Nitrous Oxide in the Atmosphere: First Measurements of a Lower Thermospheric Source

Nitrous oxide (N2O) is an important anthropogenic greenhouse gas, as well as one of the most significant anthropogenic ozone-depleting substances in the stratosphere. The satellite-based instrument Atmospheric Chemistry Experiment-Fourier Transform Spectrometer has been observing the Earth\u27s limb since 2004 and derives profiles of N2O volume mixing ratios in the upper troposphere to the lower thermosphere. The resulting climatology shows that N2O is continuously produced in the lower thermosphere via energetic particle precipitation and enhanced N2O is present at all latitudes, during all seasons. The results are consistent with an N2O production source peaking near or above 94 km via low-energy particles, as well as a polar wintertime source near 70 km via medium energy particles. N2O produced in the polar upper atmosphere descends each winter to as far down as ∼40 km. ©2016. American Geophysical Union

The SPARC Data Initiative: comparisons of CFC-11, CFC-12, HF and SF6 climatologies from international satellite limb sounders

A quality assessment of the CFC-11 (CCl3F), CFC-12 (CCl2F2), HF, and SF6 products from limb-viewing satellite instruments is provided by means of a detailed intercomparison. The climatologies in the form of monthly zonal mean time series are obtained from HALOE, MIPAS, ACE-FTS, and HIRDLS within the time period 1991–2010. The intercomparisons focus on the mean biases of the monthly and annual zonal mean fields and aim to identify their vertical, latitudinal and temporal structure. The CFC evaluations (based on MIPAS, ACE-FTS and HIRDLS) reveal that the uncertainty in our knowledge of the atmospheric CFC-11 and CFC-12 mean state, as given by satellite data sets, is smallest in the tropics and mid-latitudes at altitudes below 50 and 20 hPa, respectively, with a 1σ multi-instrument spread of up to ±5 %. For HF, the situation is reversed. The two available data sets (HALOE and ACE-FTS) agree well above 100 hPa, with a spread in this region of ±5 to ±10 %, while at altitudes below 100 hPa the HF annual mean state is less well known, with a spread ±30 % and larger. The atmospheric SF6 annual mean states derived from two satellite data sets (MIPAS and ACE-FTS) show only very small differences with a spread of less than ±5 % and often below ±2.5 %. While the overall agreement among the climatological data sets is very good for large parts of the upper troposphere and lower stratosphere (CFCs, SF6) or middle stratosphere (HF), individual discrepancies have been identified. Pronounced deviations between the instrument climatologies exist for particular atmospheric regions which differ from gas to gas. Notable features are differently shaped isopleths in the subtropics, deviations in the vertical gradients in the lower stratosphere and in the meridional gradients in the upper troposphere, and inconsistencies in the seasonal cycle. Additionally, long-term drifts between the instruments have been identified for the CFC-11 and CFC-12 time series. The evaluations as a whole provide guidance on what data sets are the most reliable for applications such as studies of atmospheric transport and variability, model–measurement comparisons and detection of long-term trends. The data sets will be publicly available from the SPARC Data Centre and through PANGAEA (doi:10.1594/PANGAEA.849223)

Hydrocarbons in the Upper Troposphere and Lower Stratosphere Observed from ACE-FTS and Comparisons with WACCM

Satellite measurements from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) are used to examine the global, seasonal variations of several hydrocarbons, including carbon monoxide (CO), ethane (C2H6), acetylene (C2H2), and hydrogen cyanide (HCN). We focus on quantifying large-scale seasonal behavior from the middle troposphere to the stratosphere, particularly in the tropics, and furthermore make detailed comparisons with the Whole Atmosphere Community Climate Model (WACCM) chemistry climate model (incorporating tropospheric photochemistry, time-varying hydrocarbon emissions, and meteorological fields nudged from reanalysis). Comparisons with Microwave Limb Sounder (MLS) measurements of CO are also included to understand sampling limitations of the ACE-FTS data and biases among observational data sets. Results show similar overall variability for CO, C2H6, and C2H2, with a semiannual cycle in the tropical upper troposphere related to seasonally varying sources and deep tropical convection, plus a maximum during Northern Hemisphere summer tied to the Asian monsoon anticyclone. These species also reveal a strong annual cycle above the tropical tropopause, tied to annual variations in the upward branch of Brewer-Dobson circulation. HCN reveals substantial differences from the other species, due to a longer photochemical lifetime and a chemical sink associated with ocean surface contact, which produces a minimum in the tropical upper troposphere not observed in the other species. For HCN, transport to the stratosphere occurs primarily through the Asian summer monsoon anticyclone. Overall, the WACCM simulation is able to reproduce most of the large-scale features observed in the ACE-FTS data, suggesting a reasonable simulation of sources and large-scale transport. The model is too low in the Southern Hemisphere subtropics during Austral spring, which indicates underestimate of biomass burning emissions and/or insufficient vertical transport in the model. © 2012. American Geophysical Union

Validation of ACE-FTS Version 3.5 NOy Species Profiles Using Correlative Satellite Measurements

