Simultaneous, in situ measurements of OH and HO_2 in the stratosphere

Stratospheric OH and HO_2 radical densities have been measured between 36 and 23 km using a balloon-borne, in situ instrument launched from Palestine, TX on August 25, 1989. OH is detected using the laser-induced fluorescence technique (LIF) employing a Cu-vapor-laser pumped dye laser coupled with an enclosed-flow detection chamber. HO_2 is detected nearly simultaneously by adding No to the sample flow to convert ambient HO_2 to OH. Observed OH and HO_2 densities ranged from 8.0 ± 2.8 × 10^6 and 1.4 ± 0.5 × 10^7 molec cm^(−3), respectively, at 36 km, to 1.4± 0.5 × 10^6 and 3.0± 1.0 × 10^6 at 23 km, where the uncertainty is ±1σ. The HO_2 density exhibits a maximum in the 34–30 km of 1.7±0.6 × 10^7. The data were obtained over a solar zenith angle variation of 51° at 36 km to 61° at 23 km. O_3 and H_2O densities also were measured simultaneously with separate instruments

Balloon borne in-situ detection of OH in the stratosphere from 37 to 23 km

The OH number density in the stratosphere has been measured over the altitude interval of 37 to 23 km at midday via a balloon-borne gondola launched from Palestine, Texas on July 6, 1988. OH radicals are detected with a laser induced fluorescence instrument employing a 17 kHz repetition rate copper vapor laser pumped dye laser optically coupled to an enclosed flow, in-situ sampling chamber. OH abundances ranged from 88±31 pptv (1.1 ± 0.4 × 10^7 molec cm^(−3)) in the 36 to 35 km interval to 0.9 ± 0.8 pptv (8.7 ± 7.7 × 10^5 molec cm^(−3)) in the 24 to 23 km interval. The stated uncertainty (±1σ) includes that from both measurement precision and accuracy. Simultaneous detection of ozone and water vapor densities was carried out with separate on-board instruments

Simultaneous, in situ measurements of OH, HO_2, O_3, and H_2O: A test of modeled stratospheric HO_x chemistry

Simultaneous, in situ measurements of OH, HO_2, H_2O, and O_3 from 37–23 km are reported. The partitioning between OH and HO_2 and the total HO_x concentration are compared with expected steady-state values. The ratio of HO_2 to OH varies from less than 2 at 36 km to more than 3 at 25 km; in the lower stratosphere this ratio is nearly a factor of two less than predicted. The data are used to calculate HO_x production and loss rates. The measured HOx mixing ratio is consistent with production dominated by the reaction of O(1D) with H_2O, and loss controlled by NO_y below 28 km and HO_x above 30 km. The steady-state concentration of H_2O_2 is inferred from the measured HO_2 concentration and calculated photolysis rate. The maximum H_2O_2 mixing ratio (at 33 km) is predicted to be less than 0.2 ppb

Are models of catalytic removal of O_3 by HO_x accurate? Constraints from in situ measurements of the OH to HO_2 ratio

Measurements of the ratio OH/HO_2, NO, O_3, ClO, and BrO were obtained at altitudes from 15–20 km and latitudes from 15–60°N. A method is presented for interpreting these simultaneous in situ observations that constrains the rates of chemical transformations that 1) are responsible for over half the ozone removal rate in the lower stratosphere via reactions of HO_2 and 2) control the abundance of HO_2 through coupling to nitrogen and halogen radicals. The results show our understanding of the chemical reactions controlling the partitioning of OH and HO_2 is complete and accurate and that the potential effects of “missing chemistry” are strictly constrained in the region of the atmosphere encompassed by the observations. The analysis demonstrates that the sensitivity of the ratio OH/HO_2 to changes in NO is described to within 12% by current models. This reduces by more than a factor of 2 the effect of uncertainty in the coupling of hydrogen and nitrogen radicals on the analysis of the potential effects of perturbations to odd nitrogen in the lower stratosphere

JNO\u3csub\u3e2\u3c/sub\u3e at high solar zenith angles in the lower stratosphere

In situ measurements of NO, NO2, O3, HO2, C1O, pressure, and temperature have been made at high solar zenith angles (SZA, 70° - 93°) in the lower stratosphere. These measurements are used to derive the photolysis rate of NO2, JNO2, using a time-dependent method. The resultant JNO2 values and the results of a multiple-scattering actinic flux model show a linear relationship throughout the SZA range. The difference of the two sets of JNO2 values of about 11% suggests that the model scattering calculation is very accurate at high SZA conditions near sunrise and sunset

The response of ClO radical concentrations to variations in NO_2 radical concentrations in the lower stratosphere

