Shyam Lal

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Tropical tropospheric chemistry and climate change

MAUSAM, 2021

A systematic programme of monitoring surface ozone and its precursor gases CH4, CO and NOx (NO + ... more A systematic programme of monitoring surface ozone and its precursor gases CH4, CO and NOx (NO + NO2) at some selected sites in the Indian region was started under ISRO's geosphere biosphere programme in 1991. Measurements have been made at Ahmedabad an urban polluted site, Gadanki a rural relatively clean site, Gurusikhar a high altitude site representative of the free troposphere and Trivandrum a coastal (relatively clean) site influenced by marine air. The data has been used to study different features of troposphere chemistry in the tropics. Some of the results from this programme relevant to the climate change problem are presented in this paper.

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Variability of trace gas concentrations over Asian region: satellite observations vs model

Nitrogen dioxide (NO_2) and Carbon Monoxide (CO) play a key role in the chemistry of the troposph... more Nitrogen dioxide (NO_2) and Carbon Monoxide (CO) play a key role in the chemistry of the tropospheric ozone and are emitted mainly by anthropogenic processes. These emissions have been increasing over Asia over the past few years due to rapid economic growth and yet there are very few systematic ground based observations of these species over this region. We have analysed ten years of data from space borne instruments: Global Ozone Monitoring Experiment (GOME), SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) and Measurements of Pollution in the Troposphere (MOPITT), which have been measuring the tropospheric abundance of these trace gases. We have examined trends over the period 1996-2008 in NO_2 and CO over a few Indian regions where high economic growth in the present decade is likely to see increased emissions for these species. However, even the highest growth rate of these species seen in the present study, is less when compared with similar po...

Transport effects on the vertical distribution of tropospheric ozone over the tropical marine regions surrounding India

Journal of Geophysical Research: Atmospheres, 2013

The measurements of vertical distribution of ozone have been made over the Bay of Bengal and the ... more The measurements of vertical distribution of ozone have been made over the Bay of Bengal and the Arabian Sea in spring 2006 during a cruise campaign, namely, the Integrated Campaign for Aerosol, Gases and Radiation Budget. The average tropospheric columnar ozone (TCO) values are found to be 36.1 ± 6.9 Dobson unit (DU) and 41.7 ± 5.0 DU over the Bay of Bengal and the Arabian Sea, respectively. In contrast to TCO, ozone mixing ratios are higher by about 10 ppbv in the lower 3 km over the Bay of Bengal due to Indo Gangetic outflow above the marine atmospheric boundary layer. Major contribution in the higher TCO value over the Arabian Sea is, possibly, by stratospheric intrusion when ozone in the upper troposphere is higher by about 20 ppbv. The lowest columnar ozone content of 22.4 DU was observed on 30 March in the central Bay of Bengal due to convective activity resulting into lower ozone throughout the troposphere, except between 6 and 10 km altitude. These near simultaneous ozone o...

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Rocket Measurements of the Vertical Structure of the Ozone Field in the Tropics

Atmospheric Ozone, 1985

A rocket ozone programme was initiated at the Physical Research Laboratory to study the height di... more A rocket ozone programme was initiated at the Physical Research Laboratory to study the height distribution of ozone in the tropical atmosphere. A solar MUV photometer for daytime measurements of ozone in the stratosphere and lower mesosphere and a lunar MUV photometer for night time measurements have been developed. A series of experiments to study the day to night changes of ozone at different levels in the tropical stratosphere and lower mesosphere were conducted during 1980–81. The experiments show that while at altitudes below 30 km night time concentrations are lower than the daytime values, at altitudes above about 40 km the night time values are larger than the daytime values by a factor which increases with increasing altitude and reaches a value of 1.5 – 1.6 at 55 km. In the intermediate region of 30–40 km the observed changes are small.

Vertical distribution of methyl bromide over Hyderabad, India

Tellus B: Chemical and Physical Meteorology, 1994

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The structure of the equatorial mesosphere at Thumba

Pure and Applied Geophysics PAGEOPH, 1980

The structure of the equatorial mesosphere is being investigated at Thumba by rocket borne ultrav... more The structure of the equatorial mesosphere is being investigated at Thumba by rocket borne ultraviolet absorption photometry as well as by the meteorological M-100 rocket launching programme. Whereas the meteorological M-100 rocket luanching programme has been regular, the UV absorption studies have been few in number and sporadic in nature. In this paper an attempt is made to consolidate the results so far obtained from both these investigations.

Seasonal variability in distribution and fluxes of methane in the Arabian Sea

Journal of Geophysical Research: Oceans, 1998

Methane, a biogeochemically important gas in Earth/s atmosphere was measured in the water column ... more Methane, a biogeochemically important gas in Earth/s atmosphere was measured in the water column and air in the Arabian Sea in different seasons, viz., northeast monsoon, intermonsoon, and southwest monsoon, as part of the Joint Global Ocean Flux Study (India). These observations record its distributions in the water column as well as its fluxes and their seasonal variations. Methane is mainly produced in subsurface water, and its supersaturation occurs in the upper 400 m. The CH4 peak concentration and its location vary with latitude and season. Below about 400 m, seawater CH4 concentrations, in general, are observed to be undersaturated, suggesting its consumption. Production of CH4 in oxygenated water appears to be under biological control; however, the peak in deep anoxic water does not show any particular relation with any single chemical, biological, or physical variable and rather suggests it to be maintained by the quasi‐horizontal transport. The average surface supersaturat...

