Near-UV Extinction of Submicron Thickness Ice Films in the 290-350 nm Range at 258 K.

Ice clouds affect the energy balance of the atmosphere through absorption, reflection, and scattering of solar radiation. We have developed a new experimental technique to simultaneously measure thin ice film extinction and its thickness (about 0.06-0.21 µm) by combining Brewster angle cavity ring-down spectroscopy and quartz crystal microbalance. The ice film serves as a proxy for ice clouds. Thin ice films were formed by water vapor deposition on a silica surface at 258 K. The average extinction cross sections of ice films were determined to be about 6.6 × 10-23, 8.1 × 10-23, 5.3 × 10-23, 5.6 × 10-23, 5.2 × 10-23, 5.1 × 10-23, and 3.9 × 10-23 cm2/molecule at wavelengths of 290, 300, 310, 320, 330, 340, and 350 nm at 258 K, respectively. Atmospheric implications of the results are discussed.

Gamma Radioactivity Detection Limits and Associated Radionuclide Intakes Study in Artificial Human Urine Using Sodium-iodide and High-purity Germanium Detectors.

ABSTRACT: The performance of several gamma detectors was investigated for emergency urine bioassay screening of two radionuclides of concern: 131 I and 137 Cs. Unspiked artificial urine samples were measured for 10 min each on four different gamma detectors: 80% relative efficiency high-purity Ge detector in standard shielding, 102% low-background high-purity Ge detector equipped with top muon shield, 78% high-purity Ge well detector in standard shielding, and 4″ × 4″ NaI well detector in standard shielding. The measured gamma spectra were analyzed in two ways: (1) for the 364-keV peak region of 131 I and 662-keV peak region of 137 Cs and (2) for the total counts in the full energy spectrum (50-2,048 keV). The results were analyzed using the principles of signal detection theory according to the Currie's formalism extended by a complete uncertainty propagation. This enabled calculation of the detection capability in terms of detection limit (Bq L -1 ) of urine, the latter referred to as minimum detectable activity. The NaI well detector had the lowest minimum detectable activities for total spectra, whereas the high-purity Ge well detector had the lowest peak minimum detectable activity values. Minimum detectable inhalation and ingestion intakes from urine bioassay were calculated from the minimum detectable activity values for urine collection 1 d, 1 wk, and 1 mo past the initial intake. The calculated intakes were compared with annual limits on intake. The results are interpreted with respect to a large-scale radiological emergency response.

Comment on "Investigations on HONO formation from photolysis of adsorbed HNO3 on quartz glass surfaces" by S. Laufs and J. Kleffmann, Phys. Chem. Chem. Phys., 2016, 18, 9616.

Laufs and Kleffmann observed that HNO3 surface photolysis rates resemble that of HNO3 in the gas phase after depositing HNO3 in air at ∼50% relative humidity onto quartz glass surfaces. They questioned the dry HNO3 coverage (1.1 × 1014 molecules per cm2 after depositing ∼15 mTorr HNO3 on silica at 0% humidity) used to derive our previously published HNO3 near-UV surface absorption cross sections. We directly determined the HNO3 coverage on a quartz surface using a quartz crystal microbalance (QCM). A similar HNO3 monolayer coverage obtained by QCM confirms that our estimated HNO3 coverage is reasonable. We also obtained an NO2 quantum yield from the 308 nm HNO3 photolysis on fused silica. In this Comment, we provide an explanation of the variance in HNO3 surface photolysis rates by clarifying the effects arising from important differences in the HNO3 coverage on quartz/silica in the presence of humidity versus those in the absence of humidity.

Photolysis of nitric acid at 308 nm in the absence and in the presence of water vapor.

We have re-examined the NOx channels from the 308 nm gas-phase photolysis of nitric acid (HNO3) by using excimer laser photolysis combined with cavity ring-down spectroscopy. The photolysis products were monitored in the 552-560 and 640-648 nm regions. Direct comparison of the photolysis product spectrum in the 640-648 nm region with literature vibronic band origins and line intensities in electronically excited NO2 (NO2*) suggests that NO2* is not formed from HNO3 photolysis at 308 nm. A comparison of the photolysis product spectrum in the 552-560 nm region with a standard NO2 spectrum indicates that ground-state NO2 is a photolysis product. We have determined the NO2 quantum yield from the 308 nm HNO3 photolysis. We also investigated HNO3 photolysis in the presence of water vapor. For equilibrated HNO3/H2O mixtures, we did not observe significant variation of product absorption around 552 nm with delay times between the firing of the photolysis and the probe lasers. Transient product absorption measurements at 342.0 and 343.5 nm (respective wavelengths where the peak and valley of HONO absorptions are located) are consistent with ground-state NO2 being the predominant NOx product from the 308 nm photolysis of a HNO3/H2O mixture. Atmospheric implications are also discussed.

