Abstract
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.
Highlights
As a critical element in cloud formation, atmospheric aerosols indirectly influence the global energy budget by affecting the atmospheric boundary structure and changing clouds' lifetime and coverage
The organic species were classified into three secondary organic aerosols (SOA) factors based on the Positive Matrix Factorization (PMF) analysis, including a fresh semivolatile 235 oxygenated organic aerosol (SV-OOA), which contributed 63% of OA mass and was strongly influenced by urban plumes transported from the W and SSW regions, a regional and more aged low-volatility oxygenated organic aerosol (LV-OOA), which was influenced by aqueous-phase processing, and a nitrogen-enriched OA (NOA), which likely composed of amine salts formed from acid-base reactions in industrial emissions (Zhou et al, 2016)
As the addition of oxygenated function groups reduces the volatility of organic species, the organics with relatively high O:C evaporated at a high thermal denuder (TD) temperature setting is expected to have a smaller molecular weight compared to those remaining in the particle phase, consistent with the decreasing κorg
Summary
As a critical element in cloud formation, atmospheric aerosols indirectly influence the global energy budget by affecting the atmospheric boundary structure and changing clouds' lifetime and coverage. In addition to size-resolved CCN activated fraction, CCN concentration spectrum was measured using a second CCN counter operated at a flow rate of 0.3 L min-1, and at seven supersaturations of 0.11%, 0.13%, 0.17%, 0.23%, 0.32%, 0.40% and 0.48% 115 corresponding to longitudinal temperature gradients of 4.3, 4.8, 5.5, 6.5, 7.9, 10, and 12 °C, respectively. The 25%- 75% percentiles of CCN concentrations at 0.5% supersaturation ranged from 1,000 to 2,500 cm-3 for all clusters, indicating no clear trend with air mass observed for CCN concentration at 0.12% These results suggest a strong sizedependency in the CCN activation properties in the aerosol particles from different regions. The E values indicate that aerosols in the long-range transported NW air masses (i.e., LRNW cluster) were not all internal mixtures and included some contribution of freshly emitted non-hygroscopic aerosol particles. The variety of aerosol sources along the LRNW trajectory paths likely contribute to the relatively large variability of the aerosol hygroscopicity dispersion for the cluster
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