Showing papers by "A. R. Ravishankara published in 1995"

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

Abstract: 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.

Role of adducts in the atmospheric oxidation of dimethyl sulfide

Abstract: Based on the results from recent laboratory studies and field observations, we suggest that adduct formation may play a key role in the atmospheric oxidation of dimethyl sulfide (DMS). Experimental evidence for the formation of the DMS · OH and CH3SOx· O2 adducts are described and used in a simple ‘box model’ to calculate the ratio of [MSA]/[SO42–] observed in the atmosphere. These calculated ratios are compared with those inferred from field measurements. Based on these comparisons, we suggest that the changes with temperature in the mechanism of the OH + DMS reaction, one of the primary initiation steps in atmospheric DMS oxidation, are unlikely to be the source of the variation in the [MSA]/[SO42–] ratio with temperature. We suggest that the competition between the thermal decomposition of adducts such as CH3SO2 and their bimolecular reactions with atmospheric species is the likely cause of the variation. Finally, a few observations needed to better our understanding of the DMS oxidation are identified.

UV/visible and IR absorption cross sections of BrONO2

Abstract: The absorption cross sections of BrONO2 between 200 and 500 nm were measured over the temperature range 298 to 220 K using a diode array spectrometer. The BrONO2 absorption cross sections are weakly dependent on temperature at wavelengths 450 nm; at 480 nm the cross section decreased by ∼35% in going from 298 K to 220 K. Our room temperature absorption cross sections are in good agreement with the measurements of Spencer and Rowland (1978) over the common wavelength range of the measurements, 200 to 390 nm. We show that wavelengths longer than 390 nm must be included in the calculation of the BrONO2 atmospheric photolysis rate. We also show that photolysis of BrONO2 could be a significant atmospheric loss process for odd oxygen (due to halogen chemistry) below about 25 km. The infrared absorption cross sections for the BrONO2 band centered at 803.3 cm−1 were also measured. The integrated band strength was (2.7±0.5) × 10−17 cm2 molecule−1 cm−1.