65 research outputs found
Absorbing aerosols at high relative humidity: linking hygroscopic growth to optical properties
One of the major uncertainties in the understanding of Earth's climate system is the interaction between solar radiation and aerosols in the atmosphere. Aerosols exposed to high humidity will change their chemical, physical, and optical properties due to their increased water content. To model hydrated aerosols, atmospheric chemistry and climate models often use the volume weighted mixing rule to predict the complex refractive index (RI) of aerosols when they interact with high relative humidity, and, in general, assume homogeneous mixing. This study explores the validity of these assumptions. A humidified cavity ring down aerosol spectrometer (CRD-AS) and a tandem hygroscopic DMA (differential mobility analyzer) are used to measure the extinction coefficient and hygroscopic growth factors of humidified aerosols, respectively. The measurements are performed at 80% and 90%RH at wavelengths of 532 nm and 355 nm using size-selected aerosols with different degrees of absorption; from purely scattering to highly absorbing particles. The ratio of the humidified to the dry extinction coefficients (<i>f</i>RH<sub>ext</sub>(%RH, Dry)) is measured and compared to theoretical calculations based on Mie theory. Using the measured hygroscopic growth factors and assuming homogeneous mixing, the expected RIs using the volume weighted mixing rule are compared to the RIs derived from the extinction measurements. <br><br> We found a weak linear dependence or no dependence of <i>f</i>RH(%RH, Dry) with size for hydrated absorbing aerosols in contrast to the non-monotonically decreasing behavior with size for purely scattering aerosols. No discernible difference could be made between the two wavelengths used. Less than 7% differences were found between the real parts of the complex refractive indices derived and those calculated using the volume weighted mixing rule, and the imaginary parts had up to a 20% difference. However, for substances with growth factor less than 1.15 the volume weighted mixing rule assumption needs to be taken with caution as the imaginary part of the complex RI can be underestimated
Evolution of the complex refractive index in the UV spectral region in ageing secondary organic aerosol
The chemical and physical properties of secondary organic aerosol (SOA)
formed by the photochemical degradation of biogenic and anthropogenic
volatile organic compounds (VOC) are as yet still poorly constrained. The evolution
of the complex refractive index (RI) of SOA, formed from purely biogenic VOC
and mixtures of biogenic and anthropogenic VOC, was studied over a diurnal
cycle in the SAPHIR photochemical outdoor chamber in Jülich, Germany.
The correlation of RI with SOA chemical and physical properties such as
oxidation level and volatility was examined. The RI was retrieved by a newly
developed broadband cavity-enhanced spectrometer for aerosol optical
extinction measurements in the UV spectral region (360 to 420 nm). Chemical
composition and volatility of the particles were monitored by a high-resolution time-of-flight aerosol mass spectrometer, and a volatility tandem
differential mobility analyzer. SOA was formed by ozonolysis of either (i) a
mixture of biogenic VOC (α-pinene and limonene), (ii) biogenic VOC
mixture with subsequent addition of an anthropogenic VOC
(<i>p</i>-xylene-d<sub>10</sub>), or (iii) a mixture of biogenic and anthropogenic VOC.
The SOA aged by ozone/OH reactions up to 29.5 h was found to be
non-absorbing in all cases. The SOA with <i>p</i>-xylene-d<sub>10</sub> showed an
increase of the scattering component of the RI correlated with an increase
of the O / C ratio and with an increase in the SOA density. There was a
greater increase in the scattering component of the RI when the SOA was
produced from the mixture of biogenic VOCs and anthropogenic VOC than from
the sequential addition of the VOCs after approximately the same ageing time.
The increase of the scattering component was inversely correlated with the
SOA volatility. Two RI retrievals determined for the pure biogenic SOA
showed a constant RI for up to 5 h of ageing. Mass spectral characterization
shows the three types of the SOA formed in this study have a significant
amount of semivolatile components. The influence of anthropogenic VOCs on
the oxygenated organic aerosol as well as the atmospheric implications are
discussed
Order and Frustration in Chiral Liquid Crystals
This paper reviews the complex ordered structures induced by chirality in
liquid crystals. In general, chirality favors a twist in the orientation of
liquid-crystal molecules. In some cases, as in the cholesteric phase, this
favored twist can be achieved without any defects. More often, the favored
twist competes with applied electric or magnetic fields or with geometric
constraints, leading to frustration. In response to this frustration, the
system develops ordered structures with periodic arrays of defects. The
simplest example of such a structure is the lattice of domains and domain walls
in a cholesteric phase under a magnetic field. More complex examples include
defect structures formed in two-dimensional films of chiral liquid crystals.
The same considerations of chirality and defects apply to three-dimensional
structures, such as the twist-grain-boundary and moire phases.Comment: 39 pages, RevTeX, 14 included eps figure
An automatic instrument for investigating the elastic peoperties and controlling the quality of refractories
Dynamic method for determining the coefficient of transverse deformation of refractories at high temperatures
Behavior of basic refractories in various spalling test conditions using the heat-cycle method
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