On the Control of Plasma Chemistry and Silicon Etching Kinetics in Ternary HBr + Cl2 + O2 Gas System: Effects of HBr/O2 and Cl2/O2 Mixing Ratios

The influences of both HBr/O2 (at constant Cl2 fraction) and Cl2/O2 (at constant HBr fraction) ratios in HBr + Cl 2 + O2 gas mixture on bulk plasma characteristics, active species densities and etching kinetics of silicon were studied. The results indicated that an increase in O2 content in a feed gas at constant Cl2 fraction in a processing gas (1) produces the stronger impact on plasma chemistry by the influence on the kinetics of electron-impact and atom-molecular reaction; and (2) provides the wider adjustments for both halogen atom flux and ion flux with the opposite tendencies with those for variable Cl2/O2 mixing ratio. The experiments demonstrated that the transition toward more oxygenated plasmas in both cases lowers the Si etching rate as well as result is decreasing effective reaction probability and etching yield. These effects may be associated with decreasing amount of adsorption sites for Cl/Br atoms as well as increasing sputtering (ion-stimulated desorption) threshold for reaction products due to the formation of the low-volatile silicon oxy-chlorides and-bromides in heterogeneous SiClx + O/OH and SiBrx + O/OH reactions.

The global impact of bacterial processes on carbon mass

Many recent studies have identified biological material as a major fraction of ambient aerosol loading. A small fraction of these bioaerosols consist of bacteria that have attracted a lot of attention due to their role in cloud formation and adverse health effects. Current atmospheric models consider bacteria as inert quantities and neglect cell growth and multiplication. We provide here a framework to estimate the production of secondary biological aerosol (SBA) mass in clouds by microbial cell growth and multiplication. The best estimate of SBA formation rates of 3.7 Tg yr−1 is comparable to previous model estimates of the primary emission of bacteria into the atmosphere, and thus this might represent a previously unrecognized source of biological aerosol material. We discuss in detail the large uncertainties associated with our estimates based on the rather sparse available data on bacteria abundance, growth conditions, and properties. Additionally, the loss of water-soluble organic carbon (WSOC) due to microbial processes in cloud droplets has been suggested to compete under some conditions with WSOC loss by chemical (OH) reactions. Our estimates suggest that microbial and chemical processes might lead to a global loss of WSOC of 8–11 and 8–20 Tg yr−1, respectively. While this estimate is very approximate, the analysis of the uncertainties and ranges of all parameters suggests that high concentrations of metabolically active bacteria in clouds might represent an efficient sink for organics. Our estimates also highlight the urgent need for more data concerning microbial concentrations, fluxes, and activity in the atmosphere to evaluate the role of bacterial processes as net aerosol sinks or sources on various spatial and temporal scales.

Atmospheric fate of a series of saturated alcohols: kinetic and mechanistic study

The atmospheric fate of a series of saturated alcohols (SAs) was evaluated through kinetic and reaction product studies with the main atmospheric oxidants. These SAs are alcohols that could be used as fuel additives. Rate coefficients (in cm3 molecule−1 s−1) measured at ∼298 K and atmospheric pressure (720±20 Torr) were as follows: k1 ((E)-4-methylcyclohexanol + Cl) = (3.70±0.16) ×10-10, k2 ((E)-4-methylcyclohexanol + OH) = (1.87±0.14) ×10-11, k3 ((E)-4-methylcyclohexanol + NO3) = (2.69±0.37) ×10-15, k4 (3,3-dimethyl-1-butanol + Cl) = (2.69±0.16) ×10-10, k5 (3,3-dimethyl-1-butanol + OH) = (5.33±0.16) ×10-12, k6 (3,3-dimethyl-2-butanol + Cl) = (1.21±0.07) ×10-10, and k7 (3,3-dimethyl-2-butanol + OH) = (10.50±0.25) ×10-12. The main products detected in the reaction of SAs with Cl atoms (in the absence/presence of NOx), OH radicals, and NO3 radicals were (E)-4-methylcyclohexanone for the reactions of (E)-4-methylcyclohexanol, 3,3-dimethylbutanal for the reactions of 3,3-dimethyl-1-butanol, and 3,3-dimethyl-2-butanone for the reactions of 3,3-dimethyl-2-butanol. Other products such as formaldehyde, 2,2-dimethylpropanal, and acetone have also been identified in the reactions of Cl atoms and OH radicals with 3,3-dimethyl-1-butanol and 3,3-dimethyl-2-butanol. In addition, the molar yields of the reaction products were estimated. The products detected indicate a hydrogen atom abstraction mechanism at different sites on the carbon chain of alcohol in the case of Cl reactions and a predominant site in the case of OH and NO3 reactions, confirming the predictions of structure–activity relationship (SAR) methods. Tropospheric lifetimes (τ) of these SAs have been calculated using the experimental rate coefficients. Lifetimes are in the range of 0.6–2 d for OH reactions, 7–13 d for NO3 radical reactions, and 1–3 months for Cl atoms. In coastal areas, the lifetime due to the reaction with Cl decreases to hours. The calculated global tropospheric lifetimes, and the polyfunctional compounds detected as reaction products in this work, imply that SAs could contribute to the formation of ozone and nitrated compounds at local, regional, and even global scales. Therefore, the use of saturated alcohols as additives in diesel blends should be considered with caution.

