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. Cloud condensation nuclei (CCN) spectrum and the CCN activatedfraction of size-resolved aerosols (SR-CCN) were measured at a rural site onLong Island during the Department of Energy (DOE) aerosol life cycleintensive operational period (ALC-IOP) from 15 July to 15 August 2011.During the last week of the ALC-IOP, the dependence of the activatedfraction on aerosol volatility was characterized by sampling downstream of athermodenuder (TD) operated at temperatures up to 100 ∘C. Here wepresent aerosol properties, including aerosol total number concentration,CCN spectrum, and the CCN hygroscopicity, for air masses of representativeorigins during the ALC-IOP. The hygroscopicity of organic species in theaerosol is derived from CCN hygroscopicity and chemical composition. Thedependence of organic hygroscopicity on the organic oxidation level (e.g.,atomic O:C ratio) agrees well with theoretical predictions and results fromprevious laboratory and field studies. The derived κorg and O:Cratio first increase as TD temperature increases from 20 ∘C (i.e.,ambient temperature) to 50 or 75 ∘C and then decrease as TD temperaturefurther increases to 100 ∘C. The initial increases of O:C andκorg with TD temperature below 50 ∘C are likely due toevaporation of more volatile organics with relatively lower O:C andhygroscopicity such as primary organic aerosol. At the high TD temperatures, thedecreases of O:C and κorg indicate that evaporated organicswere more oxygenated and had lower molecular weights. These trends aredifferent from previous laboratory experiments and field observations, whichreported that organic O:C increased monotonically with increasing TDtemperature, whereas κorg decreased with the TD temperature. Onepossible reason is that previous studies were either focused on laboratory-generated secondary organic aerosol (SOA) or based on field observations at locations moredominated by SOA.

atmospheric chemistry and physics

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)