Diana Rodríguez

Low-pressure study of the reactions of Cl atoms with acrylic acid and allyl alcohol

Abstract The kinetics of the gas-phase reactions of chlorine atoms with acrylic acid (1) and allyl alcohol (2) have been investigated at low pressure (0.5– 3 Torr ) in the temperature range 260– 333 K using the mass spectrometric discharge-flow method. The temperature dependence of the reaction rate constants at 1 Torr have been determined under pseudo-first-order conditions in excess of organic compound over Cl atoms, giving: k 1 =(1.3±0.9)×10 −13 exp (1600±200)/ T cm 3 molecule −1 s −1 and k 2 =(1.1±0.5)×10 −11 exp (470±135)/ T cm 3 molecule −1 s −1 . At room temperature and 1 Torr , products of abstraction and addition channels have been detected for reactions (1) and (2). By mass spectrometric quantification, the branching ratio for channels giving HCl, was determined as 0.13±0.05 for reaction (1) and 0.57±0.06 for reaction (2). The results obtained are related to previous studies, and the atmospheric implications are also discussed in relation to the homogeneous sinks of acrylic acid and allyl alcohol.

Kinetic and mechanistic study of Cl reactions with aliphatic thiols. Temperature dependence

The mass spectrometric discharge-flow method has been used to investigate the kinetics, mechanism and products of Cl atoms reactions with different aliphatic thiols, CH3CH2SH (1), CH3CH2CH2SH (2) and (CH3)2CHSH (3) over the temperature range 260–333 K and at low total pressure of 1 Torr. The reactions have been studied under pseudo-first order kinetic conditions in excess of organic compounds over Cl atoms. Arrhenius expressions which describe our results are (units in 10−10 cm3 molecule−1 s−1): k(1)=(3.1±0.7)exp[(−136±70)/T], k(2)=(5.4±2.1)exp[(−227±131)/T] and k(3)=(4.1±1.0)exp[(−125±21)/T]. The mechanisms of the reactions are discussed. The results obtained are related to previous studies of aliphatic thiols with other tropospheric agents and the atmospheric implications are also discussed in relation to the homogeneous sinks of these organic compounds.

Partitioning, sources and variability of regional and local oxidant (OX = O 3 + NO 2 ) in a coastal rural area in the southwest of Iberian Peninsula

The purpose of this work is to investigate the behaviour and variability of oxidant levels (OX = NO2 + O3), for the first time, in a rural coastal area in the southwest of the Iberian Peninsula, affected by several air masses types. Detailed database (built-up over the years 2008 to 2011, and containing around 500,000 data) from the Atmospheric Sounding Station “El Arenosillo” was used. The observed daily cycles of NOx and OX were influenced by air masses coming from industrial and urban area. It can be seen that the concentration of OX is made up of a NOx-independent ‘regional’ contribution (i.e. the O3 background), and a linearly NOx-dependent ‘local’ contribution from primary emissions, such as traffic. The local emission is very low in this area. Also, the regional contribution is similar to unpolluted sites and presents seasonal variation, being higher in May. However, our measurements showed that the proportion of OX in the form of NO2 increases with the increase in NOx concentration during the day. The higher proportion of NO2 observed at night must be due to the conversion of NO to NO2 by the NO + O3 reaction. With regards to the source of the local NOx-dependent contribution, it may be attributed to industrial emission, or the termolecular reaction 2NO + O2 = 2NO2, at high-NOx levels and stagnant air during several days. Finally, we estimated the photolysis rate of NO2, JNO2, an important key atmospheric reaction coupled with ozone. We also present surface plots of annual variation of the daily mean NOx and OX levels, which indicate that oxidants come from transport processes instead of local emissions associated as local photochemistry.

Atmospheric Chemistry and Environmental Assessment of Inhalational Fluroxene

Smog chamber/gas chromatography techniques are used to investigate the atmospheric degradation of fluroxene, an anesthetic, through oxidation with OH and Cl radicals at 298 K and under atmospheric pressure of N2 or air. The measured rate constants (k) are: k(fluroxene+OH(.) )=(2.96±0.61)×10(-11) and k(fluroxene+Cl(.) )=(1.62±0.19)×10(-10) cm(3)  molecule(-1)  s(-1) . The only product detected after the oxidation of fluroxene with OH radicals is 2,2,2-trifluoroethyl formate (79 % and 83 % molar yield in the absence and presence of NOx, respectively). However, after oxidation with Cl radicals, the detected products are 2,2,2-trifluoroethyl formate (78 %), 2,2,2-trifluoroethyl-1-chloroacetate (5 %), and chloroacetaldehyde (4 %), in the absence of NOx, and 2,2,2-trifluoroethyl formate (93 %), 2,2,2-trifluoroethyl-1-chloroacetate (6 %), and chloroacetaldehyde (5 %), in the presence of NOx. The results indicate that, both in the absence and presence of NOx, the main fate of fluroxene is the addition of the oxidant to the double bond and, once the alkoxy radical is formed, the main decomposition pathway is by means of degradation. Moreover, it is expected that 2,2,2-trifluoroethyl formate is the only oxidation product able to actively contribute to climate change. To successfully assess the contribution of fluroxene to global warming, we measure the infrared spectra of fluroxene and 2,2,2-trifluoroethyl formate, and calculate the radiative efficiencies (REs) to be 0.27 and 0.28 W m(-2)  ppbv(-1) , respectively. In addition, the cumulative effect owing to the formation of 2,2,2-trifluoroethyl formate is investigated, and the direct, indirect, and net global-warming potentials are calculated by using the REs and lifetimes of fluroxene and 2,2,2-trifluoroethyl formate.

