Minor Atmospheric Constituents Research Articles

A 1-D model to retrieve the vertical profiles of minor atmospheric constituents for cloud microphysical modelling: III. Disturbed weather situations

A hybrid Monte-Carlo Gear solver developed earlier has been improvised to retrieve the vertical profiles of CH4 and N2O during disturbed weather situations such as western disturbances, tropical cyclones, and heavy rainfall events over the megacities: Delhi, Kolkata, Chennai, and Mumbai. Due to rapid changes in the temperature during the passage of these systems over megacities, the percentage differences of CH4 and N2O number concentrations were more compared to the Community Long-term Infrared Microwave Coupled Atmospheric Product System (CLIMCAPS). Therefore, the hybrid solver has been modified by improving maximum likelihood estimates of vertical temperature profiles. The number concentrations of CH4 and N2O during these weather events since 2012 are obtained from the CLIMCAPS dataset for bias correction. It is found that the modified methodology has improved the retrieval of CH4 and N2O vertical profiles by reducing the error percentages during daytime and nighttime over these megacities. The percentage error in estimated number concentrations of CH4 and N2O significantly reduced during (i) the passage of the western disturbance and rainy days of August 2020 over Delhi; (ii) the rainy days of June 2020 over Kolkata; (iii) the influence of supercyclonic storm Amphan (24 and 25 Nov 2020) over Chennai and (iv) rainy days of July 2020 over Mumbai. Implementing this solver in the global model may retrieve the number concentrations more accurately.

A 1-D model to retrieve the vertical profiles of minor atmospheric constituents for cloud microphysical modeling: II. Simulation of diurnal cycle

The accurate forecast of the diurnal cycle of the number concentration of trace gases is vital due to their influence on precipitation processes by controlling the number concentration of cloud condensation nuclei (CCN). 1-D hybrid Monte Carlo-Gear solver developed to retrieve vertical profiles of the number concentration of CCNs for microphysics modeling has been tested for representation of the diurnal cycle in the present paper. The retrieved profiles of CH4 and SO2 have been tested with the Copernicus Atmosphere Monitoring Service (CAMS) model at 3-hour time intervals for four megacities: Delhi, Kolkata, Chennai, and Mumbai for rainy and non-rainy days. The retrieved profiles have shown diurnal variation up to 18 UTC at all pressure levels with lead or lag with that of the CAMS model. After 18 UTC there was a furious increase in the number concentrations. During non-rainy days, the 1-D model slightly overestimated (underestimated) the maximum (minimum) number concentrations of CH4 over Delhi whereas concentrations are overestimated over Kolkata, Chennai, and Mumbai. Forecasted CH4 has a good (weak) correlation over Chennai (Mumbai) respectively. The 1-D model overestimated (overestimated) the maximum (minimum) number concentrations of SO2 over Delhi but the maximum (minimum) concentrations are underestimated (overestimated) over Kolkata, Chennai, and Mumbai. The number concentrations of SO2 have shown a good correlation for all megacities except Delhi. CH4 number concentration is overestimated during rainy days. Delhi and Kolkata show a good correlation of CH4 during rainy days. SO2 during rainy days is underestimated except over Chennai and both models show a good correlation except over Mumbai. Overall, it can be stated that the 1-D hybrid solver is successful in simulating the monthly mean diurnal variation of vertical profiles of CH4 and SO2, and its implementation in the global model may estimate the number concentrations with better accuracies.

