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Atmospheric abundances of black carbon aerosols and their radiative impact over an urban and a rural site in SW India
Atmospheric Environment xxx (2015) 1e8
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Atmospheric abundances of black carbon aerosols and their radiative impact over an urban and a rural site in SW India M.P. Raju a, P.D. Safai a, *, K. Vijayakumar a, P.C.S. Devara b, C.V. Naidu c, P.S.P. Rao a, G. Pandithurai a a b c
Indian Institute of Tropical Meteorology, Dr. Homi Bhabha Road, Pune 411008, India Amity Centre for Ocean-Atmospheric Science and Technology (ACOAST), Amity University Haryana, Gurgaon (Manesar) 122 413, India Department of Meteorology and Oceanography, Andhra University, Visakhapatnam, India
h i g h l i g h t s Long term seasonal BC climatology at two environmentally different sites in SW India. Three times more BC at Pune (urban site) than at Sinhagad (rural site). BC enhancement in winter in terms of concentration and mass fraction to TSP. Significant cooling at SUF and warming at ATM leading to high heating rates.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 February 2015 Received in revised form 1 September 2015 Accepted 2 September 2015 Available online xxx
Observations on black carbon (BC) aerosols over an urban site (Pune) and a rural, high altitude site (Sinhagad) during summer and winter seasons over the period of 2009e2013 are reported. Apart from the temporal variation of BC over both the sites, its mass fraction to total suspended particulates (TSP) is studied. Finally, using the chemical composition of TSP and BC in the OPAC model, season-wise optical properties of aerosols are obtained which are further used in the SBDART model to derive the aerosol radiative forcing (ARF) at surface and top of the atmosphere and thereby the atmospheric forcing and heating rates in each season over both the sites. BC mass concentration and its mass fraction to TSP (Mf BC) were higher at Pune than at Sinhagad, indicating impact of more anthropogenic sources. At both the sites winter season witnessed higher BC concentrations than summer as well as higher Mf BC which is due to the prevailing favorable meteorological conditions in winter. Diurnal variation of BC showed different patterns at Pune and Sinhagad in terms of strength and occurrence of high and low values that could be attributed to varying local boundary layer conditions and source activities at both the sites. Negative ARF indicated cooling at top of the atmosphere and at surface leading to warming of the atmosphere at both the sites. However, surface cooling and atmospheric warming was more dominant at Pune leading to higher atmospheric heating rates, underlining the impact of absorbing BC aerosols which were about three times more at Pune than Sinhagad. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Urban site Rural site Black carbon Temporal variations Aerosol radiative forcing Atmospheric heating rates
1. Introduction The spatial and temporal variability in aerosol physical and chemical characteristics results in large uncertainty in the assessment of their contribution towards aerosol radiative forcing and consequently in climate change (IPCC, 2013 and references therein).
* Corresponding author. E-mail address: [email protected] (P.D. Safai).
