Empirical determinations of the longwave and shortwave radiative forcing efficiencies of wildfire smoke

Abstract

[1] The surface radiation budget (SRB) is modulated by smoke aerosols that attenuate solar and emit thermal infrared radiation. Direct radiative impacts of smoke depend on several factors that lead to large uncertainties in assessing how wildfires influence climate. To quantify the impact of smoke on the SRB, evaluations of the longwave (LW) as well as the shortwave (SW) radiative forcing efficiencies (RFElw and RFEsw) are needed. Radiative forcing efficiency (RFE) is defined as the change in net irradiance per unit increase in aerosol optical depth at 500 nm (AOD500). An opportunity to evaluate RFElw and RFEsw of smoke presented itself on 6–7 September 2010, when a wildfire burned west of Boulder, Colorado. Smoke passed over sites where SRB, atmospheric state, and AOD measurements were being made. Values of RFE were derived empirically from coincident SRB and AOD measurements made over a range of Sun angles. RFEsw ranged between −65 and −194 Wm−2AOD500−1 as solar zenith angles decreased from 73° to 34° (at solar noon). RFElw averaged ∼10 (±7) Wm−2AOD500−1 throughout the daylight hours. During the event, the diurnally integrated value of net RFE was −51.5 Wm−2AOD500−1, revealing the dominance of SW cooling over LW warming attributed to the smoke. In response, the surface under the smoke plume cooled by 2°–5°C during the day, depending on the smoke's opacity. No evaluation of nighttime effects were possible, although very slight warming may have occurred owing to thermal emissions from the plume.