A 1-D model to retrieve the vertical profiles of minor atmospheric constituents for cloud microphysical modeling: I. Formulation and validation

Determining the number concentration of minor constituents in the atmosphere is very important as it determines the whole tropospheric chemistry process. These constituents may act as cloud condensation nuclei (CCN) and ice nuclei (IN), impacting heterogeneous nucleation inside the cloud. However, the estimations of the number concentration of CCN/IN in cloud microphysical parameters are associated with uncertainties. In the present work, a hybrid Monte Carlo Gear solver has been developed to retrieve profiles of CH4, N2O, and SO2. The idealized experiments have been carried out using this solver for retrieving vertical profiles of these constituents over four megacities, viz., Delhi, Mumbai, Chennai, and Kolkata. Community Long-term Infrared Microwave Coupled Atmospheric Product System (CLIMCAPS) dataset around 0800 UTC (2000UTC) has been used for initializing the number concentration of CH4, N2O, and SO2 for daytime (nighttime). The daytime (nighttime) retrieved profiles have been validated using 2000 UTC (next day 0800 UTC) CLIMCAPS products. ERA5 temperature dataset has been used to estimate the kinematic rate of reactions with 1000 perturbations determined using Maximum Likelihood Estimation (MLE). The retrieved profiles and CLIMCAPS products are in very good agreement, as evidenced by the percentage difference between them within the range of 1.3 × 10−5–60.8 % and the coefficient of determination mainly within the range between 81 and 97 %. However, during the passage of tropical cyclone and western disturbance, its value became as low as 27 and 65 % over Chennai and Kolkata, respectively. The enactment of synoptic scale systems such as western disturbances, tropical cyclone Amphan, and easterly waves caused disturbed weather over these megacities—the retrieved profiles during disturbed weather cause large deviations of vertical profiles of N2O. However, the profiles of CH4 and SO2 have less deviation. It is inferred that incorporating this methodology in the dynamical model will be useful to simulate the realistic vertical profiles of the minor constituents in the atmosphere.

Potential for Phototrophy in Venus' Clouds.

We show that solar irradiances calculated across Venus' clouds support the potential for Earth-like phototrophy and that treatment of Venus' aerosols containing neutralized sulfuric acid favor a habitable zone. The phototrophic potential of Venus' atmosphere was assessed by calculating irradiances (200-2000 nm, 15° solar zenith angle, local noon) using a radiative transfer model that accounted for absorption and scattering by the major and minor atmospheric constituents. Comparisons to Earth's surface (46 W m-2, 280-400 nm) suggest that Venus' middle and lower clouds receive ∼87% less normalized UV flux (6-7 W m-2) across 200-400 nm, yet similar normalized photon flux densities (∼4400-6200 μmol m-2 s-1) across 350-1200 nm. Further, Venus' signature phototrophic windows and subwindows overlap with the absorption profiles of several photosynthetic pigments, especially bacteriochlorophyll b from intact cells and phycocyanin. Therefore, Venus' light, with limited UV flux in the middle and lower clouds, is likely quite favorable for phototrophy. We additionally present interpretations to refractive index and radio occultation measures for Venus' aerosols that suggest the presence of lower sulfuric abundances and/or neutralized forms of sulfuric acid, such as ammonium bisulfate. Under these considerations, the aerosols in Venus' middle clouds could harbor water activities (≥0.6) and buffered acidities (Hammett acidity factor, H0 -0.1 to -1.5) that lie within the limits of acidic cultivation (≥H0 -0.4) and are tantalizingly close to the limits of oxygenic photosynthesis (≥H0 0.1). Together, these photophysical and chemical considerations support a potential for phototrophy in Venus' clouds.

Two‐Year Observations of the Jupiter Polar Regions by JIRAM on Board Juno

AbstractWe observed the evolution of Jupiter's polar cyclonic structures over two years between February 2017 and February 2019, using polar observations by the Jovian InfraRed Auroral Mapper, JIRAM, on the Juno mission. Images and spectra were collected by the instrument in the 5‐μm wavelength range. The images were used to monitor the development of the cyclonic and anticyclonic structures at latitudes higher than 80° both in the northern and the southern hemispheres. Spectroscopic measurements were then used to monitor the abundances of the minor atmospheric constituents water vapor, ammonia, phosphine, and germane in the polar regions, where the atmospheric optical depth is less than 1. Finally, we performed a comparative analysis with oceanic cyclones on Earth in an attempt to explain the spectral characteristics of the cyclonic structures we observe in Jupiter's polar atmosphere.