Absorbing carbonaceous aerosols i.e. black carbon (BC), especially from the south/south east Asian region have become one of the crucial entity for atmospheric warming and related changes in atmospheric thermodynamics as well as changes in precipitation patterns over this region (Ramanathan et al., 2001; Menon et al., 2002). Lack of systematic and continuous long term physical and chemical data of aerosols from regions representing different environments is a major limitation and source of uncertainty in climate models (IPCC, 2013; Bond et al., 2013). The uniqueness of BC aerosols lies in their inert chemical
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Please cite this article in press as: Raju, M.P., et al., Atmospheric abundances of black carbon aerosols and their radiative impact over an urban and a rural site in SW India, Atmospheric Environment (2015), http://dx.doi.org/10.1016/j.atmosenv.2015.09.023
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character, fine size and high absorbing potential. These properties together with the increasing sources for BC make it imperative to give proper and sincere attention to monitor these aerosols from different regions. So far, various studies on BC aerosols and related impacts on atmospheric radiation are reported from Indian region. However, continuous data for a longer period of time are still few (Safai et al., 2013 and references therein). In this paper, we present a most comprehensive data set on BC aerosols, collected for a period of 5 years covering both summer and winter seasons from an urban site (Pune) and a rural high altitude site (Sinhagad) located in western India. These observations have been in progress for the past decade under the ISRO-GBP-ARFI project. Even though both these sites are separated by aerial distance of about 20 km; the difference in their altitude as well as that in the surroundings and also in the presence of anthropogenic sources, makes it interesting to study the character of BC aerosols in varying seasons over these sites. Apart from results related with temporal variations of BC, their impact on major aerosol optical properties and atmospheric short wave radiative forcing along with atmospheric heating rates are presented for both the sites in summer as well as winter season during the period of 5 years from 2009 to 2013. 2. Details on sampling sites and methodology 2.1. Sampling sites at Pune and Sinhagad Observations on BC aerosols were conducted in summer (MarcheApril) and winter (DecembereJanuary) during 2009e2013 at two sites, namely, Pune (18.53 N, 73.8 E, 559 m AMSL), one of the rapidly expanding metropolises in India and Sinhagad (18.36 N, 73.75 E, 1450 m AMSL), a rural, high altitude site near Pune. Fig. 1 depicts the sampling locations of two sites at Pune and Sinhagad. The site at Sinhagad is located at about 45 km by road, to the south-west of Pune city. Sampling site, situated on top of the hill is a flat terrain with an area of about 0.5 km2 and is surrounded by other hill-tops of comparable heights. The sampling site is surrounded by a few scattered hamlets with very meager population. However being a historical fort, Sinhagad attracts tourist activity through out the year which is in fact increasing significantly during
the last few years. During winter through summer, the surrounding area is generally dry and production of major pollutants is from local sources such as wood burning carried out for cooking and also some grass burning or charcoal-making. In addition, sometimes agricultural burning activity also takes place in the nearby surrounding villages during certain period. However, there is no major anthropogenic activity carried out in the nearby vicinity of the sampling site. Overall, Sinhagad is considered as a rural background location. Observations were carried out in the campus of telecom departments (BSNL, Government of India) Microwave Tower which is a protected area and does not face human interference. Pune is a rapidly expanding metropolitan city situated at about 100 km inland from the west coast of India. The environment in the immediate vicinity of the station is urban, with several industries nearby. The sampling site lies to the north east of the main city of Pune at about 10 km from city center and has been experiencing tremendous growth in terms of urbanization as well as vehicular activity in the last few years. Observations were carried out on the terrace of the Institute's building at about 12 m above the surface. The weather at both the sites during summer season is very hot with mostly gusty surface winds. High temperature gives rise to more convective activity during this season. During this season, winds are mostly from west/north west, where Arabian Sea and parts of arid region (Thar Desert) are located. Fair-weather conditions, with clear sky and very low relative humidity exist during the winter season. Low-level inversions during morning and evening hours, and dust haze during morning hours, occur during this season. Winds in the winter season are observed mainly from Indian continent; mostly from north east/east sector (Pune falls in the up-wind region for Sinhagad during winter). More details on the environmental and meteorological conditions for these two sites have been mentioned elsewhere (Raju et al., 2011; Vijayakumar and Devara, 2013; Safai et al., 2013, 2014a). 2.2. Monitoring of BC aerosols Observations on BC mass concentration were made using an Aethalometer (Magee Sci. Inc. USA, Model AE-42) at Pune and Sinhagad during summer and winter season. Atmospheric air was sampled at the flow rate of 3 L/min and at the data was collected at 5 min interval. Details of the instrument and its operation have
Fig. 1. Location of sampling sites at Pune and Sinhagad in the map of India.