Atmospheric fate of a series of Methyl Saturated Alcohols (MSA): Kinetic and Mechanistic study

The atmospheric fate of a series of Methyl Saturated Alcohols (MSA) has been evaluated through the kinetic and reaction product studies with the main atmospheric oxidants. Rate coefficients (in cm3 molecule−1 s−1 unit) measured at ~ 298 K and atmospheric pressure (~ 740 Torr) were as follows: (3.71 ± 0.53) × 10−10, (1.91 ± 0.65) × 10−11 and (2.92 ± 1.38) × 10−15 for reaction of E-4-methyl-cyclohexanol with Cl, OH and NO3, respectively. (2.70 ± 0.55) × 10−10 and (5.57 ± 0.66) × 10−12 for reaction of 3,3-dimethyl-1-butanol with Cl and OH radical respectively and (1.21 ± 0.37) × 10−10 and (10.51 ± 0.81) × 10−12 for reaction of 3,3-dimethyl-2-butanol with Cl and OH radical respectively. The main detected products were 4-methylcyclohexanone, 3,3-dimethylbutanal and 3,3-dimethyl-2-butanone for the reactions of E-4-methyl-cyclohexanol, 3,3-dimethyl-1-butanol and 3,3-dimethyl-2-butanol respectively with the three oxidants. A tentative estimation of yields have been done obtaining the following ranges (25–60) % for 4-methylcyclohexanone, (40–60) % for 3,3-dimethylbutanal and (40–80) % for 3,3-dimethyl-2-butanone. Other products as HCOH, 2,2-dimethylpropanal and acetone have been identified in the reaction of 3,3-dimethyl-1-butanol and 3,3-dimethyl-2-butanol. The yields of these products indicate a hydrogen abstraction mechanism at different sites of the alkyl chain in the case of Cl reaction and a predominant site in the case of OH and NO3 reactions, supported by SAR methods prediction. Tropospheric lifetimes (τ) of these MSA have been calculated using the experimental rate coefficients. Lifetimes are in the range of 0.6–2 days for OH reactions, 8–13 days for NO3 radical reactions and 1–3 months for Cl atoms. In coastal areas the lifetime due to the reaction with Cl decreases to hours. The global tropospheric lifetimes calculated, and the polyfunctional compounds detected as reaction products in this work, imply that the Methyl Saturated Alcohols could contribute to ozone and nitrated compound formation at local, but also regional and even to global scale. Therefore, the use of large saturated alcohols as additives in biofuels must be taken with caution.

