Molecular Understanding of the Enhancement in Organic Aerosol Mass at High Relative Humidity
Mihnea Surdu, Houssni Lamkaddam, Dongyu Wang, David M. Bell, Mao Xiao, Chuan Ping Lee, Dandan Li, Lucía Caudillo, Guillaume Marie, Wiebke Scholz, Mingyi Wang, Brandon Lopez, Ana A. Piedehierro, Farnoush Ataei, Rima Baalbaki, Barbara Bertozzi, Pia Bogert, Zoé Brasseur, Lubna Dada, Jonathan Duplissy, Henning Finkenzeller, Xu‐Cheng He, Kristina Höhler, Kimmo Korhonen, Jordan Krechmer, Katrianne Lehtipalo, Naser Mahfouz, Hanna E. Manninen, Ruby Marten, Dario Massabò, Roy L. Mauldin, Tuukka Petäjä, Joschka Pfeifer, Maxim Philippov, Birte Rörup, Mario Simon, Jiali Shen, Nsikanabasi Silas Umo, Franziska Vogel, Stefan K. Weber, Marcel Zauner-Wieczorek, Rainer Volkamer, Harald Saathoff, Ottmar Möhler, J. Kirkby, Douglas R. Worsnop, Markku Kulmala, Frank Stratmann, Armin Hansel, Joachim Curtius, André Welti, Matthieu Riva, Neil M. Donahue, Urs Baltensperger, Imad El Haddad
Abstract
The mechanistic pathway by which high relative humidity (RH) affects gas–particle partitioning remains poorly understood, although many studies report increased secondary organic aerosol (SOA) yields at high RH. Here, we use real-time, molecular measurements of both the gas and particle phase to provide a mechanistic understanding of the effect of RH on the partitioning of biogenic oxidized organic molecules (from α-pinene and isoprene) at low temperatures (243 and 263 K) at the CLOUD chamber at CERN. We observe increases in SOA mass of 45 and 85% with increasing RH from 10–20 to 60–80% at 243 and 263 K, respectively, and attribute it to the increased partitioning of semi-volatile compounds. At 263 K, we measure an increase of a factor 2–4 in the concentration of C 10 H 16 O 2–3, while the particle-phase concentrations of low-volatility species, such as C 10 H 16 O 6–8, remain almost constant. This results in a substantial shift in the chemical composition and volatility distribution toward less oxygenated and more volatile species at higher RH (e.g., at 263 K, O/C ratio = 0.55 and 0.40, at RH = 10 and 80%, respectively). By modeling particle growth using an aerosol growth model, which accounts for kinetic limitations, we can explain the enhancement in the semi-volatile fraction through the complementary effect of decreased compound activity and increased bulk-phase diffusivity. Our results highlight the importance of particle water content as a diluting agent and a plasticizer for organic aerosol growth.