V.K. Soni

Carbonaceous aerosols and pollutants over Delhi urban environment: temporal evolution, source apportionment and radiative forcing.

Particulate matter (PM2.5) samples were collected over Delhi, India during January to December 2012 and analysed for carbonaceous aerosols and inorganic ions (SO4(2-) and NO3(-)) in order to examine variations in atmospheric chemistry, combustion sources and influence of long-range transport. The PM2.5 samples are measured (offline) via medium volume air samplers and analysed gravimetrically for carbonaceous (organic carbon, OC; elemental carbon, EC) aerosols and inorganic ions (SO4(2-) and NO3(-)). Furthermore, continuous (online) measurements of PM2.5 (via Beta-attenuation analyser), black carbon (BC) mass concentration (via Magee scientific Aethalometer) and carbon monoxide (via CO-analyser) are carried out. PM2.5 (online) range from 18.2 to 500.6μgm(-3) (annual mean of 124.6±87.9μgm(-3)) exhibiting higher night-time (129.4μgm(-3)) than daytime (103.8μgm(-3)) concentrations. The online concentrations are 38% and 28% lower than the offline during night and day, respectively. In general, larger night-time concentrations are found for the BC, OC, NO3(-)and SO4(2-), which are seasonally dependent with larger differences during late post-monsoon and winter. The high correlation (R(2)=0.74) between OC and EC along with the OC/EC of 7.09 (day time) and 4.55 (night-time), suggest significant influence of biomass-burning emissions (burning of wood and agricultural waste) as well as secondary organic aerosol formation during daytime. Concentrated weighted trajectory (CWT) analysis reveals that the potential sources for the carbonaceous aerosols and pollutants are local emissions within the urban environment and transported smoke from agricultural burning in northwest India during post-monsoon. BC radiative forcing estimates result in very high atmospheric heating rates (~1.8-2.0Kday(-1)) due to agricultural burning effects during the 2012 post-monsoon season.

An early South Asian dust storm during March 2012 and its impacts on Indian Himalayan foothills: A case study

The impacts of an early South Asian dust storm that originated over the western part of the Middle East and engulfed northwest parts of India during the third week of March 2012 have been studied at four different stations covering India and Pakistan. The impacts of this dust storm on aerosol optical properties were studied in detail at Delhi, Jodhpur, Lahore and Karachi. The impact could also be traced up to central Himalayan foothills at Manora Peak. During dust events, the aerosol optical depth (AOD) at 500 nm reached a peak value of 0.96, 1.02, 2.17 and 0.49 with a corresponding drop in Ångström exponent (AE for 440-870 nm) to 0.01, -0.02, 0.00 and 0.12 at Delhi, Jodhpur, Lahore and Karachi, respectively. The single scattering albedo (SSA) at 675 nm was relatively lower at Delhi (0.87) and Jodhpur (0.86), with absorption Ångström exponent (AAE) less than 1.0, but a large value of SSA was observed at Lahore (0.98) and Karachi (0.93), with AAE value greater than 1.0 during the event. The study of radiative impact of dust aerosols revealed a significant cooling at the surface and warming in the atmosphere (with corresponding large heating rate) at all the stations during dust event. The effect of this dust storm was also seen at Manora Peak in central Himalayas which showed an enhancement of ~28% in the AOD at 500 nm. The transport of dust during such events can have severe climatic implications over the affected plains and the Himalayas.

