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Technical description

A common way to observe airborne particles produced by biomass burning is through aerosol optical properties, for instance, using aerosol optical depth from satellites or low-cost sensors that measure scattered light. Since health effects are associated to aerosol mass concentrations, a conversion factor between them is needed. Here we use in-plume measurements collected from an aircraft to show that aging processes alone can produce a factor of 2–3 change in aerosol extinction per unit of aerosol mass concentration. We also find that these changes are driven not only due to changes in aerosol size, but also due to changes in the material properties of aerosols. These results are relevant as fires are becoming more common and extreme, and thus these changes in smoke properties need to be taken into consideration in many fields of study such as assimilating satellite smoke into atmospheric composition models, satellite-based smoke impacts on health, and corrections for low-cost PM2.5 sensors.

1.Introduction

Smoke from wildfires can have a wide range of impacts, including adverse effects on human health (O’Dell et al., 2021; C. E. Reid et al., 2016), reduction in visibility (Spracklen et al., 2007), and climate implications (Randerson et al., 2006). The severity of wildfires has been predicted to increase with climate change (Jin et al., 2015; Spracklen et al., 2009), and recent wildfire seasons across the world are confirming this by being devastating by many measures (Deb et al., 2020; Higuera & Abatzoglou, 2021).

Aerosol optical properties play an important role in observing and quantifying smoke and its impacts. For instance, satellite-derived aerosol optical depth (AOD), as well as aerosol extinction and backscattering coef-ficient can be used to provide constraints on smoke emissions (Ichoku & Ellison, 2014; Peterson et al., 2021; Saide et al., 2015) and to estimate surface PM2.5 when surface networks are not available (Cheeseman et al., 2020; Mirzaei et al., 2020). Also, increasingly common low-cost sensors quantify PM2.5 by measuring light scattering (Delp & Singer, 2020), which can be useful to extend the coverage of reference networks (Forehead et al., 2020).

These examples show that, while optical properties are sampled, many times the quantity of interest is a meas-ure of smoke mass (e.g., concentrations or emissions). Thus, a key parameter to understand is the efficiency at which a unit of aerosol mass concentration produces aerosol extinction (scattering + absorption) or scattering, generally referred to as mass extinction efficiency (MEE) and mass scattering efficiency (MSE). Laboratory and field measurements have found a large range in mid-visible MSE for fresh and aged smoke (1.5–6 m2/g for wavelengths on the 532–550 nm range), with values being correlated with changes in aerosol mean diameter as expected from Mie theory (Kleinman et al., 2020; Laing et al., 2016; Levin et al., 2010; J. S. Reid et al., 1998; J. S. Reid, Eck, et al., 2005). Aerosol size increase with age is the result of a combination of coagulation and gas-to-particle conversion of semi-volatile material (Martins et al., 1996; J. S. Reid, Koppmann, et al., 2005), likely primarily coagulation (Hodshire et al., 2021; June et al., 2022).The complex refractive index is an intrinsic property of the aerosols that describes how light interacts with the material in terms of the scattering (real part) and absorption (imaginary part) (Moosmüller et al., 2009), and thus also influences MEE and MSE. Numerous laboratory and field estimates of the real refractive index have been made for biomass burning. Average real refractive index in mid-visible wavelengths for ambient wildfire smoke has been found to be in the 1.52–1.55 range, while the whole range of smoke measurements tend to be within 1.42–1.61 (Aldhaif et al., 2018; Bian et al., 2020; Espinosa et al., 2017). Similar averages and ranges are found in laboratory studies (Levin et al., 2010; Sumlin et al., 2018; Womack et al., 2021). While these studies sampled a mixture of smoke ages, the evolution of refractive index and dependence on other smoke aerosol intrinsic properties was not explored. While there is a relevant body of literature studying changes of real refractive index of organic aerosol (other than smoke) with aging (Moise et al., 2015 and references therein), there is limited literature on the topic for smoke aerosols.Here we use observations from the 2019 NOAA-NASA FIREX-AQ field campaign to assess the evolution of MEE with smoke age. FIREX-AQ occurred during a below-average (compared to recent years) fire season, which enabled tracking of individual plumes for much longer time periods and without plumes of interest mixing with smoke from other fires. In the following, we describe the observations and computational methods, analyze and discuss results, and provide future directions.

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