Daniel S. McKenna

Assessing future nitrogen deposition and carbon cycle feedback using a multimodel approach: Analysis of nitrogen deposition

n this study, we present the results of nitrogen deposition on land from a set of 29 simulations from six different tropospheric chemistry models pertaining to present-day and 2100 conditions. Nitrogen deposition refers here to the deposition (wet and dry) of all nitrogen-containing gas phase chemical species resulting from NOx (NO + NO2) emissions. We show that under the assumed IPCC SRES A2 scenario the global annual average nitrogen deposition over land is expected to increase by a factor of ∼2.5, mostly because of the increase in nitrogen emissions. This will significantly expand the areas with annual average deposition exceeding 1 gN/m2/year. Using the results from all models, we have documented the strong linear relationship between models on the fraction of the nitrogen emissions that is deposited, regardless of the emissions (present day or 2100). On average, approximately 70% of the emitted nitrogen is deposited over the landmasses. For present-day conditions the results from this study suggest that the deposition over land ranges between 25 and 40 Tg(N)/year. By 2100, under the A2 scenario, the deposition over the continents is expected to range between 60 and 100 Tg(N)/year. Over forests the deposition is expected to increase from 10 Tg(N)/year to 20 Tg(N)/year. In 2100 the nitrogen deposition changes from changes in the climate account for much less than the changes from increased nitrogen emissions.

Severe chemical ozone loss in the Arctic during the winter of 1995-96

Severe stratospheric ozone depletion is the result of perturbations of chlorine chemistry owing to the presence of polar stratospheric clouds (PSCs) during periods of limited exchange of air between the polar vortex and midlatitudes and partial exposure of the vortex to sunlight. These conditions are consistently encountered over Antarctica during the austral spring. In the Arctic, extensive PSC formation occurs only during the coldest winters, when temperatures fall as low as those regularly found in the Antarctic,,. Moreover, ozone levels in late winter and early spring are significantly higher than in the corresponding austral season,,, and usually strongly perturbed by atmospheric dynamics. For these reasons, chemical ozone loss in the Arctic is difficult to quantify. Here we use the correlation between CH4 and O3 in the Arctic polar vortex to discriminate between changes in ozone concentration due to chemical and dynamical effects. Our results indicate that 120–160 Dobson units (DU) of ozone were chemically destroyed between January and March 1996—a loss greater than observed in Antarctica in 1985, when the ‘ozone hole’ was first reported,. This loss outweighs the expected increase in total ozone over the same period through dynamical effects, leading to an observed net decrease of about 50 DU. This ozone loss arises through the simultaneous occurrence of extremely low Arctic stratospheric temperatures, and large stratospheric chlorine loadings. Comparable depletion is likely to recur because stratospheric cooling, and elevated chlorine concentrations, are expected to persist for several decades.

Fast in situ stratospheric hygrometers: A new family of balloon‐borne and airborne Lyman α photofragment fluorescence hygrometers

A new set of hygrometers based on the Lyman α photofragment fluorescence technique has been developed for operation on aircraft and balloons in the stratosphere and upper troposphere. They combine technical details from existing fluorescence hygrometers with several improvements in order to achieve the highest data quality and to minimize maintenance and operational procedures. With these instruments, stratospheric H2O measurements can be accomplished with a precision of < 0.2 ppmv at 1 s integration time, as has been demonstrated both in the laboratory and under field deployment. The design enables a rapid exchange of the air sample of the order of 1 s for fast measurements of small-scale variations of the H2O mixing ratio in the atmosphere. The hygrometer is calibrated using a laboratory calibration bench with approximately 4% accuracy. Measurements made by the hygrometer are compared with a frost point hygrometer during an aircraft mission at H2O mixing ratios from 280 to 8 ppmv, yielding an agreement between both techniques within the instrumental errors.

