Abstract
The role of the elemental carbon (EC), in synergy with hygroscopic ionic species, was investigated to study the formation of electrical bridging phenomena once the aerosol deliquescence is achieved. Ambient aerosol samples were collected on hydrophobic surfaces in urban and rural sites in Northern Italy; their conductance was measured in an Aerosol Exposure Chamber (AEC) while varying the relative humidity. An electric signal was detected on 64% of the collected samples with conductance values (11.20 ± 7.43 μS) above the failure threshold (1 μS) of printed circuit boards. The ionic content was higher for non-electrically conductive samples (43.7 ± 5.6%) than for electrically conductive ones (37.1 ± 5.6%). Conversely, EC was two times higher for electrically conductive samples (26.4 ± 4.1 μg cm−2; 8.4 ± 1.7%) than for non-electrical ones (12.0 ± 4.1 μg cm−2; 5.2 ± 1.9%) suggesting that the synergy between the ionic and carbonaceous fractions is necessary to promote a bridging phenomenon. Synthetic aerosols (EC only, saline only, mixed saline and EC) were generated in laboratory and their conductance was measured in the AEC to verify the ambient results. Only in case of a contemporary presence of both EC and ionic components the bridging phenomenon occurred in keeping with the theoretical deliquescence values of each salt (R2 = 0.996).
Highlights
The physical state of atmospheric aerosols determines their physical–chemical properties responsible for their behavior in various environmental processes, such as atmospheric corrosion and bridging phenomena [1,2,3,4,5], interaction with solar radiation and satelliteAppl
Conductance and chemical analysis on samples collected at the urban site of MI-TS and at the rural site OB are first presented in Sections 3.1 and 3.2, respectively; the aim is to highlight the different electrical behavior in function of the ionic and elemental carbon (EC) content presents on ambient aerosol samples in different sites
62 samples were characterized and exposed to relative humidity (RH) cycles in the Aerosol Exposure Chamber (AEC) for the determination of their Deliquescence relative humidity (DRH) and crystallization relative humidity (CRH) together with their conductance. 50 of them were collected at the urban site of MI-TS and 12 at the rural site OB
Summary
The physical state (wet or dry) of atmospheric aerosols determines their physical–chemical properties responsible for their behavior in various environmental processes, such as atmospheric corrosion and bridging phenomena [1,2,3,4,5], interaction with solar radiation and satelliteAppl. The critical points of the phase transition of an atmospheric particle are the deliquescence and the crystallization relative humidity (DRH and CRH). DRH and CRH determine whether in certain atmospheric conditions aerosol particles are solid or liquid (i.e., their soluble components are in solution) in function of the variation of the relative humidity (RH) of the surrounding atmosphere. Starting from a RH < DRH, the aerosol is dry until RH reaches DRH; from this point the aerosol absorbs water producing a saturated aqueous solution. A reduction in RH (starting from a value above DRH) leads to evaporation until the CRH is reached promoting the aerosol crystallization and bringing again the aerosol to a dry state [13,14,15,16]. The aforementioned cycle is known as hysteresis cycle of the aerosol [14,18,19]
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