Plant stress alters emissions of volatile organic compounds.
However,
little is known about how this could influence climate-relevant properties
of secondary organic aerosol (SOA), particularly from complex mixtures
such as real plant emissions. In this study, the chemical composition
and viscosity were examined for SOA generated from real healthy and
aphid-stressed Canary Island pine (Pinus canariensis) trees, which are commonly used for landscaping in Southern California.
Healthy Canary Island pine (HCIP) and stressed Canary Island pine
(SCIP) aerosols were generated in a 5 m3environmental
chamber at 35–84% relative humidity and room temperature via
OH-initiated oxidation. Viscosities of the collected particles were
measured using an offline poke-flow method, after conditioning the
particles in a humidified air flow. SCIP particles were consistently
more viscous than HCIP particles. The largest differences in particle
viscosity were observed in particles conditioned at 50% relative humidity
where the viscosity of SCIP particles was an order of magnitude larger
than that of HCIP particles. The increased viscosity for the aphid-stressed
pine tree SOA was attributed to the increased fraction of sesquiterpenes
in the emission profile. The real pine SOA particles, both healthy
and aphid-stressed, were more viscous than α-pinene SOA particles,
demonstrating the limitation of using a single monoterpene as a model
compound to predict the physicochemical properties of real biogenic
SOA. However, synthetic mixtures composed of only a few major compounds
present in emissions (<10 compounds) can reproduce the viscosities
of SOA observed from the more complex real plant emissions.