Photochemistry Of Ozone Loss In The Arctic Region In Summer Research Articles

Validation and data characteristics of water vapor profiles observed by the Improved Limb Atmospheric Spectrometer (ILAS) and processed with the version 5.20 algorithm

Vertical profiles of water vapor concentration at high latitudes (57–72°N; 64–89°S) were observed by the Improved Limb Atmospheric Spectrometer (ILAS) solar occultation sensor aboard the Advanced Earth Observing Satellite (ADEOS). These measurements were made continuously from November 1996 through June 1997 with some additional periods in September to October 1996. A validation study of the water vapor data processed with the version 5.20 ILAS retrieval algorithm is presented in this paper. Uncertainty and general characteristics of the ILAS water vapor measurements are briefly reviewed. Comparisons are made with data obtained by (1) the ILAS validation balloon campaigns at Kiruna, Sweden and at Fairbanks, Alaska; (2) the aircraft measurements under the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaign; and (3) available satellite measurements of the version 19 Halogen Occultation Experiment (HALOE) and the version 6 Stratospheric Aerosol and Gas Experiment II (SAGE II). The agreement between ILAS water vapor and all independent reliable correlative measurements in the altitude region of 15–60 km is better than 10% for the majority of cases and better than 20% for all comparisons, with the exception of some isolated cases detailed in this paper. Climatological comparisons of ILAS data with Upper Atmosphere Research Satellite (UARS) climatology and HALOE data show the overall consistency of ILAS water vapor data considering the known features of atmospheric circulation. The characteristics of ILAS measurements, i.e., high sampling frequency in polar latitudes with high vertical resolution along with the good data quality, make the ILAS water vapor data set valuable for various polar stratospheric research applications.

A High-Precision Fast-Response Airborne CO2Analyzer for In Situ Sampling from the Surface to the Middle Stratosphere

Two in situ CO2 analyzers have been developed for deployment on the NASA ER-2 aircraft and on stratospheric balloons. The ER-2 instrument has had more than 150 flights during 21 deployments from 1992 to 2000, resulting in a dataset with nearly pole-to-pole coverage that includes data from all seasons in both hemispheres except austral summer. In-flight calibrations show that the typical long-term (i.e., flight-to-flight) precision of the instruments is better than ±0.1 ppmv. The flight standards are traceable to standards held by the Scripps Institute of Oceanography and the National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostics Laboratory. The balloon instrument has had eight balloon flights since September 1996, providing the first in situ observations of CO2 above ∼21 km. In addition, the balloon instrument has been flown on board a Cessna Citation II aircraft for sampling between the surface and 10 km. In this paper, the instrumentation and calibration procedures for both instruments are described in detail. An intercomparison of the two instruments during the Photochemistry of Ozone Loss in the Arctic Region In Summer (POLARIS) project showed that, on average, the instruments agreed to within 0.05 ppmv.

The NOx−HNO3System in the Lower Stratosphere: Insights from In Situ Measurements and Implications of theJHNO3−[OH] Relationship

During the 1997 Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) mission, simultaneous in situ observations of NO_x and HO_x radicals, their precursors, and the radiation field were obtained in the lower stratosphere. We use these observations to evaluate the primary mechanisms that control NO_x−HNO_3 exchange and to understand their control over the partitioning between NO_2 and HNO_3 in regions of continuous sunlight. We calculate NO_x production (P_(NO)_x) and loss (L_(NO)_x) in a manner directly constrained by the in situ measurements and current rate constant recommendations, using approaches for representing albedo, overhead O_3 and [OH] that reduce model uncertainty. We find a consistent discrepancy of 18% between modeled rates of NO_x production and loss (L_(NO)_x = 1.18P_(NO)_x) which is within the measurement uncertainty of ±27%. The partitioning between NO_x production processes is [HNO_3 + OH (41 ± 2)%; HNO+3 + hν (59 ± 2)%] and between NO_x loss processes is [NO_2 + OH, 90% to >97%; BrONO_2 + H_2O, 10% to <3%]. The steady-state description of NO_x−HNO_3 exchange reveals the significant influence of the tight correlation between the photolysis rate of HNO_3 and [OH] established by in situ measurements throughout the lower stratosphere. Parametrizing this relationship, we find (1) the steady-state value of [NO_2]_(24h-avg)/[HNO_3] in the continuously sunlit, lower stratosphere is a function only of temperature and number density, and (2) the partitioning of NO_x production between HNO_3 + OH and HNO_3 + hν is nearly constant throughout most of the lower stratosphere. We describe a methodology (functions of latitude, day, temperature, and pressure) for accurately predicting the steady-state value of [NO_2]_(24h-avg)/[HNO_3] and the partitioning of NO_x production within these regions. The results establish a metric to compare observations of [NO_2]_(24h-avg)/[HNO_3] within the continuously sunlit region and provide a simple diagnostic for evaluating the accuracy of models that attempt to describe the coupled NO_x−HO_x photochemistry in the lower stratosphere.