308 nm photolysis of nitric acid in the gas phase, on aluminum surfaces, and on ice films.

We have studied the photolysis of nitric acid (HNO(3)) in the gas phase at 253 and 295 K, on aluminum surfaces at 253 and 295 K, and on ice films at 253 K, by using 308 nm excimer laser photolysis combined with cavity ring-down spectroscopy. We monitored both the ground-state NO(2) and the electronically excited NO(2), NO(2)*, produced from the HNO(3) photolysis. NO(2)* + OH is a predominant photolysis pathway (if not the only photolysis pathway) from the gas-phase photolysis of HNO(3) at 308 nm. The NO(2)* quantum yields from the HNO(3) photolysis on aluminum surfaces are 0.80 +/- 0.15 at 295 K and 0.92 +/- 0.26 at 253 K, where errors quoted represent 2sigma measurement uncertainty. The corresponding NO(2)* quantum yield from the HNO(3) photolysis on ice films is 0.60 +/- 0.34 at 253 K. The 308 nm absorption cross sections of HNO(3) on Al surfaces and on ice films have been directly measured. Absorption cross sections of HNO(3) on Al surface at 308 nm are (4.19 +/- 0.17) x 10(-18) and (4.23 +/- 0.45) x 10(-18) cm(2)/molecule at 253 and at 295 K, whereas the corresponding absorption cross section of HNO(3) on ice films is (1.21 +/- 0.31) x 10(-18) cm(2)/molecule at 253 K (errors quoted represent 2sigma measurement uncertainty). Atmospheric implications of the results are discussed.

Interactions of oxalic acid and ice on Cu surface.

The interactions between oxalic acid (C 2H 2O 4) and H 2O on a polycrystalline Cu surface have been investigated by reflection-absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) methods. The desorption of H 2O and C 2H 2O 4 was studied; we found that the ice desorption temperature increases with the ice-film thickness. Desorption of the C 2H 2O 4 layer involves a structural modification and sublimation. The H 2O/C 2H 2O 4 and C 2H 2O 4/H 2O interfaces and the codeposited C 2H 2O 4+H 2O were prepared on the Cu surface by varying deposition sequences of gaseous C 2H 2O 4 and H 2O at 155 K. We found that the interaction between ice and C 2H 2O 4 does not lead to the H 2O-induced deprotonation of C 2H 2O 4 in a temperature range 155-283 K. However, H-bonding interactions between H 2O and C 2H 2O 4 can lead to the formation of a metastable oxalic acid-ice complex in the C 2H 2O 4/H 2O and C 2H 2O 4+H 2O systems during the TPD process. Desorption of H 2O from the C 2H 2O 4/H 2O/Cu system is suggested to involve the diffusion of H 2O through the top C 2H 2O 4 layer. H 2O desorption is followed by a rearrangement of C 2H 2O 4 to form a C 2H 2O 4 adlayer on Cu in the C 2H 2O 4+H 2O system. These experimental findings suggest that C 2H 2O 4 is not ionized on snow and ice in the polar boundary layer and at upper tropospheric temperatures ( approximately 240 K).

Uptake of NH(3) and NH(3) + HOBr reaction on ice surfaces at 190 K.

The uptake of NH3 and the heterogeneous reaction of NH3 + HOBr --> products on ice surfaces at 190 K have been investigated in a flow reactor coupled with a differentially pumped quadrupole mass spectrometer. The uptake coefficient gammat for NH3 was determined to be (3.8 +/- 1.4) x 10(-4) on ice films at 189.8 K, for a partial pressure of NH3 in the range of 7.0 x 10(-7) to 3.8 x 10(-6) torr. The amount of NH3 uptake on the ice film was determined to be >2.9 x 10(15) molecules/cm(2), based on the total ice surface area at 189.2 K. The heterogeneous reaction of NH3 + HOBr on ice surfaces has been studied at 190 K. The reaction probability gammat was determined to be (5.3 +/- 2.2) x 10(-4) and was found to vary insignificantly as HOBr surface coverage changes from 2.1 x 10(13) to 2.1 x 10(14) molecules/cm(2). A reaction pathway is proposed on the basis of experimental observations.