Please cite this article in press as: Raju, M.P., et al., Atmospheric abundances of black carbon aerosols and their radiative impact over an urban and a rural site in SW India, Atmospheric Environment (2015), http://dx.doi.org/10.1016/j.atmosenv.2015.09.023
M.P. Raju et al. / Atmospheric Environment xxx (2015) 1e8
been discussed elsewhere (Raju et al., 2011; Safai et al., 2013). An absorption efficiency value of 16.6 m2/g was used to determine the BC mass concentration. In the Aethalometer, BC concentration is calculated from the rate of change of attenuation (ATN). However, it is observed that as the filter tape advances, the BC concentrations apparently rise (Arnott et al., 1999; Hansen et al., 2007; Virkkula et al., 2007) and therefore, the relationship between ATN change and BC concentration is not linear (Bond et al., 1999; Weingartner et al., 2003). Therefore, as the ATN increases, the measured BC concentration becomes underestimated. In order to account for this loading effect, we have used the correction technique described by Virkkula et al. (2007) and also employed by Park et al. (2010). Also at Sinhagad, being a high altitude site, pressure-temperature related corrections were carried out (Moorthy et al., 2004). The BC obtained from the Aethalometer correlated very well with EC from OCEC Analyzer for the entire one year period of 2012e13 at Pune (Safai et al., 2014b) indicating a good comparison between optical and thermal techniques for BC monitoring. The uncertainty involved in the BC mass concentration measurements is about 10% (Ramachandran and Rajesh, 2007). 2.3. Model derived optical and radiative properties Along with the BC measurements, samples for total suspended particulates (TSP) were collected in summer and winter season at Pune and Sinhagad using High Volume Air Sampler at an average flow rate of 1.1 m3/min. Daily one to two samples were collected with the duration of about 6e8 h each. Whatman-41 filter papers of 800 1000 size were used for collection of TSP. Details on sampling and chemical analyses of TSP are mentioned elsewhere (Safai et al., 2010). The mass concentration of water soluble and water insoluble ions obtained from TSP samples was used along with BC mass concentration in the OPAC (Optical properties of Aerosol and Cloud) model to obtain optical properties such as aerosol optical depth (AOD), single scattering albedo (SSA) and asymmetric parameter (ASP) as detailed by Hess et al. (1998). The measured optical properties were reconstructed by constraining the number densities of water soluble (WS), water insoluble (WIS) and BC (soot) components which were derived from measured mass concentrations. The AOD and SSA are reconstructed in a way until the modeled and observed/satellite derived values matched within ±5% deviation. The optical parameters were estimated at different wavelengths in spectral range of 0.2e3.0 mm. Season wise mixing layer heights for both the stations were adjusted using Padmanabha Murthy (1984). The OPAC-derived AODs were validated with those measured using MICROTOPS observations which were carried out only on clear sky days. Observations were taken in the triplet-mode in order to eliminate cloud contamination to the data from spectral variation of AOD. Details on these measurements are reported earlier by Vijayakumar and Devara (2013). The absolute uncertainty in AOD measurement from MICROTOPS varies between ±0.01 and 0.02 (Smirnov et al., 2000). However, when ever sufficient data on AOD was not available from MICROTOPS due to technical problems, we have used the Moderate Resolution Imaging Spectroradiometer (MODIS) derived AOD values at 500 nm. MODIS is a remote sensor onboard the two Earth Observing System (EOS) Terra and Aqua satellites. Level 3 MODIS Collection V005 daily AOD at a 1 1 grid from Terra and Aqua at 550 nm were utilized in the present study. MODIS Terra-and Aqua-derived aerosol products over land and oceans are validated, and used extensively to investigate spatiotemporal variations in aerosol optical characteristics (Remer et al., 2005; Kaskaoutis et al., 2008). The predicted retrieval uncertainty of MODIS-derived AODs is found to be