Inorganic chlorine partitioning in the summer lower stratosphere: Modeled and measured [ClONO2]/[HCl] during POLARIS

We examine inorganic chlorine (Cly,) 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 [ClONO2]/[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 HO2 chemistry. We find that inorganic chlorine photochemistry alone overestimates observed [ClONO2]/[HCl] by approximately 55–60% at mid and high latitudes. On the basis of POLARIS studies of the inorganic chlorine budget, [ClO]/[ClONO2], and an intercomparison with balloon observations, the most direct explanation for the model‐measurement discrepancy in Cly, 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 ClONO2 measurement (too low). The high precision of our simulation (±15% 1σ for [ClONO2]/[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.

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

In situ observations Of NO2, NO, NOy, ClONO2, OH, O3, 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 NO2 and the slower chemistry connecting NOx and HNO3. 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/NO2 analysis suggests that reducing the activation energy for the NO+O3 reaction by 1.7 kJ/mol will improve model representation of the temperature dependence of the NO/NO2 ratio in the range 215–235 K. The NOx/HNO3 analysis shows that systematic errors in the relative rate coefficients used to describe NOx loss by the reaction OH + NO2 → HNO3 and by the reaction set NO2 + O3 → NO3; NO2 + NO3 → N2O5; N2O5 + H2O → 2HNO3 are in error by +8.4% (+30/−45%) (OH+NO2 too fast) in models using the Jet Propulsion Laboratory 1997 recommendations [DeMore et al., 1997]. Models that use recommendations for OH+NO2 and OH+HNO3 based on reanalysis of recent and past laboratory measurements are in error by 1.2% (+30/−45%) (OH+NO2 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 NOx production reactions HNO3 + OH → H2O + NO3 and HNO3 + hν → OH + NO2. Even under the assumption that all other aspects of the model are perfect, the POLARIS observations only constrain the rate coefficient for OH+HNO3 to a range of 65% around the currently recommended value.

Tropical stratospheric gravity wave activity and relationships to clouds

Wind measurements from NASA's ER‐2 aircraft in the stratosphere are used to obtain information on the momentum flux carried by gravity waves with horizontal wavelengths between 5 and 150 km. Tropical data are compared with the cloud brightness temperature below the aircraft as an indicator of deep convective activity. A striking correlation between cold, high clouds and large gravity wave momentum flux is seen in data from the Stratosphere‐Troposphere Exchange Project (STEP) tropical campaign during the monsoon season over northern Australia and Indonesia. There is an enhancement in the flux carried by waves propagating against the background wind in these observations. The same analysis was performed with data from more recent ER‐2 flights over the tropical Pacific Ocean during the Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA), Stratospheric Tracers of Atmospheric Transport (STRAT), and Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaigns which took place in 1994, 1995–1996, and 1997, respectively. These data also show a correlation between gravity wave momentum flux and deep convective clouds, but the relationship is much weaker, and the magnitudes of the momentum flux over the deepest clouds are about 7 times smaller than those seen in the STEP data. The reasons for these differences remain uncertain, but possibilities include both real geophysical differences and differences associated with the flight paths during the 1987 versus later campaigns.

Comparison of in situ N2O and CH4 measurements in the upper troposphere and lower stratosphere during STRAT and POLARIS

Nitrous oxide (N2O) and methane (CH4) were measured in the upper troposphere and lower stratosphere by multiple instruments aboard the NASA ER‐2 aircraft during the 1995–1996 Stratospheric Tracers of Atmospheric Transport (STRAT) and 1997 Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaigns. Differences between coincidental, in situ measurements are examined to evaluate the agreement and variability in the agreement between these instruments during each flight. Mean N2O measurement differences for each flight were much smaller than limits calculated from quoted values of N2O measurement accuracy and for all but two flights were ≤8.7 ppb (3.5%). Mean CH4 measurement differences for flights were similarly much smaller than calculated limits and for all but three flights were ≤65 ppb (4.4%). Typical agreement between instruments during flights averaged 6.2 ppb (2.5%) for N2O and 43 ppb (2.9%) for CH4. In contrast, for about half of the flights, the variability of N2O and CH4 measurement differences exceeded limits calculated from quoted values of measurement precision. The typical measurement difference variability (1σ) during a flight averaged ±8.0 ppb (3.2%) for N2O and ±43 ppb (2.9%) for CH4. For some flights, large differences or variations in differences are attributable to the poor measurement accuracy or precision of one instrument. It is demonstrated that small offsets between the computer clocks of these instruments can result in significant differences between their “coincidental” N2O and CH4 data, especially when there is high spatial variability in tracer abundance along a flight track.