Variability in optical properties of atmospheric aerosols and their frequency distribution over a mega city “New Delhi,” India

The role of atmospheric aerosols in climate and climate change is one of the largest uncertainties in understanding the present climate and in capability to predict future climate change. Due to this, the study of optical properties of atmospheric aerosols over a mega city “New Delhi” which is highly polluted and populated were conducted for two years long to see the aerosol loading and its seasonal variability using sun/sky radiometer data. Relatively higher mean aerosol optical depth (AOD) (0.90u2009±u20090.38) at 500xa0nm and associated Angstrom exponent (AE) (0.82u2009±u20090.35) for a pair of wavelength 400–870xa0nm is observed during the study period indicating highly turbid atmosphere throughout the year. Maximum AOD value is observed in the months of June and November while minimum is in transition months March and September. Apart from this, highest value of AOD (AE) value is observed in the post-monsoon [1.00u2009±u20090.42 (1.02u2009±u20090.16)] season followed by the winter [0.95u2009±u20090.36 (1.02u2009±u20090.20)] attributed to significance contribution of urban as well as biomass/crop residue burning aerosol which is further confirmed by aerosol type discrimination based on AOD vs AE. During the pre-monsoon season, mostly dust and mixed types aerosols are dominated. AODs value at shorter wavelength observed maximum in June and November while at longer wavelength maximum AOD is observed in June only. For the better understanding of seasonal aerosol modification process, the aerosol curvature effect is studied which show a strong seasonal dependency under a high turbid atmosphere, which are mainly associated with various emission sources. Five days air mass back trajectories were computed. They suggest different patterns of particle transport during the different seasons. Results suggest that mixtures of aerosols are present in the urban environment, which affect the regional air quality as well as climate. The present study will be very much useful to the modeler for validation of satellite data with observed data during estimation of radiative effect.

Tethered balloon-born and ground-based measurements of black carbon and particulate profiles within the lower troposphere during the foggy period in Delhi, India

The ground and vertical profiles of particulate matter (PM) were mapped as part of a pilot study using a Tethered balloon within the lower troposphere (1000m) during the foggy episodes in the winter season of 2015-16 in New Delhi, India. Measurements of black carbon (BC) aerosol and PM <2.5 and 10μm (PM2.5 & PM10 respectively) concentrations and their associated particulate optical properties along with meteorological parameters were made. The mean concentrations of PM2.5, PM10, BC370nm, and BC880nm were observed to be 146.8±42.1, 245.4±65.4, 30.3±12.2, and 24.1±10.3μgm-3, respectively. The mean value of PM2.5 was ~12 times higher than the annual US-EPA air quality standard. The fraction of BC in PM2.5 that contributed to absorption in the shorter visible wavelengths (BC370nm) was ~21%. Compared to clear days, the ground level mass concentrations of PM2.5 and BC370nm particles were substantially increased (59% and 24%, respectively) during the foggy episode. The aerosol light extinction coefficient (σext) value was much higher (mean: 610Mm-1) during the lower visibility (foggy) condition. Higher concentrations of PM2.5 (89μgm-3) and longer visible wavelength absorbing BC880nm (25.7μgm-3) particles were observed up to 200m. The BC880nm and PM2.5 aerosol concentrations near boundary layer (1km) were significantly higher (~1.9 and 12μgm-3), respectively. The BC (i.e BCtot) aerosol direct radiative forcing (DRF) values were estimated at the top of the atmosphere (TOA), surface (SFC), and atmosphere (ATM) and its resultant forcing were - 75.5Wm-2 at SFC indicating the cooling effect at the surface. A positive value (20.9Wm-2) of BC aerosol DRF at TOA indicated the warming effect at the top of the atmosphere over the study region. The net DRF value due to BC aerosol was positive (96.4Wm-2) indicating a net warming effect in the atmosphere. The contribution of fossil and biomass fuels to the observed BC aerosol DRF values was ~78% and ~22%, respectively. The higher mean atmospheric heating rate (2.71Kday-1) by BC aerosol in the winter season would probably strengthen the temperature inversion leading to poor dispersion and affecting the formation of clouds. Serious detrimental impacts on regional climate due to the high concentrations of BC and PM (especially PM2.5) aerosol are likely based on this study and suggest the need for immediate, stringent measures to improve the regional air quality in the northern India.

