to be more efficient scatters. Averaged over nine flights, mass scattering efficiency (MSE) increased in ∼ 2 h by 56 % and doubled in one flight. Mechanisms for redistributing mass from small to large particles are discussed. Coagulation is effective at moving aerosol from the Aitken to accumulation modes but yields only a minor increase in MSE. As absorption remained nearly constant with age, the time evolution of single scatter albedo was controlled by age-dependent scattering. Near-fire aerosol had a single scatter albedo (SSA) of 0.8-0.9. After 1 to 2 h of aging SSAs were typically 0.9 and greater. Assuming global-average surface and atmospheric conditions, the observed age dependence in SSA would change the direct radiative effect of a wildfire plume from near zero near the fire to a cooling effect downwind.
UR - http://www.scopus.com/inward/record.url?scp=85096123562&partnerID=8YFLogxK
U2 - 10.5194/acp-20-13319-2020
DO - 10.5194/acp-20-13319-2020
M3 - Article
AN - SCOPUS:85096123562
SN - 1680-7316
VL - 20
SP - 13319
EP - 13341
JO - Atmospheric Chemistry and Physics
JF - Atmospheric Chemistry and Physics
IS - 21
M1 - 684
ER -