Atmospheric ammonia retrieval from the TANSO-FTS/GOSAT thermal infrared sounder

Abstract. Hyperspectral thermal infrared sounders enable us to grasp the global behavior of minor atmospheric constituents. Ammonia, which imparts large impacts on the atmospheric environment by reacting with other species, is one of them. In this work, we present an ammonia retrieval system that we developed for the Greenhouse Gases Observing Satellite (GOSAT) and the estimates of global atmospheric ammonia column amounts that we derived from 2009 to 2014. The horizontal distributions of the seasonal ammonia column amounts represent significantly high values stemming from six anthropogenic emission source areas and four biomass burning ones. The monthly mean time series of these sites were investigated, and their seasonality was clearly revealed. A comparison with the Infrared Atmospheric Sounding Interferometer (IASI) ammonia product showed good agreement spatially and seasonally, though there are some differences in detail. The values from GOSAT tend to be slightly larger than those from IASI for low concentrations, especially in spring and summer. On the other hand, they are lower for particularly high concentrations during summer, such as eastern China and northern India. In addition, the largest differences were observed in central Africa. These differences seem to stem from the temporal gaps in observations and the fundamental differences in the retrieval systems.

Atmospheric test environments for planetary in-situ missions: Never quite “Test as you fly”

The planetary atmospheric and surface environments that planetary probes, landers and rovers may encounter cannot be perfectly replicated in tests on Earth. The temperature, pressure and composition of atmospheric test environments for previous missions are reviewed, and the differences between the conditions used in tests and the actual conditions at the target body are discussed. Generally, it has been the practice to replicate only those few key parameters that determine the phenomena of interest, and the effects of gravity and of minor atmospheric constituents are rarely simulated explicitly. Typically tests have been performed in nitrogen atmospheres (rather than carbon dioxide for Mars and Venus) or Helium (instead of hydrogen for Jupiter): exceptions are a handful of specific tests where the composition was considered critical. In-flight thermal anomalies are generally attributable to differences between the static conditions in a test chamber and the dynamic environment of flight, rather than to the composition of test atmospheres.

Seasonal changes in Saturn’s stratosphere inferred from Cassini/CIRS limb observations

We present temperature and hydrocarbons abundances (C2H6, C2H2, C3H8) retrieved from Cassini/CIRS limb spectra, acquired during northern spring in 2010 (LS=12°) and 2012 (LS=31°). We compare them to the previous limb measurements performed by Guerlet et al. (Guerlet, S. et al. [2009]. Icarus 203, 214–232) during northern winter. The latitudinal coverage (from 79°N to 70°S) and the sensitivity of our observations to a broad range of pressure levels (from 20hPa to 0.003hPa) allow us to probe the meridional and vertical structure of Saturn’s stratosphere during northern spring. Our results show that in the northern hemisphere, the lower stratosphere (1hPa) has experienced the strongest warming from northern winter to spring (11±0.91.1K), while the southern hemisphere exhibits weak variations of temperature at the same pressure level. We investigate the radiative contribution in the thermal seasonal evolution by comparing these results to the radiative–convective model of Guerlet et al. (Guerlet, S. et al. [2014]. Icarus 238, 110–124). We show that radiative heating and cooling by atmospheric minor constituents is not always sufficient to reproduce the measured variations of temperature (depending on the pressure level). The measurements of the hydrocarbons abundances and their comparison with the predictions of the 1D photochemical model of Moses and Greathouse (Moses, J.I., Greathouse, T.K. [2005]. J. Geophys. Res. (Planets) 110, 9007) give insights into large scale atmospheric dynamics. At 1hPa, C2H6, C2H2, and C3H8 abundances are remarkably constant from northern winter to spring. At the same pressure level, C2H6 and C3H8 exhibit homogeneous meridional distributions unpredicted by this photochemical model, unlike C2H2. This is consistent with the existence of a meridional circulation at 1hPa, as suggested by previous studies.

