Pavan S. Kulkarni

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.

Temporal variation of urban NOx concentration in India during the past decade as observed from space

Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) tropospheric NO2 retrievals (average during March–April–May 2003–2006) were used to locate the major urban sources of NO2 in India. To show that tropospheric NO2 measurements from satellites can provide useful information on day-to-day changes in urban NO2 concentration, we compared 2-year (October 2004–October 2006) records of surface air NO2 concentrations obtained at one of the measurement sites in Delhi to retrievals of tropospheric NO2 columns from Ozone Monitoring Instrument (OMI) observations. The quality of correlation between satellite observations at the urban scale and surface measurements (45%) provided confidence to investigate the temporal changes in the local emission over major urban locations. Combined Global Ozone Monitoring Experiment (GOME) and SCIAMACHY measurements for the period March 1996–December 2007 were used to compute the trends over the selected urban locations. A statistical analysis was applied to derive trends for this period over these locations. The time series of the monthly NO2 columns for these 11 years were fitted with a linear function superposed on an annual seasonal cycle. We found a statistically significant increase in tropospheric NO2 levels over the major urban cities in India. Notable urban locations of positive trends were found to be the cities where the urban population is growing rapidly.

Regional CO pollution over the Indian-subcontinent and various transport pathways as observed by MOPITT

We used day-side Measurement of Pollution in the Troposphere (MOPITT) carbon monoxide (CO) retrievals (2000–2007) to examine the regional CO emission and its transport pathways during the summer/winter monsoon, with a specific focus on the Indian-subcontinent. It is observed that MOPITT CO retrievals at 850 hPa level in general show large scale features of CO emission in India, as reflected in the bottom-up inventory. In particular, high CO mixing ratios over the eastern north-eastern part of India, along the Indo-Gangetic (IG) region, and low CO mixing ratios over central India are generally captured from the MOPITT data. A strong plume with enhanced CO mixing ratios at 350 hPa is observed during the summer monsoon, demonstrating large scale vertical transport of the boundary layer CO from the Indian region into the upper troposphere. During winter outflow CO from the Indian region is found to be transported over the Arabian Sea and Bay of Bengal and reaches up to Saudi Arabia and north-eastern Africa. It is observed that emissions from Southeast Asia and the eastern north-eastern Indian region have the greatest impact over the Bay of Bengal and the eastern Indian Ocean, while emissions from the rest of India dominate over the Arabian Sea and the western Indian Ocean.

Anomalous low tropospheric column ozone over Eastern India during the severe drought event of monsoon 2002: a case study

Background, aim, and scopeThe present study is an attempt to examine some of the probable causes of the unusually low tropospheric column ozone observed over eastern India during the exceptional drought event in July 2002.MethodWe examined horizontal wind and omega (vertical velocity) anomalies over the Indian region to understand the large-scale dynamical processes which prevailed in July 2002. We also examined anomalies in tropospheric carbon monoxide (CO), an important ozone precursor, and observed low CO mixing ratio in the free troposphere in 2002 over eastern India.Results and discussionIt was found that instead of a normal large-scale ascent, the air was descending in the middle and lower troposphere over a vast part of India. This configuration was apparently responsible for the less convective upwelling of precursors and likely caused less photochemical ozone formation in the free troposphere over eastern India in July 2002.ConclusionThe insight gained from this case study will hopefully provide a better understanding of the process controlling the distribution of the tropospheric ozone over the Indian region.

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.

Ozone Seasonal Variation with Ground-Based and Satellite Equipments at Évora Observatory: Portugal During 2007–2010

The present study deals with the retrieval and analysis of O3 total columns over the Evora Observatory (South of Portugal) for the period 2007–2010. The data-set presented in this paper is derived from spectral measurements carried out with the UV–Vis. Spectrometer for Atmospheric Tracers Measurements—SPATRAM, installed at the Observatory of the Geophysics Centre of Evora (CGE) –Portugal (38.5oN; 7.9 oW, 300 m asl). The results obtained applying Differential Optical Absorption Spectroscopy (DOAS) methodology to the SPATRAM measurements of zenith sky scattered radiation are presented in terms of seasonal variations of O3. The O3 retrieved with SPATRAM instrument confirms the typical seasonal cycle for middle latitudes reaching the maximum during the spring and the minimum during the autumn. The ground-based results obtained for O3 column are also compared with data from Ozone Monitoring Instrument (OMI) instrument onboard Aura Satellite.