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±(0.05 þ 0.15AOD) over land and ±(0.03 þ 0.05AOD) over ocean respectively (Remer et al., 2005). The OPAC derived SSA at 500 nm was validated with SSA derived from the AURA-OMI satellite (http://gdata1.sci.gsfc.nasa.gov/daac-bin/G3/gui.cgi?instance_ id¼omi). The optical parameters deduced from the OPAC model were then fed into the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model (Ricchiazzi et al., 1998) for the forcing estimations. This algorithm includes multiple scattering in a vertically inhomogeneous, non isothermal plane-parallel media, and is shown to be computationally efficient in reliably resolving the radiative transfer equation. This radiative transfer computer code characterizes atmospheric aerosol radiative effects using as input the solar zenith angle, the spectral AOD, the spectral SSA, and the spectral ASP. Columnar water vapor (CWV) from MODIS and columnar ozone from OMI were used to derive net fluxes in the spectral range 0.2e3.0 mm. The surface albedo used in the present study is a combination of ocean, sand and vegetation. The aerosol radiative forcing for 33 atmospheric layers calculated at 10 zenith interval were used to determine the diurnal averages. The average aerosol radiative forcing for surface (SUF) and top of the atmosphere (TOA) were determined and atmospheric forcing (ATM) was calculated as the difference between forcing at TOA and that at SUF. Valenzuela et al. (2012) have reported a good agreement over a common spectral interval between the simulated SBDART global irradiances at surface and those provided by AERONET and CM-11 pyranometer measurements. Panicker et al. (2010) have reported overall uncertainty in aerosol radiative forcing calculations from SBDART within the range of 20%, considering uncertainties in BC measurement, OPAC simulations, meteorology and spectral albedo used in SBDART. 3. Results and discussion 3.1. Temporal variation of BC and its mass fraction to TSP at Pune and Sinhagad The hourly variation of mean BC mass concentration in summer and winter at Pune and Sinhagad for the entire observational period during 2009e2013 is shown in Fig. 1. BC mass concentration was enhanced by about 2.5e3 times in winter than in summer at both the sites. However, the range of variation in BC concentration was comparatively more at Pune than at Sinhagad indicating towards the impact of varying anthropogenic emissions at Pune. The major reason behind the observed enhancement of BC in winter is the prevailing regional meteorological conditions that are conducive for not only BC but most of the other aerosol components during this season (Safai et al., 2010, 2013; Raju et al., 2011). Overall the BC mass concentration at Pune was about 3 times more than that at Sinhagad during the period of study across both summer and winter seasons. It is also seen from Fig. 2 that BC concentrations at Pune exhibited a dominant and significant peak during morning (around 09.00) and a weaker plateau around evening hours (20.00e22.00), implying the influence of anthropogenic emissions (mainly high traffic density) as well as changes in the local boundary layer. The pollutants that are accumulated since previous night hours that are residing at the surface, start rising in the morning hours as the boundary layer is lifted up which enhances their concentration near the surface. This is called “Fumigation effect” (Stull, 1988). Similar results have been reported by Babu and Moorthy (2002). The diurnal variation of BC at Sinhagad was quite different in terms of the times of occurrence of BC peaks and lows as well as in terms of the strength of these peaks. There was a peak around noon hours (12.00e13.00) and that around night hours (20.00e23.00).
Please cite this article in press as: Raju, M.P., et al., Atmospheric abundances of black carbon aerosols and their radiative impact over an urban and a rural site in SW India, Atmospheric Environment (2015), http://dx.doi.org/10.1016/j.atmosenv.2015.09.023
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Fig. 2. Hourly variation of BC during summer and winter at Pune and Sinhagad during 2009e2013 (please note the difference in scale for both a and b).
