B. C. Arya

Observational study of surface ozone at New Delhi, India

Surface ozone has been measured over New Delhi, an urban site, a region of intensive anthropogenic activity since 1997. Seasonal variations in ozone concentration show pronounced maxima in the summer and autumn seasons and minima in monsoon and winter seasons. Diurnal patterns in ozone concentration show daytime in situ photochemical production throughout the year. The high ozone episode days were associated with meteorological parameters such as sunny and warm weather, stagnant wind patterns and low relative humidity. The monthly average maximum concentration in summer was found to be in the range of 62–95 ppb whereas, it was found to be 50–82 ppb in the autumn (October and November). The analysis of hourly averaged surface ozone data illustrates that on a large number of days the surface ozone values at Delhi exceed the World Health Organization (WHO) ambient air quality standards (hourly average of 80 ppb) for ozone. On some occasions, night‐time increases of surface ozone concentration have been observed under stable boundary layer conditions and during thunderstorms.

Variation in aerosol black carbon concentration and its emission estimates at the mega-city Delhi

Simultaneous measurements of aerosol black carbon (BC) mass concentration using an Aethalometer Model AE-42 and mixing layer height (MLH) using a monostatic sonic detection and ranging (SODAR) system were carried out from January 2006 to January 2007 at the mega-city Delhi. The BC concentration generally had a typical diurnal variation with morning and late-afternoon/night peaks. The average BC concentration during the whole period of observation was fairly high at 14.75 μg m−3. The BC concentration nearly doubled during cloudy-sky conditions compared to that during clear-sky conditions. The seasonal variation showed a maximum average concentration during the winter (25.5 μg m−3) and a minimum during the monsoon season (7.7 μg m−3), with post- and pre-monsoon values at 13.7 and 9.4 μg m−3, respectively. The average BC concentrations were strongly affected by the ventilation coefficient, a product of average wind speed (WS) and average MLH, and were found to be strongly anticorrelated. A simple model of BC concentration along with the MLH and WS was applied to estimate the average BC emission, which was found to vary in the range 11 000–17 000 kg of BC per day. The maximum emission during the day averaged every hour for different months lay in the range 1000–2100 kg h−1. The mean monthly emission varied in the range 0.35–0.52 Gg per month, giving rise to an annual estimated emission of 4.86 Gg in the year 2006 over Delhi.

On some aspects of tropospheric ozone variability over the Indo-Gangetic (IG) basin, India

To study tropospheric ozone variability over the Indo-Gangetic (IG) basin, monthly tropospheric ozone residual (TOR) data has been analysed for the 1979–2004 period. Tropospheric column ozone has been observed to have a maximum during late summer (48 ± 4.1 DU) and a minimum during late winter (30 ± 4.2 DU) over the IG basin. The amplitude of the seasonal cycle has been observed to be comparatively larger over the western part of the IG region (∼51 ± 2.3 DU) than over the central (∼47 ± 3.2 DU) and eastern parts (∼47 ± 3.2 DU) of the region. Similarly, the seasonal variation in tropospheric ozone has been observed to be comparatively larger over the western part of the IG region (∼22 DU) than over the central (∼18 DU) and eastern parts (∼17 DU) of the region. The difference in tropospheric ozone amount over the eastern and western parts of the IG region also shows seasonal variation with a large difference (up to 4 DU) during the monsoon season. The monsoon system plays an important role in the seasonal variation of the tropospheric ozone over the different parts of the IG region.

Global distribution of tropospheric ozone and its precursors: a view from space

Satellite-borne tropospheric ozone measurements obtained from the tropospheric ozone residual (TOR) method, CO from the MOPITT (at 850 hPa level) measurements and NO2 from the SCIAMACHY measurements for the three-year period 2003–2005 have been utilized to examine the distribution of the pollutant sources and long-range transport on a global scale. Elevated tropospheric ozone columns have been observed over regions of high NO2 and CO concentrations in the northern and southern hemispheres. High levels of the tropospheric ozone column have been observed below about 5°S in the vicinity of the biomass burning regions and extend from continents out over the Atlantic during October. The seasonal distribution of tropospheric O3 and its precursors in the southern hemisphere shows the strong correlation with the seasonal variation of biomass burning in Africa and South America. Northern hemisphere summer shows the widespread ozone and CO pollution throughout the middle latitudes. The inter-hemispheric gradient of ozone and CO found to be decreased during October. Large-scale transport of the ozone and CO over the Atlantic and Pacific Oceans has been clearly identified. Strong inter-continental transport has been observed to occur from west to east along with the mid-latitude winds in the northern hemisphere.

Depolarization ratio measurement using single photomultiplier tube in micropulse lidar

The conventional dual polarization micropulse lidar uses two separate photomultiplier tubes (PMT) to detect both the copolarized and cross-polarized beam. The prominent sources of error in the depolarization ratio measurement are mismatch in PMT, improper selection of discriminator threshold and unequal PMT high voltage. In the present work a technique for the measurement of lidar depolarization ratio using only one PMT sensor has been developed. The same PMT detects both copolarized and cross-polarized lidar backscatter. A stepper motor is used along with the mirrors to bring both the received polarization signals over the PMT window. Application of the same PMT minimizes the error caused in the depolarization ratio measurement due to error in photon counting of an individual channel. The design description of this technique along with the preliminary results depicting its functionality has been mentioned in this article.