Modeling the concentration of pollutants using the WRF-ARW atmospheric model and CHIMERE chemistry transport model

Described is a system for analyzing and forecasting the air quality in the central regions of Russia, During the operation of the system, the detailed meteorological information provided by the WRF-ARW model is used by the CHIMERE chemistry transport model for simulating the processes of transport, chemical transformation, and deposition of atmospheric minor constituents. Considered is the quality of retrieved and forecasted (with the lead time up to three days) concentrations of O3, NO2, NO, CO, and PM10. The presented verification scores of pollutant concentrations demonstrate a relative success of the system. Demonstrated is a need in improving the data on the emissions of the air pollutants used for simulations. A procedure for the statistical correction of computed concentrations is described and verification scores of its results are given.

Overview and early results of the Superconducting Submillimeter‐Wave Limb‐Emission Sounder (SMILES)

The Superconducting Submillimeter‐Wave Limb‐Emission Sounder (SMILES) was successfully launched and attached to the Japanese Experiment Module (JEM) on the International Space Station (ISS) on 25 September 2009. It has been making atmospheric observations since 12 October 2009 with the aid of a 4 K mechanical cooler and superconducting mixers for submillimeter limb‐emission sounding in the frequency bands of 624.32–626.32 GHz and 649.12–650.32 GHz . On the basis of the observed spectra, the data processing has been retrieving vertical profiles for the atmospheric minor constituents in the middle atmosphere, such as O3 with isotopes, HCl, ClO, HO2, BrO, and HNO3. Results from SMILES have demonstrated its high potential to observe atmospheric minor constituents in the middle atmosphere. Unfortunately, SMILES observations have been suspended since 21 April 2010 owing to the failure of a critical component.

Seasonality of Peroxyacetyl nitrate (PAN) in the upper troposphere and lower stratosphere using the MIPAS-E instrument

Abstract. The Michelson Interferometer for Passive Atmospheric Sounding onboard ENVISAT (MIPAS–E) offers the opportunity to detect and spectrally resolve many atmospheric minor constituents affecting atmospheric chemistry. In this paper, we retrieve global, seasonal PAN volume mixing ratio (vmr) data from MIPAS-E measurements made in January, March, August and October 2003 and present results from this scheme between approximately 300 and 150 hPa. The total error on a single PAN retrieval is better than 20% outside the tropics and better than 50% in the tropics where uncertainties in water vapor dominate the total error budget. We observe clear differences in the seasonal cycle of PAN in our data, linked closely to biomass burning regions and growing seasons. Highest Northern Hemisphere mid-latitude PAN vmrs were observed in August (300–600 pptv on average) compared with the January and October data (less than 250 pptv on average). In the March 2003 data we observe highest PAN vmrs in the tropics with evidence of vmrs between 600 and 1000 pptv over Eastern Asia and over the Central Pacific at 333 hPa. The vertical distribution of PAN as a function of latitude (i.e. the zonal mean) highlights the strong inter-annual variability of PAN in the upper troposphere and lower stratosphere (UTLS), most pronounced poleward of 40° N (up to 400 pptv over the year). The variability of PAN in the tropical UTLS is also significant and we derive a variability of up to 250 pptv in the averages between January and October 2003. These results represent the first seasonal observations of PAN in the UTLS.

Phase acceleration: a new important parameter in GPS occultation technology

Based on 40 years of radio-occultation (RO) experiments, it is now recognized that the phase acceleration of radio waves (equal to the time derivative of the Doppler shift), derived from analysis of high-stability Global Positioning System (GPS) RO signals, is as important as the Doppler frequency. The phase acceleration technique allows one to convert the phase and Doppler frequency changes into refractive attenuation variations. From such derived refractive attenuation and amplitude data, one can estimate the integral absorption of radio waves. This is important for future RO missions when measuring water vapor and minor atmospheric gas constituents, because the difficulty of removing the refractive attenuation effect from the amplitude data can be avoided. The phase acceleration technique can be applied also for determining the location and inclination of sharp layered plasma structures (including sporadic E s layers) in the ionosphere. The advantages of the phase acceleration technique are validated by analyzing RO data from the Challenging Minisatellite Payload (CHAMP) and the FORMOSA Satellite Constellation Observing Systems for Meteorology, Ionosphere, and Climate missions (FORMOSAT-3/COSMIC).

