A method for the in-situ measurement of the water content of atmospheric particles

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Τσιλιγιάννης, Επαμεινώνδας
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The hygroscopic behavior of atmospheric aerosols influences their size, composition, lifetime, chemical reactivity, and light scattering. Hygroscopic growth plays an important role in a number of air pollution problems including visibility impairment, climate change, acid deposition, long­range transport and the ability of particles to penetrate the human respiratory system. The absorption of water by aerosol particles often exhibits a hysteresis. Thus, the physical state (liquid or solid) of particles and the amount of aerosol water at a specific relative humidity (RH) are uncertain, as they depend on the history of these particles. In this work, the reduced version of the Dry Ambient Aerosol Size Spectrometer (DAASS) that measures the water content of atmospheric aerosols has been redesigned and optimized. The DAASS measures the number distribution of the aerosols at ambient conditions and at low RH thus drying the particles. A comparison of these distributions allows the determination of the physical state of the particles and their water content. The new version of the DAASS is capable of operating at higher RH values than its predecessor. The instrument has been characterized regarding particle wall losses, in a set of smog chamber experiments using (NH4)2SO4 particles. An algorithm checking the consistency of the measurements and the applicability of the assumptions used in the data analysis was developed. The water concentrations observations were compared to the predictions of the aerosol thermodynamics model E­AIM. Ambient measurements, using the original and the improved version of DAASS, were conducted during two different time periods in a suburban area in Patras. These tests allowed the testing and assessment of the operation of the DAASS and also the examination of the hygroscopic behavior of particulate matter. The original version was used during a moderate RH period, in which the water content of the aerosol represented 0-­50% of the fine aerosol mass. The improved DAASS operated during high RH conditions. The particles retained water throughout the duration of the measurements. The measured volume growth factors were quite higher than the measured ones during the moderate RH period. The water concentrations observations were compared to the predictions of the aerosol thermodynamics model E­AIM.
Atmospheric particles, Hygroscopic growth, Water content, Deliquescence - efflorescence