Kinetic Study of the Gas-Phase Reactions of Nitrate Radicals with Methoxyphenol Compounds: Experimental and Theoretical Approaches.

The gas-phase reactions of five methoxyphenols (three disubstituted and two trisubstituted) with nitrate radicals were studied in an 8000 L atmospheric simulation chamber at atmospheric pressure and 294 ± 2 K. The NO3 rate constants were investigated with the relative kinetic method using PTR-ToF-MS and GC-FID to measure the concentrations of the organic compounds. The rate constants (in units of cm(3) molecule(-1) s(-1)) determined were: 2-methoxyphenol (guaiacol; 2-MP), k(2-MP) = (2.69 ± 0.57 × 10(-11); 3-methoxyphenol (3-MP), k(3-MP) = (1.15 ± 0.21) × 10(-11); 4-methoxyphenol (4-MP), k(4-MP) = (13.75 ± 7.97) × 10(-11); 2-methoxy-4-methylphenol, k(2-M-4-MeP) = (8.41 ± 5.58) × 10(-11) and 2,6-dimethoxyphenol (syringol; 2,6-DMP), k(2,6-DMP) = (15.84 ± 8.10) × 10(-11). The NO3 rate constants of the studied methoxyphenols are compared with those of other substituted aromatics, and the differences in the reactivity are construed regarding the substituents (type, number and position) on the aromatic ring. This study was also supplemented by a theoretical approach of the methoxyphenol reactions with nitrate radicals. The upper limits of the NO3 overall rate constants calculated were in the same order of magnitude than those experimentally determined. Theoretical calculations of the minimum energies of the adducts formed from the reaction of NO3 radicals with the methoxyphenols were also performed using a DFT approach (M06-2X/6-31G(d,p)). The results indicate that the NO3 addition reactions on the aromatic ring of the methoxyphenols are exothermic, with energy values ranging between -13 and -21 kcal mol(-1), depending on the environment of the carbon on which the oxygen atom of NO3 is attached. These energy values allowed identifying the most suitable carbon sites for the NO3 addition on the aromatic ring of the methoxyphenols: at the exception of the 3-MP, the NO3 ipso-addition to the hydroxyl group is one of the favored sites for all the studies compounds.

Heterogeneous interaction of H2O2 with Arizona Test Dust.

The heterogeneous interaction of H2O2 with solid films of Arizona Test Dust (ATD) was investigated under dark conditions and in presence of UV light using a low pressure flow tube reactor coupled with a quadrupole mass spectrometer. The uptake coefficients were measured as a function of the initial concentration of gaseous H2O2 ([H2O2]0 = (0.18 - 5.1) × 10(12) molecules cm(-3)), irradiance intensity (JNO2 = 0.002 - 0.012 s(-1)), relative humidity (RH = 0.002 - 69%), and temperature (T = 268 - 320 K). The initial uptake coefficient was found to be independent of the concentration of H2O2 and UV irradiation intensity and to decrease with increasing RH and temperature according to the following expressions: γ0 = 4.8 × 10(-4)/(1+ RH(0.66)) at T = 275 K and γ0 = 3.2 × 10(-4)/(1 + 2.5 × 10(10)exp(-7360/T)) at RH = 0.35% (calculated using BET surface area, estimated conservative uncertainty of 30%). By contrast, the steady state uptake coefficient was found to be independent of temperature, to increase upon UV irradiation of the surface, and to be inversely (γSS ∼ [H2O2](-0.6)) dependent on the concentration of H2O2. The RH independent steady state uptake coefficient was measured under dark and UV irradiation conditions: γSS(dark) = (0.95 ± 0.30) × 10(-5) and γSS(UV) = (1.85 ± 0.55) × 10(-5), for RH = (2 - 69)% and [H2O2]0 ≅ 1.0 × 10(12) molecules cm(-3). The present experimental data support current considerations that uptake of H2O2 on mineral aerosol is potentially an important atmospheric process.

Kinetics and products of heterogeneous reaction of HONO with Fe2O3 and Arizona Test Dust.

