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Closure study between chemical composition and hygroscopic growth of aerosol particles during TORCH2

by M. Gysel, J. Crosier, D. O. Topping, J. D. Whitehead, K. N. Bower, M. J. Cubison, P. I. Williams, M. J. Flynn, G. B. McFiggans, H. Coe show all authors
Atmospheric Chemistry and Physics ()

Abstract

Measurements of aerosol properties were made in aged polluted and\nclean background air masses encountered at the North Norfolk (UK)\ncoastline as part of the TORCH2 field campaign in May 2004. Hygroscopic\ngrowth factors (GF) at 90% relative humidity (RH) for D0=27-217 nm\nparticles and size-resolved chemical composition were simultaneously\nmeasured using a Hygroscopicity Tandem Differential Mobility Analyser\n(HTDMA) and an Aerodyne aerosol mass spectrometer (Q-AMS), respectively.\nBoth hygroscopic properties and chemical composition showed pronounced\nvariability in time and with particles size. With this data set we\ncould demonstrate that the Zdanovskii-Stokes-Robinson (ZSR) mixing\nrule combined with chemical composition data from the AMS makes accurate\nquantitative predictions of the mean GF of mixed atmospheric aerosol\nparticles possible. In doing so it is crucial that chemical composition\ndata are acquired with high resolution in both particle size and\ntime, at least matching the actual variability of particle properties.\nThe closure results indicate an ensemble GF of the organic fraction\nof ~1.20+-0.10 at 90% water activity. Thus the organics contribute\nsomewhat to hygroscopic growth, particularly at small sizes, however\nthe inorganic salts still dominate. \n\n\nFurthermore it has been found that most likely substantial evaporation\nlosses of NH4NO3 occurred within the HTDMA instrument, exacerbated\nby a long residence time of ~1 min. Such an artefact is in agreement\nwith our laboratory experiments and literature data for pure NH4NO3,\nboth showing similar evaporation losses within HTDMAs with residence\ntimes of ~1 min. Short residence times and low temperatures are hence\nrecommended for HTDMAs in order to minimise such evaporation artefacts.

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