Measurement report: Cloud condensation nuclei activity and its variation with organic oxidation level and volatility observed during an aerosol life cycle intensive operational period (ALC-IOP)
2021
Abstract. Cloud condensation nuclei (CCN) spectrum and the CCN activated
fraction of size-resolved aerosols (SR-CCN) were measured at a rural site on
Long Island during the Department of Energy (DOE) aerosol life cycle
intensive operational period (ALC-IOP) from 15 July to 15 August 2011.
During the last week of the ALC-IOP, the dependence of the activated
fraction on aerosol volatility was characterized by sampling downstream of a
thermodenuder (TD) operated at temperatures up to 100 ∘ C. Here we
present aerosol properties, including aerosol total number concentration,
CCN spectrum, and the CCN hygroscopicity, for air masses of representative
origins during the ALC-IOP. The hygroscopicity of organic species in the
aerosol is derived from CCN hygroscopicity and chemical composition. The
dependence of organic hygroscopicity on the organic oxidation level (e.g.,
atomic O:C ratio) agrees well with theoretical predictions and results from
previous laboratory and field studies. The derived κorg and O:C
ratio first increase as TD temperature increases from 20 ∘ C (i.e.,
ambient temperature) to 50 or 75 ∘ C and then decrease as TD temperature
further increases to 100 ∘ C. The initial increases of O:C and
κorg with TD temperature below 50 ∘ C are likely due to
evaporation of more volatile organics with relatively lower O:C and
hygroscopicity such as primary organic aerosol. At the high TD temperatures, the
decreases of O:C and κorg indicate that evaporated organics
were more oxygenated and had lower molecular weights. These trends are
different from previous laboratory experiments and field observations, which
reported that organic O:C increased monotonically with increasing TD
temperature, whereas κorg decreased with the TD temperature. One
possible reason is that previous studies were either focused on laboratory-generated secondary organic aerosol (SOA) or based on field observations at locations more
dominated by SOA.
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