These observed variations in the strength and time of occurrence of peaks and lows in BC are attributed to the surrounding topography and associated boundary layer evolution; and also to the different source activities i.e. more fossil fuel burning at Pune and more biomass/bio-fuel burning at Sinhagad. It is interesting to note that the fumigation effect was not observed at Sinhagad where no morning peak was observed. Results from other high altitude sites in India (Nainital-Dumka et al., 2010 and Darjeeling-Sarkar et al., 2015) also report the similar feature with no morning peak. The diurnal variation of BC at Sinhagad is also different than that reported at other high altitude sites in India, e.g. Nainital (Pant et al., 2006; Dumka et al., 2010) at 1950 m AMSL in western Himalayan foothills and Darjeeling (Sarkar et al., 2015) at 2250 m AMSL in eastern Himalayan foothills. This suggests the importance of changes in local BC sources (biomass/bio-fuel burning for domestic
and agricultural purposes) apart from the changes in local boundary layer parameters. The mass fraction of BC to TSP (Mf BC) was significantly higher during winter at both the sites. The mean Mf BC at Pune was 1.9% and 9.0% during summer and winter, respectively whilst Mf BC was 0.8% and 4.3% during summer and winter respectively, at Sinhagad. This is not only due to the high BC concentrations in winter but also the TSP concentration was comparatively less in winter (78.6 ± 18.2 mg/m3 at Pune and 65.0 ± 19.4 mg/m3 at Sinhagad) than in summer (160.3 ± 15.2 mg/m3 at Pune and 130.1 ± 25.5 mg/m3 at Sinhagad). Mf BC values of 10e15% have been reported particularly in winter from South Indian region (Babu and Moorthy, 2002; Latha and Badrinath, 2003) which is also due to low TSP concentration with substantial BC mass concentration. Mf BC values of about 5% are reported in the suburban regions of Europe and North America,
Please cite this article in press as: Raju, M.P., et al., Atmospheric abundances of black carbon aerosols and their radiative impact over an urban and a rural site in SW India, Atmospheric Environment (2015), http://dx.doi.org/10.1016/j.atmosenv.2015.09.023
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(Ramanathan and Crutzen, 2003). 3.2. Aerosol radiative forcing and heating rates at Pune and Sinhagad The aerosol optical properties such as AOD, SSA, ASP, each at 500 nm, and Angstrom Exponent were computed using the TSP chemical composition and BC from Aethalometer in the OPAC model as described in section 2.3. These parameters along with CWV from MODIS and columnar ozone from OMI were used to compute the ARF over the spectral range of 0.25e4.0 mm at SUF, TOA and ATM for each season at Pune and Sinhagad during the period of 2009e2013. The OPAC derived AOD and SSA values were found to match reasonably well with those obtained from MICROTOPS and OMI, respectively at mid-visible wavelength of 500 nm (Fig. 3). The mean difference in AOD values was 0.02 whereas in SSA it was 0.03, which are well within the retrieval uncertainties. The mean OPAC derived AOD during summer season (0.48 and 0.36 at Pune and Sinhagad, respectively) was found to be more than that in winter (0.32 and 0.25 at Pune and Sinhagad, respectively). This corroborates well with the observed TSP values which were 161 and 130 mg/m3, respectively at Pune and Sinhagad during summer and, 78.6 and 65 mg/m3, respectively at Pune and Sinhagad during winter. BC showed a very good anti-correlation with SSA at both the sites (R2 ¼ 0.52, p ¼ 0.002) indicating its impact on this vital climate variable. Mean ARF for the entire period of 2009e2013 was more in the summer season over Pune (52.0, 9.7 and 42.3 W/m2 at SUF, TOA and ATM, respectively) than that in winter (35.4, 6.3 and 29.3 W/m2 at SUF, TOA and ATM, respectively). Similarly over Sinhagad, mean ARF was more in summer (29.4, 11.8 and 17.5 W/ m2 at SUF, TOA and ATM, respectively) than that in winter (22.5, 8.0 and 14.5 W/m2 at SUF, TOA and ATM, respectively). More atmospheric forcing in summer may be due to the observed higher TSP and AOD values during summer for both the sites. As observed from Fig. 4, negative forcing values observed at SUF and TOA which implies a net cooling effect at surface as well as at the top of the atmosphere. The difference between the ARF at TOA and the SUF gives the atmospheric forcing (ATM). The higher forcing at ATM over Pune in summer indicates more absorbing dust as BC values are moderate in this season. However at Sinhagad, there was no significant difference between ARF at ATM in summer and that in winter which shows nearly equal impact from absorbing