Kinetics and products of the reaction of HONO with solid films of Fe2O3 and Arizona Test Dust (ATD) were investigated using a low pressure flow reactor (1 - 10 Torr) combined with a modulated molecular beam mass spectrometer. The reactive uptake of HONO was studied as a function of HONO concentration ([HONO]0 = (0.6 - 15.0) × 10(12) molecules cm(-3)), relative humidity (RH = 3 × 10(-4) - 84.1%) and temperature (T = 275 - 320 K). Initial reactive uptake coefficients were found to be similar under dark conditions and in the presence of UV irradiation (JNO2 = 0.012 s(-1)) and independent of the HONO concentration and temperature. In contrast, the relative humidity (RH) was found to have a strong impact on the uptake coefficients: γ (ATD) = 3.8 × 10(-6) (RH)(-0.61) and γ (Fe2O3) = 1.7 × 10(-6) (RH)(-0.62) (γ calculated with BET surface area, 30% conservative uncertainty). In both reactions of HONO studied, NO2 and NO were observed as gaseous products with yields of (60 ± 9) and (40 ± 6) %, respectively, independent of relative humidity, temperature, concentration of HONO and UV irradiation intensity. The observed data point to minor importance of the HONO uptake on mineral aerosol compared with other known sinks of HONO in the atmosphere, which are its dry deposition and photolysis in night-time and during the day, respectively.

Interaction of OH radicals with Arizona Test Dust: uptake and products.

Kinetics and products of the interaction of OH radicals with solid films of Arizona Test Dust (ATD) were studied using a low pressure flow reactor (0.5-3 Torr) combined with a modulated molecular beam mass spectrometer for monitoring of the gaseous species involved. The reactive uptake coefficient of OH was measured from the kinetics of OH consumption on Pyrex rods coated with ATD as a function of OH concentration ((0.4-5.2) × 10(12) molecules cm(-3)), relative humidity (RH = 0.03-25.9%), temperature (T = 275-320 K), and UV irradiance intensity (J(NO(2)) = 0-0.012 s(-1)). Deactivation of ATD surface upon exposure to OH was observed. The initial uptake coefficient was found to be independent of temperature and irradiation conditions and to decrease with relative humidity: γ(0) = 0.2/(1 + RH(0.36)) (calculated using geometric surface area, with 30% estimated conservative uncertainty). H(2)O(2) and H(2)O were observed in the gas phase as products of the OH reaction with ATD surface with yields of (10 ± 3) and (98 ± 25) %, respectively.

Interaction of NO2 with TiO2 surface under UV irradiation: products study.

The interaction of NO(2) with TiO(2) solid films was studied under UV irradiation using a low pressure flow reactor (1-10 Torr) combined with a modulated molecular beam mass spectrometer for monitoring of the gaseous species involved. HONO, NO, and N(2)O were observed as the products of the reactive uptake of NO(2) to the illuminated TiO(2) surface with the sum of their yields corresponding nearly to 100% of the nitrogen mass balance. The yield of the products was measured as a function of different parameters such as irradiance intensity, relative humidity (RH), temperature, and concentrations of NO(2) and O(2). The yield of N(2)O was found to be 0.15 ± 0.05 independent of the experimental conditions. The distribution of the products between NO and HONO was found to be independent of temperature in the range T = 280-320 K and was governed by relative humidity: increase in RH led to lower NO and higher HONO yield, with a maximum of nearly 65% reached at ~5% RH. Presence of molecular oxygen was shown to shift the HONO/NO distribution to HONO at low RH (<5%) with no effect at higher RH where the HONO yield is maximum. The following values for the yield of the products of NO(2) interaction with pure TiO(2) under real atmospheric conditions can be recommended from this work: 0.65 ± 0.10, 0.05 ± 0.05, and 0.15 ± 0.05 for HONO, NO, and N(2)O, respectively. The mechanism of the photoinitiated heterogeneous reaction and possible atmospheric implications of the obtained results are discussed.

Reactive uptake of HONO to TiO2 surface: "dark" reaction.

