Abstract
The frequency and intensity of new particle formation (NPF) over remote
forest regions in the temperate and boreal zones, and thus the
importance of NPF for the aerosol budget and life cycle in the pristine
atmosphere, remains controversial. Whereas NPF has been shown to occur
relatively frequently at several sites in Scandinavia [1], it was
found to be nearly absent at a mid-continental site in Siberia [2].
To explore this issue further, we made measurements of aerosol size
distributions between 10 and 300 nm diameter at two remote sites in the
transition region between temperate and boreal forest in British
Columbia, Canada. The measurements covered 23 days during the month of
June 2019, at the time when NPF typically reaches its seasonal maximum
in remote mid-latitude regions. These are the first such measurements on
the North American continent. Although the sites were only 150 km apart,
there were dramatic differences in NPF frequency and intensity between
them. At the Eagle Lake site, NPF occurred almost daily and nucleation
mode particle concentrations reached above 5000 cm-3. In contrast, at
the Nazko River site, there were only 6 NPF events in 11 days and
nucleation mode particle concentrations reached only about 800 cm-3. The
reasons for this difference are under investigation. They may include
airmass origins, pre-existing aerosols, density of forest cover in the
surrounding regions, and proximity to the Pacific coast. Our results
suggest that measurement campaigns in the remote forest regions of North
America to investigate the role of NPF with a more comprehensive set of
instrumentation are essential for a deeper scientific understanding of
this important process. [1] Nieminen et al., Atmos. Chem. Phys. 18
(2018) 14737-14756. [2] Wiedensohler et al., Atmospheric
Environment, 200 (2019) 167-169.