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The vertical distribution of CH4, N2O, CFC-12 and CFC-11 in the middle atmosphere at mid-latitudes

Journal of Atmospheric and Terrestrial Physics, 1989

Abatract-CH,, N20, CFC-12 and CFC-11 data analysed from samples collected by means of cryogenic s... more Abatract-CH,, N20, CFC-12 and CFC-11 data analysed from samples collected by means of cryogenic samplers from five balloon flights conducted during the 1982-1984 period, are presented. The vertical distributions of these gases show structures with dominant scale size of 16 f 2.5 km. The amplitude of the perturbation is larger for the species having steeper gradient. A comparison of the present results with the results of earlier measurements representing the period of 1977-1979 do not show any appreciable change in the mixing ratios of CH, and NzO. However, an average tropospheric increase of (6.5+ l)% per year is found for CFC-12 and CFC-11. This annual change in the mixing ratios of CFC-12 and CFC-11 is found to increase with height suggesting a time lag of about 6 years between the troposphere and the altitude region around 25 km. The average mixing ratios of CH, and N,O are compared with the available satellitejspacelab data. This data set shows a satisfactory agreement with the present in situ measurements for the height region above 30 km.

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Variabilities in the fluxes and annual emissions of nitrous oxide from the Arabian Sea

Global Biogeochemical Cycles, 1998

Extensive measurements of nitrous oxide (N20) have been made during April-May 1994 (intermonsoon)... more Extensive measurements of nitrous oxide (N20) have been made during April-May 1994 (intermonsoon), February-March 1995 (northeast monsoon), July-August 1995 and August 1996 (southwest monsoon) in the Arabian Sea. Low N20 supersaturations in the surface waters are observed during intermonsoon compared to those in northeast and southwest monsoons. Spatial distributions of supersaturations manifest the effects of larger mixing during winter cooling and wind-driven upwelling during monsoon period off the Indian west coast. A net positive flux is observable during all the seasons, with no discernible differences from the open ocean to coastal regions. The average ocean-to-atmosphere fluxes of N20 are estimated, using wind speed dependent gas transfer velocity, to be of the order of 0.26, 0.003, and 0.51, and 0.78 pg (pico grams) cm-2 s-1 during northeast monsoon, intermonsoon, and southwest monsoon in 1995 and 1996, respectively. The lower range of annual emission of N20 is estimated to be 0.56-0.76 Tg N20 per year which constitutes 13-17% of the net global oceanic source. However, N20 emission from the Arabian Sea can be as high as 1.0 Tg N20 per year using different gas transfer models. of ocean-to-atmosphere exchange depend primarily on the degree of saturation in the surface water and the transfer velocity of the gas under study across the airwater interface. A recent comprehensive analysis shows that the budget of N20 emission can be affected significantly depending on the choice of gas transfer model used in calculating the fluxes [Nevison et al., 1995]. Available data sets suggest that the Arabian Sea is one of the high N20 flux producing areas among the world oceans [Law and Owens, 1990; Naqvi and Novonha, 1991; Bange et al., 1996] because of its large

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Changes in surface ozone amount and its diurnal and seasonal patterns, from 1954-55 to 1991-93, measured at Ahmedabad (23 N), India

Geophysical Research Letters, 1996

... Manish Naja. Physical Research Laboratory, Ahmedabad, India. Shyam Lal. Physical Research Lab... more

Seasonal and spatial variability in N2O distribution in the Arabian Sea

Deep Sea Research Part I: Oceanographic Research Papers, 1999

Extensive measurements of nitrous oxide (N O) were made in the central and eastern Arabian Sea du... more Extensive measurements of nitrous oxide (N O) were made in the central and eastern Arabian Sea during the northeast monsoon (February-March), intermonsoon (April-May) and southwest monsoon (July-August) seasons. The latitudinal and longitudinal variations, along with seasonal changes with respect to winter convection and coastal upwelling, are clearly discernible. Vertical profiles collected down to 1000 m show that the Arabian Sea water column is supersaturated with N O at all depths. However, N O consumption at intermediate depths, coincident with the oxygen minimum and associated with sediment-water interfaces, and in the denitrifying zone, coincident with NO\ secondary maxima, are also apparent. The N O concentration varies from &10 nM near the surface to about 80 nM in the secondary peak region (+800 m). Interrelationships with chemical parameters suggest nitrification to be the main process for the production of N O in the oceanic water. Plots of apparent oxygen utilization vs production of N O indicate a consistent linear relationship for AOU between 0 and 200 M.

