Technical Description


Instrumentation


Research Interests

HEC: CCN Activity of Secondary Organic Aerosol

The cloud condensation nucleus (CCN) activity of organic-sulfate particles was investigated using the HEC. The organic component consisted of secondary organic aerosol (SOA) generated in the dark from 24 ± 2 ppb α-pinene for conditions of 300 ± 5 ppb ozone, 40 ± 2% relative humidity, and 25 ± 1°C, with the organic mass loading in the chamber ranging from 23 to 37 μg m^-3. CCN analysis was performed for 80- to 150-nm particles having variable organic-sulfate volume fractions, which were estimated from the diameter of the organic-sulfate particle relative to that of the seed as well as independently from mass spectra. Critical supersaturation, which increased for greater SOA volume fraction and smaller particle diameter, was well predicted by a Köhler model having two components, one for ammonium sulfate and another for SOA. The entire data set could be successfully modeled by a single suite of effective chemical parameters for SOA. The results suggest that the effects of limited organic solubility in mixed SOA-sulfate particles may be reliably omitted in the treatment of cloud droplet formation.

Representative CCN activation curves of SOA particles having ammonium sulfate cores of 51-nm diameter. Critical supersaturation, Sc, is determined as the supersaturation intersecting the dotted line, where Fa = 0.5.

Sc of SOA particles internally mixed with sulfate. Data are shown for four particle mobility diameters for increasing organic volume fraction (ε^d_SOA). Curves represent modeled values using a single set of parameters. Insets: (a) Comparison of modeled Sc to observed Sc for all particle diameters. Comparison to the shown 1:1 line yields an r-squared value of 0.99. (b) Modeled Sc values of 100-nm mixed SOA-sulfate particles for a limited-solubility system with varying values of C_sat,SOA.

Our most recent experiments show that Sc values also shift at very low levels of organic mass loading, where the amount of reacted α-pinene is atmospherically relevant.