Stephen George Donnelly

Extreme deuterium enrichment in stratospheric hydrogen and the global atmospheric budget of H2.

Molecular hydrogen (H2) is the second most abundant trace gas in the atmosphere after methane (CH4). In the troposphere, the D/H ratio of H2 is enriched by 120‰ relative to the worlds oceans. This cannot be explained by the sources of H2 for which the D/H ratio has been measured to date (for example, fossil fuels and biomass burning). But the isotopic composition of H2 from its single largest source—the photochemical oxidation of methane—has yet to be determined. Here we show that the D/H ratio of stratospheric H2 develops enrichments greater than 440‰, the most extreme D/H enrichment observed in a terrestrial material. We estimate the D/H ratio of H2 produced from CH4 in the stratosphere, where production is isolated from the influences of non-photochemical sources and sinks, showing that the chain of reactions producing H2 from CH4 concentrates D in the product H2. This enrichment, which we estimate is similar on a global average in the troposphere, contributes substantially to the D/H ratio of tropospheric H2.

Partitioning of the Reactive Nitrogen Reservoir in the lower stratosphere of the southern hemisphere: Observations and modeling

Measurements of nitric oxide (NO), nitrogen dioxide (NO2), and total reactive nitrogen (NOy = NO + NO2 + NO3 + HNO3 + ClONO2 + 2N2O5 + …) were made during austral fall, winter, and spring 1994 as part of the NASA Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft mission. Comparisons between measured NO2 values and those calculated using a steady state (SS) approximation are presented for flights at mid and high latitudes. The SS results agree with the measurements to within 8%, suggesting that the kinetic rate coefficients and calculated NO2 photolysis rate used in the SS approximation are reasonably accurate for conditions in the lower stratosphere. However, NO2 values observed in the Concorde exhaust plume were significantly less than SS values. Calculated NO2 photolysis rates showed good agreement with values inferred from solar flux measurements, indicating a strong self-consistency in our understanding of UV radiation transmission in the lower stratosphere. Model comparisons using a full diurnal, photochemical steady state model also show good agreement with the NO and NO2 measurements, suggesting that the reactions affecting the partitioning of the NOy reservoir are well understood in the lower stratosphere.

Subsidence, mixing, and denitrification of Arctic polar vortex air measured during POLARIS

We determine the degree of denitrification that occurred during the 1996-1997 Arctic winter using a technique that is based on balloon and aircraft borne measurements of NO y , N 2 O, and CH 4 . The NO 3 /N 2 O relation can undergo significant change due to isentropic mixing of subsided vortex air masses with extravortex air due to the high nonlinearity of the relation. These transport related reductions in NO y can be difficult to distinguish from the effects of denitrification caused by sedimentation of condensed HNO 3 . In this study, high-altitude balloon measurements are used to define the properties of air masses that later descend in the polar vortex to altitudes sampled by the ER-2 aircraft (i.e., ∼20 km) and mix isentropically with extravortex air. Observed correlations of CH 4 and N 2 O are used to quantify the degree of subsidence and mixing for individual air masses. On the basis of these results the expected mixing ratio of NO y resulting from subsidence and mixing, defined here as NO y ** , is calculated and compared with the measured mixing ratio of NO y . Values of NO y and NO y ** agree well during most parts of the flights. A slight deficit of NO y versus NO y ** is found only for a limited region during the ER-2 flight on April 26, 1997. This deficit is interpreted as indication for weak denitrification (∼2-3 ppbv) in that air mass. The small degree of denitrification is consistent with the general synoptic-scale temperature history of the sampled air masses, which did not encounter temperatures below the frostpoint and had relatively brief encounters with temperatures below the nitric acid trihydrate equilibrium temperature. Much larger degrees of denitrification would have been inferred if mixing effects had been ignored, which is the traditional approach to diagnose denitrification. Our analysis emphasizes the importance of using other correlations of conserved species to be able to accurately interpret changes in the NO y /N 2 O relation with respect to denitrification.

Multiscale simulations of tropospheric chemistry in the eastern Pacific and on the U.S. West Coast during spring 2002