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A study of the atmospheric photochemical loss of N2O based on trace gas measurements

Chemosphere - Global Change Science, 2000

The budget of nitrous oxide (N 2 O) in the atmosphere has been under debate for some time now. Th... more The budget of nitrous oxide (N 2 O) in the atmosphere has been under debate for some time now. The global estimates of the source emissions of N 2 O have undergone constant revisions recently, trying to balance the sources and sinks of N 2 O. Despite these eorts, there have been proposals for new atmospheric sources and sinks to explain the isotopic anomaly observed in N 2 O. It thus becomes important to verify such prepositions. One useful method, used in the present work, is to study the distribution of N 2 O in the atmosphere. The observations are then compared with results from models which use the standard photochemistry for N 2 O. The extent of deviations in the comparison study are used to infer the possibility of a new source/sink, hitherto undiscovered.

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Study of a high SO2 event observed over an urban site in western India

Asia-Pacific Journal of Atmospheric Sciences, 2012

Continuous measurements of SO 2 , NO x and O 3 along with sampling based measurements of CO, CH 4... more Continuous measurements of SO 2 , NO x and O 3 along with sampling based measurements of CO, CH 4 , NMHCs and CO 2 were carried out during May, 2010 at Ahmedabad. The diurnal variations of SO 2 in ambient air exhibited elevated values during the night and lower levels during the sunlit hours. The mean concentration of SO 2 during the study period was 0.95 ± 0.88 ppbv. However, the ambient SO 2 exceeded 17 ppbv in the night of 20 May, 2010. On the same day, tropospheric columnar SO 2 from OMI showed almost 350% increase corroborating the surface observations over an extended height regime. This was also the highest columnar value of SO 2 during the summer of 2010. Columnar loadings were also found to be high for formaldehyde, precipitable water vapor and aerosol optical depth on 20 May. Elevated concentrations were also recorded for other trace gases like NO 2 and O 3. Analysis of related data of trace gases indicated characteristics of fresh emissions with dominant contributions from mobile sources during the study period. However, SO 2 /NO 2 ratio of 0.36 during the event period on 20th May connotes non-local influences. Analyses of meteorological parameters suggest combined impacts of transport and inversion causing higher levels of SO 2 and other pollutants during 20-21 May. Episodes of such enhancements may perturb chemical and radiative balance of the atmosphere.

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Long-range transport of volcanic SO2 over Northern India

ABSTRACT Anthropogenic activities contribute to more than 70% of global sulfur dioxide (SO2) emis... more ABSTRACT Anthropogenic activities contribute to more than 70% of global sulfur dioxide (SO2) emissions. Among the natural sources of SO2, volcanic emissions can be significant but sporadic contributors. Apart from releasing a plethora of pollutant gases, volcanoes can also impact atmospheric temperatures and radiation balance as was observed after El Chichon (1982) and Mt. Pinatubo (1991) eruptions. The present study deals with episodes of long-range transport of SO2 from Africa to Northern India using multiple satellite observations. Monthly averaged SO2 from OMI were of the order of 0.6-0.9 DU during November, 2008 over the Indo-Gangetic Plain (IGP). In contrast, SO2 monthly averages retrieved from observations across different locations over North India had never exceeded 0.3 DU during the six year period of 2005-2010. The columnar SO2 loadings were a factor of 10 higher than background levels over most of the IGP on the day of impact. These enhanced SO2 levels were, however, not reciprocated in satellite derived NO2 or CO columns, indicating transport from a non-anthropogenic source of SO2. Back-trajectory analysis revealed strong winds in the free troposphere, which originated from a volcanic eruption over Ethiopia. Wind streams and stable atmospheric conditions were conducive to the long-range transport of volcanic plume over the IGP. Increase in aerosol optical depths were observed both from ground-based measurements as well as satellite observations. A well separated layer of aerosols above the boundary layer was observed from CALIPSO, most likely as a result of this transport. Apart from known anthropogenic sources, the additional transport of volcanic SO2 over the IGP region would have implications to air quality and radiation balance over this region. Details of this episode of transport in terms of meteorology and dynamics will be presented during the assembly.

Comparison of satellite observed tropospheric NO2 over India with model simulations

Atmospheric Environment, 2010

Nitrogen dioxide (NO 2) plays a key role in the chemistry of the atmosphere and is emitted mainly... more Nitrogen dioxide (NO 2) plays a key role in the chemistry of the atmosphere and is emitted mainly by combustion processes. These emissions have been increasing over India over the past few years due to rapid economic growth and yet there are very few systematic ground based observations of NO 2 over this region. We thus take recourse to satellite data and compare tropospheric NO 2 column abundances simulated by a chemical transport model, MOZART, with data from the Global Ozone Monitoring Experiment (GOME) for a few locations in India that have seen a rapid economic growth in the last decade. The model generally simulates higher columnar abundances of NO 2 compared to GOME observations and does not reproduce the features of the observed seasonal behaviour. The combined uncertainties of the emission inventory and retrieval of the satellite data could be contributing factors to the discrepancies. It may be thus worthwhile to develop emission inventories for India at a higher resolution to include local level activity data. The ten year data (1996e2006) from GOME and SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) show increasing trends for Indian cities where rapid industrial and vehicular traffic growth has been observed during this period.

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