The ACE-FTS (Atmospheric Chemistry Experiment - Fourier Transform Spectrometer) instrument on the Canadian SCISAT satellite, which has been in operation for over 12 years, has the capability of deriving stratospheric profiles of many of the NOy (N + NO + NO2 + NO3 + 2 x N2O5 + HNO3 + HNO4 + ClONO2 + BrONO2) species. Version 2.2 of ACE-FTS NO, NO2, HNO3, N2O5, and ClONO2 has previously been validated, and this study compares the most recent version (v3.5) of these five ACE-FTS products to spatially and temporally coincident measurements from other satellite instruments - GOMOS, HALOE, MAESTRO, MIPAS, MLS, OSIRIS, POAM III, SAGE III, SCIAMACHY, SMILES, and SMR. For each ACE-FTS measurement, a photochemical box model was used to simulate the diurnal variations of the NOy species and the ACE-FTS measurements were scaled to the local times of the coincident measurements. The comparisons for all five species show good agreement with correlative satellite measurements. For NO in the altitude range of 25-50 km, ACE-FTS typically agrees with correlative data to within -10%. Instrument-averaged mean relative differences are approximately -10% at 30-40 km for NO2, within ± 7% at 8-30km for HNO3, better than -7 % at 21-34 km for local morning N205, and better than -8% at 21-34 km for ClONO2. Where possible, the variations in the mean differences due to changes in the comparison local time and latitude are also discussed

Assessment of the quality of ACE-FTS stratospheric ozone data

For the past 17 years, the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) instrument on the Canadian SCISAT satellite has been measuring profiles of atmospheric ozone. The latest operational versions of the level 2 ozone data are versions 3.6 and 4.1. This study characterizes how both products compare with correlative data from other limb-sounding satellite instruments, namely MAESTRO, MLS, OSIRIS, SABER, and SMR. In general, v3.6, with respect to the other instruments, exhibits a smaller bias (which is on the order of similar to 3 %) in the middle stratosphere than v4.1 (similar to 2 %-9 %); however, the bias exhibited in the v4.1 data tends to be more stable, i.e. not changing significantly over time in any altitude region. In the lower stratosphere, v3.6 has a positive bias of about 3 %-5 % that is stable to within +/- 1 % per decade, and v4.1 has a bias on the order of -1 % to +5 % and is also stable to within +/- 1 % per decade. In the middle stratosphere, v3.6 has a positive bias of similar to 3 % with a significant negative drift on the order of 0.5 %-2.5 % per decade, and v4.1 has a positive bias of 2 %-9 % that is stable to within +/- 0.5 % per decade. In the upper stratosphere, v3.6 has a positive bias that increases with altitude up to similar to 16 % and a significant negative drift on the order of 2 %-3 % per decade, and v4.1 has a positive bias that increases with altitude up to similar to 15 % and is stable to within +/- 1 % per decade. Estimates indicate that both versions 3.6 and 4.1 have precision values on the order of 0.1-0.2 ppmv below 20 km and above 45 km (similar to 5 %-10 %, depending on altitude). Between 20 and 45 km, the estimated v3.6 precision of similar to 4 %-6 % is better than the estimated v4.1 precision of similar to 6 %-10 %

Global carbonyl sulfide (OCS) measured by MIPAS/Envisat during 2002–2012

We present a global carbonyl sulfide (OCS) data set covering the period June 2002 to April 2012, derived from FTIR (Fourier transform infrared) limb emission spectra measured with the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on the ENVISAT satellite. The vertical resolution is 4–5 km in the height region 6–15 km and 15 at 40 km altitude. The total estimated error amounts to 40–50 pptv between 10 and 20 km and to 120 pptv at 40 km altitude. MIPAS OCS data show no systematic bias with respect to balloon observations, with deviations mostly below ±50 pptv. However, they are systematically higher than the OCS volume mixing ratios of the ACE-FTS instrument on SCISAT, with maximum deviations of up to 100 pptv in the altitude region 13–16 km. The data set of MIPAS OCS exhibits only moderate interannual variations and low interhemispheric differences. Average concentrations at 10 km altitude range from 480 pptv at high latitudes to 500–510 pptv in the tropics and at northern mid-latitudes. Seasonal variations at 10 km altitude amount to up to 35 pptv in the Northern and up to 15 pptv in the Southern Hemisphere. Northern hemispheric OCS abundances at 10 km altitude peak in June in the tropics and around October at high latitudes, while the respective southern hemispheric maxima were observed in July and in November. Global OCS distributions at 250 hPa (∼ 10–11 km) show enhanced values at low latitudes, peaking during boreal summer above the western Pacific and the Indian Ocean, which indicates oceanic release. Further, a region of depleted OCS amounts extending from Brazil to central and southern Africa was detected at this altitude, which is most pronounced in austral summer. This depletion is related to seasonally varying vegetative uptake by the tropical forests. Typical signatures of biomass burning like the southern hemispheric biomass burning plume are not visible in MIPAS data, indicating that this process is only a minor source of upper tropospheric OCS. At the 150 hPa level (∼ 13–14 km) enhanced amounts of OCS were also observed inside the Asian monsoon anticyclone, but this enhancement is not especially outstanding compared to other low latitude regions at the same altitude. At the 80 hPa level (∼ 17–18 km), equatorward transport of mid-latitude air masses containing lower OCS amounts around the summertime anticyclones was observed. A significant trend could not be detected in upper tropospheric MIPAS OCS amounts, which points to globally balanced sources and sinks. Simulations with the ECHAM-MESSy model reproduce the observed latitudinal cross sections fairly well

Asian Monsoon Transport of Pollution to the Stratosphere

Transport of air from the troposphere to the stratosphere occurs primarily in the tropics, associated with the ascending branch of the Brewer-Dobson circulation. Here we identify the transport of air masses from the surface, through the Asian monsoon, and deep into the stratosphere, using satellite observations of hydrogen cyanide (HCN), a tropospheric pollutant produced in biomass burning. A key factor in this identification is that HCN has a strong sink from contact with the ocean; much of the air in the tropical upper troposphere is relatively depleted in HCN, and hence broad tropical upwelling cannot be the main source for the stratosphere. The monsoon circulation provides an effective pathway for pollution from Asia, India and Indonesia to enter the global stratosphere