D.S. Bisht

Water-soluble organic carbon aerosols during a full New Delhi winter: Isotope-based source apportionment and optical properties

Water-soluble organic carbon (WSOC) is a major constituent (~ 20–80%) of the total organic carbon aerosol over the Indian subcontinent during the dry winter season. Due to its multiple primary and secondary formation pathways, the sources of WSOC are poorly characterized. In this study, we present radiocarbon constraints on the biomass versus fossil sources of WSOC in PM2.5 for the 2010/2011 winter period for the megacity Delhi, situated in the northern part of the heavily polluted Indo-Gangetic Plain. The fossil fuel contribution to Delhi WSOC (21 ± 4%) is similar to that recently found at two South Asian background sites. In contrast, the stable carbon isotopic composition of Delhi WSOC is less enriched in 13C relative to that at the two receptor sites. Although potentially influenced also by source variability, this indicates that near-source WSOC is less affected by atmospheric aging. In addition, the light absorptive properties of Delhi WSOC were studied. The mass absorption cross section at 365 nm (MAC365) was 1.1–2.7 m2/g with an Absorption Angstrom Exponent ranging between 3.1 and 9.3. Using a simplistic model the relative absorptive forcing of the WSOC compared to elemental carbon in 2010/2011 wintertime Delhi was estimated to range between 3 and 11%. Taken together, this near-source study shows that WSOC in urban Delhi comes mainly (79%) from biomass burning/biogenic sources. Furthermore, it is less influenced by photochemical aging compared to WSOC at South Asian regional receptor sites and contributes with a relatively small direct absorptive forcing effect.

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.

Visibility degradation during foggy period due to anthropogenic urban aerosol at Delhi, India

Fog occurs more frequently over urban areas than rural areas. It may occur due to increased air pollution emanating from variety of sources in the urban areas. The increased pollution levels may lead to the atmospheric reactions resulting into the formation of secondary pollutants that may also lead to the needed cloud condensation nuclei. Northern regions of India experience severe foggy conditions during the winter period (November–January) each year. In this study, we have simultaneously measured the particulate mass concentration (0.23 µm to 20 µm), meteorological parameters and atmospheric visibility in Mega city Delhi during a winter month of the years 2007– 2008 in order to have an improved understanding of their role in fog formation. The effects of aerosols on fog formation are discussed through an analysis of trends in fog frequency and comparison with meteorological parameters, and visibility as an indicator of aerosol load. This satisfies the precondition for using these relations. The association between the meteorological parameters (visibility, depression temperature) and air pollutants are examined. The Windows software SPSS (version 17.0) is used to fit a linear regression model. The model explained the variation in visibility due to depression temperature and aerosols load.

Variability in radiative properties of major aerosol types: A year-long study over Delhi—An urban station in Indo-Gangetic Basin

Aerosol measurements over an urban site at Delhi in the western Ganga basin, northern India, were carried out during 2009 using a ground-based automatic sun/sky radiometer to identify their different types and to understand their possible radiative implications. Differentiation of aerosol types over the station was made using the appropriate thresholds for size-distribution of aerosols (i.e. fine-mode fraction, FMF at 500 nm) and radiation absorptivity (i.e. single scattering albedo, SSA at 440 nm). Four different aerosol types were identified, viz., polluted dust (PD), polluted continent (PC), mostly black carbon (MBC) and mostly organic carbon (MOC), which contributed ~48%, 32%, 11% and 9%, respectively to the total aerosols. Interestingly, the optical properties for these aerosol types differed considerably, which were further used, for the first time, to quantify their radiative implications over this station. The highest atmospheric forcing was observed for PC aerosol type (about +40 W m(-2), along with the corresponding atmospheric heating rate of 1.10 K day(-1)); whereas the lowest was for MBC aerosol type (about +25 W m(-2), along with the corresponding atmospheric heating rate of 0.69 K day(-1)).

Boundary layer aerosol characteristics at Mahabubnagar during CAIPEEX-IGOC: Modeling the optical and radiative properties

An Integrated Ground Observational Campaign (IGOC) was conducted at Mahabubnagar--a tropical rural station in the southern peninsular India, under the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) program during the period from July to November 2011. Measured chemical composition and carbonaceous aerosols from PM2.5 samples were used in an aerosol optical model to deduce crucial aerosol optical properties, which were then used in a radiative transfer model for radiative forcing estimations. The model derived aerosol optical depth (AOD at 500 nm), varied from 0.13 to 0.76 (mean of 0.40 ± 0.18) whereas Ångström exponent (AE) between 0.10 and 0.65 (mean of 0.33 ± 0.17) suggests relative dominance of coarse particles over the station. On the other hand, single scattering albedo (SSA at 500 nm) was found to vary from 0.78 to 0.92 (mean of 0.87 ± 0.04) during the measurement period. The magnitude of absorption Ångström exponent (AAE), varied from 0.83 to 1.33 (mean of 1.10 ± 0.15), suggests mixed type aerosols over the station. Aerosol direct radiative forcing was estimated and found to vary from -8.9 to -49.3 W m(-2) (mean of -27.4 ± 11.8 W m(-2)) at the surface and +9.7 to +44.5 W m(-2) (mean of +21.3 ± 9.4 W m(-2)) in the atmosphere during the course of measurements. The atmospheric forcing was observed to be ~30% higher during October (+ 29 ± 9 W m(-2)) as compared to August (+21 ± 7 W m(-2)) when the station is mostly influenced by continental polluted aerosols. The result suggests an additional atmospheric heating rate of 0.24 K day(-1) during October, which may be crucial for various boundary layer processes in favorable atmospheric conditions.

