G. B. Osterman

Measurements of reactive nitrogen in the stratosphere

We present volume mixing ratio profiles of NO, NO2, HNO3, HNO4, N2O5, and ClNO3 and their composite budget (NOy), from 20 to 39 km, measured remotely in solar occultation by the Jet Propulsion Laboratory MkIV Interferometer during a balloon flight from Fort Sumner, New Mexico (35°N), on September 25, 1993. In general, observed profiles agree well with values calculated using a photochemical steady state model constrained by simultaneous MkIV observations of long-lived precursors and aerosol surface area from the Stratospheric Aerosol and Gas Experiment II. The measured variation of concentrations of NOx (= NO + NO2) and N2O5 between sunrise and sunset reveals the expected ∼2:1 stoichiometry at all altitudes. Despite relatively good agreement between theory and observation for profiles of NO and HNO3 the observed concentration of NO2 becomes progressively higher than model values below 30 km, with the discrepancy reaching ∼30% at 22 km. This suggests an incomplete understanding of factors that regulate the NO/NO2 and NO2/HNO3 ratios below 30 km. Observations obtained during September 1990, prior to the June 1991 eruption of Mount Pinatubo, as well as during April 1993 and September 1993 provide a test of our understanding of the affect of aerosol surface area on the NOx/NOy ratio at midlatitudes. The observations reveal a decrease in the NOx/NOy ratio for increasing aerosol surface area that is consistent with the heterogeneous hydrolysis of N2O5 being the dominant sink of between altitudes of 18 and 24 km for the conditions encountered (e.g., surface areas as high as 14 μm2 cm−3 and temperatures from 209 to 219 K).

Implementation of cloud retrievals for Tropospheric Emission Spectrometer (TES) atmospheric retrievals: part 1. Description and characterization of errors on trace gas retrievals

terms of a set of frequency-dependent nonscattering optical depths and a cloud height. These cloud parameters are retrieved jointly with surface temperature, emissivity, atmospheric temperature, and trace gases such as ozone from spectral data. We demonstrate the application of this approach using data from the Tropospheric Emission Spectrometer (TES) and test data simulated with a scattering radiative transfer model. We show the value of this approach in that it results in accurate estimates of errors for trace gas retrievals, and the retrieved values improve over the initial guess for a wide range of cloud conditions. Comparisons are made between TES retrievals of ozone, temperature, and water to model fields from the Global Modeling and Assimilation Office (GMAO), temperature retrievals from the Atmospheric Infrared Sounder (AIRS), tropospheric ozone columns from the Goddard Earth Observing System (GEOS) GEOS-Chem, and ozone retrievals from the Total Ozone Mapping Spectrometer (TOMS). In each of these cases, this cloud retrieval approach does not introduce observable biases into TES retrievals.

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.

Balloon-Borne Measurements of Stratospheric Radicals and their Precursors: Implications for the Production and Loss of Ozone

Measurements of hydrogen, nitrogen and chlorine radicals from a balloon flight on 25 September 1993 from Ft. Sumner, NM provide an opportunity to quantify photochemical production and loss of stratospheric ozone. Ozone loss rates determined using measured radical concentrations agree fairly well with loss rates calculated using a photochemical model. Catalytic cycles involving OH and HO2 are shown to dominate photochemical loss of ozone for altitudes between 44 and 50 km. Reactions involving NO and NO2 are the dominant sink for ozone between 25 and 38 km. The total ozone loss rate determined from the measurements balances calculated production rates for altitudes between 30 and 40 km. However, loss of ozone exceeds production by ∼35% between 42 and 50 km. The imbalance between production and loss of ozone above 42 km is larger than the uncertainty of any one of the critical kinetic parameters or species concentrations. No single adjustment to any of these parameters can simultaneously resolve the imbalance and satisfy constraints imposed by measured OH, HO2, NO2 and ClO. Our results are consistent with an additional mechanism for ozone production above 40 km other than photolysis of ground state O2.