[ 1] Regional modeling analysis for the Intercontinental Transport and Chemical Transformation 2002 (ITCT 2K2) experiment over the eastern Pacific and U. S. West Coast is performed using a multiscale modeling system, including the regional tracer model Chemical Weather Forecasting System (CFORS), the Sulfur Transport and Emissions Model 2003 (STEM-2K3) regional chemical transport model, and an off-line coupling with the Model of Ozone and Related Chemical Tracers ( MOZART) global chemical transport model. CO regional tracers calculated online in the CFORS model are used to identify aircraft measurement periods with Asian influences. Asian-influenced air masses measured by the National Oceanic and Atmospheric Administration (NOAA) WP-3 aircraft in this experiment are found to have lower DeltaAcetone/DeltaCO, DeltaMethanol/DeltaCO, and DeltaPropane/DeltaEthyne ratios than air masses influenced by U. S. emissions, reflecting differences in regional emission signals. The Asian air masses in the eastern Pacific are found to usually be well aged (> 5 days), to be highly diffused, and to have low NOy levels. Chemical budget analysis is performed for two flights, and the O-3 net chemical budgets are found to be negative ( net destructive) in the places dominated by Asian influences or clear sites and positive in polluted American air masses. During the trans-Pacific transport, part of gaseous HNO3 was converted to nitrate particle, and this conversion was attributed to NOy decline. Without the aerosol consideration, the model tends to overestimate HNO3 background concentration along the coast region. At the measurement site of Trinidad Head, northern California, high-concentration pollutants are usually associated with calm wind scenarios, implying that the accumulation of local pollutants leads to the high concentration. Seasonal variations are also discussed from April to May for this site. A high-resolution nesting simulation with 12-km horizontal resolution is used to study the WP-3 flight over Los Angeles and surrounding areas. This nested simulation significantly improved the predictions for emitted and secondary generated species. The difference of photochemical behavior between the coarse (60-km) and nesting simulations is discussed and compared with the observation.

Measurements of the NO y ‐N2O correlation in the lower stratosphere: Latitudinal and seasonal changes and model comparisons

The tracer species nitrous oxide, N2O, and the reactive nitrogen reservoir, NOy, were measured in situ using instrumentation carried aboard the NASA ER-2 high altitude aircraft as part of the NASA Airborne Southern Hemisphere Ozone Expedition/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA) and Stratospheric Tracers of Atmospheric Transport (STRAT) missions. Measurements were made throughout the latitude range of 70°S to 60°N over the time period of March to October 1994 and October 1995 to January 1996, which includes the period when the Antarctic polar vortex is most intense. The correlation plots of NOy with N2O reveal compact, near-linear curves throughout data obtained in the lower stratosphere (50 mbar to 200 mbar). The average slope of the correlation, ΔNOy/ΔN2O, in the southern hemisphere (SH) exhibited a much larger seasonal variation during this time period than was observed in the northern hemisphere (NH). Between March and October in the potential temperature range of 400 K to 525 K, the correlation slope in the SH midlatitudes increased by 28%. A smaller but still positive increase in the correlation slope was observed for higher-latitude data obtained within or near the edge of the SH polar vortex. At NH midlatitudes the correlation slope did not significantly change between March and October, while between October and January the slope increased by +7%. The larger SH midlatitude increase is consistent with ongoing descent throughout the winter and spring and also suggests that denitrification, the irreversible loss of HNO3 through sedimentation of cloud particles, is not a significant term (<10–15%) in the budget of NOy at SH midlatitudes during the wintertime. A secular increase in the correlation slope is ruled out by comparison with SH data obtained during the 1987 Airborne Antarctic Ozone Expedition (AAOE) aircraft campaign. These results suggest that a seasonal cycle exists in the correlation slope for both hemispheres, with the SH correlation slope returning to the April value during the SH spring and summer. Changes in stratospheric circulation also probably play a role in both the SH and the NH correlation slope seasonal cycles. Comparisons with two-dimensional model results suggest that the slope decreases when the denitrified Antarctic vortex is diluted into midlatitudes upon vortex breakup in the spring and that through the descent of stratospheric air, the slope recovers during the following fall/winter period.

Evaluating the role of NAT, NAD, and liquid H2SO4/H2O/HNO3 solutions in Antarctic polar stratospheric cloud aerosol: Observations and implications

Airborne measurements of total reactive nitrogen (NOy) and polar stratospheric cloud (PSC) aerosol particles were made in the Antarctic (68°S) as part of the NASA Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAES A) campaign in late July 1994. As found in both polar regions during previous studies, substantial PSC aerosol volume containing NOy was observed at temperatures above the frost point, confirming the presence of particles other than water ice. The composition of the aerosol particles is evaluated using equilibrium expressions for nitric acid trihydrate (NAT), nitric acid dihydrate (NAD), and the supercooled ternary solution (STS) composed of nitric acid (HNO3), sulfuric acid (H2SO4), and water (H2O). The equilibrium abundance of condensed HNO3 is calculated for each phase and compared to estimates made using observations of aerosol volume and NOy. The best agreement is found for STS composition, using criteria related to the onset and abundance of aerosol volume along the flight track. Throughout the PSC region, a comparison of the number of particles between 0.4 and 4.0 μm diameter with the number of available nuclei indicates that a significant fraction of the background aerosol number participates in PSC growth. Modeled STS size distributions at temperatures below 191 K compare favorably with measured size distributions of PSC aerosol. Calculations of the heterogeneous loss of chlorine nitrate (ClONO2) show that the reactivity of the observed PSC surface area is 30 to 300% greater with STS than with NAT composition for temperatures less than 195 K. The total volume of STS PSCs is shown to be more sensitive than NAT to increases in H2O, HNO3, and H2SO4 from supersonic aircraft fleet emissions. Using the current observations and perturbations predicted by the current aircraft assessments, an increase of 50 to 260% in STS aerosol volume is expected at the lowest observed temperatures (190 to 192 K), along with an extension of significant PSC activity to regions ∼0.7 K higher in temperature. These results improve our understanding of PSC aerosol formation in polar regions while strengthening the requirement to include STS aerosols in studies of polar ozone loss and the effects of aircraft emissions.

