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| Titel |
Combined volatility and mass spectrometric measurements of biogenic secondary organic aerosol |
| VerfasserIn |
E. Emanuelsson, A. Buchholz, M. Hallquist, A. Kiendler-Scharr, T. Mentel, C. Spindler |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250025405
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| Zusammenfassung |
| The volatility of secondary organic aerosol (SOA) from the oxidation of mixtures of biogenic
Volatile Organic Compounds (VOC) has been investigated in the SAPHIR facility in
Forschungszentrum Jülich, Germany, by using a Volatility Tandem Differential
Mobility Analyser (VTDMA). The standard VTDMA setup comprises three main
parts: 1) An initial DMA, where a nearly monodisperse size fraction of the aerosol
particles is selected (typically 100 or 150 nm), 2) the oven unit, i.e. four ovens
in parallel where each oven includes a heating and adsorption section where the
evaporation and adsorption of the volatile fraction occurs and 3) a final SMPS (Scanning
Mobility Particle Sizer) system where the residual particle number distribution is
measured. For this measurement campaign the set-up also contained a Quadrupole
Aerosol Mass Spectrometer (Aerodyne QAMS). The temperature of the ovens can
be varied between 298 and well above 573 K. In parallel to the final SMPS the
AMS was used for chemical composition and density measurements. When the
system was dedicated for AMS measurements the initial DMA was bypassed to
improve the aerosol concentration. However, the produced SOA has a narrow size
distribution still making it possible to follow small changes in the aerosol peak
diameter. A general feature of the thermo-denuder system is that a less volatile
SOA gives a larger residual particle size distribution compared to more volatile
SOA.
The experiments conducted were based on photochemical oxidation of selected terpene
mixtures. A reference boreal mixture of terpenes, consisting of α-pinene, β-pinene,
limonene, δ-3-carene, and ocimene was used as base case. Secondary organic aerosol was
formed from the precursor compounds by reaction with O3/H2O/OH in SAPHIR on the first
day. The particles were kept in the chamber for up to two further days and were
exposed to natural sunlight and OH radicals to initiate close to natural chemical
ageing.
The VTDMA results show that SOA becomes less volatile during ageing and this ageing
was further enhanced when the mixtures were exposed to sunlight. The volatility was also
affected by changes in the terpene mixtures.
With the AMS we measured mass spectra of the organic aerosol particles at a reference
temperature of 298 K and two additional elevated temperatures. Size distributions of the
particles were obtained from the particle-time-of-flight mode of the AMS at sixteen
representative m/z values. The residual total mass measured at the elevated oven
temperatures was related to the total mass at the reference temperature to obtain the mass
fraction remaining (MFR), which is higher for less volatile SOA. In agreement
with the decreasing volatility during aging measured with the VTDMA, the MFR
increases with time. An effective density of the particles was calculated comparing the
mode position of the size distributions measured with the AMS and the SMPS. The
effective density increases with ongoing photochemical ageing. In addition, the
density of the low volatile residual particles that passed a high-temperature oven is
higher than the density of particles at reference temperature. In order to investigate if
the observed changes in density and volatility can be attributed to changes of the
chemical composition of the particles, the mass spectra obtained at different oven
temperatures and different chemical age were compared. We found that the ratio of
heavy fragments (m/z > 90) increases with higher temperatures. Furthermore the
fraction of the CO2+-fragment at m/z 44 to the total mass increases during the ageing
process. |
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