CCN Activity of Organic Aerosols Observed Downwind of Urban Emissions during CARES

Journal Article · Tue Dec 17 04:00:00 EST 2013 · Atmospheric Chemistry and Physics

DOI:https://doi.org/10.5194/acp-13-12155-2013· OSTI ID:1130264

Mei, Fan; Setyan, Ari; Zhang, Qi; Wang, J. X.

During the Carbonaceous Aerosols and Radiative Effects Study (CARES), activation fraction of size-resolved aerosol particles and aerosol chemical composition were characterized at the T1 site (~60 km downwind of Sacramento, California) from 10 June to 28 June 2010. The hygroscopicity of CCN-active particles (K_CCN) with diameter from 100 to 170 nm, derived from the size-resolved activated fraction, varied from 0.10 to 0.21, with an average of 0.15, which was substantially lower than that proposed for continental sites in earlier studies. The low K_CCN value was due to the high organic volume fraction, averaged over 80% at the T1 site. The derived K_CCN exhibited little diurnal variation, consistent with the relatively constant organic volume fraction observed. At any time, over 90% of the size selected particles with diameter between 100 and 171nm were CCN active, suggesting most particles within this size range were aged background particles. Due to the large organic volume fraction, organic hygroscopicity (K_org) strongly impacted particle hygroscopicity and therefore calculated CCN concentration. For vast majority of the cases, an increase of K_org from 0.03 to 0.18, which are within the typical range, doubled the calculated CCN concentration. Organic hygroscopicity was derived from K_CCN and aerosol chemical composition, and its variations with the fraction of total organic mass spectral signal at m/z 44 (f₄₄) and O:C were compared to results from previous studies. Overall, the relationships between K_org and f₄₄ are quite consistent for organic aerosol (OA) observed during field studies and those formed in smog chamber. Compared to the relationship between K_org and f₄₄, the relationship between K_org and O:C exhibits more significant differences among different studies, suggesting korg may be better parameterized using f₄₄. A least squares fit yielded K_org = 2.10 (±0.07) × f₄₄ -0.11 (±0.01) with the Pearson R² value of 0.71. One possible explanation for the stronger correlation between K_org and f₄₄ is that the m/z 44 signal (mostly contributed by the CO₂⁺ ion) is more closely related to organic acids, which may dominate the overall korg due to their relatively high water solubility and hygroscopicity.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (US)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1130264
• Report Number(s): PNNL-SA-94932; KP1704010
• Journal Information: Atmospheric Chemistry and Physics, Journal Name: Atmospheric Chemistry and Physics Journal Issue: 24 Vol. 13; ISSN 1680-7316
• Publisher: European Geosciences Union
• Country of Publication: United States
• Language: English

Related Subjects

CARES
CNN
aerosols
emissions
organic
urban