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, NO_(2), O_(3), ClO, and HO_(2) 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, NO_(2) and NO are in steady state through NO2 photolysis and reactions involving O_(3), ClO, BrO, and HO_(2). By combining observations of O_(3), ClO, and HO_(2), observed and modeled values of the NO_(2) photolysis rate coefficient (JNO_(2)), and model estimates of BrO, several comparisons are made between steady state and measured values of both NO_(2) and JNO_(2). 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 NO_(2)/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 + O_(3) → NO_(2) + O_(2) would aid future analyses of these or similar atmospheric observations.

Fossil-fueled power plants as a source of atmospheric carbon monoxide

Elevated carbon monoxide (CO) mixing ratios in excess of those derived from emissions inventories have been observed in plumes from one gas- and coal-fired power plant and three of four lignite coal-fired electric utility power plants observed in east and central Texas. Observations of elevated CO on days characterized by differing wind directions show that CO emissions from the lignite plants were relatively constant over time and cannot be ascribed to separate sources adjacent to the power plants. These three plants were found to be emitting CO at rates 22 to 34 times those tabulated in State and Federal emissions inventories. Elevated CO emissions from the gas- and coal-fired plant were highly variable on time scales of hours to days, in one case changing by a factor of 8 within an hour. Three other fossil-fueled power plants, including one lignite-fired plant observed during this study, did not emit substantial amounts of CO, suggesting that a combination of plant operating conditions and the use of lignite coal may contribute to the enhanced emissions. Observed elevated CO emissions from the three lignite plants, if representative of average operating conditions, represent an additional 30% of the annual total CO emissions from point sources for the state of Texas.

Observations of APAN during TexAQS 2000

Measurements of peroxycarboxylic nitric anhydrides (PANs) made in Houston, Texas during TexAQS (Texas Air Quality Study) 2000 showed a relatively abundant PAN compound that had not been identified in previous studies in North America [cf Williams et al., 2000]. This compound was hypothesized to be peroxyacrylic nitric anhydride {CH 2 =CH-C(O)OONO 2 , APAN} based on the work of Tanimoto and Akimoto, [2001]. APAN was synthesized and characterized on one of the two GC systems used to make those measurements, subsequent to the TexAQS 2000 field study, confirming that APAN was observed during TexAQS 2000, both on the ground and in airborne measurements. Mixing ratios of APAN were estimated from the response of the system to PAN and PPN and ranged up to 502 pptv, which was 30% of PAN. High APAN values were associated with the precursor species 1,3-butadiene and acrolein, which had local petrochemical sources. The presence of APAN at these unprecedented levels demonstrates the impact of these reactive VOC species, and may have associated health effects.

Influence of Antarctic denitrification on two‐dimensional model NO y /N2O correlations in the lower stratosphere

The mechanisms responsible for latitudinal and seasonal variations in the stratospheric NOy/N2O correlation, represented by the effective NOy yield from N2O loss, or FNOy, are explored using the Garcia-Solomon two-dimensional model. The model is run with and without Antarctic denitrification. Model results are compared to in situ NOy/N2O measurements taken onboard the NASA ER-2 high-altitude aircraft in the lower stratosphere during the 1994 Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft campaign, and to global-scale measurements taken onboard the Upper Atmosphere Research Satellite (UARS) from 1992 to 1993. The southern hemisphere midlatitude seasonal cycle observed by the ER-2 and the latitudinal gradients observed by UARS are consistent with the results of the denitrified model, although some aspects of the model results are sensitive to prescribed and/or calculated horizontal diffusion coefficients. The consistency with observations supports the models prediction of a seasonal cycle in which FNOy, increases at southern midlatitudes during winter due to descent of FNOy-enriched air from above and decreases in spring due to mixing with FNOy-depleted air from the denitrified polar vortex. Antarctic denitrification appears to affect midlatitudes mainly by a one-time dilution of the polar vortex following the final warming rather than by flow-through vortex processing during the winter. Because of the high concentrations of NO3, at polar latitudes before denitrification a large fraction of total stratospheric NOy can be removed by a one-time dilution of the denitrified polar vortex. The nondenitrified model results generally do not agree well with observations, suggesting that denitrification strongly influences latitudinal and seasonal variations in FNOy in the southern hemisphere.