How emissions uncertainty influences the distribution and radiative impacts of smoke from fires in North America

Abstract

<p><strong>Abstract.</strong> Fires and the aerosols that they emit impact air quality, health, and climate, but the abundance and properties of carbonaceous aerosol (both black carbon and organic carbon) from biomass burning (BB) remain uncertain and poorly constrained. We aim to quantify the uncertainties associated with fire emissions and their air quality and radiative impacts from underlying dry matter consumed and emissions factors. To explore this, we compare model simulations from a global chemical transport model, GEOS-Chem, driven by a variety of fire emission inventories with surface and airborne observations of black carbon (BC) and organic aerosol (OA) concentrations and satellite-derived aerosol optical depth (AOD). We focus on two fire detection/burned area-based (FD/BA) inventories using burned area and active fire counts, respectively: the Global Fire Emissions Database version 4 (GFED4s) with small fires and the Fire INventory from NCAR version 1.5 (FINN1.5) and two fire radiative power (FRP)-based approaches: the Quick Fire Emission Dataset version 2.4 (QFED2.4) and the Global Fire Assimilation System version 1.2 (GFAS1.2). We show that, across the inventories, emissions of BB aerosol (BBA) differ by a factor of 4 to 7 over North America and that dry matter differences, not emissions factors, drive this spread. We find that simulations driven by QFED2.4 generally overestimate BC and, to a lesser extent, OA concentrations observations from two fire-influenced aircraft campaigns in North America (ARCTAS and DC3) and from the Interagency Monitoring of Protected Visual Environments (IMPROVE) network, while simulations driven by FINN1.5 substantially underestimate concentrations. The GFED4s and GFAS1.2-driven simulations provide the best agreement with OA and BC mass concentrations at the surface (IMPROVE), BC observed aloft (DC3 and ARCTAS), and AOD observed by MODIS over North America. We also show that a sensitivity simulation including an enhanced source of secondary organic aerosol (SOA) from fires based on the NOAA Fire Lab 2016 experiments produces substantial additional OA; however, the spread in the primary emissions estimates implies that this magnitude of SOA cannot be either confirmed or ruled out when comparing the simulations against the observations explored here. Given the substantial uncertainty in fire emissions, as represented by these four emission inventories, we find a sizeable range in BBA population-weighted exposure over Canada and the contiguous United States (0.5 to 1.6 µg m<sup>−3</sup>). We also show that the range in the estimated global direct radiative effect of carbonaceous aerosol from fires (−0.11 to −0.048 W m<sup>−2</sup>) is large and comparable to the direct radiative forcing of OA (−0.09 W m<sup>−2</sup>) estimated in AR5. Our analysis suggests that fire emissions uncertainty challenges our ability to accurately characterize the impact of smoke on air quality and climate.</p>

Summer PM2.5 Pollution Extremes Caused by Wildfires Over the Western United States During 2017–2018

“…Several studies using GFED emissions show that they were able to capture mean surface PM 2.5 observed at IMPROVE sites; however, they emit all fire emissions within the boundary layer (Carter et al, 2020; O'Dell et al, 2019; Spracklen et al, 2007). Our GFED_SFC simulation with fire emissions emitted in surface air also captures the observed surface PM 2.5 levels but still substantially underestimates satellite‐derived AOD and aerosol extinction profiles (blue lines in Figures 3a–3d and S10).…”

“…The 2017–2018 extremes provide a rare opportunity to evaluate chemistry‐climate models needed to assess the impacts of wildfires in a warming climate. However, fire emissions used in these models are highly uncertain: Organic carbon emissions in six datasets differ by a factor of 10 over North America (Carter et al, 2020; Pan et al, 2020). Uncertainties also exist in the vertical distribution of fire smoke (Paugam et al, 2016; Rémy et al, 2017; Zhu et al, 2018) and in the representation of fire plume chemistry (Posner et al, 2019; Shrivastava et al, 2015).…”

Spatial and Temporal Variability of Brown Carbon in the United States: Implications for Direct Radiative Effects

“…Observed OC peaks during summer and fall in the NE, NW, and SW regions and peaks during fall in the SE region. To a large extent, this is driven by wildfire emissions (Carter et al, 2020), as shown in Figure S10. In particular, Southern Appalachian Fires occurred from October to December 2016 in Georgia and North Carolina due to abnormal drought, leading to OC peaks during fall in this region.…”

The changing risk and burden of wildfire in the United States

“…However, generating accurate exposure estimates from CTMs requires surmounting several major uncertainties in the pathway between source and receptor. First, large uncertainties in wildfire emissions inventories have been shown to lead to many-fold differences in wildfire-attributed (particulate matter with diameter <2.5 μm) concentrations across the United States (and 20× regional differences in high fire years) when different inventories are used as input to the same CTM ( 15 , 16 ), and integration of satellite observations only slightly improves performance ( 17 ). Second, the detailed conditions surrounding emissions such as the height of emissions injections and very localized meteorology and their transport may not be captured by models and can dramatically impact downstream exposure estimates ( 18 , 19 ).…”