Chemistry Reference
In-Depth Information
Figure
1c
shows the variability of the submicron aerosol volume distribution
(calculated from number size distribution using assumption of spherical particles)
as the function of size. As the particle volume concentration is closely connected
with particle mass concentration, two key findings come apparent: (1) as the aerosol
volume variability is dominated by larger accumulation particles in the diameter
range from 300 to 700 nm, the variabilities of most mass-based aerosol meas-
urements (especially PM
1
, as it is not dependent on supermicron coarse mode, but
in lesser extent also PM
2.5
and PM
10
) are very sensitive to changes in these particles
sizes, and in comparison (2) the variability of particle number concentration is
rapidly diminishing above 100 nm in diameter, show that the variability of that
aerosol number concentrations are not strongly influenced by changes in accumu-
lation mode particle number, and that the used integral particle number concen-
trations (N
30-50
,N
50
and N
100
) are mostly sensitive to changes in the smallest
particle sizes in each diameter range.
In short, the mass-based measurements are often more sensitive to aerosol
concentration changes near the maximum diameter of the size range measured,
whereas the number-based measurements are more sensitive on aerosol concentra-
tion near the minimum diameter measured.
2 Measurement and Monitoring in Europe
2.1 Measuring the Aerosol Particle Number Size Distributions
Two types of particle mobility particle size spectrometers for measuring
submicrometer particle number size distributions are generally used: the differen-
tial mobility particle sizer (or DMPS), and the scanning mobility particle sizer
(SMPS) [
14
]. Both are very similar instruments in their operation: they aspirate
dried air, use ionizing radiation to establish an equilibrium bipolar charge-
distribution in the sampled aerosol, use a cylindrical differential mobility analyzer
to select particles based on their electrical mobility, and use a condensation particle
counter to measure the resulting concentrations in each of the selected electrical
mobility. The main difference between these instruments is the mode of operation,
as the DMPS keeps the differential mobility analyzer voltage constant during
measurement of a single size interval and the SMPS scans with continuously
differing voltages. The size range and time resolution of a DMPS or an SMPS
system depends on the system architecture (e.g., the physical dimensions of the
instrument and the flow rates used) and on user choice (more size channels vs. time
resolution). One key factor is that the measured particle properties are measured for
dry conditions
. The sample air is dried to relative humidity RH
40% before size
selection, meaning that most of the water is evaporated from the particles, and thus
they are most likely smaller than in ambient conditions.
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