P.D. Safai

Scavenging of aerosols and their chemical species by rain

Abstract Washout or scavenging coefficients have been widely used to study the wet deposition processes quantitatively. In the present theoretical study, the washout coefficients are computed for the aerosols of diameters in the range of 0.02–10xa0μm having various densities in accordance with their chemical compositions for heavy rain regime. The theoretical scavenging rates are applied to the observed average particle size distributions of pre-monsoon months of the year 1998 and 1999 for Pune and 1999 for Himalayan regions. The evolution of particle size distributions at different time intervals for the non-hygroscopic particles of CaCO3, MgCO3, Zn and Mn indicates that the inertial impaction mechanism is the dominant one in removing particles of all sizes for the heavy rain regime. The size dependence of aerosols as a function of relative humidity is considered for the estimation of washout coefficients of hygroscopic particles such as NaCl and (NH4)2SO4. The washout coefficients are found to be highly dependent on relative humidity for hygroscopic particles. The rainwater concentrations are predicted as a function of rainfall depth and a comparison is made with the observed rainwater concentrations of sequential samples collected on 27 June 2001 in a single rain event to support the results of this theoretical work. The predicted rainwater concentrations for RH=50% are about two times larger than that for RH=95% in the case of hygroscopic particles.

Spread of acid rain over India

Abstract Rain water and aerosol samples were collected at a few locations representative of urban and non-urban regions in India. Also, rain water samples were collected in and around a coal-fired power plant. All the rain water and aerosol samples were analyzed for major chemical components along with pH. The rain water at all the places of measurement, except near the industrial sources, has been found to be alkaline and was characterized by the presence of excess cations, particularly by Ca2+. The acid rain near the industrial sources was associated with excess anions, especially SO42−. The atmospheric aerosols at all the places of measurement were found rich with basic components, suggesting that the alkaline soil dust and fly ash are responsible at present for preventing the spread of acid rain in India.

Influence of alkaline particulates on pH of cloud and rain water in India

Abstract Cloud and rain water samples were collected on board aircraft by specially designed equipment, during three monsoon seasons, 1983, 1984 and 1985 in the Pune region in India. The samples were analyzed for major ionic components and pH, and the concentrations of all the ionic components were found to be significantly higher (35–161%) in cloud water than in rain water. In cloud water Cl contributed most (35%) to the total ionic concentration followed by Ca(21%) and Na(17%). Sulphate and nitrate concentrations, on the average, were low and were found to account for only 6% of the total ionic concentration. pH of cloud water and rain water was substantially higher than that of the CO2-equilibrated value (5.6). The findings suggest that influence of anthropogenic sources is negligible and that of soil dust which is alkaline is substantial on the pH and chemistry of cloud/rain water in India.

Study of surface ozone behaviour at urban and forested sites in India

Abstract Surface ozone concentrations were measured continuously at Pune for a period of one year during 1991–1992 and for a period of 10 days in January 1992 at Upper Kargudi and in April 1992 at Bandipur, core zones of the Nilgiri Biosphere Reserve forests located in Tamil Nadu and Karnataka States, respectively, in south India. There is a marked diurnal variation in the concentration of surface ozone which clearly follows the diurnal variation of surface temperature. The monthly maximum concentration was observed during the summer season (March–May) and minimum during the monsoon season (June–September). The annual average concentration of ozone was 27 ppb and hourly values varied between 2 and 69 ppb at Pune. However, in the forest environment the 10-day average concentration was 15 ppb and hourly values varied between 5 and 31 ppb. The values of surface ozone at Pune and forest environment are below the U.S. EPA standard (113 ppb, maximum 1 h permissible concentration).

Studies of Wet Deposition and Dustfall at Pune, India

Rain water and dustfall deposition samples were collected at Pune, an urban site (1992–98) and at Sinhagad, a rural site (1992–94). The samples were collected with wet-only and bulk collectors at Pune and with bulk collector at Sinhagad. The samples were analyzed for major ions, pH and conductivity. The study showed that the rain water at both places is alkaline (pH > 5.6). The average pH at Pune was 6.1. Neutralising components, indicated by Ca and non sea salt (nss) Mg have higher concentrations than the acidifying components SO4 and NO3. The wet deposition fluxes of all the ionic components were higher than the dustfall fluxes. Relative contribution from dustfall was largest for K, Ca, Mg and NO3. Dustfall was greater at Pune, compared to Sinhagad for all components and up to double for Ca.