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.

Aerosol characteristics during the coolest June month over New Delhi, northern India

June 2008, which is also the transition month between two major seasons for Indo-Gangetic Basin (IGB), has been identified the coolest June over New Delhi during the past century, showing mean temperature of 31.6 ± 1.7°C, which was found to be ∼2°C less than its climatological mean (33.9°C). Aerosol optical properties for this month and thus obtained physical parameters have been studied using data from the CIMEL sun/sky radiometer, installed in New Delhi under the Aerosol Robotic Network (AERONET) programme. Results reveal bimodal aerosol volume size distribution. The monthly mean values for aerosol optical depth (AOD) at 500 nm (0.96 ± 0.31) and Ångström exponent at the wavelength pair of 440–870 nm (0.79 ± 0.42) show significant lower values whereas single scattering albedo at 675 nm shows a significantly larger value (0.94 ± 0.04) compared with previous measurements over the station. Results suggest dominance of scattering-type particles such as water-soluble aerosols from anthropogenic sources and dust aerosols from natural sources with higher relative humidity over the station. Radiative forcing caused due to the aerosols for the month of June 2008, which have been computed using the radiative-transfer model, informs low forcing at the top of atmosphere (TOA, +14 W m−2) as well as at surface (−33 W m−2). The resultant atmospheric forcing (+47 W m−2) indicates warming effect that caused heating of lower atmosphere at the rate of 0.89 K day−1.

Interannual and Intraseasonal Variability in Fine Mode Particles over Delhi: Influence of Meteorology

Fine mode particles (i.e., PM2.5) were collected at Delhi, India, for three consecutive years from January 2007 to December 2009 and were statistically analyzed. Daily mean mass concentration of PM2.5 was found to be 108.81 ± 75.5 μg m−3 ranged from 12 to 367.9 μg m−3, which is substantially higher than the Indian National Ambient Air Quality Standards (NAAQS). Among the measurements, ~69% of PM2.5 samples exceeded 24 h Indian NAAQS of PM2.5 level ( µg m−3); however, ~85% samples exceeded its annual level (40 µg m−3). Approximately 30% of PM2.5 mass was in the range of 40–80 μg m−3, indicating abundance of fine particles over Delhi. Intraseasonal variability of PM2.5 indicates highest mass concentration during postmonsoon (154.31 ± 81.62 μg m−3), followed by winter (150.81 ± 74.65 μg m−3), summer (70.86 ± 29.31 μg m−3), and monsoon (45.06 ± 18.40 μg m−3). In interannual variability, it was seen that in 2008, the fine mode particle was ~23% and ~36% higher as compared to 2007 and 2009, respectively. Significantly negative correlation was found between PM2.5 and temperature (−0.59) as well as wind speed (−0.38). Higher concentration of PM2.5 (173.8 μg m−3) was observed during calm conditions whereas low concentration (79.18 μg m−3) was observed when wind speed was >5 Km/hr. In winter, greater exposure risk is expected, as the pollutant often gets trapped in lower atmosphere due to stable atmospheric conditions.

Seasonal inhomogeneity of soot particles over the central Indo-Gangetic Plains, India: Influence of meteorology

Black carbon (BC) particles play a unique and important role in earth’s climate system. BC was measured (in-situ) in the central part of the Indo-Gangetic Plains (IGP) at Varanasi, which is a highly populated and polluted region due to its topography and extensive emission sources. The annual mean BC mass concentration was 8.92 ± 7.0 µg m -3, with 34% of samples exceeding the average value. Seasonally, BC was highest during the post-monsoon and winter periods (approximately 18 µg m -3) and lower in the premonsoon/ monsoon seasons (approximately 6 µg m -3). The highest frequency (approximately 46%) observed for BC mass was in the interval from 5 to 10 µg m -3. However, during the post-monsoon season, the most common values (approximately 23%) were between 20 and 25 µg m -3. The nighttime concentrations of BC were approximately twice as much as the daytime values because of lower boundary layer heights at nighttime. The Ångström exponent was significantly positively correlated (0.55) with ground-level BC concentrations, indicating the impact of BC on the columnar aerosol properties. The estimated mean absorption Ångström exponent was 1.02 ± 0.08 µg m -3, indicating that the major source of BC was from fossil fuel combustion. Significant negative correlations between BC mass and meteorological parameters indicate a pronounced effect of atmospheric dynamics on the BC mass in this region. The highest mean BC mass concentration (18.1 ± 6.9 µg m -3) as a function of wind speed was under calm wind conditions (38% of the time).

Radiative Impact of Fireworks at a Tropical Indian Location: A Case Study

During Diwali festival, extensive burning of crackers and fireworks is made. Weeklong intensive observational campaign for aerosol study was carried out at a representative urban location in the eastern Indo-Gangetic Plain (IGP), Varanasi (25.3°N, 83.0°E), from October 29 to November 04, 2005 (Diwali on November 01, 2005), to investigate behavioral change of aerosol properties and radiative forcing between firework affected and nonaffected periods. Results show a substantial increase (~27%) in aerosol optical depth, aerosol absorption coefficients, and aerosol scattering coefficients during affected period as compared to non-affected periods. Magnitudes of radiative forcing at top of atmosphere during affected and non-affected periods are found to be