Tropospheric ozone variability over the Indian coastline and adjacent land and sea

A tropospheric ozone variability study is carried out to investigate the spatial and temporal distribution over the coastline of the Indian peninsula and adjacent land and sea using NASA Langley Tropospheric Ozone Residual data set for the period 1979–2005. A strong seasonal cycle has been observed with large variation (∼ 55%) over the upper eastern coast, followed by the upper and lower western coast, compared to the lower eastern coast (∼ 33%). A negative gradient in ozone concentration is observed along eastern and western coasts during summer (slope ∼ –0.78 and –0.65) and a positive gradient (slope ∼ 0.16 and 0.21) during winter. The same is observed over the adjacent land and sea along the coastline with slight variation. This change in gradient can be attributed to the anthropogenic emission of precursor gases that reinforce localized photochemical production of ozone. In addition, topography, transport, seasonality of emission of precursor gases and the solar insolation cycle play a vital role.

Observations and model calculations of direct solar UV irradiances in the Schirmacher region of east Antarctica

Measurements of direct UV irradiances (using a MICROTOPS II Sunphotometer) carried out from a high‐latitude site, Antarctica are presented. The instantaneous irradiances at 305±0.9, 312±0.9 and 320±1.0 nm during a no‐ozone‐hole (13 December 2004) and an ozone‐hole (4 October 2004) period have been observed to be about 0.031, 0.150 and 0.299 W m−2 and 0.010, 0.049 and 0.102 W m−2, respectively at local noontime. The observations of the direct UV irradiances at 305±0.9, 312±0.9 and 320±1.0 nm are compared with tropospheric ultraviolet visible (TUV) radiation transfer model calculations. The model estimate shows that, during the ozone‐hole period, a loss of ozone of the order of 44% leads to an increase in irradiance of the order of 410%, 90% and 25% at 305±0.9, 312±0.9 and 320±1.0 nm, respectively. The relationship between change in UV irradiance due to a change in column ozone is obtained using a TUV model and irradiances thus estimated from this relationship are found to compare well with the observed irradiances.

Comparative study of the total ozone column over Maitri, Antarctica during 1997, 2002 and 2003

The total ozone (TOZ) column was measured using a Microtop Sun‐photometer at Maitri (70° 45′ S, 11° 44′ E), Antarctica during the 16th, 21st and 22nd Indian Scientific Expedition to Antarctic. A comparative study of the TOZ column at Maitri, Antarctica was made to understand the behaviour of the ozone hole. The observations showed a direct relationship between the stratospheric temperature anomaly and year‐to‐year variation in TOZ. The minimum TOZ observed at Maitri, Antarctica during spring was 135 DU (Dobson unit), 185 DU and 126 DU in 1997, 2002 and 2003, respectively. The ozone hole in 2003 was much deeper and had a longer duration. The observations showed that chemical loss of ozone over Maitri during the ozone hole period in 2003 was increased by 18.6% when compared with 1997 and 42% when compared with 2002. The observations at Maitri also showed an event of major stratospheric warming along with surface warming during 2002. A temporary sudden rise in the TOZ column before the recovery period (days 300–315) was also observed and found to be overlapping in all the observational years.

Design and development of micro pulse lidar for cloud and aerosol studies

A micro pulse lidar (MPL) has been indigenously designed and developed at the National Physical Laboratory, New Delhi using a 532 nm, 500 pico second pulsed laser having average power of 50mW (at 7.5 KHz PRR). Photon counting technique has been incorporated using the conventional optics, multichannel scaler (Stanford Research Systems SR430) and high sensitive photomultiplier tube. The sensitivity, range and bin etc are computer controlled in the present system. The interfacing between MPL and computer has been achieved by serial (RS232) and parallel printer port. The necessary software and graphical user interface has been developed using visual basic. In addition to this the telescope cover status sensing circuit has been incorporated to avoid conflict between dark count and background acquisition. The micro pulse lidar will be used for the aerosol, boundary layer and the cloud studies at a bin resolution of 6 meters. In the present communication the details of the system and preliminary results will be presented.

Monitoring of vertical aerosol profiles using micropulse lidar

Tropospheric aerosol play an important role in regional meteorology and energy balance of radiation. Specially in huge urban areas like New Delhi, India a large amount of aerosols from anthropogenic origins is continuously produced and released in the atmospheric boundary layer. The effect of aerosols on atmospheric energy balance is a key global change problem. Aerosol vertical distribution monitoring can be significantly improved using active remote sensing by Lidar. Micro-pulse lidar proved to be an important state of art tool providing a detailed picture of the vertical structure of boundary layer and elevated dust or tiny aerosol. Aerosols are spatially and temporarily varied in short period. The movement of the pollutants can be tracked or mapped out as a function of time by the help of Lidar which is very important to understand the dynamics of particulate matters. The in-situ measurements of aerosol at ground will not be a true representation of total aerosol and its vertical distribution in the atmosphere, therefore the monitoring of vertical profiles of aerosol is very important and timely which is not possible by conventional methods. In view of the above a micro pulse lidar is being setup at NPL, New Delhi to get vertical profiles of aerosol to study the radiative forcing and characterization of aerosols using depolarization ratio. In the present communication details of the system and some preliminary results will be presented.