Heterogeneous reactions of SO2 with HOCl and HOBr on ice surfaces.

The heterogeneous reactions of SO2 + HOX (X = Cl or Br) --> products on ice surfaces at low temperature have been investigated in a flow reactor coupled with a differentially pumped quadrupole mass spectrometer. Pseudo-first-order loss of SO2 over the ice surfaces has been measured under the conditions of concurrent HOX flow. The initial uptake coefficient of SO2 reaction with HOX has been determined as a function of HOX surface coverage, theta(HOX), on the ice. The initial uptake coefficients increase as the HOX coverage increases. The uptake coefficient can be expressed as gamma(t) = k(h)theta(HOX), where k(h) is an overall rate constant of SO2 + HOCl, which was determined to be (2.3 +/- 0.6) x 10(-19) and (1.7 +/- 0.5) x 10(-19) molecules(-1) x cm2 at 190 and 210 K, and k(h) of SO2 + HOBr is (6.1 +/- 2.0) x 10(-18) molecules(-1) x cm2 at 190 K. theta( HOX) is in the range 8.1 x 10(13)-9.1 x 10(14) molecules x cm(-2). The kinetic results of the heterogeneous reaction of SO2 + HOX on ice surface are interpreted using the Eley-Rideal mechanism. The activation energy of the heterogeneous reaction of SO2 with HOCl on ice surface was determined to be about -37 +/- 10 kJ/mol in the 190-238 K range.

Uptake of SO2 on HOBr-ice surfaces.

The uptake of SO2 on HOBr-treated ice surfaces has been studied using a flow reactor coupled with a differentially pumped quadrupole mass spectrometer at 190-240 K. The initial uptake coefficient was determined as a function of HOBr surface coverage, theta(HOBr), on the ice. The uptake coefficients increase as the HOBr coverage increases. The uptake coefficient can be expressed as gamma(t) = k(h)theta(HOBr), where k(h) = 1.5 x 10(-19) molecules(-1) cm(-2) at 191 K and k(h) = 6.4 x 10(-21) molecules(-1) cm(-2) at 210 K and theta(HOBr) is in the range of 8 x 10(13) to 1.2 x 10(15) molecules cm(-2). The effects of temperature and film thickness on the uptake coefficients of SO2 by the HOBr-treated ice films were also studied. The activation energy E(a) of SO(2) on HOBr-ice surfaces is approximately -81 +/- 8 kJ/mol in the 190-215 K range. Kinetic results were interpreted in terms of the Eley-Rideal mechanism. This study suggests that the uptake of SO2 on ice/snow surfaces is enhanced by the presence of HOBr near the ice surface. The implication for atmospheric chemistry is that HOBr-ice surfaces may not provide a significant pathway to oxide S(IV) in the boundary layer due to both lower uptake coefficient and smaller HOBr surface coverage at T > 220 K.

Heterogeneous reactions of HX + HONO and I2 on ice surfaces: kinetics and linear correlations.

Reaction probabilities of gaseous nitrous acid, HONO, with HCl, HBr, and HI treated ice surfaces have been investigated in a fast flow-tube reactor coupled with a differentially pumped quadrupole mass spectrometer (QMS) at 191 K. The reaction probability increases with the HX surface coverage, and the rate is the highest for the HONO reaction on the HI-treated ice surface. Relative rate constants are correlated to the nucleophilic parameter, according to the linear free-energy relationship for this series of heterogeneous reactions on ice surfaces. The correlation was also extended to HOCl + HX(ad) reactions on the ice surface, and it can be used to treat other heterogeneous atmospheric and catalytic reactions. The reaction products ClNO and BrNO were determined by the QMS. INO was found to rapidly convert to I2 on surfaces, and I2 was observed from the reaction of HONO + HI. The uptake coefficient of I2 on the HI-treated ice surface is higher than that for I2 on the water-ice surface.