Ranajit K. Talukdar

Investigation of the loss processes for peroxyacetyl nitrate in the atmosphere: UV photolysis and reaction with OH

The UV absorption cross sections of peroxyacetyl nitrate (PAN), CH 3 C(O)O 2 NO 2 , have been measured as a function of temperature (298, 273, and 250 K) between 195 and 345 nm using a diode array spectrometer. The absorption cross sections decrease monotonically with increasing wavelength. The cross sections also decrease as the temperature is lowered. Upper limits for the rate coefficients for the reaction of OH with PAN at atmospheric temperatures were determined to be <3×10 −14 cm 3 molecule −1 s −1 using the pulsed photolysis laser induced fluorescence technique. Photolysis becomes the most important atmospheric loss process for PAN above ∼7 km, and the OH reaction is found to be unimportant throughout the troposphere. These results are compared with previous measurements, and the significance of the revised values on the atmospheric loss rates of PAN is discussed.

Rate constants for the reaction OH+NO2+M → HNO3+M under atmospheric conditions

Abstract We present rate constants for the title reaction, k 1 , measured using a pulsed photolysis–laser induced fluorescence technique between 220 and 250 K in 20–250 Torr of N 2 and 20–50 Torr of O 2 . Our measured k 1 agree with literature values at low temperatures but show that the current recommendations for atmospheric modeling overestimate k 1 by 10–30% in the falloff region below 250 K. The revised values of k 1 help to better define the role of NO 2 in the stratosphere.

Reactive uptake of NO3 on pure water and ionic solutions

The reactive uptake coefficients (γ) of NO 3 onto pure water and dilute solutions of NaCl, NaBr, and NaNO 2 were measured using a wetted-wall flow-tube setup combined with a long-path absorption cell for the detection of NO 3 . The measured γ values were in the range 1.5 × 10 -4 - 6 × 10 -3 , depending on the salt concentration in the water. By measuring γ as a function of salt concentration, HD l 0.5 for NO 3 in water was determined to be (1.9±0.4)×10 -3 M atm -1 cm s -0.5 at 273 K, assuming that the rate coefficient for the reaction of NO 3 with Cl - is 2.76×10 6 M -1 s -1 at 273 K. The Henrys law coefficient for NO 3 in water is estimated to be 0.6±0.3 M atm -1 , assuming that the diffusion coefficient of NO 3 in water is D l = (1.0±0.5)×10 -5 cm 2 s -1 . Uptake of NO 3 on pure water is interpreted as due to reaction of NO 3(aq) with H 2 O (l) to produce HNO 3 and OH in the liquid phase. Implications of these findings to the chemistry of NO 3 in the troposphere are also discussed.

Temperature dependence of the ozone absorption spectrum over the wavelength range 410 to 760 nm

The ozone, O3, absorption cross sections between 410 and 760 nm, the Chappuis band, were measured at 220, 240, 260, and 280 K relative to that at room temperature using a diode array spectrometer. The measured cross sections varied very slightly, <1%, with decreasing temperature between 550 and 660 nm, near the peak of the Chappuis band. At wavelengths away from the peak, the absorption cross sections decreased with decreasing temperature; e.g., ∼40% at 420 nm between 298 and 220 K. These results are compared with previous measurements and the impact on atmospheric measurements are discussed.

Atmospheric fate of methyl vinyl ketone and methacrolein

The rate coefficients for the reaction of OH with methyl vinyl ketone (MVK,CH3C(O)CHCH2) and methacrolein (MACR, CH2C(CH3)CHO) between 232 and 378 K were measured using the pulsed laser photolysis-pulsed laser induced fluorescence (PP-PLIF) technique. The rate coefficient data can be expressed in the Arrhenius form as k1(OH + MVK) = 2.67±0.45) × 10−12exp((452±130)/T) and k2(OH + MACR) = (7.73±0.65) × 10−12exp((379±46)/T) cm3 molecule −1 s−1, where the error limits are 2σ and include estimated systematic error. The UV absorption cross-sections of MVK and MACR were measured over the wavelength range 250–395 nm using a diode array spectrometer. Absolute quantum yields for loss of MVK and MACR were measured at 308, 337, and 351 nm. The MACR quantum yield, ФMACR, was <0.01. The MVK quantum yield was both pressure and wavelength dependent and is represented by the expression:: Ф0(λ,P) < exp[ −0.055 (λ−308)]/(5.5 + 9.2 × 10−19N) where λ is measured in nm and N is the number density in molecule cm−3. Atmospheric loss rate calculations using these results show that the primary loss process for both MVK and MACR is the reaction with OH radicals throughout the troposphere.