Variability of Atmospheric Aerosols Over India

Atmospheric aerosols play a significant role in climate change due to their ability to scatter and absorb the incoming and outgoing radiation (direct effect). In addition to this, aerosols can also impact climate through modifying cloud properties, such as droplet size distribution and cloud lifetime, a process known as “indirect effect.” Recent studies using long-term data on aerosols (>25 years in some locations) obtained from the ARFINET have revealed a statistically significant seasonally dependent increasing trend. Comparison with measurements taken about 50 years ago indicates the phenomenal nature of the increase in aerosol loading. The rate of increase is high during December to March (dry months) over the entire region. However, the trends are incoherent during April to May (pre-monsoon) and June to September (summer monsoon period). The characteristic features of the spectral variation in aerosol optical depth (AOD) clearly demonstrate the impact of anthropogenic activities on the increasing trend in aerosol loading. Data from a remote coastal location in the southern peninsula (Thiruvananthapuram), on the concentration of BC, normally considered as a tracer for human impact, show a decreasing trend of ~250 ng m−3 per year. This is particularly perceptible after 2004. CAIPEEX data reveal that during the monsoon season, aerosol number concentration showed strong vertical gradient with a transition between the boundary layer and free troposphere.

Assessment of total columnar ozone climatological trends over the Indian sub-continent

ABSTRACT In the present study, long term satellite and Dobson spectrophotometer Total Column Ozone (TCO) data have been used to study the interannual variability and also to assess climatological trends in TCO over different geographical locations of Indian sub-continent. TCO data were analyzed for the period 1957 to 2015 over New Delhi (28.63° N, 77.18° E), Varanasi (25.30° N, 83.02° E), Pune (18.53° N, 73.84° E) and Kodaikanal (10.0° N, 77.47° E). An extensive validation was performed for Total Ozone Mapping Spectrometer (TOMS) and Ozone Monitoring Instrument (OMI) retrieved TCO data independently with Dobson Spectrophotometer TCO measurements over New Delhi, Varanasi, Pune and Kodaikanal. The results of this exercise showed good correlation coefficient (r) of 0.87 (0.88), 0.84 (0.82), 0.91 (0.80) and 0.84 (Data not available) respectively. Climatological mean TCO over New Delhi, Varanasi, Pune and Kodaikanal are 275.02 ± 6.44 DU, 269.03 ± 7.34 DU, 260.78 ± 5.07 DU and 258.71 ± 6.36 DU respectively for the period 1957 to 2015. An increasing trend over New Delhi (0.20 DU year–1), Pune (0.18 DU year–1), Kodaikanal (0.14 DU year–1) and decreasing trend over Varanasi (0.01 DU year–1) were observed. High significance of TCO trend was found at New Delhi (p-value < 0.0001), Pune (p-value = 0.002) and Kodaikanal (p-value = 0.003) with negligible trend over Varanasi with p-value of 0.84. The TCO variations at different geographical locations associated with upper atmospheric meteorological parameters such as lower Stratospheric Temperature (ST) at 65 hPa and Tropopause Height (TH) were also addressed. Annual lower stratospheric temperature shows positive relationship with TCO and Stratospheric ozone over the study sites. Further, decadal variability in TCO with respect to solar activity at New Delhi was also analyzed.

Aerosol columnar characteristics and their heterogeneous nature over Varanasi, in the central Ganges valley