Influence of air mass histories on radical species during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) mission

The Goddard trajectory chemistry model was used with ER‐2 aircraft data to test our current knowledge of radical photochemistry during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaign. The results of the trajectory chemistry model with and without trajectories are used to identify cases where steady state does not accurately describe the measurements. Over the entire mission, using trajectory chemistry reduces the variability in the modeled NOx comparisons to data by 25% with respect to the same model simulating steady state. Although the variability is reduced, NOx/NOy trajectory model results were found to be systematically low relative to the observations by 20–30% as seen in previous studies. Using new rate constants for reactions important in NOy partitioning improves the agreement of NOx/NOy with the observations but a 5–10% bias still exists. OH and HO2 individually are underpredicted by 15% of the standard steady state model and worsen with the new rate constants. Trajectory chemistry model results of OH/HO2 were systematically low by 10–20% but improve using the new rates constants because of the explicit dependence on NO. This suggests that our understanding of NOx is accurate to the 20% level and HOx chemistry is accurate to the 30% level in the lower stratosphere or better for the POLARIS regime. The behavior of the NOx and HOx comparisons to data using steady state versus trajectory chemistry and with updated rate coefficients is discussed in terms of known chemical mechanisms and lifetimes.

An examination of the inorganic chlorine budget in the lower stratosphere

We use the first simultaneous in situ measurements of ClONO2, ClO, and HCl acquired using the NASA ER‐2 aircraft during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) mission to test whether these three compounds quantitatively account for total inorganic chlorine (Cly) in the lower stratosphere in 1997. We find (ClO + ClONO2 + HCl)/Cly = 0.92±0.10, where Cly is inferred from in situ measurements of organic chlorine source gases. These observations are consistent with our current understanding of the budget and partitioning of Cly in the lower stratosphere. We find no evidence in support of missing inorganic chlorine species that compose a significant fraction of Cly. We apply the analysis to earlier ER‐2 observations dating from 1991 to investigate possible causes of previously observed discrepancies in the inorganic chlorine budget. Using space shuttle, satellite, balloon, and aircraft measurements in combination with ER‐2 data, we find that the discrepancy is unlikely to have been caused by missing chlorine species or an error in the photolysis rate of chlorine nitrate. We also find that HCl/Cly is not significantly controlled by aerosol surface area density in the lower stratosphere.

An examination of chemistry and transport processes in the tropical lower stratosphere using observations of long‐lived and short‐lived compounds obtained during STRAT and POLARIS

A suite of compounds with a wide range of photochemical lifetimes (3 months to several decades) was measured in the tropical and midlatitude upper troposphere and lower stratosphere during the Stratospheric Tracers of Atmospheric Transport (STRAT) experiment (fall 1995 and winter, summer, and fall 1996) and the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) deployment in late summer 1997. These species include various chlorofluorocarbons, hydrocarbons, halocarbons, and halons measured in whole air samples and CO measured in situ by tunable diode laser spectroscopy. Mixing ratio profiles of long‐lived species in the tropical lower stratosphere are examined using a one‐dimensional (1‐D) photochemical model that includes entrainment from the extratropical stratosphere and is constrained by measured concentrations of OH. Profiles of tracers found using the 1‐D model agree well with all the observed tropical profiles for an entrainment time scale of 8.5−4+6 months, independent of altitude between potential temperatures of 370 and 500 K. The tropical profile of CO is used to show that the annually averaged ascent rate profile, on the basis of a set of radiative heating calculations, is accurate to approximately ±44%, a smaller uncertainty than found by considering the uncertainties in the radiative model and its inputs. Tropical profiles of ethane and C2Cl4 reveal that the concentration of Cl is higher than expected on the basis of photochemical model simulations using standard gas phase kinetics and established relationships between total inorganic chlorine and CFC‐11. Our observations suggest that short‐lived organic chlorinated compounds and HCl carried across the tropical tropopause may provide an important source of inorganic chlorine to the tropical lower stratosphere that has been largely unappreciated in previous studies. The entrainment timescale found here is considerably less than the value found by a similar study that focused on observations obtained in the lower stratosphere during 1994. Several possible explanations for this difference are discussed.

