Abstract
Abstract. A short, but severe, wildfire smoke episode in July 2015, with an aerosol optical depth (AOD) approaching 9, is shown to strongly impact radiation budgets across four distinct land-use types (forest, field, urban and wetland). At three of the sites, impacts on the energy balance are also apparent, while the event also appears to elicit an ecosystem response with respect to carbon fluxes at the wetland and a forested site. Greatest impacts on radiation and energy budgets were observed at the forested site where the role of canopy architecture and the complex physiological responses to an increase in diffuse radiation were most important. At the forest site, the arrival of smoke reduced both sensible and latent heat flux substantially but also lowered sensible heat flux more than the latent heat flux. With widespread standing water, and little physiological control on evapotranspiration, the impacts on the partitioning of turbulent fluxes were modest at the wetland compared to the physiologically dominated fluxes at the forested site. Despite the short duration and singular nature of the event, there was some evidence of a diffuse radiation fertilization effect when AOD was near or below 2. With lighter smoke, both the wetland and forested site appeared to show enhanced photosynthetic activity (a greater sink for carbon dioxide). However, with dense smoke, the forested site was a strong carbon source. Given the extensive forest cover in the Pacific Northwest and the growing importance of forest fires in the region, these results suggest that wildfire aerosol during the growing season potentially plays an important role in the regional ecosystem response to smoke and ultimately the carbon budget of the region.
Highlights
Wildfire activity is projected to increase in frequency and duration over the century in western North America, primarily as a result of the increased summer temperatures, persistent drought, and reduced snowpack accompanying climate change (IPCC, 2014; Setelle et al, 2014)
The wildfire smoke episode of early July 2015 in southwestern British Columbia had a significant impact on air quality, the radiation budget, and turbulent fluxes of latent and sensible heat
It appeared to elicit an ecosystem response with respect to net ecosystem exchange (NEE) of land ecosystems, this response depended on the overall concentration, and we observed enhancements and reductions
Summary
Wildfire activity is projected to increase in frequency and duration over the century in western North America, primarily as a result of the increased summer temperatures, persistent drought, and reduced snowpack accompanying climate change (IPCC, 2014; Setelle et al, 2014). At the plant canopy scale, Yamosoe et al (2006) have focused on the impact of biomass burning aerosol on Amazonian forests and have noted an increase in diffuse radiation within the canopy combined with a reduction in total photosynthetically active radiation (PAR) at the top of the canopy These impacts affected sensible and latent fluxes as well as net ecosystem exchange (NEE) of carbon dioxide (CO2). Niyogi et al (2004), in an examination of six AmeriFlux sites, conclude that aerosols can exert a significant impact on net CO2 exchange (perhaps more so than clouds), whereby the CO2 sink is increased with aerosol loading for forest and croplands This effect has become known as the diffuse radiation fertilization effect (DRF), whereby an increase in photosynthesis results from a trade-off between decreased solar radiation and increased light scattering during clouds or smoke (Park et al, 2018, and references therein). Commercial aircraft soundings (Aircraft Meteorological Data Relay – AMDAR) from Vancouver International Airport (YVR) departures and arrivals show development of a strong surface-based inversion late on 4 July that persisted through 5 July 2015 and coincided with smoke arrival midafternoon on 5 July over the western edge of the lower Fraser
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