Vibrational Overtones and Rotational Structures of HCl in Rare Gas Matrices

The rotational structure in the vibrational transitions from ν=0 to ν=1,2,3,4 of H35Cl and H37Cl is studied in Xe, Kr and Ar matrices with high spectral resolution. A consistent set of rotational constants Bv for the vibrational levels ν=0 to 4 is derived. B0 decreases with the tightness of the cage from 9.78 cm-1 in Xe to 8.83 cm-1 in Ar for H35Cl (gas phase 10.44 cm-1). The values for B0 to B4 decrease linearly with v due to the vibration-rotation-coupling constant α which increases from 0.37 cm-1 in Xe to 0.479 cm-1 in Ar (gas 0.303) according to the cage tightness. The splitting of the R(1) transition which originates from the hindering of rotation is analyzed in Xe using the T2g-T1u and T2g-Eg transition energies. A comparison with force field calculations yields a dominant contribution of the 6th spherical harmonic YA1g6 of the octahedral matrix potential. The modulation of the potential takes a value of K6/B=17 which corresponds to a barrier for the rotation of 160 cm-1. The splitting increases with the vibrational level v which can be interpreted as a weak admixture of the YA1g4 spherical harmonic. A large isotope effect and a reduction of the T1u-A1g transition energy (R(0)-transition) beyond the crystal field value are attributed to an excentric rotation with a displacement of the center of mass of the order of 0.05 Å. The vibrational energies ωe show an opposite trend with matrix atom size and decrease with polarizability from 2970 cm-1 in Ar to 2945.4 cm-1 in Xe (gas 2989.9 cm-1) while the anharmonicity ωeχe of the free molecule lies close to the Kr value and thus between that of Ar and Xe

Inversion of droplet aerosol analyzer data for long-term aerosol–cloud interaction measurements

The droplet aerosol analyzer (DAA) was developed to study the influence of aerosol properties on clouds. It measures the ambient particle size of individual droplets and interstitial particles, the size of the dry (residual) particles after the evaporation of water vapor and the number concentration of the dry (residual) particles. A method was developed for the evaluation of DAA data to obtain the three-parameter data set: ambient particle diameter, dry (residual) particle diameter and number concentration. First results from in-cloud measurements performed on the summit of Mt. Brocken in Germany are presented. Various aspects of the cloud–aerosol data set are presented, such as the number concentration of interstitial particles and cloud droplets, the dry residue particle size distribution, droplet size distributions, scavenging ratios due to cloud droplet formation and size-dependent solute concentrations. This data set makes it possible to study clouds and the influence of the aerosol population on clouds

Cloud condensation nuclei in polluted air and biomass burning smoke near the mega-city Guangzhou, China – Part 2: Size-resolved aerosol chemical composition, diurnal cycles, and externally mixed weakly CCN-active soot particles

Size-resolved chemical composition, mixing state, and cloud condensation nucleus (CCN) activity of aerosol particles in polluted mega-city air and biomass burning smoke were measured during the PRIDE-PRD2006 campaign near Guangzhou, China, using an aerosol mass spectrometer (AMS), a volatility tandem differential mobility analyzer (VTDMA), and a continuous-flow CCN counter (DMT-CCNC). The size-dependence and temporal variations of the effective average hygroscopicity parameter for CCN-active particles (κa) could be parameterized as a function of organic and inorganic mass fractions (forg, finorg) determined by the AMS: κa,p=κorg·forg + κinorg·finorg. The characteristic κ values of organic and inorganic components were similar to those observed in other continental regions of the world: κorg≈0.1 and κinorg≈0.6. The campaign average κa values increased with particle size from ~0.25 at ~50 nm to ~0.4 at ~200 nm, while forg decreased with particle size. At ~50 nm, forg was on average 60% and increased to almost 100% during a biomass burning event. The VTDMA results and complementary aerosol optical data suggest that the large fractions of CCN-inactive particles observed at low supersaturations (up to 60% at S≤0.27%) were externally mixed weakly CCN-active soot particles with low volatility (diameter reduction <5% at 300 °C) and effective hygroscopicity parameters around κLV≈0.01. A proxy for the effective average hygroscopicity of the total ensemble of CCN-active particles including weakly CCN-active particles (κt) could be parameterized as a function of κa,p and the number fraction of low volatility particles determined by VTDMA (φLV): κt,p=κa,p−φLV·(κa,p−κLV). Based on κ values derived from AMS and VTDMA data, the observed CCN number concentrations (NCCN,S≈102–104 cm−3 at S = 0.068–0.47%) could be efficiently predicted from the measured particle number size distribution. The mean relative deviations between observed and predicted CCN concentrations were ~10% when using κt,p, and they increased to ~20% when using only κa,p. The mean relative deviations were not higher (~20%) when using an approximate continental average value of κ≈0.3, although the constant κ value cannot account for the observed temporal variations in particle composition and mixing state (diurnal cycles and biomass burning events). Overall, the results confirm that on a global and climate modeling scale an average value of κ≈0.3 can be used for approximate predictions of CCN number concentrations in continental boundary layer air when aerosol size distribution data are available without information about chemical composition. Bulk or size-resolved data on aerosol chemical composition enable improved CCN predictions resolving regional and temporal variations, but the composition data need to be highly accurate and complemented by information about particle mixing state to achieve high precision (relative deviations <20%)

