Vm. Kerminen et al., FORMATION OF PARTICULATE MSA - DEDUCTIONS FROM SIZE DISTRIBUTION MEASUREMENTS IN THE FINNISH ARCTIC, Tellus. Series B, Chemical and physical meteorology, 49(2), 1997, pp. 159-171
Formation of particulate MSA was studied by conducting a field campaig
n in the Finnish Arctic. Based on continuous mass size distributions e
xtracted from impactor measurements, 4 MSA modes were identified: an a
ccumulation mode centered between 0.3-0.5 mu m of particle aerodynamic
diameter, an Aitken mode below; 0.1 mu m, and 2 supermicron modes tha
t peaked at 2-3 and 7-10 mu m, respectively. The lower supermicron mod
e resulted primarily from the reaction of gaseous MSA with sea salt pa
rticles, and the upper mode probably from its reaction with soil-deriv
ed particles. From 70 to 90% of the MSA was found in the accumulation
mode, where it was distributed very similarly to ammonium. The overall
MSA to nss-SO42- ratio, R, ranged from 2 to 34%, with most of this va
riation resulting from different degrees of anthropogenic perturbation
in the measured air masses. When comparing different-size particles,
R was clearly the highest in the Aitken mode, suggesting that MSA and
sulfate contribute with comparable magnitudes to nuclei condensational
growth at high latitudes during the summer. The evident variation of
R with particle size, together with potential enhanced MSA production
in air influenced by pollution, demonstrate further that one needs to
be extremely careful when using observed MSA to nss-SO42- ratios for e
stimating the contribution of biogenic sources to total particulate su
lfate in different environments. Strong indications on the saturation
of MSA over accumulation-mode particles were found. The saturation eff
ect was hypothesized to explain, in part, the observed large partition
ing of MSA in the supermicron mode in warm and very acidic aerosol sys
tems. The distribution of submicron MSA, including potential saturatio
n, was shown to be affected significantly by cloud processing.