Closure study between chemical composition and hygroscopic growth of aerosol particles during TORCH2 M. Gysel1,2, J. Crosier1, D. O. Topping1, J. D. Whitehead1, K. N. Bower1, M. J. Cubison1,*, P. I. Williams1, M. J. Flynn1, G. B. McFiggans1, and H. Coe1 1Atmospheric Sciences Group, SEAES, University of Manchester, P.O.~Box 88, Manchester, M60 1QD, UK 2Labor für Atmsphärenchemie, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland *now at: Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, USA
Abstract. Measurements of aerosol properties were made in aged polluted and clean background air masses encountered at the North
Norfolk (UK) coastline as part of the TORCH2 field campaign in May 2004. Hygroscopic growth factors (GF) at 90%
relative humidity (RH) for D0=27–217 nm particles and size-resolved chemical composition were
simultaneously measured using a Hygroscopicity Tandem Differential Mobility Analyser (HTDMA) and an Aerodyne aerosol
mass spectrometer (Q-AMS), respectively. Both hygroscopic properties and chemical composition showed pronounced
variability in time and with particles size. With this data set we could demonstrate that the
Zdanovskii-Stokes-Robinson (ZSR) mixing rule combined with chemical composition data from the AMS makes accurate
quantitative predictions of the mean GF of mixed atmospheric aerosol particles possible. In doing so it is crucial that
chemical composition data are acquired with high resolution in both particle size and time, at least matching the
actual variability of particle properties. The closure results indicate an ensemble GF of the organic fraction of
~1.20±0.10 at 90% water activity. Thus the organics contribute somewhat to hygroscopic growth, particularly
at small sizes, however the inorganic salts still dominate.
Furthermore it has been found that most likely substantial evaporation losses of NH4NO3 occurred within the
HTDMA instrument, exacerbated by a long residence time of ~1 min. Such an artefact is in agreement with
our laboratory experiments and literature data for pure NH4NO3, both showing similar evaporation losses within
HTDMAs with residence times of ~1 min. Short residence times and low temperatures are hence recommended
for HTDMAs in order to minimise such evaporation artefacts.
Citation: Gysel, M., Crosier, J., Topping, D. O., Whitehead, J. D., Bower, K. N., Cubison, M. J., Williams, P. I., Flynn, M. J., McFiggans, G. B., and Coe, H.: Closure study between chemical composition and hygroscopic growth of aerosol particles during TORCH2, Atmos. Chem. Phys., 7, 6131-6144, doi:10.5194/acp-7-6131-2007, 2007.