Growth rates of stratospheric HCFC-22

Abstract. The Michelson Interferometer for Passive Atmospheric Sounding onboard ENVISAT (MIPAS-E) offers the opportunity to detect and spectrally resolve many atmospheric minor constituents affecting atmospheric chemistry. In this paper, we describe an algorithm produced to retrieve HCFC–22 profiles from MIPAS-E measurements made in 2003 and present results from this scheme between 300 and 50 mb. By comparison with ATMOS (AT–3) version 3 data, we find a mean Northern Hemisphere mid-latitude (20–50° N) HCFC–22 growth rate between 1994 and 2003 of 5.4±0.7 pptv/yr in the lower stratosphere (LS) and a mean LS Southern Hemisphere growth rate (60–80° S) of 6.0±0.7 pptv/yr in the same period. We test the feasibility of using a global data set to estimate the chemical lifetime of HCFC–22 in the LS and we derive this for two regions: 20–50° N (246±38 years) and 60–80° S (274±34 years). From these data we note a global LS lifetime of 260±25 years, significantly longer than previous estimates.

Retrieval of minor constituents in a cloudy atmosphere with remote-sensing millimetre-wave measurements

The presence of clouds in remote-sensing measurements is often the cause of information loss about the minor atmospheric constituent profiles that are the goal of the measurement. Millimetre- and submillimetre-wave sounding can be used to abate the problems of cloud contamination. A quantitative assessment of this possibility is provided with the systematic simulation of the retrieval of minor atmospheric constituents in the presence of ice cirrus clouds of different density and particle size. The simulations are made for the particular case of the MARSCHALS instrument which is a heterodyne limb-sounding spectrometer that measures the atmospheric thermal emission in the millimetre- and submillimetre-wave regions operating on board a stratospheric aircraft. Four types of measuring conditions are identified: clouds that do not modify the observation, clouds that are adequately described with an atmospheric continuum absorption model, clouds for which an error is introduced when the limited vertical resolution of the sounding does not resolve the sharp discontinuity of the cloud, and clouds which require the modelling of scattering effects for a correct retrieval. When a rigorous modelling of the cloud is performed, the retrieval of minor atmospheric constituent is marginally affected by the presence of the cloud. We find that in the worst case the cloud causes a loss, with respect to clear-sky conditions, of two degrees of freedom from about 90 and of 7% of information content relative to climatological information. Copyright © 2007 Royal Meteorological Society

SPICAV on Venus Express: Three spectrometers to study the global structure and composition of the Venus atmosphere

