James B. Burkholder

The photochemistry of acetone in the upper troposphere: A source of odd-hydrogen radicals

This paper summarizes measured photodissociation quantum yields for acetone in the 290-320 nm wavelength region for pressures and temperatures characteristic of the upper troposphere. Calculations combine this laboratory data with trace gas concentrations obtained during the NASA and NOAA sponsored Stratospheric Tracers of Atmospheric Transport (STRAT) field campaign, in which measurements of OH, HO_(2), odd-nitrogen, and other compounds were collected over Hawaii, and west of California during fall and winter of 1995/1996. OH and HO_(2) concentrations within 2 to 5 km layers just below the tropopause are ∼50% larger than expected from O_(3), CH_(4), and H_(2)O chemistry alone. Although not measured during STRAT, acetone is inferred from CO measurements and acetone-CO correlations from a previous field study. These inferred acetone levels are a significant source of odd-hydrogen radicals that can explain a large part of the discrepancy in the upper troposphere. For lower altitudes, the inferred acetone makes a negligible contribution to HO_(x) (HO+HO_(2)), but influences NO_(y) partitioning. A major fraction of HO_(x) production by acetone is through CH_(2)O formation, and the HO_(x) discrepancy can also be explained by CH_(2)O levels in the 20 to 50 pptv range, regardless of the source.

Bias in Filter-Based Aerosol Light Absorption Measurements Due to Organic Aerosol Loading: Evidence from Laboratory Measurements

Light absorption by soot or nigrosin dye aerosol particles were measured in the laboratory using a particle soot absorption photometer (PSAP) and a photo-acoustic spectrometer (PAS) to assess the influence of non-absorbing organic aerosol (OA) on the PSAP measurements. For the PSAP, particle light absorption is measured after collection on a filter, whereas for the PAS light absorption is measured while the particles remain suspended in the gas phase. OA was generated from the reaction of α -pinene with ozone. It was observed that the presence of this OA in an external mixture of absorbing aerosol and OA can cause an increase in the light absorption measured by the PSAP, relative to that measured by the PAS, by more than a factor of two. This enhancement in the PSAP absorption was found to increase as the amount of OA increased relative to the absorbing compound. Additionally, experiments where absorbing aerosol was deposited on a PSAP filter prior to addition of OA demonstrated that the non-absorbing OA can actually appear as if it were absorbing, with measured single scattering albedo values as low as 0.92. These results indicate that filter-based measurement techniques may significantly overestimate light absorption by aerosols in the atmosphere under conditions where the organic loading is large, with consequent implications for understanding and calculating the Earths radiation budget. These laboratory experiments aid in the interpretation of results from a recent field study, discussed in a companion article (Lack et al. 2008).

Photochemistry of acetone under tropospheric conditions

Abstract The absorption cross sections of acetone were measured between 215 and 350 nm over the temperature range of 235 to 298 K using a diode array spectrometer. The quantum yield for the photodissociation of acetone was measured as a function of pressure at nine discrete wavelengths between 248 and 337 nm. At wavelengths longer than 270 nm, the quantum yields were found to decrease with increasing pressure in accordance with the Stern–Volmer mechanism. The zero pressure quantum yield was found to increase as the wavelength decreased and reached a value of unity near 290 nm. The quantum yields at 308 nm were found to be nearly independent of temperature between 298 and 195 K. These results were used to calculate an expression for the variation of the acetone photodissociation quantum yield with pressure and wavelength. The calculated atmospheric photolysis lifetime of acetone shows the photolysis to be the main loss process for acetone in the upper troposphere, while reaction with the OH radicals dominates its loss rate near the Earths surface.

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.

H+O3 Fourier-transform infrared emission and laser absorption studies of OH (X 2Π) radical: An experimental dipole moment function and state-to-state Einstein A coefficients

The relative intensities of 88 pairs of rovibrational transitions of OH (X 2Π) distributed over 16 vibrational bands (v’≤9, Δv=−1,−2) have been measured using Fourier transform infrared (FTIR) emission/absorption spectroscopy. Each pair of transitions originates from a common vibrational, rotational, and spin–orbit state, so that the measured relative intensities are independent of the OH number density and quantum state distribution. These data are combined with previous v=1←0 relative intensity absorption measurements and v=0, 1, and 2 permanent dipole moments to determine the OH dipole moment function as a cubic polynomial expanded about re, the equilibrium bond length. The relative intensities provide detailed information about the shape of the OH dipole moment function μ(r) and hence the absolute Einstein A coefficients.The intensity information is inverted through a procedure which takes full account of the strong rotation–vibration interaction and spin uncoupling effects in OH to obtain the dipole ...

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.

Temperature dependence of UV absorption cross sections and atmospheric implications of several alkyl iodides

The ultraviolet absorption spectra of a number of alkyl iodides which have been found in the troposphere, CH3I, C2H5I, CH3CH2CH2I, CH3CHICH3, CH2I2, and CH2ClI, have been measured over the wavelength range 200–380 nm and at temperatures between 298 and 210 K. The absorption spectra of the monoiodides CH3I, C2H5I, CH3CH2CH2I, and CH3CHICH3 are nearly identical in shape and magnitude and consist of single broad bands centered near 260 nm. The addition of a chlorine atom in CH2ClI shifts its spectrum to longer wavelengths (σmax at 270 nm). The spectrum of CH2I2 is further red-shifted, reaching a maximum of 3.85×10−18 cm2 molecule−1 at 288 nm and exhibiting strong absorption in the solar actinic region, λ>290 nm. Atmospheric photolysis rate constants, J values, have been calculated assuming quantum efficiencies of unity for different solar zenith angles as a function of altitude using the newly measured cross sections. Surface photolysis rate constants, calculated from the absorption cross sections measured at 298 K, range from 3×10−6 s−1 for CH3I to 5×10−3 s−1 for CH2I2 at a solar zenith angle of 40°.

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.