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| Titel |
To what extent can aerosol water explain the discrepancy between model calculated and gravimetric PM10 and PM2.5? |
| VerfasserIn |
S. G. Tsyro |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 5, no. 2 ; Nr. 5, no. 2 (2005-02-16), S.515-532 |
| Datensatznummer |
250002361
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| Publikation (Nr.) |
 copernicus.org/acp-5-515-2005.pdf |
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| Zusammenfassung |
| Inter-comparisons of European air quality models show that regional
transport models, including the EMEP (Co-operative Programme for monitoring
and evaluation of the long-range transmission of air pollutants in Europe)
aerosol model, tend to underestimate the observed concentrations of
PM10 and PM2.5. Obviously, an accurate representation of the
individual aerosol constituents is a prerequisite for adequate calculation
of PM concentrations. On the other hand, available measurements on the
chemical characterization of ambient particles reveal that full chemical PM
mass closure is rarely achieved. The fraction unaccounted for by chemical
analysis can comprise as much as 30-40% of gravimetric PM10 or
PM2.5 mass. The unaccounted PM mass can partly be due to non-C atoms in
organic aerosols and/or due to sampling and measurement artefacts. Moreover,
a part of the unaccounted PM mass is likely to consist of water associated
with particles. Thus, the gravimetrically measured particle mass does not
necessarily represent dry PM10 and PM2.5 mass. This is thought to
be one of the reasons for models under-prediction of observed PM, if
calculated dry PM10 and PM2.5 concentrations are compared with
measurements. The EMEP aerosol model has been used to study to what extent
particle-bound water can explain the chemically unidentified PM mass in
filter-based particle samples. Water content of PM2.5 and PM10 has
been estimated with the model for temperature 20°C and relative humidity
50%, which are conditions required for equilibration of dust-loaded
filters according to the Reference method recommended by the European
Committee for Standardization (CEN). Model calculations for Europe show
that, depending on particle composition, particle-bound water constitutes
20-35% of the annual mean PM10 and PM2.5 concentrations, which
is consistent with existing experimental estimates. At two Austrian sites,
in Vienna and Streithofen, where daily measurements of PM2.5 mass and
chemical composition are available, calculated PM2.5 water content is
found to be about 75-80% of the undetermined PM2.5 mass and there is
correlation between them. Furthermore, accounting for aerosol water has
improved the agreement between modelled and measured daily PM2.5
concentrations, whilst model calculated dry PM2.5 concentrations appear
to agree quite well with the total identified PM2.5 mass. No
information on the composition of PM measured at EMEP sites is presently
available. Given that PM10 and PM2.5 concentrations are measured
at EMEP stations with gravimetric methods they are likely to contain water.
We show that the levels of modelled PM10 and PM2.5 concentrations
with aerosol water included agree with measurements better than dry PM
concentrations. As expected, the spatial correlation has not changed
significantly, whereas the temporal correlation of daily PM10 and
PM2.5 with monitoring data has slightly improved at most of the EMEP
sites. Our results suggest that aerosol water should be accounted for in
modelled PM10 and PM2.5 when compared with filter-based
gravimetric measurements. |
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