vT/vt ¼ g/Cp*DF/DP where vT/vt is the heating rate (K/day), Cp is the specific heat capacity of air at constant pressure, g is the acceleration due to gravity
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AOD MICROTPS AOD OPAC SSA Satellite SSA OPAC
0.6 0.5
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Sinhagad Su m 09 W in 09 Su m 10 W in 10 Su m 11 W in 11 Su m 12 W in 12 Su m 13 W in 13
Pune Su m 09 W in 09 Su m 10 W in 10 Su m 11 W in 11 Su m 12 W in 12 Su m 13 W in 13
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SSA Satellite SSA OPAC
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0.4
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AOD MICROTOPS AOD OPAC
0.7 AOD/SSA
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dust and BC. The absorbing dust at both the sites could have its origin either in the local soil or in long range transport of desert dust from West African/Arabian region, especially during the summer when winds are generally westerly/north westerly. Pandithurai et al. (2004) have reported 0.0, 33.0 and 33.0 W m2 ARF, respectively for TOA, SUF and ATM at Pune during November 2001 to April 2002. Similarly, Kumar and Devara (2012) have reported 6.8, 40.0 and 32.4 W m2 ARF, respectively at TOA, SUF and ATM in the summer and 6.0, 36.5 and 30.8 W m2 ARF, respectively at TOA, SUF and ATM in the winter during 2004e2009. The atmospheric ARF observed in the present study at Pune is more during the summer and almost similar during the winter as compared to that reported earlier by Kumar and Devara (2012). The ATM forcing at Pune during summer (42.3 W/m2) and winter (29.3 W/m2) was comparable with that reported at Ahmedabad (48.0 W/m2 in summer and 28.0 W/m2 in winter, Ganguly and Jayaraman, 2006). However, it was comparatively more during summer and less during winter than that reported at Dibrugarh (35.7 W/m2 in summer and 33.3 W/m2 in winter, Pathak et al., 2010), Thiruvananthapuram (37.6e32.8 W/m2 in summer and 52.9 to 46.6 W/m2 in winter, Babu et al., 2007) and Visakhapatnam (20.8 W/m2 in summer and 44.2 W/m2 in winter, Sreekanth et al., 2007). The ATM forcing at Sinhagad in summer (17.5 W/m2) and winter (14.5 W/m2) was comparatively higher than that reported at another high altitude location in western India, Gurushikhar (9.0 W/m2 in summer and 4.3 W/m2 in winter, Ramachandran and Kedia, 2011). However, it was less in summer and high in winter as compared with that at Nainital, a high altitude location in Western Himalayan foothills (28.4 W/m2 in summer and 6.0 in W/m2 in winter, Srivastava et al., 2015). The ARF at ATM was used to compute atmospheric heating rate which is direct indicator of warming potential of aerosols at a given location. Heating rate (K/day) has direct implications for climate change and subsequent impacts on atmospheric circulation as well as hydrological cycle (Ramanathan et al., 2001). High forcing at ATM indicates towards excess energy trapped in the atmosphere that reflects in more heating rates. The average heating rate of the atmosphere due to aerosols is defined as
1.0
0.8
5
Fig. 3. Comparison between OPAC derived AOD and SSA with MICROTOPS derived AOD and OMI derived SSA for Pune and Sinhagad.
Please cite this article in press as: Raju, M.P., et al., Atmospheric abundances of black carbon aerosols and their radiative impact over an urban and a rural site in SW India, Atmospheric Environment (2015), http://dx.doi.org/10.1016/j.atmosenv.2015.09.023
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M.P. Raju et al. / Atmospheric Environment xxx (2015) 1e8
60 50 40
Pune
Sum m er W inter
30 20
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10 0 -10
TOA
SUF
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-20 -30 -40 -50 -60
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TO A
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-2 0 -3 0 -4 0 -5 0 -6 0 Fig. 4. Seasonal mean aerosol radiative forcing at TOA, SUF and ATM during summer and winter over Pune and Sinhagad during 2009e2013.
