Atmos. Chem. Phys., vol. 13, issue 6 (2013) pp. 3149-3161
Over the last decade, extensive beetle outbreaks in western North
America have destroyed over 100 000 km(2) 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.
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