Atmospheric Chemistry and Physics, vol. 7, issue 24 (2007) pp. 6131-6144
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.
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