Airborne measurements of the photolysis frequency of NO2

A set of photoelectric detectors for airborne measurements of the photolysis frequency of NO2, i.e., JNO2, was developed and integrated aboard the research aircraft Hercules C-130 operated by the U.K. Meteorological Office. The instrument consists of two separate sensors, each of which provides an isotropic response over a solid angle of 2π steradian (sr). The sensors are mounted on top and below the aircraft, respectively, to obtain a field of view of 4π sr, and permit the discrimination of the upwelling and downwelling components of the actinic flux. From experimental tests and model calculations it is demonstrated that small differences between the spectral sensitivity of the sensors and the spectral response of JNO2 can lead to significant errors in the determination of JNO2, especially under cloudy conditions. We present correction factors for clear sky conditions and suggest the use of a new filter combination in the sensors which requires only small corrections and provides acceptable accuracy, even under cloudy conditions. A climatology of JNO2 values is presented from a series of flights made in 1993 at latitudes of 36°–59°N. For clear sky conditions and solar zenith angles of 33°–35°, JNO2 was 8.3 × 10−3 s−1 at sea level and increased with altitude to values of 13 × 10−3 s−1 at 7.5 km altitude. Above clouds, JNO2 reached maximum values of 24 × 10−3 s−1, and peak values of 29 × 10−3 s−1 were observed for very short periods in the uppermost layers of clouds. Enhancement of the actinic flux due to light scattered from clouds was also observed at altitudes below 0.5 km. Comparison of the clear sky data with predictions from different radiative transfer models reveals the best agreement for models of higher angular resolution. The Delta Eddington method underpredicts the measurements significantly, whereas the JNO2 values predicted by the discrete ordinate method and multidirectional model are only about 5% smaller than our measurements, a difference that is within the experimental uncertainties.

Ozone loss rates in the Arctic stratosphere in the winter 1991/92: Model calculations compared with match results

We present box model calculations of ozone loss rates corresponding to the results of the Match experiment 1991/92. The Match technique infers chemical ozone depletion from an analysis of pairs of balloon soundings that measure ozone in the same airparcel at different points of a calculated trajectory. It allows a quantitative comparison with model results because the exposure of the observed air-masses to sunlight is well known. The model significantly underestimates the loss rates inferred for January to mid-February. Extensive sensitivity studies show that the discrepancy between model and Match results cannot be explained by the known uncertainties in the model parameters.

Photochemical trajectory modeling studies of the North Atlantic region during August 1993

A Lagrangian photochemical trajectory model has been used to assess the factors affecting O-3 production during transport of polluted air masses across the North Atlantic Ocean. Sensitivity studies have been performed along idealized trajectories, and it is found that the potential impact of North American emission sources is maximized by transport of air at high altitudes, in drier conditions and in conditions where mixing of the air with background air masses is relatively limited. Measurements taken from the NCAR King Air aircraft as part of the North Atlantic Regional Experiment (NARE) August 1993 intensive have been used to initialize forward trajectories, calculated using European Centre for Medium-Range Weather Forecasting analyzed wind fields, from eastern North America to assess O-3 production over the Atlantic during this period. The effects of dilution of a polluted air parcel with air from the upper troposphere have also been studied, and the contribution of photochemical O-3 production to the air mass composition is found to be smaller than that of dilution, particularly for long trajectories and for conditions where dilution is relatively rapid or involves air from the stratosphere. Measurements taken from the Meteorological Research Flight Hercules aircraft over the eastern Atlantic as part of the Oxidizing Capacity of the Tropospheric Atmosphere campaign have been examined in the light of these studies. A backward trajectory analysis has been performed from one of the vertical profiles taken off the coast of Portugal on August 31, 1993, to assess the origin of the different air masses intercepted. While the lower levels are characteristic of air from the European boundary layer advected over the ocean, the upper levels show strong evidence for anthropogenic influence from North American sources, with elevated levels of O-3, NOy, CO, and aerosol. Although it cannot be concluded that this air mass definitely originated from over North America, the measured concentrations are shown to be consistent with those for an air mass from this source region experiencing some mixing with air masses in the upper troposphere.

