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| Titel |
Estimation of aerosol water and chemical composition from AERONET Sun–sky radiometer measurements at Cabauw, the Netherlands |
| VerfasserIn |
A. J. van Beelen, G. J. H. Roelofs, O. P. Hasekamp, J. S. Henzing, T. Röckmann |
| 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 ; 14, no. 12 ; Nr. 14, no. 12 (2014-06-18), S.5969-5987 |
| Datensatznummer |
250118814
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| Publikation (Nr.) |
 copernicus.org/acp-14-5969-2014.pdf |
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| Zusammenfassung |
| Remote sensing of aerosols provides important information on atmospheric
aerosol abundance. However, due to the hygroscopic nature of aerosol
particles observed aerosol optical properties are influenced by atmospheric
humidity, and the measurements do not unambiguously characterize the aerosol
dry mass and composition, which complicates the comparison with aerosol
models. In this study we derive aerosol water and chemical composition by a
modeling approach that combines individual measurements of remotely sensed
aerosol properties (e.g., optical thickness, single-scattering albedo,
refractive index and size distribution) from an AERONET (Aerosol Robotic
Network) Sun–sky radiometer with radiosonde measurements of relative
humidity. The model simulates water uptake by aerosols based on the chemical
composition (e.g., sulfates, ammonium, nitrate, organic matter and black
carbon) and size distribution. A minimization method is used to calculate
aerosol composition and concentration, which are then compared to in situ
measurements from the Intensive Measurement Campaign At the Cabauw Tower
(IMPACT, May 2008, the Netherlands). Computed concentrations show good
agreement with campaign-average (i.e., 1–14 May) surface observations (mean
bias is 3% for PM10 and 4–25% for the individual compounds). They
follow the day-to-day (synoptic) variability in the observations and are in
reasonable agreement for daily average concentrations (i.e., mean bias is
5% for PM10 and black carbon, 10% for the inorganic salts and
18% for organic matter; root-mean-squared deviations are 26% for
PM10 and 35–45% for the individual compounds). The modeled water
volume fraction is highly variable and strongly dependent on composition.
During this campaign we find that it is >0.5 at approximately 80% relative humidity
(RH) when the aerosol composition is dominated by hygroscopic inorganic salts, and
<0.1 when RH is below 40%, especially when the composition is
dominated by less hygroscopic compounds such as organic matter. The
scattering enhancement factor (f(RH), the ratio of the scattering coefficient
at 85% RH and its dry value at 676 nm) during 1–14 May is
2.6 ± 0.5. The uncertainty in AERONET (real) refractive index
(0.025–0.05) is the largest source of uncertainty in the modeled aerosol
composition and leads to an uncertainty of 0.1–0.25 (50–100%) in aerosol
water volume fraction. Our methodology performs relatively well at Cabauw,
but a better performance may be expected for regions with higher aerosol
loading where the uncertainties in the AERONET inversions are smaller. |
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