The Indo–Gangetic Basin (IGB) experiences one of the highest aerosol loading over the globe with pronounced inter-/intra-seasonal variability. Four-year (January 2011–December 2014) continuous MICROTOPS-II sun-photometer measurements at Varanasi, central Ganges valley, provide an opportunity to investigate the aerosol physical and optical properties and their variability. A large variation in aerosol optical depth (AOD: from 0.23 to 1.89, mean of 0.82u2009±u20090.31) and Ångström exponent (AE: from 0.19 to 1.44, mean of 0.96u2009±u20090.27) is observed, indicating a highly turbid atmospheric environment with significant heterogeneity in aerosol sources, types and optical properties. The highest seasonal means of both AOD and AE are observed in the post-monsoon (October–November) season (0.95u2009±u20090.31 for AOD and 1.16u2009±u20090.14 for AE) followed by winter (December, January, February; 0.97u2009±u20090.34 for AOD and 1.09u2009±u20090.20 for AE) and are mainly attributed to the accumulation of aerosols from urban and biomass/crop residue burning emissions within a shallow boundary layer. In contrast, during the pre-monsoon and monsoon seasons, the aerosols are mostly coming from natural origin (desert and mineral dust) mixed with pollution in several cases. The spectral dependence of AE, the aerosol “curvature” effect and other graphical techniques are used for the identification of the aerosol types and their mixing processes in the atmosphere. Furthermore, the aerosol source–apportionment assessment using the weighted potential source contribution function (WPSCF) analysis reveals the different aerosol types, emission sources and transport pathways.

Trends in Radiative Fluxes Over the Indian Region

The amount of solar radiation reaching the Earth’s surface is a major component of the surface energy balance and drives a large number of diverse surface as well as atmospheric processes. Over India, aerosols and clouds are the two major controlling factors that affect the shortwave and long-wave radiative fluxes. The present chapter reviews all the major studies carried out in India with respect to long-term variations in solar radiative fluxes in a changing climate scenario. Ground measurements of global irradiance data over India from 1981 to 2006 suggested a significant decreasing trend of 0.89 W m−2 year−1 under all-sky conditions, called solar/global dimming. Further, the long-term analysis of nearly four decades data from 1971 to 2010 showed a decreasing trend of 0.6 W m−2 year−1 during the period 1971–2000 and 0.2 W m−2 year−1 during the period 2001–2010. The lower trend value in the latter part is due to the reversal trends observed at some stations after 2001. This is also consistent with the decreasing trend in total cloud cover at some stations and suggests brightening of solar radiation.

Wet Deposition of Nitrogen at Different Locations in India

The wet deposition data for Pune (2000–2007), for the other locations representing different environments (i.e., urban, rural, industrial, high altitude, marine, traffic etc.) for different time periods during 2001–2007, and for ten Global Atmospheric Watch (GAW) locations in India for a period of 8 years (2000–2007) are considered in this chapter. All the rain water samples were analyzed for pH, conductivity, anions (Cl, SO4 and NO3) and cations (NH4, Na, K, Ca and Mg). In general, in India the rain water was found to be in the alkaline range. Out of ten GAW stations, the 8 years average pH was slightly acidic (pH 5.15–5.36) at only three locations. At the remaining seven locations the pH was alkaline (pH > 5.65). This alkaline nature is due to high dust levels. Neutralization factors indicated that calcium (Ca) is the major neutralizing cation in wet deposition. Calcium concentrations were higher in north and northwestern regions and lower in southern and northeastern regions. Non-sea salt component and back trajectory analyses showed that Ca and SO4 aerosols were transported to the Indian sub-continent from North African and Gulf countries. The wet deposition fluxes were estimated for all the ionic components including nitrogen (N). The 8 year average annual wet deposition of N for ten locations varied between 4.7 and 34.3 kg N ha−1 year−1 and yearly depositions varied between 1.8 and 57 kg N ha−1 year−1. At all the locations, the NO3-N depositions were higher compared to NH4-N. At some of the locations, even though the concentrations are low, the depositions were higher due to the high rainfall amounts. In regional perspective, the excess SO4-S deposition was higher at an industrial location and the N deposition was higher at a traffic junction in Pune region. At a high altitude rural location (Sinhagad) nearby Pune, the concentrations of excess SO4, NO3 and NH4 were lower but their depositions were higher due to higher rainfall amounts. The total N deposition at four different locations in Pune region varied from 10.4 to 13.2 kg N ha−1 year−1.