Rapid evolution of aerosol particles and their optical properties downwind of wildfires in the western US
<p>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...
| Main Authors: | , , , , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2020-11-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/20/13319/2020/acp-20-13319-2020.pdf |
| Summary: | <p>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, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NH</mi><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="8aeb386a576ed6c8280ae774099f80e4"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-13319-2020-ie00001.svg" width="24pt" height="15pt" src="acp-20-13319-2020-ie00001.png"/></svg:svg></span></span> is
approximately equivalent to <span class="inline-formula">NO<sub>3</sub></span>. After 2 h of daytime aging, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NH</mi><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="f83a9f1907f38a5589c34b239e10518b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-13319-2020-ie00002.svg" width="24pt" height="15pt" src="acp-20-13319-2020-ie00002.png"/></svg:svg></span></span> increased and is approximately equivalent to
the sum of Cl, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">SO</mi><mn mathvariant="normal">4</mn><mn mathvariant="normal">2</mn></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="22pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="fac0ae929f4b2d8ca7c7776e5fe7ffc7"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-13319-2020-ie00003.svg" width="22pt" height="16pt" src="acp-20-13319-2020-ie00003.png"/></svg:svg></span></span>, and <span class="inline-formula">NO<sub>3</sub></span>. Particle size increased with downwind distance,
causing particles to be more efficient scatters. Averaged over nine flights,
mass scattering efficiency (MSE) increased in <span class="inline-formula">∼</span> 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.</p> |
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| ISSN: | 1680-7316 1680-7324 |