Partitioning of NOy species in the summer Arctic stratosphere

Volume mixing ratio profiles of the quantitatively significant NO y species NO, NO 2 , HNO 3 , HNO 4 , ClNO 3 and N 2 O 5 were measured remotely from 8 to 38 km by the JPL MkIV FTIR solar absorption spectrometer during balloon flights from Fairbanks, Alaska (64.8°N, 147.6°W) on May 8 and July 8. 1997. The observed ratio of NO x (NO+NO 2 ) to NO y (total reactive nitrogen) is 10 to 30% greater than calculated by a steady state model using standard photochemistry constrained by MkIV measurements of long lived precursors (e.g., H 2 O, CH 4 , CO and N 2 O) and SAGE II aerosol surface area. The persistence of this discrepancy to 38 km altitude suggests that processes involving aerosols, such as the reduction of HNO 3 on the surface of soot particles, cannot be the sole explanation. The most likely resolution to this discrepancy is that the rate of NO 2 +OH +M→HNO 3 +M (the dominant sink of NO x in the Arctic stratosphere during times of near continuous solar illumination) is significantly slower than the currently recommended rate.

Quantifying spatial and seasonal variability in atmospheric ammonia with in situ and space-based observations

Ammonia plays an important role in many biogeochemical processes, yet atmospheric mixing ratios are not well known. Recently, methods have been developed for retrieving NH3 from space-based observations, but they have not been compared to in situ measurements. We have conducted a field campaign combining co-located surface measurements and satellite special observations from the Tropospheric Emission Spectrometer (TES). Our study includes 25 surface monitoring sites spanning 350 km across eastern North Carolina, a region with large seasonal and spatial variability in NH3. From the TES spectra, we retrieve a NH3 representative volume mixing ratio (RVMR), and we restrict our analysis to times when the region of the atmosphere observed by TES is representative of the surface measurement. We find that the TES NH3 RVMR qualitatively captures the seasonal and spatial variability found in eastern North Carolina. Both surface measurements and TES NH3 show a strong correspondence with the number of livestock facilities within 10 km of the observation. Furthermore, we find that TES NH3 RVMR captures the month-to-month variability present in the surface observations. The high correspondence with in situ measurements and vast spatial coverage make TES NH3 RVMR a valuable tool for understanding regional and global NH3 fluxes.

Ground‐based observations of Arctic O3 loss during spring and summer 1997

Ground-based solar absorption spectra were measured from Fairbanks, Alaska (65°N, 148°W) from March to September 1997 by the Jet Propulsion Laboratory (JPL) MkIV Fourier transform infrared (FTIR) spectrometer. The derived column abundances of 03 declined by 35% over this period (20% in April and May, and 15% during the summer), whereas those of HF, a long-lived tracer, changed by less than 5%. High-latitude, summertime balloon observations reveal similar shapes for the volume mixing ratio profiles of O 3 and HF in the lower stratosphere, where most of their column abundance resides. Vertical transport should therefore have similar effects on the column abundances of O 3 and HF. Data from the Halogen Occultation Experiment (HALOE) show a poleward decrease in the O 3 /HF ratio at all stratospheric altitudes, so that any reductions in column O 3 due to horizontal meridional transport would have been accompanied by even larger reductions in column HF. Therefore the observed column O 3 decrease must be the result of chemical loss processes. Column measurements of other atmospheric gases show a summertime maximum in the NO x /NO y column ratio and little change in the chlorine partitioning. We conclude that most of the reduction in column O 3 over Fairbanks from March to September 1997 was likely driven by NO x chemistry. These conclusions are supported by the similar behavior of column abundances measured by another ground-based FTIR spectrometer based in Ny Alesund, Spitsbergen, (79°N, 12°E).