A sensitivity study of photolysis rate coefficients during POLARIS

Recent improvements in the agreement between observation‐derived and modeled photolysis rate coefficients (j‐values) have allowed for the close examination of the sensitivity of j‐values to variations in physical parameters influencing their magnitude and temporal(spatial variability. Altitude and solar zenith angle profiles of j‐values for two photolytic processes, NO2 → NO + O(3P) and O3 → O2 + O(1D), are modeled, varying surface albedo, atmospheric baseheight, total column ozone, and ozone and temperature altitude profiles over the ranges observed during the NASA Photochemistry of Ozone Loss in the Arctic Region In Summer (POLARIS) high‐altitude ER‐2 aircraft campaign. The effect of atmospheric refraction at high solar zenith angles is also addressed. Modeled j‐values using measured ozone/albedo input from the Composition and Photodissociative Flux Measurement (CPFM) spectroradiometer on board the ER‐2 exceed those derived from CPFM flux measurements by 6% for jNO2 and 14% for jO3, within experimental uncertainties. The individual effects of albedo, baseheight, and ozone on j‐values along specific ER‐2 flight tracks are modeled and related to the temporal and spatial variability observed. For jNO2, surface albedo has the greatest effect; for jO3, the ozone above the aircraft and surface albedo are the most important.

Intercomparison of total ozone observations at Fairbanks, Alaska, during POLARIS

The pattern of seasonal ozone loss over Fairbanks, Alaska (AK), during the NASA Photochemistry of Ozone Loss in the Arctic Region In Summer (POLARIS) campaign in the spring and summer of 1997 is defined. Five independent data sets of total ozone observations at Fairbanks are presented, from the Earth Probe and ADEOS Total Ozone Mapping Spectrometer (TOMS) satellite instruments, balloon‐borne electrochemical concentration cell ozonesondes, and ground‐based (Brewer spectroradiometer, Dobson spectrophotometer, and the Jet Propulsion Laboratory MkIV infrared interferometer) instruments. The excellent agreement between different observational techniques lends confidence to the observed rate of summertime loss of total ozone at high latitudes. In addition, the small offsets between the data sets are well understood.

Comparison of modeled and observed values of NO2 and JNO2 during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) mission

Stratospheric measurements of NO, NO2, O3, ClO, and HO2 were made during spring, early summer, and late summer in the Arctic region during 1997 as part of the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) field campaign. In the sunlit atmosphere, NO2 and NO are in steady state through NO2 photolysis and reactions involving O3, ClO, BrO, and HO2. By combining observations of O3, ClO, and HO2, observed and modeled values of the NO2 photolysis rate coefficient (JNO2), and model estimates of BrO, several comparisons are made between steady state and measured values of both NO2 and JNO2. An apparent seasonal dependence in discrepancies between calculated and measured values was found; however, a source for this dependence could not be identified. Overall, the mean linear fits in the various comparisons show agreement within 19%, well within the combined uncertainties (±50 to 70%). These results suggest that photochemistry controlling the NO2/NO abundance ratio is well represented throughout much of the sunlit lower stratosphere. A reduction in the uncertainty of laboratory determinations of the rate coefficient of NO + O3 → NO2 + O2 would aid future analyses of these or similar atmospheric observations.

Preface [to special section on Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS)

Preface [to special section on Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS)

Comparison of MkIV balloon and ER‐2 aircraft measurements of atmospheric trace gases

On May 8, 1997, vertical profiles of over 30 different gases were measured remotely in solar occultation by the Jet Propulsion Laboratory MkIV Interferometer during a balloon flight launched from Fairbanks, Alaska. These gases included H2O, N2O, CH4, CO, NOx, NOy, HCl, ClNO3, CCl2F2, CCl3F, CCl4, CHClF2, CClF2CCl2F, SF6, CH3Cl, and C2H6, all of which were also measured in situ by instruments on board the NASA ER‐2 aircraft, which was making flights from Fairbanks during this same early May time period as part of the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) experiment. A comparison of the gas volume mixing ratios in the upper troposphere and lower stratosphere reveals agreement better than 5% for most gases. The three significant exceptions to this are SF6 and CCl4 for which the remote measurements exceed the in situ observations by 15–20% at all altitudes, and H2O for which the remote measurements are up to 30% smaller than the in situ observations near the hygropause.