HALOE observations of the vertical structure of chemical ozone depletion in the Arctic vortex during winter and early spring 1996-1997

We discuss observations by the Halogen Occultation Experiment on the Upper Atmosphere Research Satellite in the lower stratosphere in the Arctic vortex during winter and spring 1996-1997. Using HF as a chemically conserved tracer, we identify chemical ozone depletion and chlorine activation, despite variations caused by dynamical processes. For the Arctic vortex region, significant chemical ozone loss (up to two thirds around 475 K potential temperature) due to extensive activation of the inorganic chlorine reservoir is deduced, as observed similarly for previous winters. Chemical reductions in column ozone of up to 70-80 Dobson units (DU) in the lower stratosphere are calculated. Both chlorine activation and ozone loss inside the vortex, however, are more variable than observed in previous years.

Ozone loss rates in the Arctic stratosphere in the winter 1994/1995: Model simulations underestimate results of the Match analysis

We present box model simulations of ozone loss rates in the Arctic lower stratosphere for the winter 1994/1995. The ozone loss was simulated along each of the trajectories of the Match data set for 1994/1995 to conduct a quantitative comparison with the Match results. The simulated ozone loss rates reach their maximum value of ≈ 4 ppb per sunlit hour at the end of January. For this period and for potential temperatures below 475 K the model results are in good agreement with the Match results, but for potential temperatures above 475 K the model underestimates the ozone loss rate by up to a factor of 2. This difference cannot be explained by known uncertainties of the model. Enhanced ozone loss has also been observed in March 1995 for potential temperatures below 475 K. These loss rates are also substantially underestimated by the model, but are within the range of the model uncertainties. In particular, the ozone loss rates simulated for March 1995 strongly depend on the extent of denitrification.

A test of our understanding of the ozone chemistry in the Arctic polar vortex based on in situ measurements of ClO, BrO, and O3 in the 1994/1995 winter

We present an analysis of in situ measurements of ClO, BrO, O3, and long-lived tracers obtained on a balloon flight in the Arctic polar vortex launched from Kiruna, Sweden, 68°N, on February 3, 1995. Using the method of tracer correlations, we deduce that the air masses sampled at an altitude of 21 km (480 K potential temperature), where a layer of enhanced ClO mixing ratios of up to 1150 parts per trillion by volume was observed, experienced a cumulative chemical ozone loss of 1.0±0.3 ppmv between late November 1994 and early February 1995. This estimate of chemical ozone loss can be confirmed using independent data sets and independent methods. Calculations using a trajectory box model show that the simulations underestimate the cumulative ozone loss by approximately a factor of 2, although observed ClO and BrO mixing ratios are well reproduced by the model. Employing additional simulations of ozone loss rates for idealized conditions, we conclude that the known chlorine and bromine catalytic cycles destroying odd oxygen with the known rate constants and absorption cross sections do not quantitatively account for the early winter ozone losses infered for air masses observed at 21 km.

Balloon-borne in situ measurements of stratospheric H2O, CH4 and H2 at midlatitudes

Using a new Lyman α hygrometer, balloon-borne measurements of H 2 O were performed on September 20, 1993 at midlatitudes (43° N). A cryogenic whole air sampler to measure CH 4 and H 2 amongst other long-lived trace gases was operated on the same payload. The profile of the water vapor mixing ratio showed a well-pronounced hygropause of 4 ppmv at θ = 420 K increasing to 6.0 ppmv at θ = 920 K. For θ > 400 K, total hydrogen ΣH 2 = H 2 O + 2.CH 4 + H 2 appeared to be constant at a mean value of 7.72 ± 0.13 ppmv; the overall accuracy of this value is 0.65 ppmv. From the correlation functions of CH4 with H 2 and H 2 O the water vapor yield from methane oxidation is determined with high precision to be 1.975 ± 0.030. The globally averaged water vapor mixing ratio entering the stratosphere from the troposphere was found to be 3.9 ± 1.0 ppmv. Intercomparison with H 2 O and CH 4 profiles obtained nearby from the Halogen Occultation Experiment (HALOE) on board the UARS satellite showed good agreement within the experimental errors.