Frequent new particle formation at remote sites in the subboreal forest of North America

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, it was found to be nearly absent at a mid-continental site in Siberia. To explore this issue further, we made measurements of aerosol size distributions between 10 and 420 nm diameter at two remote sites in the transition region between temperate and boreal forest in British Columbia, Canada. The measurements covered 23 d during the month of June 2019, at the time when NPF typically reaches its seasonal maximum in remote midlatitude regions. These are the first such measurements in a near-pristine region on the North American continent. Although the sites were only 150 km apart, there were clear differences in NPF frequency and intensity between them. At the Eagle Lake site, NPF occurred daily, and nucleation-mode particle concentrations reached above 5000 cm−3. In contrast, at the Nazko River site, there were only six NPF events in 11 d, and the nucleation-mode particle concentrations only reached about 800 cm−3. The reasons for this difference could not be conclusively resolved with the available data; they may include air mass origins, preexisting aerosols, and the density and type of forest cover in the surrounding regions. In contrast to observations in other temperate and boreal environments, we found that NPF at our sites occurred at nighttime just as frequently as during daytime. Together with the lack of identifiable sources of sulfuric acid precursor species in the fetch region of our sites, this suggests that nucleation of extremely low volatility organics was the predominant NPF mechanism. Our results indicate that extended measurement campaigns with a more comprehensive set of instrumentation to investigate the role of NPF in the remote forest regions of North America are essential for a deeper scientific understanding of this important process and its role in the global aerosol budget.