Influence of synoptic waves on column ozone during Arctic summer 1997

The influence of synoptic waves on column ozone over Alaska during the summer of 1997 is explored using observations of ozone and meteorological quantities, a nonhydrostatic three‐dimensional model, and theory. From early May to early August, column ozone over Fairbanks declined from 450 to 250 Dobson Units, while 300 hPa heights rose from 8500 to 9500 m. Time series of daily column ozone and 300 hPa heights are correlated at −0.78. Week‐to‐week variations contribute strongly to this anticorrelation, with column ozone increasing as a trough approaches. Case studies of synoptic scale waves show that they disturb the lower stratosphere, stretching and folding tropospheric and stratospheric air during their life cycles. Mesoscale resolution simulations with the University of Wisconsin Nonhydrostatic Modeling System (UW‐NMS) were carried out in support of NASA's Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaigns. Layered ozone structures in an ER2 stairstep flight are represented fairly well by the UW‐NMS. Two ozone fields were transported in the model, with one subject to a sink below the dynamical tropopause. UW‐NMS ozone distributions, a simple box model, and a Stokes drift argument support the idea that synoptic waves cause a poleward and downward flow of ozone through the high latitude tropopause throughout the year. Planetary wave transport of ozone from the tropics is much reduced in the summer. Net transport out of the high latitude stratosphere likely contributes to column ozone decline during summer.

On the accuracy of in situ water vapor measurements in the troposphere and lower stratosphere with the Harvard Lyman‐α hygrometer

In an effort to better constrain atmospheric water vapor mixing ratios and to understand the discrepancies between different measurements of water vapor in the stratosphere and troposphere, we have carefully examined data from the Harvard Lyman‐α photofragment fluorescence hygrometer, which has flown on the NASA ER‐2 aircraft from 1992 through 1998. The instrument is calibrated in the laboratory before and after each deployment, and the calibration is checked by direct absorption measurements in the troposphere. On certain flights, the ER‐2 flew level tracks during which water vapor varied by up to 80 ppmv, under nearly constant atmospheric conditions; These flights provide a stringent test of our calibration via direct absorption and indicate agreement to within 3%. During the 1997 Photochemistry of Ozone Loss in the Arctic Region In Summer (POLARIS) mission, our Lyman‐α instrument was compared with a new diode laser hygrometer from the Jet Propulsion Laboratory. Overall agreement was 5% during the June/July deployment and 1% for potential temperatures of 490 to 540 K. The accuracy of our instrument is shown to be ±5%, with an additional offset of at most 0.1 ppmv. Data from this instrument, combined with simultaneous measurements of CH4 and H2, are therefore ideal for studies of the hydrogen budget of the lower stratosphere.

Closure of the total hydrogen budget of the northern extratropical lower stratosphere

Methane (CH4), molecular hydrogen (H2), and water vapor (H2O) were measured concurrently on board the NASA ER‐2 aircraft during the 1995–1996 Stratospheric Tracers of Atmospheric Transport (STRAT) and 1997 Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaigns. Correlations between these three main hydrogen reservoirs in the northern extratropical lower stratosphere are examined to evaluate H2O production from CH4 and H2 oxidation. The expected ratio of stratospheric H2O production (PH2O)to CH4 destruction (LCH4) = −1.973±0.003 is calculated from an evaluation of CH4 and H2 oxidation reactions and the relationship between H2 and CH4 mixing ratios measured during STRAT. Correlations between H2O and CH4 were tight and linear only for air masses with mean ages ≥3.8 years, restricting this analysis predominantly to latitudes between 40° and 90°N and potential temperatures between 470 and 540 K. The mean observed ΔH2O/CH4 (−2.15±0.18) is in statistical agreement with the expected PH2O/LCH4. The annual mean stratospheric entry mixing ratio for H2O calculated from this slope is 4.0 ± 0.3 ppm. The quantity H2O + 2·CH4 is quasi‐conserved at 7.4 ± 0.5 ppm in older air masses in the northern extratropical lower stratosphere. Significant departure of H2O + 2·CH4 from the mean value is a sensitive indicator of processes which influence H2O without affecting CH4, such as dehydration in a polar vortex or near the tropical tropopause. No significant trend is observed in ER‐2 aircraft data for H2O + 2·CH4 in the lower stratosphere from 1993 through 1997.