Increased CCN production from isoprene photooxidation in progressively drier air

 

Authors

Jian Wang — Washington University in St. Louis
Alexander Laskin — Purdue University
Alla Zelenyuk-Imre — Pacific Northwest National Laboratory
Daniel James Cziczo — Massachusetts Institute of Technology
Timothy B Onasch — Aerodyne Research, Inc.
Rahul Zaveri — Pacific Northwest National Laboratory
Jerome D Fast — Pacific Northwest National Laboratory
Qi Zhang — University of California, Davis
Dick C Easter — Pacific Northwest National Laboratory
Douglas R Worsnop — Aerodyne Research, Inc.
Chongai Kuang — Brookhaven National Laboratory
John E Shilling — Pacific Northwest National Laboratory
Ari Setyan — University of California, Davis
Joel Thornton — University of Washington
Jacqueline Mary Wilson — Pacific Northwest National Laboratory
Bingbing Wang — Pacific Northwest National Laboratory
Jiumeng Liu — Pacific Northwest National Laboratory

Category

Secondary Organic Aerosol

Description

Isoprene produced in forest ecosystems is the most abundantly emitted non-methane volatile organic compound in the Earth’s atmosphere. Recent laboratory and field studies suggest that photooxidation of isoprene forms appreciable amounts of secondary organic aerosol (SOA), which likely affects climate by scattering solar radiation and by acting as cloud condensation nuclei (CCN). As these climate effects depend on both aerosol size and number concentration, it is crucial to understand how SOA partitioning influences particle growth kinetics. Recent studies indicate that biogenic SOA becomes increasingly viscous as relative humidity decreases, with the theoretical implication that its reduced bulk diffusivity slows down further growth via condensation of semivolatile organic compounds. We present here evidences from chamber experiments and the CARES field campaign for such bulk diffusion-limited growth of isoprene SOA formed in the presence of pre-existing Aitken and accumulation mode aerosols and relative humidity ≤ 50%. Kinetic modeling of the aerosol size distribution evolution suggests that the condensing isoprene photooxidation products are semivolatile and the bulk diffusivity is on the order 10^-15 cm2 s-1. The model also successfully reproduces the evaporation kinetics of size-selected chamber particles using similar estimates of bulk diffusivity and volatility for isoprene SOA. As the bulk diffusion timescale decreases a hundred-fold with a ten-fold decrease in size, the hindered growth of viscous accumulation mode particles effectively promotes the growth of the Aitken mode particles that are competing to absorb the semivolatile vapors. Isoprene SOA formation over forests thus represents a Gaia-like self-regulating system that produces CCN more efficiently under progressively drier conditions.