Understanding the Evolution of Smoke Mass Extinction Efficiency Using Field Campaign Measurements

Abstract

Aerosol mass extinction efficiency (MEE) is a key aerosol property used to connect aerosol optical properties with aerosol mass concentrations. Using measurements of smoke obtained during the Fire Influence on Regional to Global Environments and Air Quality (FIREX‐AQ) campaign we find that mid‐visible smoke MEE can change by a factor of 2–3 between fresh smoke (<2 hr old) and one‐day‐old smoke. While increases in aerosol size partially explain this trend, changes in the real part of the aerosol refractive index (real(n)) are necessary to provide closure assuming Mie theory. Real(n) estimates derived from multiple days of FIREX‐AQ measurements increase with age (from 1.40 – 1.45 to 1.5–1.54 from fresh to one‐day‐old) and are found to be positively correlated with organic aerosol oxidation state and aerosol size, and negatively correlated with smoke volatility. Future laboratory, field, and modeling studies should focus on better understanding and parameterizing these relationships to fully represent smoke aging.

Optical properties of biomass burning aerosol during the 2021 Oregon fire season: comparison between wild and prescribed fires

“…A similarly rapid increase in MSE was observed in FIREX-AQ data. 59 We also observed a negative correlation between MSE and SAE (r = −0.73). The SAE describes the wavelength dependence of scattering and is inversely related to particle size.…”

“…Our results suggest that about 50% of the observed variability in MSE is explained by particle size (based on the R 2 between SAE and MSE), and that particle growth towards the measurement wavelength of 550 nm is more efficient in denser plumes. Additional potential drivers of MSE include changes in refractive index and aerosol density, 59 which we are unable to quantify in the current study but warrant further investigation.…”

“…Additional potential drivers of MSE include changes in refractive index and aerosol density, 59 which we are unable to quantify in the current study but warrant further investigation.…”

“…While these properties show little variation across replicate laboratory burns, 58 there is evidence that both aerosol density and refractive index can change with smoke age. 59 The OPC was factory-calibrated prior to deployment with polydisperse dolomite dust. Uncertainty based on flow rate and accuracy errors is estimated by the manufacturer at ±5%, although the true uncertainty of our PM 1 measurement is likely larger considering the above approximations.…”

“…This range is higher than previous observations of MSE at MBO (2.5-4.8 m 2 g −1 ), 47,48 although MSEs > 6 m 2 g −1 at visible wavelengths have also been observed in BB smoke elsewhere. 59,[120][121][122] Because of the uncertainties and assumptions associated with using OPC data to estimate MSE (see Section 2.3), we caution against making direct comparisons between our MSE estimates and those made with more accurate mass concentration measurements or calculated using Mie theory. That said, we found robust relationships that explain some of the observed variability in MSE, which is important to understand given that this property is used to convert remote sensing observations of aerosol optical properties to mass concentrations, 123 as well as a correction factor for ground-based light scattering instruments, including low-cost sensors.…”