The impact of bark beetle infestations on monoterpene emissions and secondary organic aerosol formation in western North America
Over the last decade, extensive beetle outbreaks in western North\nAmerica have destroyed over 100 000 km(2) of forest throughout British\nColumbia and the western United States. Beetle infestations impact\nmonoterpene emissions through both decreased emissions as trees are\nkilled (mortality effect) and increased emissions in trees under attack\n(attack effect). We use 14 yr of beetle-induced tree mortality data\ntogether with beetle-induced monoterpene emission data in the National\nCenter for Atmospheric Research (NCAR) Community Earth System Model\n(CESM) to investigate the impact of beetle-induced tree mortality and\nattack on monoterpene emissions and secondary organic aerosol (SOA)\nformation in western North America.\nRegionally, beetle infestations may have a significant impact on\nmonoterpene emissions and SOA concentrations, with up to a 4-fold\nincrease in monoterpene emissions and up to a 40% increase in SOA\nconcentrations in some years (in a scenario where the attack effect is\nbased on observed lodgepole pine response). Responses to beetle attack\ndepend on the extent of previous mortality and the number of trees under\nattack in a given year, which can vary greatly over space and time.\nSimulated enhancements peak in 2004 (British Columbia) and 2008 (US).\nResponses to beetle attack are shown to be substantially larger (up to a\n3-fold localized increase in summertime SOA concentrations) in a\nscenario based on bark-beetle attack in spruce trees. Placed in the\ncontext of observations from the IMPROVE network, the changes in SOA\nconcentrations due to beetle attack are in most cases small compared to\nthe large annual and interannual variability in total organic aerosol\nwhich is driven by wildfire activity in western North America. This\nindicates that most beetle-induced SOA changes are not likely detectable\nin current observation networks; however, these changes may impede\nefforts to achieve natural visibility conditions in the national parks\nand wilderness areas of the western United States.