Size-resolved measurement of the mixing state of soot in the megacity Beijing, China: diurnal cycle, aging and parameterization

Soot particles are the most efficient light absorbing aerosol species in the atmosphere, playing an important role as a driver of global warming. Their climate effects strongly depend on their mixing state, which significantly changes their light absorbing capability and cloud condensation nuclei (CCN) activity. Therefore, knowledge about the mixing state of soot and its aging mechanism becomes an important topic in the atmospheric sciences. &lt;br&gt;&lt;br&gt; The size-resolved (30–320 nm diameter) mixing state of soot particles in polluted megacity air was measured at a suburban site (Yufa) during the CAREBeijing 2006 campaign in Beijing, using a volatility tandem differential mobility analyzer (VTDMA). Particles in this size range with non-volatile residuals at 300 &amp;deg;C were considered to be soot particles. On average, the number fraction of internally mixed soot in total soot particles (&lt;i&gt;F&lt;/i&gt;&lt;sub&gt;in&lt;/sub&gt;), decreased from 0.80 to 0.57 when initial &lt;i&gt;D&lt;/i&gt;&lt;sub&gt;p&lt;/sub&gt; increased from 30 to 320 nm. Further analysis reveals that: (1) &lt;i&gt;F&lt;/i&gt;&lt;sub&gt;in&lt;/sub&gt; was well correlated with the aerosol hygroscopic mixing state measured by a CCN counter. More externally mixed soot particles were observed when particles showed more heterogeneous features with regard to hygroscopicity. (2) &lt;i&gt;F&lt;/i&gt;&lt;sub&gt;in&lt;/sub&gt; had pronounced diurnal cycles. For particles in the accumulation mode (&lt;i&gt;D&lt;/i&gt;&lt;sub&gt;p&lt;/sub&gt; at 100–320 nm), largest &lt;i&gt;F&lt;/i&gt;&lt;sub&gt;in&lt;/sub&gt; were observed at noon time, with "apparent" turnover rates (&lt;i&gt;k&lt;/i&gt;&lt;sub&gt;ex &amp;rarr; in&lt;/sub&gt;) up to 7.8% h&lt;sup&gt;−1&lt;/sup&gt;. (3) &lt;i&gt;F&lt;/i&gt;&lt;sub&gt;in&lt;/sub&gt; was subject to competing effects of both aging and emissions. While aging increases &lt;i&gt;F&lt;/i&gt;&lt;sub&gt;in&lt;/sub&gt; by converting externally mixed soot particles into internally mixed ones, emissions tend to reduce &lt;i&gt;F&lt;/i&gt;&lt;sub&gt;in&lt;/sub&gt; by emitting more fresh and externally mixed soot particles. Similar competing effects were also found with air mass age indicators. (4) Under the estimated emission intensities, actual turnover rates of soot (&lt;i&gt;k&lt;/i&gt;&lt;sub&gt;ex &amp;rarr; in&lt;/sub&gt;) up to 20% h&lt;sup&gt;−1&lt;/sup&gt; were derived, which showed a pronounced diurnal cycle peaking around noon time. This result confirms that (soot) particles are undergoing fast aging/coating with the existing high levels of condensable vapors in the megacity Beijing. (5) Diurnal cycles of &lt;i&gt;F&lt;/i&gt;&lt;sub&gt;in&lt;/sub&gt; were different between Aitken and accumulation mode particles, which could be explained by the faster growth of smaller Aitken mode particles into larger size bins. &lt;br&gt;&lt;br&gt; To improve the &lt;i&gt;F&lt;/i&gt;&lt;sub&gt;in&lt;/sub&gt; prediction in regional/global models, we suggest parameterizing &lt;i&gt;F&lt;/i&gt;&lt;sub&gt;in&lt;/sub&gt; by an air mass aging indicator, i.e., &lt;i&gt;F&lt;/i&gt;&lt;sub&gt;in&lt;/sub&gt; = &lt;i&gt;a&lt;/i&gt; + &lt;i&gt;bx&lt;/i&gt;, where &lt;i&gt;a&lt;/i&gt; and &lt;i&gt;b&lt;/i&gt; are empirical coefficients determined from observations, and &lt;i&gt;x&lt;/i&gt; is the value of an air mass age indicator. At the Yufa site in the North China Plain, fitted coefficients (&lt;i&gt;a&lt;/i&gt;, &lt;i&gt;b&lt;/i&gt;) were determined as (0.57, 0.21), (0.47, 0.21), and (0.52, 0.0088) for &lt;i&gt;x&lt;/i&gt; (indicators) as [NO&lt;sub&gt;z&lt;/sub&gt;]/[NO&lt;sub&gt;y&lt;/sub&gt;], [E]/[X] ([ethylbenzene]/[m,p-xylene]) and ([IM] + [OM])/[EC] ([inorganic + organic matter]/[elemental carbon]), respectively. Such a parameterization consumes little additional computing time, but yields a more realistic description of &lt;i&gt;F&lt;/i&gt;&lt;sub&gt;in&lt;/sub&gt; compared with the simple treatment of soot mixing state in regional/global models