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)

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.

Measurement report: CCN activity and its variation with organic oxidation level and volatility observed during aerosol life cycle intensive operational period (ALC-IOP)

Cloud condensation nuclei (CCN) spectrum and the CCN activated fraction of size selected 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 July 15 to August 15, 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 operated at temperatures up to 100 ⁰C. 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 thermal denuder (TD) temperature increases from 20 ℃ (i.e., ambient temperature) to 50 or 75 ℃, then decreases as TD temperature further increases to 100 ℃. 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. The initial increases of O:C and κorg with TD temperature below 50 ℃ are likely due to the evaporation of more volatile organics with relatively lower O:C and hygroscopicity such as primary OA. Previous studies were either focused on laboratory-generated SOA or based on field observations at locations dominated by SOA.

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