The budget and partitioning of stratospheric chlorine during the 1997 Arctic summer

Volume mixing ratio profiles of HCl, HOCl, ClNO 3 , CH 3 Cl, CFC-12, CFC-11, CCl 4 , HCFC-22, and CFC-113 were measured simultaneously from 9 to 38 km by the Jet Propulsion Laboratory MkIV Fourier Transform Infrared solar absorption spectrometer during two balloon flights from Fairbanks, Alaska (64.8°N), on May 8 and July 8, 1997. The altitude variation of total organic chlorine (CCl y ), total inorganic chlorine (Cl y ), and the nearly constant value (3.7±0.2 ppbv) of their sum (Cl TOT ) demonstrates that the stratospheric chlorine species available to react with O 3 are supplied by the decomposition of organic chlorinated compounds whose abundances are well quantified. Measured profiles of HCl and ClNO 3 agree well with profiles found by photochemical model (differences < 10% for altitudes below 35 km) constrained by various other constituents measured by MkIV. The production of HCl by ClO + OH plays a relatively small role in the partitioning of HCl and ClNO 3 for the sampled air masses. However, better agreement with the measured profiles of HCl and ClNO 3 is obtained when this source of HCl is included in the model. Both the measured and calculated [ClNO 3 ]/[HCl] ratios exhibit the expected near linear variation with [O 3 ] 2 /[CH 4 ] over a broad range of altitudes. MkIV measurements of HCl, ClNO 3 , and CCl y agree well with ER-2 in situ observations of these quantities for directly comparable air masses. These results demonstrate good understanding of the budget of stratospheric chlorine and that the partitioning of inorganic chlorine is accurately described by photochemical models that employ JPL97 reaction rates and production of HCl from ClO + OH for the environmental conditions encountered: relatively warm temperatures, long periods of solar illumination, and relatively low aerosol surface areas.

Lightning and anthropogenic NOx sources over the United States and the western North Atlantic Ocean: Impact on OLR and radiative effects

The migration of enhancements in NO_2 concentration, outgoing longwave radiation (OLR), and radiative effects associated with the onset of the North American Monsoon in July 2005 has been investigated using satellite data and the Regional Chemical Transport Model (REAM). The satellite data include the tropospheric NO2 columns, tropospheric O_3 profiles, and OLR from OMI, TES and NOAA-16 satellite, respectively, for June and July 2005. The simulated OLR captures the spatial distribution of the remotely sensed OLR fields with relatively small biases (≤5.7%) and high spatial correlations (R ≥ 0.88). This study reveals that the lightning-generated NOx exerts a larger, by up to a factor of three, impact on OLR (up to 0.35 Wm^(−2)) and radiative effects (up to 0.55 Wm^(−2)) by enhancing O_3 in the upper troposphere than anthropogenic NO_x that increases O_3 in the lower troposphere, despite the fact that the lightning-generated NO_x and O_3 are much smaller than those from the anthropogenic emissions. The radiative effect by lightning-derived upper tropospheric O_3 over the convective outflow regions is affected by the changes in lightning frequency. Thus the changes in convection due to global warming may alter the geographical distribution and magnitude of the radiative effect of lightning-derived O3, and this paper is a first step in quantifying the current radiative impact.

A global comparison of carbon monoxide profiles and column amounts from Tropospheric Emission Spectrometer (TES) and Measurements of Pollution in the Troposphere (MOPITT)

[1] In this study, we compare carbon monoxide (CO) products from the Measurements of Pollution in the Troposphere (MOPITT) and Tropospheric Emission Spectrometer (TES) and investigate the possible causes of the differences between retrievals for these two data sets. Direct comparisons of CO retrievals for July 2006 show that TES CO concentrations are consistently biased lower than those of MOPITT by 25 ppbv near the surface and by 20 ppbv at 150 hPa, primarily due to different a priori profiles and covariance matrices used in the TES and MOPITT CO retrievals. To reduce the effects of different a priori constraints, we apply TES a priori profiles and covariance matrices to a modified MOPITT retrieval algorithm. The mean TES-MOPITT CO difference decreases from � 25 to � 10 ppbv near the surface. To further account for retrieval smoothing errors due to different TES and MOPITT averaging kernels, TES averaging kernels are used to smooth MOPITT CO profiles to derive TES-equivalent CO profiles. Compared to these, TES CO profiles are biased 1 ppbv lower near the surface and 4–9 ppbv lower in the troposphere, and the mean absolute TES and TES-equivalent CO column difference is less than 6.5%. The mean TES and MOPITT CO differences due to smoothing errors are close to zero, and the remaining bias is primarily due to the combined effects of radiance biases, forward model errors, and the spatial and temporal mismatches of TES and MOPITT pixels.