Abstract
Abstract. Over the last decade, extensive beetle outbreaks in western North America have destroyed over 100 000 km2 of forest throughout British Columbia and the western United States. Beetle infestations impact monoterpene emissions through both decreased emissions as trees are killed (mortality effect) and increased emissions in trees under attack (attack effect). We use 14 yr of beetle-induced tree mortality data together with beetle-induced monoterpene emission data in the National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM) to investigate the impact of beetle-induced tree mortality and attack on monoterpene emissions and secondary organic aerosol (SOA) formation in western North America. Regionally, beetle infestations may have a significant impact on monoterpene emissions and SOA concentrations, with up to a 4-fold increase in monoterpene emissions and up to a 40% increase in SOA concentrations in some years (in a scenario where the attack effect is based on observed lodgepole pine response). Responses to beetle attack depend on the extent of previous mortality and the number of trees under attack in a given year, which can vary greatly over space and time. Simulated enhancements peak in 2004 (British Columbia) and 2008 (US). Responses to beetle attack are shown to be substantially larger (up to a 3-fold localized increase in summertime SOA concentrations) in a scenario based on bark-beetle attack in spruce trees. Placed in the context of observations from the IMPROVE network, the changes in SOA concentrations due to beetle attack are in most cases small compared to the large annual and interannual variability in total organic aerosol which is driven by wildfire activity in western North America. This indicates that most beetle-induced SOA changes are not likely detectable in current observation networks; however, these changes may impede efforts to achieve natural visibility conditions in the national parks and wilderness areas of the western United States.
Highlights
Earth System SciencesIn the last decade, western North America has experienced the largest bark beetle outbreaks in recorded history (Taylor et al, 2006)
MPB infestation peaked in British Columbia (BC) in 2007 and the western United States (US) in 2009, the MPB had the largest impact on volatile organic compounds (VOCs) emissions in BC and the western US in 2004 and 2008, respectively
Changes in secondary organic aerosol (SOA) concentrations estimated under the spruce scenario (∼ 1 μg m−3) are comparable to the natural aerosol levels estimated for the western United States (1.21 μg m−3 US Environmental Protection Agency (EPA), 2003), suggesting that achieving “natural visibility” may not be possible in forests impacted by beetle infestation
Summary
Western North America has experienced the largest bark beetle outbreaks in recorded history (Taylor et al, 2006). A few studies have examined and quantified monoterpene concentration and emission changes under beetle attack for specific tree species including lodgepole pine, ponderosa pine, and Engelmann spruce. Despite growing evidence that bark beetle attack can enhance monoterpene emissions from vegetation, in addition to the role that these beetles play in changing land cover and density, no study has quantified the impact of these changes on regional air quality and visibility. We use beetle-caused tree mortality data from 1997–2010 and beetle-induced monoterpene data from the recent literature in the National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM) to study the impact of beetle infestation on monoterpene emissions and SOA formation in western North America. We compare two scenarios based on beetle-induced monoterpene data from lodgepole pine and Engelmann spruce and focus on the spatial and temporal evolution of monoterpene emissions and the SOA formed due to cumulative beetle attack and subsequent forest mortality in the model
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