Abstract
Biogenic organic aerosols can affect cloud condensation nuclei (CCN) properties, and subsequently impact climate change. Large uncertainties exist in how the difference in the types of terrestrial biogenic sources and the abundance of organics relative to sulfate affect CCN properties. For the submicron water-soluble aerosols collected for two years in a cool-temperate forest in northern Japan, we show that the hygroscopicity parameter κCCN (0.44 ± 0.07) exhibited a distinct seasonal trend with a minimum in autumn (κCCN = 0.32–0.37); these κCCN values were generally larger than that of ambient particles, including water-insoluble fractions. The temporal variability of κCCN was controlled by the water-soluble organic matter (WSOM)-to-sulfate ratio (R2 > 0.60), where the significant reduction of κCCN in autumn was linked to the increased WSOM/sulfate ratio. Positive matrix factorization analysis indicates that α-pinene-derived secondary organic aerosol (SOA) substantially contributed to the WSOM mass (~75%) in autumn, the majority of which was attributable to emissions from litter/soil microbial activity near the forest floor. These findings suggest that WSOM, most likely α-pinene SOA, originated from the forest floor can significantly suppress the aerosol CCN activity in cool-temperate forests, which have implications for predicting climate effects by changes in biogenic emissions in future.
Highlights
Atmospheric aerosols play a key role in the climate system as they act as cloud condensation nuclei (CCN), and impact cloud formation[1]
Previous field studies conducted in areas dominated by biogenic organic aerosol (OA), such as the Amazon[8, 9], boreal forests[10,11,12] or mountainous forests[13, 14], have reported κ values below 0.3
The objective of our research is to elucidate the impact of seasonal changes in the types and relative amounts of biogenic secondary organic aerosols (BSOAs) and primary biological aerosol particles (PBAPs) on the cloud forming potential of aerosols in forest environments
Summary
Atmospheric aerosols play a key role in the climate system as they act as cloud condensation nuclei (CCN), and impact cloud formation[1]. Because a large fraction of this biogenic OA is water-soluble[7], the formation and abundance of biogenic OA relative to sulfate can modify CCN activity when organics coat sulfate particles. This type of coating can change the surface composition and water affinity of the particle. Several field studies have shown lower hygroscopicity for particles in the accumulation mode in summer, which has been attributed to a large fraction of organic mass, likely of biogenic origin[11, 13]. We present evidence for modification of the aerosol hygroscopicity by specific types of biogenic emissions and discuss their implications for climate
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