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

摘要

Measurements of aerosol properties were made in aged polluted and clean background air masses encountered at the NorthNorfolk (UK) coastline as part of the TORCH2 field campaign in May 2004. Hygroscopic growth factors (GF) at 90%relative humidity (RH) for =27–217 nm particles and size-resolved chemical composition weresimultaneously measured using a Hygroscopicity Tandem Differential Mobility Analyser (HTDMA) and an Aerodyne aerosolmass spectrometer (Q-AMS), respectively. Both hygroscopic properties and chemical composition showed pronouncedvariability in time and with particles size. With this data set we could demonstrate that theZdanovskii-Stokes-Robinson (ZSR) mixing rule combined with chemical composition data from the AMS makes accuratequantitative predictions of the mean GF of mixed atmospheric aerosol particles possible. In doing so it is crucial thatchemical composition data are acquired with high resolution in both particle size and time, at least matching theactual 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, particularlyat small sizes, however the inorganic salts still dominate.Furthermore it has been found that most likely substantial evaporation losses of NHNO occurred within theHTDMA instrument, exacerbated by a long residence time of ~1 min. Such an artefact is in agreement withour laboratory experiments and literature data for pure NHNO, both showing similar evaporation losses withinHTDMAs with residence times of ~1 min. Short residence times and low temperatures are hence recommendedfor HTDMAs in order to minimise such evaporation artefacts.