Sink mechanism for significantly low level of ozone over the Arabian Sea during monsoon

[1]xa0Measurement of surface ozone over the Arabian Sea during the southwest monsoon season (June–September) of 2002 has shown an unusually low level of ozone with an overall average of 9 nmol/mol. Such a low level of ozone could not be explained by simulations using a three-dimensional chemistry transport model, Model for Ozone and Related Tracers (MOZART), which accounts for the known processes of advective transport and includes a standard photochemical mechanism. Thus, for the Arabian Sea region, we propose for the first time that destruction of ozone by reactive halides released from sea salt aerosols is the sink mechanism which played a crucial role in ensuring the significantly low ozone level over the Arabian Sea. Theoretical calculations constrained by observations have shown that, on average, ozone losses due to catalytic action of halogens and due to photolysis plus chemical reaction amount to 2.15 nmol mol−1d−1 and 4.64 nmol mol−1d−1, respectively.

Influence of alkaline particulates on the chemistry of fog water at Delhi, North India

Chemical analysis of fog water collected by impaction in a pre-cooled chamber in Delhi has shown high concentrations of major ions. The dominant ions measured were Na+, K+, Ca+2, and Mg+2. The concentrations of acidic ions, SO4−2 and NO3−, were low. The pH values in the fog water were highly alkaline and varied from 6.4 to 7.6. The high concentrations of soil-oriented components, especially Ca+2, were more than sufficient to neutralize the small acidity in the fog water and were responsible for maintaining high alkaline pH. The pH values of fog water did not attain the low values in India (Delhi), as found in other polluted environments of Europe and the northeast United States.

Chemical composition of fresh snow from Gulmarg, North India

The chemical composition and pH of 30 fresh snow samples collected during December 1986 to May 1987 at Gulmarg (34 degrees 03 N, 74 degrees 24 E, 2655 m above mean sea level), a remote place in north India, were studied. The snow samples were, by and large, alkaline in nature and were largely influenced by non-marine aerosols. The concentrations of cations (Ca(2+), K(+) and Mg(2+)) were more than the anions (SO(2-)(4) and NO(-)(3)). Factor analysis indicated that most of the ionic components were transported into the region during the period of measurements. The transport of ionic components could be attributed to the passage of western disturbances over this region. The comparison of concentrations of anions and cations in the snow samples at Gulmarg with those reported from a few countries in the west revealed that the composition of Gulmarg snow largely differs in the concentrations of cations rather than anions. Among the cations, the concentration of Ca(2+) was high at Gulmarg and this could be responsible for buffering the pH of snow in the alkaline range.

Precipitation chemistry at Sinhagad : a hill station in India

The chemistry of precipitation in remote sites such as mountain tops is of interest in the study of atmospheric pollution and acid rain. The chemical composition measured at mountain site which is away from industrial and urban areas is useful as a reference level and it allows to determine the extent of anthropogenic contamination. Hence, rain water samples were collected at Sinhagad (18°21′N, 73°45′E, 1450 m asl during the monsoon season (June-September) of 1992 and were analysed for major ions. The precipitation samples collected at Sinhagad were alkaline in nature and pH values ranged between 5.9 to 6.76. The ionic composition was dominated by soil dust The concentration of Ca2+ was highest among all the ions. The concentrations of excess SO32− and NO4− were small (23.8 and 15.2 μeq l−1 respectively) compared to the values of polluted regions in India. The correlation coefficient between the ions and pH values was calculated and it was found to be maximum in case of Ca2+. Precipitation samples collected at Sinhagad were alkaline owing to higher concentration of Ca2+ and lower levels of acidic pollutants (SO42− and NO3−).

Origin of calcium in marine aerosol over the arabian sea near the west Coast of India

Abstract We report the influence of soil dust on marine aerosols inferred from the analysis of aerosol samples collected on board research vessel ‘Gaveshani’ for a period of 11 days during May 1983 over the sea areas around the Indian subcontinent. The number distribution curve over the Arabian Sea was disturbed due to the presence of soil particles in the large size aerosols ( r ⩾ 1 μm) as compared to that over the Bay of Bengal. The atmospheric concentration of non sea-salt calcium was positively correlated with the concentration of Fe. The Ca/Na ratios varied widely from the Arabian Sea to the Bay of Bengal with respect to ratio in sea-water. All the excess calcium in the marine aerosols is found to originate from the soil particles of the Arabian Desert and is transported towards the west coast of India.