Spectroscopy for the investigation of the characteristics of the atmosphere of Venus (SPICAV) is a suite of three spectrometers in the UV and IR range with a total mass of 13.9 kg flying on the Venus Express (VEX) orbiter, dedicated to the study of the atmosphere of Venus from ground level to the outermost hydrogen corona at more than 40,000 km. It is derived from the SPICAM instrument already flying on board Mars Express (MEX) with great success, with the addition of a new IR high-resolution spectrometer, solar occultation IR (SOIR), working in the solar occultation mode. The instrument consists of three spectrometers and a simple data processing unit providing the interface of these channels with the spacecraft. A UV spectrometer (118–320 nm, resolution 1.5 nm) is identical to the MEX version. It is dedicated to nadir viewing, limb viewing and vertical profiling by stellar and solar occultation. In nadir orientation, SPICAV UV will analyse the albedo spectrum (solar light scattered back from the clouds) to retrieve SO 2, and the distribution of the UV-blue absorber (of still unknown origin) on the dayside with implications for cloud structure and atmospheric dynamics. On the nightside, γ and δ bands of NO will be studied, as well as emissions produced by electron precipitations. In the stellar occultation mode the UV sensor will measure the vertical profiles of CO 2, temperature, SO 2, SO, clouds and aerosols. The density/temperature profiles obtained with SPICAV will constrain and aid in the development of dynamical atmospheric models, from cloud top (∼60 km) to 160 km in the atmosphere. This is essential for future missions that would rely on aerocapture and aerobraking. UV observations of the upper atmosphere will allow studies of the ionosphere through the emissions of CO, CO +, and CO 2 +, and its direct interaction with the solar wind. It will study the H corona, with its two different scale heights, and it will allow a better understanding of escape mechanisms and estimates of their magnitude, crucial for insight into the long-term evolution of the atmosphere. The SPICAV VIS-IR sensor (0.7–1.7 μm, resolution 0.5–1.2 nm) employs a pioneering technology: an acousto-optical tunable filter (AOTF). On the nightside, it will study the thermal emission peeping through the clouds, complementing the observations of both VIRTIS and Planetary Fourier Spectrometer (PFS) on VEX. In solar occultation mode this channel will study the vertical structure of H 2O, CO 2, and aerosols. The SOIR spectrometer is a new solar occultation IR spectrometer in the range λ=2.2–4.3 μm, with a spectral resolution λ/Δ λ>15,000, the highest on board VEX. This new concept includes a combination of an echelle grating and an AOTF crystal to sort out one order at a time. The main objective is to measure HDO and H 2O in solar occultation, in order to characterize the escape of D atoms from the upper atmosphere and give more insight about the evolution of water on Venus. It will also study isotopes of CO 2 and minor species, and provides a sensitive search for new species in the upper atmosphere of Venus. It will attempt to measure also the nightside emission, which would allow a sensitive measurement of HDO in the lower atmosphere, to be compared to the ratio in the upper atmosphere, and possibly discover new minor atmospheric constituents.

MARC: A code for the retrieval of atmospheric parameters from millimeter-wave limb measurements

A new data analysis software is presented that has been developed for the retrieval of atmospheric minor constituents from limb-sounding observations made in the millimeter and sub-millimeter spectral regions. The code, which is called MARC (Millimetre-wave Atmospheric-Retrieval Code), has been designed to analyze the observations of the MARSCHALS (Millimetre-wave Airborne Receivers for Spectroscopic CHaracterisation in Atmospheric Limb-Sounding) instrument which operates on the M-55 stratospheric aircraft. The main objective of the analysis of MARSCHALS observations will be to assess long-wave measurement capabilities for the study of the upper troposphere and lower stratosphere regions. The key questions will be the accuracy and spatial resolution that can be achieved by long-wave measurements in presence of clouds and horizontal gradients. MARC performs a global-fit multi-target retrieval, in which optimal estimation is used and errors of the forward model parameters are taken into account for the definition of the cost function minimized in the retrieval. With these features it is easy to use the variables of the problem as either forward model constant parameters or retrieved unknowns with minimum impact on the stability of the retrieval. MARC can perform a wide spectral-band analysis of the observations without a selection of the analyzed channels, and the retrieval process provides an error budget of the retrieved unknowns that includes both the forward model errors and the measurement errors. The error budget obtained in this way is smaller than that obtained when accounting a posteriori for the systematic errors. The new combination of the retrieval features makes possible an efficient and optimal exploitation of the information content of the observations.

Compact high-resolution spaceborne echelle grating spectrometer with acousto-optical tunable filter based order sorting for the infrared domain from 22 to 43 μm

A new compact spaceborne high-resolution spectrometer developed for the European Space Agency's Venus Express spacecraft is described. It operates in the IR wavelength range of 2.2 to 4.3 microm and measures absorption spectra of minor constituents in the Venusian atmosphere. It uses a novel echelle grating with a groove density of 4 lines/mm in a Littrow configuration in combination with an IR acousto-optic tunable filter for order sorting and an actively cooled HgCdTe focal plane array of 256 by 320 pixels. It is designed to obtain an instrument line profile of 0.2 cm(-1). First results on optical and spectral properties are reported.