Atmospheric fate of several alkyl nitrates Part2UV absorption cross-sectionsand photodissociation quantum yields

The UV absorption cross-sections of methyl, ethyl and isopropyl nitrate between 233 and 340 nm have been measured using a diode array spectrometer in the temperature range 240–360 K. The absorption cross-sections of these alkyl nitrates decrease with increasing wavelength and decrease with decreasing temperature for λ>280 nm. The photodissociation quantum yield for CH 3 ONO 2 to produce NO 2 and CH 3 O was found to be essentially unity at 248 nm using transient UV absorption methods. Production of O and H atoms in the photodissociation of methyl nitrate at 248 and 308 nm were found to be negligible using resonance fluorescence detection of the atoms. High quantum yields for O atoms were measured following 193 nm photolysis of methyl nitrate. The OH radical was measured to be a photoproduct with a very small quantum yield. Using the OH rate coefficients reported in the accompanying paper and the UV absorption cross-sections and the photodissociation quantum yields measured here, the first-order rate constants for atmospheric loss of methyl, ethyl and isopropyl nitrate were calculated. Photolysis was found to be the dominant atmospheric loss process for the three alkyl nitrates.

Temperature dependence of the HNO3 UV absorption cross sections

The temperature dependence of the HNO3 absorption cross sections between 240 and 360 K over the wavelength range 195 to 350 nm has been measured using a diode array spectrometer. Absorption cross sections were determined using both (1) absolute pressure measurements at 298 K and (2) a dual absorption cell arrangement, in which the absorption spectrum at various temperatures is measured relative to the room temperature absorption spectrum. The HNO3 absorption spectrum showed a temperature dependence which is weak at short wavelengths but stronger at longer wavelengths which are important for photolysis in the lower stratosphere. The 298 K absorption cross sections were found to be larger than the values currently recommended for atmospheric modeling (DeMore et al., 1992). Our absorption cross section data are critically compared with the previous measurements of both room temperature and temperature-dependent absorption cross sections. Temperature-dependent absorption cross sections of HNO3 are recommended for use in atmospheric modeling. These temperature dependent HNO3 absorption cross sections were used in a two-dimensional dynamical-photochemical model to demonstrate the effects of the revised absorption cross sections on loss rate of HNO3 and the abundance of NO2 in the stratosphere.

Rate coefficients for O(1D) + H2, D2, HD reactions and H atom yield in O(1D) + HD reaction

Abstract Rate coefficients, in units of 10 −10 cm 3 molecule −1 s −1 , for the O( 1 D) reactions with H 2 , D 2 and HD at 298 K were determined to be 1.2 ± 0.1, 1.1 ± 0.1, and 1.2 ± 0.1, respectively. The H atom yield in the O ( 1 D ) + HD reaction, relative to that in the O ( 1 D ) + H 2 reaction, was measured to be 0.57 ± 0.03. The temporal profiles of O( 3 P) and H( 2 S), following pulsed photolytic generation of O( 1 D), were measured using cw resonance fluorescence detection to determine the rate coefficients and product yield.

Atmospheric fate of hydrofluoroethanes and hydrofluorochloroethanes: 1. Rate coefficients for reactions with OH

The rate coefficients for the reactions of OH with five halocarbons [CF3CH2F (HFC 134a), CF3CHClF (HCFC 124), CF3CHCl2 (HCFC 123), CH3CHF2 (HFC 152a), and CH3CF2Cl (HCFC 142b)], which are proposed as alternatives to chlorofluoromethanes, have been measured. A pulsed photolysis system and a discharge flow apparatus were used to measure the rate coefficients between approximately 210 and 425 K. Use of the complementary techniques enabled identification of systematic errors and minimization of these errors. The obtained values are compared with values previously measured by other groups. This data base is used in the subsequent paper to calculate the atmospheric lifetimes of the five compounds.

Role of nitrogen oxides in the stratosphere: A reevaluation based on laboratory studies

The changes in chemical partitioning and stratospheric O3 abundance due to the recently measured rate coefficients for the O + NO2, OH + HNO3, and OH + NO2 reactions are examined using a two-dimensional model. The rate constant changes increase NOx abundance (up to 40%) and NOx-catalyzed O3 destruction, and extend down by several kilometers the altitude region where NOx dominates catalytic O3 destruction. Reductions in the abundance of HOx (10–30%) and ClOx (20-40–) in the lower stratosphere partially buffer the effect on column O3 amount. Column O3 at middle and high latitudes is reduced by 2–10% depending on season for current halogen levels. The model derived long-term O3 trend at midlatitudes due to increases in anthropogenic halogens is reduced by approximately 30%.