The response of ClO concentrations to changes in NO_2 concentrations has been inferred from simultaneous observations of [ClO], [NO], [NO_2] and [O_3] in the mid-latitude lower stratosphere. This analysis demonstrates that [ClO] is inversely correlated with [NO_2], consistent with formation and photolysis of [ClONO_2]. A factor of ten range in the concentration of NO_2 was sampled (0.1 to 1×10^9 mol/cm^3), with a comparable range in the ratio of [ClO] to total available inorganic chlorine (1% ≤ [ClO]/[Cl_y] ≤ 5%). This analysis leads to an estimate of [ClONO_2]/[Cl_y] = 0.12 (×/÷2), in the mid-latitude, lower-stratospheric air masses sampled

Observations of large reductions in the NO/NO_y ratio near the mid-latitude tropopause and the role of heterogeneous chemistry

During the 1993 NASA Stratospheric Photochemistry, Aerosols and Dynamics Expedition (SPADE), anomalously low nitric oxide (NO) was found in a distinct sunlit layer located above the mid-latitude tropopause. The presence of a significant amount of reactive nitrogen (NO_y) in the layer implies the systematic removal of NO, which is without precedent in stratospheric in situ observations. Large increases in measured chlorine monoxide (ClO) and the hydroperoxyl radical (HO_2) also were observed in the layer. Heterogeneous reaction rate constants of chlorine nitrate (ClONO_2) with hydrogen chloride (HCl) and H_2O to form nitric acid (HNO_3) on sulfate aerosol are enhanced in the NO removal layer by local increases in H_2O and aerosol surface area. The associated conversion of NO_x (= NO + NO_2) to HNO_3 is the most likely cause of the observed low NO and NO_x/NO_y values and high ClO values

Quantitative constraints on the atmospheric chemistry of nitrogen oxides: An analysis along chemical coordinates

In situ observations Of NO_2, NO, NO_y, ClONO_2, OH, O_3, aerosol surface area, spectrally resolved solar radiation, pressure and temperature obtained from the ER-2 aircraft during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) experiments are used to examine the factors controlling the fast photochemistry connecting NO and NO_2 and the slower chemistry connecting NO_x and HNO_3. Our analysis uses “chemical coordinates” to examine gradients of the difference between a model and precisely calibrated measurements to provide a quantitative assessment of the accuracy of current photochemical models. The NO/NO_2 analysis suggests that reducing the activation energy for the NO+O_3 reaction by 1.7 kJ/mol will improve model representation of the temperature dependence of the NO/NO_2 ratio in the range 215–235 K. The NO_x/HNO_3 analysis shows that systematic errors in the relative rate coefficients used to describe NO_x loss by the reaction OH + NO_2 → HNO_3 and by the reaction set NO_2 + O_3 → NO_3; NO_2 + NO_3 → N_(2)O_5; N_(2)O_5 + H_(2)O → 2HNO_3 are in error by +8.4% (+30/−45%) (OH+NO_2 too fast) in models using the Jet Propulsion Laboratory 1997 recommendations [DeMore et al., 1997]. Models that use recommendations for OH+NO2 and OH+HNO_3 based on reanalysis of recent and past laboratory measurements are in error by 1.2% (+30/−45%) (OH+NO_2 too slow). The +30%/−45% error limit reflects systematic uncertainties, while the statistical uncertainty is 0.65%. This analysis also shows that the POLARIS observations only modestly constrain the relative rates of the major NO_x production reactions HNO3 + OH → H_(2)O + NO_3 and HNO_3 + hν → OH + NO_2. Even under the assumption that all other aspects of the model are perfect, the POLARIS observations only constrain the rate coefficient for OH+HNO_3 to a range of 65% around the currently recommended value

Inorganic chlorine partitioning in the summer lower stratosphere: Modeled and measured [ClONO_2]/[HCl] during POLARIS

We examine inorganic chlorine (Cl_y,) partitioning in the summer lower stratosphere using in situ ER-2 aircraft observations made during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaign. New steady state and numerical models estimate [ClONO_2]/[HCl] using currently accepted photochemistry. These models are tightly constrained by observations with OH (parameterized as a function of solar zenith angle) substituting for modeled HO_2 chemistry. We find that inorganic chlorine photochemistry alone overestimates observed [ClONO_2]/[HCl] by approximately 55–60% at mid and high latitudes. On the basis of POLARIS studies of the inorganic chlorine budget, [ClO]/[ClONO_2], and an intercomparison with balloon observations, the most direct explanation for the model-measurement discrepancy in Cl_y, partitioning is an error in the reactions, rate constants, and measured species concentrations linking HCl and ClO (simulated [ClO]/[HCl] too high) in combination with a possible systematic error in the ER-2 ClONO_2 measurement (too low). The high precision of our simulation (±15% 1σ for [ClONO_2]/[HCl], which is compared with observations) increases confidence in the observations, photolysis calculations, and laboratory rate constants. These results, along with other findings, should lead to improvements in both the accuracy and precision of stratospheric photochemical models