Detectability of minor constituents in the martian atmosphere by infrared and submillimeter spectroscopy

Spectroscopic remote sensing in the infrared and (sub)millimeter range is a powerful technique that is well suited for detecting minor species in planetary atmospheres (Planet Space Sci. 43(1995) 1485). Yet, only a handful of molecules in the Mars atmosphere ( CO 2 , CO and H 2 O along with their isotopic species, O 3 , and more recently H 2 O 2 and CH 4 ) have been detected so far by this method. New high performance spectroscopic instruments will become available in the future in the infrared and (sub)millimeter range, for observations from the ground (infrared spectrometers on 8 m class telescopes, large millimeter and submillimeter interferometers) and from space, in particular the Planetary Fourier Spectrometer (PFS) aboard Mars Express (MEx), and the Heterodyne Instrument for the Far-Infrared (HIFI) aboard the Herschel Space Observatory (HSO). In this paper we will present results of a study that determines detectability of minor species in the atmosphere of Mars, taking into account the expected performance of the above spectroscopic instruments. In the near future, a new determination of the D/H value is expected with the PFS, especially during times of maximum H 2 O abundance in the martian atmosphere. PFS is also expected to place constraints on the abundance of several minor species ( H 2 O 2 , CH 4 , CH 2 O , SO 2 , H 2 S , OCS, HCl) above any local outgassing sources, the hot spots. It will be possible to obtain complementary information on some minor species ( O 3 , H 2 O 2 , CH 4 ) from ground-based infrared spectrometers on large telescopes. In the more distant future, HIFI will be ideally suited for measuring the isotopic ratios with unprecedented accuracy. Moreover, it should be able to observe O 2 , which has not yet been detected spectroscopically in the IR/submm range, as well as H 2 O 2 . HIFI should also provide upper limits for several species that have not yet been detected (HCl, NH 3 , PH 3 ) in the atmosphere of Mars. Some species (SO, SO 2 , H 2 S , OCS, CH 2 O ) that may be observable from the ground could be searched for with present single-dish antennae and arrays, and in the future with the Atacama Large Millimeter Array (ALMA) submillimeter interferometer.

Mesospheric ozone response to changes in water vapor

Observations made by the Upper Atmosphere Research Satellite (UARS) now extend in time for almost a decade and allow for an improved understanding of seasonal and interannual variations of middle atmospheric minor constituents. In particular, measurements of mesospheric ozone and water vapor made by the UARS Halogen Occultation Experiment (HALOE) reveal significant seasonal and decadal variability. At midlatitudes, the observed variation in water vapor is primarily an annual cycle that peaks in summertime driven by changes in the residual circulation. In contrast to earlier satellite observations, HALOE ozone sunset observations also have an annual cycle that is anticorrelated with the water vapor cycle. The anticorrelation is a consequence of water vapor photolysis producing hydrogen species that destroy ozone. However, HALOE sunrise observations of ozone show no seasonal cycle. Analysis of monthly anomalies between 1991 and 2001 show a long‐term water vapor increase of approximately 1%/year. The observed response of ozone depends on altitude and time of day and is strongly negative (up to −4%/year) for sunset observations around 80 km. Again, the response of ozone at sunrise is minimal. Sensitivity tests using a three‐dimensional global chemical transport model show good agreement with the observations and confirm that the observed decrease in sunset ozone is a response to increasing water vapor.

Satellite techniques for trace gas measurements

Satellite measurements of atmospheric minor constituents are very useful as they provide frequent global coverage over a long period of time. Due to technical developments measurements of ozone and other related trace gases are becoming possible since the past decade even in the troposphere. Measurements in the infrared and microwave are being utilised with the advent of new technology. Different types of viewing geometries of satellite observations and retrieval algorithms are discussed with examples for temperature and ozone, respectively. Since India is capable of launching satellites, it is suggested that monitoring some of the key pollutants along with meteorological parameters will be important to understand their transport and chemistry in the tropical region using a low inclination orbit satellite.