Showing papers by "James B. Burkholder published in 1999"

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

Abstract: 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%.

Temperature dependent rate coefficient for the reaction o(3p) + no2 no + o2

Abstract: Nitrogen oxides, NO and NO{sub 2} (collectively called No{sub x}), play a crucial role in atmospheric ozone chemistry: they lead to photochemical ozone production in the troposphere and catalytic ozone destruction in the stratosphere. The rate coefficient (k{sub 1}) for the reaction O({sup 3}P) + NO{sub 2} {r_arrow} O{sub 2} + NO was measured under pseudo-first-order conditions in O({sup 3}P) atom concentration over the temperature range 220--412 K. Measurements were made using pulsed laser photolysis of NO{sub 2} to produce oxygen atoms and time-resolved vacuum UV resonance fluorescence detection of O atoms. The NO{sub 2} concentration was measured using three techniques: flow rate, UV absorption, and chemical titration (NO + O{sub 3} {r_arrow} NO{sub 2} + O{sub 2}). The NO{sub 2} UV absorption cross section at 413.4 nm was determined as a function of temperature using the chemical titration and flow methods. Including the low-temperature data of Harder et al., the temperature-dependent No{sub 2} cross section is given by {sigma}{sub 413.4}(T) = (9.49 {minus} 0.00549 T) {times} 10{sup {minus}19} cm{sup 2} molecule{sup {minus}1}. The measured rate coefficients for reaction 1 can be expressed as k{sub 1}(T) = (5.26 {+-} 0.60) {times} 10{sup {minus}12} exp[(209 {+-} 35)/T] cm{sup 3} molecule{sup {minus}1}more » s{sup {minus}1}, where the quoted uncertainties are 2{sigma} and include estimated systematic errors. This result is compared with previously reported measurements of k{sub 1}.« less

Rate Coefficient Measurements for the Reaction OH + ClO → Products

Abstract: The rate coefficient for the reaction OH + ClO → products (1) was measured under pseudo-first-order conditions in OH. A discharge flow system was used to produce ClO, and its concentration was measured by UV/visible absorption. OH was produced by pulsed laser photolysis of O3 (or ClO) at 248 nm in the presence of H2O and was monitored by laser-induced fluorescence. The value of k1 between 234 and 356 K is given by k1(T) = (8.9 ± 2.7) × 10-12 exp[(295 ± 95)/T] cm3 molecule-1 s-1, where uncertainties are 95% confidence limits and include estimated systematic uncertainties. Our value is compared with those from previous investigations.