Over the last decade, extensive beetle outbreaks in western North Americahave destroyed over 100 000 km of forest throughout British Columbiaand the western United States. Beetle infestations impact monoterpeneemissions through both decreased emissions as trees are killed (mortalityeffect) and increased emissions in trees under attack (attack effect). Weuse 14 yr of beetle-induced tree mortality data together with beetle-inducedmonoterpene emission data in the National Center for Atmospheric Research(NCAR) Community Earth System Model (CESM) to investigate the impact ofbeetle-induced tree mortality and attack on monoterpene emissions andsecondary organic aerosol (SOA) formation in western North America.Regionally, beetle infestations may have a significant impact on monoterpeneemissions and SOA concentrations, with up to a 4-fold increase inmonoterpene emissions and up to a 40% increase in SOA concentrations insome years (in a scenario where the attack effect is based on observedlodgepole pine response). Responses to beetle attack depend on the extent ofprevious mortality and the number of trees under attack in a given year,which can vary greatly over space and time. Simulated enhancements peak in2004 (British Columbia) and 2008 (US). Responses to beetle attack are shownto be substantially larger (up to a 3-fold localized increase in summertimeSOA concentrations) in a scenario based on bark-beetle attack in sprucetrees. Placed in the context of observations from the IMPROVE network, thechanges in SOA concentrations due to beetle attack are in most cases smallcompared to the large annual and interannual variability in total organicaerosol which is driven by wildfire activity in western North America. Thisindicates that most beetle-induced SOA changes are not likely detectable incurrent observation networks; however, these changes may impede efforts toachieve natural visibility conditions in the national parks and wildernessareas of the western United States.
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