The interaction of HONO with TiO(2) solid films was studied under dark conditions using a low pressure flow reactor (1-10 Torr) combined with a modulated molecular beam mass spectrometer for monitoring of the gaseous species involved. The reactive uptake of HONO to TiO(2) was studied as a function of HONO concentration ([HONO)(0) = (0.3-3.3) × 10(12) molecules cm(-3)), water concentration (RH = 3 × 10(-4) to 13%), and temperature (T = 275-320 K). TiO(2) surface deactivation upon exposure to HONO was observed. The measured initial uptake coefficient of HONO on TiO(2) surface was independent of the HONO concentration and showed slight negative temperature dependence (activation factor = -1405 ± 110 K). In contrast, the relative humidity (RH) was found to have a strong impact on the uptake coefficient: γ(0) = 1.8 × 10(-5) (RH)(-0.63) (calculated using BET surface area, 40% uncertainty) at T = 300 K. NO(2) and NO were observed as products of the HONO reaction with TiO(2) surface with sum of their yields corresponding to nearly 100% of the nitrogen mass balance. The yields of the NO and NO(2) products were found to be 42 ± 7% and 60 ± 9%, respectively, independent of relative humidity, temperature, and concentration of HONO under experimental conditions used. The contribution of aerosol to the total HONO loss in the boundary layer (calculated with initial uptake data for HONO on TiO(2) surface) showed the unimportance of this process in the atmosphere. In addition, the diffusion coefficient of HONO in He was determined to be D(HONO-He) = 490 ± 50 Torr cm(2) s(-1) at T = 300 K.

Heterogeneous interaction of H2O2 with TiO2 surface under dark and UV light irradiation conditions.

The heterogeneous interaction of H(2)O(2) with TiO(2) surface was investigated under dark conditions and in the presence of UV light using a low pressure flow tube reactor coupled with a quadrupole mass spectrometer. The uptake coefficients were measured as a function of the initial concentration of gaseous H(2)O(2) ([H(2)O(2)](0) = (0.17-120) × 10(12) molecules cm(-3)), irradiance intensity (J(NO(2)) = 0.002-0.012 s(-1)), relative humidity (RH = 0.003-82%), and temperature (T = 275-320 K). Under dark conditions, a deactivation of TiO(2) surface upon exposure to H(2)O(2) was observed, and only initial uptake coefficient of H(2)O(2) was measured, given by the following expression: γ(0)(dark) = 4.1 × 10(-3)/(1 + RH(0.65)) (calculated using BET surface area, estimated conservative uncertainty of 30%) at T = 300 K. The steady-state uptake coefficient measured on UV irradiated TiO(2) surface, γ(ss)(UV), was found to be independent of RH and showed a strong inverse dependence on [H(2)O(2)] and linear dependence on photon flux. In addition, slight negative temperature dependence, γ(ss)(UV) = 7.2 × 10(-4) exp[(460 ± 80)/T], was observed in the temperature range (275-320) K (with [H(2)O(2)] ≈ 5 × 10(11) molecules cm(-3) and J(NO(2)) = 0.012 s(-1)). Experiments with NO addition into the reactive system provided indirect evidence for HO(2) radical formation upon H(2)O(2) uptake, and the possible reaction mechanism is proposed. Finally, the atmospheric lifetime of H(2)O(2) with respect to the heterogeneous loss on mineral dust was estimated (using the uptake data for TiO(2)) to be in the range of hours during daytime, i.e., comparable to H(2)O(2) photolysis lifetime (~1 day), which is the major removal process of hydrogen peroxide in the atmosphere. These data indicate a strong potential impact of H(2)O(2) uptake on mineral aerosol on the HO(x) chemistry in the troposphere.

Contributions of local and regional anthropogenic sources of metals in PM2.5 at an urban site in northern France.

PM2.5 have been related to various adverse health effects, mainly due to their ability to penetrate deeply and to convey harmful chemical components, such as metals inside the body. In this work, PM2.5 were sampled at Saint-Omer, a medium-sized city located in northern France, in March-April 2011 and analyzed for their total carbon, water-soluble ions, major and trace elements. More specifically, the origin of 15 selected elements was examined using different tools including enrichment factors, conditional bivariate probability function (CBPF) representations, diagnostic ratios and receptor modelling. The results indicated that PM2.5 metal composition is affected by both emissions of a local glassmaking factory and an integrated steelworks located at a distance of 35 km from the sampling site. For the first time, diagnostic ratios were proposed for the glassmaking activity. Therefore, metals in PM2.5 could be attributed to the following anthropogenic sources: (i) local glassmaking industry for Sn, As, Cu and Cr, (ii) distant integrated steelworks for Ag, Fe, Cd, Mn, Rb and Pb, (iii) heavy fuel oil combustion for Ni, V and Co and (iv) non-exhaust traffic for Zn, Pb, Mn, Sb, and Cu. The impact of such sources on metal concentrations in PM2.5 was assessed using a constrained receptor model. Despite their low participation to PM2.5 concentration (2.7%), the latter sources were found as the main contributors (80%) to the overall concentration levels of the 15 selected elements in PM2.5.