The global impact of bacterial processes on carbon mass

Many recent studies have identified biological material as a major fraction of ambient aerosol loading. A small fraction of these bioaerosols consist of bacteria that have attracted a lot of attention due to their role in cloud formation and adverse health effects. Current atmospheric models consider bacteria as inert quantities and neglect cell growth and multiplication. We provide here a framework to estimate the production of secondary biological aerosol (SBA) mass in clouds by microbial cell growth and multiplication. The best estimate of SBA formation rates of 3.7 Tg yr-1 are comparable to previous model estimates of the primary emission of bacteria into the atmosphere, and thus might represent a previously unrecognized source of biological aerosol material. We discuss in detail the large uncertainties associated with our estimates based on the rather sparse available data on bacteria abundance, growth conditions and properties. Additionally, the loss of water-soluble organic carbon (WSOC) due to microbial processes in cloud droplets has been suggested to compete under some conditions with WSOC loss by chemical (OH) reactions. Our estimates suggest that microbial and chemical processes might lead to a global loss of WSOC of 8–11 Tg yr-1 and 8–20 Tg yr-1, respectively. While also this estimate is very approximate, the analysis of the uncertainties and ranges of all parameters gives hints about the conditions under which microbial processes cannot be neglected as organic carbon sinks in clouds. Our estimates also highlight the urgent needs for more data concerning microbial concentrations, fluxes and activity in the atmosphere to evaluate the role of bacterial processes as net aerosol sink or source on various spatial and temporal scales.

Estimation of rate coefficients and branching ratios for gas-phase reactions of OH with aliphatic organic compounds for use in automated mechanism construction

Reaction with the hydroxyl (OH) radical is the dominant removal process for volatile organic compounds (VOCs) in the atmosphere. Rate coefficients for reactions of OH with VOCs are therefore essential parameters for chemical mechanisms used in chemistry transport models, and are required more generally for impact assessments involving the estimation of atmospheric lifetimes or oxidation rates for VOCs. Updated and extended structure–activity relationship (SAR) methods are presented for the reactions of OH with aliphatic organic compounds, with the reactions of aromatic organic compounds considered in a companion paper. The methods are optimized using a preferred set of data including reactions of OH with 489 aliphatic hydrocarbons and oxygenated organic compounds. In each case, the rate coefficient is defined in terms of a summation of partial rate coefficients for H abstraction or OH addition at each relevant site in the given organic compound, so that the attack distribution is defined. The information can therefore guide the representation of the OH reactions in the next generation of explicit detailed chemical mechanisms. Rules governing the representation of the subsequent reactions of the product radicals under tropospheric conditions are also summarized, specifically their reactions with O2 and competing processes.

Estimation of rate coefficients and branching ratios for gas-phase reactions of OH with aromatic organic compounds for use in automated mechanism construction

Reaction with the hydroxyl (OH) radical is the dominant removal process for volatile organic compounds (VOCs) in the atmosphere. Rate coefficients for the reactions of OH with VOCs are therefore essential parameters for chemical mechanisms used in chemistry transport models, and are required more generally for impact assessments involving estimation of atmospheric lifetimes or oxidation rates for VOCs. A structure–activity relationship (SAR) method is presented for the reactions of OH with aromatic organic compounds, with the reactions of aliphatic organic compounds considered in the preceding companion paper. The SAR is optimized using a preferred set of data including reactions of OH with 67 monocyclic aromatic hydrocarbons and oxygenated organic compounds. In each case, the rate coefficient is defined in terms of a summation of partial rate coefficients for H abstraction or OH addition at each relevant site in the given organic compound, so that the attack distribution is defined. The SAR can therefore guide the representation of the OH reactions in the next generation of explicit detailed chemical mechanisms. Rules governing the representation of the reactions of the product radicals under tropospheric conditions are also summarized, specifically the rapid reaction sequences initiated by their reactions with O2.

Estimation of rate coefficients and branching ratios for gas-phase reactions of OH with aliphatic organic compounds for use in automated mechanism construction

Reaction with the hydroxyl (OH) radical is the dominant removal process for volatile organic compounds (VOCs) in the atmosphere. Rate coefficients for reactions of OH with VOCs are therefore essential parameters for chemical mechanisms used in chemistry-transport models, and are required more generally for impact assessments involving estimation of atmospheric lifetimes or oxidation rates for VOCs. Updated and extended structure-activity relationship (SAR) methods are presented for the reactions of OH with aliphatic organic compounds, with the reactions of aromatic organic compounds considered in a companion paper. The methods are optimized using a preferred set of data including reactions of OH with 489 aliphatic hydrocarbons and oxygenated organic compounds. In each case, the rate coefficient is defined in terms of a summation of partial rate coefficients for H abstraction or OH addition at each relevant site in the given organic compound, so that the attack distribution is defined. The information can therefore guide the representation of the OH reactions in the next generation of explicit detailed chemical mechanisms. Rules governing the representation of the subsequent reactions of the product radicals under tropospheric conditions are also summarized, specifically their reactions with O2 and competing processes.