Kinetic and products of the BrO + CH3SH reaction: temperature and pressure dependence

Abstract The kinetics and the mechanism of the reaction BrO + CH 3 SH → P (1) have been studied using the mass spectrometric discharge-flow method over the temperature range 259–333 K and at low total pressure between 0.5 and 3 Torr. The temperature dependence of the reaction rate constant has been determined under pseudo-first-order kinetic conditions in excess of CH3SH over BrO radicals, k=(2.2±1.9)×10 −15 exp [(827±255)/T] cm 3 molecule −1 s −1 . The reaction has been found to be pressure dependent in the pressure range studied. The experimental evidences suggest an addition mechanism. The studied reaction constitutes an efficient atmospheric sink for the reduced sulfur compound, CH3SH.

Gas-phase reaction of chlorine atoms with acrylonitrile. Temperature and pressure dependence

Abstract The kinetics and mechanism of the reaction of chlorine atoms with acrylonitrile (CH2CHCN) (1) have been studied by the mass spectrometric discharge-flow method in the temperature range (260–333) K and at a total pressure between 0.5 and 3 Torr. At 1.0 Torr the following Arrhenius expression for the rate constant has been obtained from the kinetics of Cl radical consumption in excess of CH2CHCN: k1=(5.2±1.6)×10−14exp[(1400±100/T)] cm3 molecule−1 s−1. The reaction is found to proceed through an addition mechanism involving a relatively stable adduct. The atmospheric significance of the present results for acrylonitrile is also discussed.

The environmental impact of unsaturated fluoroesters: atmospheric chemistry towards OH radicals and Cl atoms, radiative behavior and cumulative ozone creation

Smog chamber/GC techniques were used to investigate the atmospheric degradation of two hydrofluoroesters (allyl trifluoroacetate (CF3C(O)OCH2CHCH2) and vinyl trifluoroacetate (CF3C(O)OCHCH2)) by oxidation with OH radicals and Cl atoms at 298 K and an atmospheric pressure of N2 or air. The measured rate coefficients were (in units of cm3 per molecule per s): kallyl trifluoroacetate+OH = (9.27 ± 3.81) × 10−12; kvinyl trifluoroacetate+OH = (8.07 ± 1.92) × 10−12; kallyl trifluoroacetate+Cl = (1.75 ± 0.21) × 10−10 and kvinyl trifluoroacetate+Cl = (2.08 ± 0.16) × 10−10. In the OH-initiated oxidation of allyl trifluoroacetate, the identified product can arise from OH addition to both carbons in the double bond and the later decomposition of the alkoxy radical formed. However, in the reactions of both fluoroesters with Cl atoms, the main product detected arises from Cl addition to the terminal carbon atom and the subsequent reaction of the chloroalkoxy radical formed with O2. Infrared spectra of the studied esters were collected and their contribution to global warming was assessed by calculating their radiative efficiencies. Combining these results with the kinetic data we found that their global warming potentials are negligible. Finally, the photochemical ozone creation potentials were calculated, obtaining values lower than those of non-fluorinated unsaturated hydrocarbons.

Kinetic and mechanistic study of the atmospheric reaction of MBO331 with Cl atoms

The present work deals with the reaction of 3-methyl-3-buten-1-ol (MBO331) with Cl atoms, which has been investigated by gas chromatography with flame ionization detection (GC-FID) at atmospheric pressure in N2 or air, using the relative rate technique. The rate constant reaction at 298 ± 1 K was found to be (5.01 ± 0.70) × 10−10 cm3 molecule−1 s−1, using cyclohexane, octane and 1-butene as a reference compounds. The temperature dependence for the reaction was studied within the 298−333 K range. Additionally, a product identification under atmospheric conditions has been performed for the first time by GC-MS, with 3-methyl-3-butenal, methacrolein and chloroacetone being observed as degradation products. A theoretical study on the reaction at the QCISD(T)/6-311G**//MP2/6-311G** level was also carried out to obtain more information on the mechanism. From the theoretical study it can be predicted that Cl addition to the double bond proceeds through lower energy barriers than H-abstraction pathways and therefore is energetically favoured. Finally, atmospheric implications of the results obtained are discussed.