and DP is the atmospheric pressure difference between top and bottom boundary of each layer (Liou, 2002). In this study, we calculated the heating rate for the whole atmospheric column, which is the difference in heating rates between an aerosol-laden and an aerosol-free atmosphere. Large differences between forcing at TOA and SUF during summer and winter indicate the trapping of large energy in the atmosphere over Pune and Sinhagad which is responsible for the increase in atmospheric heating at Pune and Sinhagad (Fig. 5). Atmospheric heating rates over Pune were 1.2 and 0.82 K/day in summer and winter, respectively whereas those at Sinhagad were 0.49 and 0.41 K/day, respectively. Similar results have been reported for Pune earlier by Panicker et al. (2010) and Kumar and Devara (2012). This indicates the considerable influence of absorbing BC aerosols, particularly in winter as well as that of absorbing dust, particularly in summer over both Pune and Sinhagad. Such strong atmospheric absorption of incident solar
radiation plays an important role in altering the atmospheric thermodynamic conditions and thereby affects atmospheric circulation considerably (Chou et al., 2002; Lau and Kim, 2006). Atmospheric heating rates during different seasons are reported earlier over Ahmedabad (0.6e1.13 K/day), Trivandrum (0.62e1.51 K/day), Visakhapatnam (0.09e1.23 K/day), and Kanpur (~1 K/day) by Ganguly and Jayaraman (2006), Babu et al. (2007), Sreekanth et al. (2007), and Dey and Tripathi (2008), respectively. Over Ranchi, Latha et al. (2014) reported seasonal mean aerosol heating rate was maximum in summer (~1.15 K/day) and minimum in winter (~0.45 K/day). Srivastava et al. (2015) have reported heating rates for Nainital, a high altitude location in Western Himalayan foothills (~2000 m AMSL) as 0.17 K/day and 0.80 K/day during winter and summer, respectively. The seasonal heating rate differs from place to place depending on abundance of aerosols, seasons, location, underlying surface, etc.
Please cite this article in press as: Raju, M.P., et al., Atmospheric abundances of black carbon aerosols and their radiative impact over an urban and a rural site in SW India, Atmospheric Environment (2015), http://dx.doi.org/10.1016/j.atmosenv.2015.09.023
M.P. Raju et al. / Atmospheric Environment xxx (2015) 1e8 1 .6
Pune S in h a g a d
Heating Rate (K/Day)
1 .4 1 .2 1 .0 0 .8 0 .6 0 .4 0 .2
Wi n2 013
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Su m2 012
Wi n2 011
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Su m2 010
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Fig. 5. Atmospheric heating rates computed for Pune and Sinhagad during 2009e2013.
4. Conclusions The five-year seasonal study on BC aerosols at an urban (Pune) and a rural high altitude (Sinhagad) site during 2009e2013 under the ISRO-GBP/ARFI project revealed the following results: BC mass concentration at Pune was predominantly more than that at Sinhagad during the period of study across both summer and winter seasons. Enhancement in the BC mass concentration was about 2.5e3 times in winter than in summer at both sites mainly due to conducive meteorological conditions. The diurnal variation of BC at Pune differed from that at Sinhagad in terms of time of occurrence of peaks and lows, and their strengths due to difference in topography and surrounding source activities. Mf BC was significantly higher during winter at both the sites due to comparatively high BC and low TSP concentrations in winter. During both the seasons and at both the sites, negative forcing at SUF and TOA indicated cooling effect and positive forcing at ATM indicated warming of the atmosphere. Higher cooling at SUF and warming at ATM, particularly in more summer led to higher heating rates at Pune mainly due to higher BC mass concentrations at this site. Increasing concentrations of BC aerosols, especially related with fossil fuel burning at Pune and bio-fuel burning at Sinhagad; along with high incursion of dust aerosols in summer, have shown their impact on atmospheric heating over both the sites. Acknowledgments Authors are thankful to the Director, IITM for all the encouragement and support to undertake this work. Thanks are also due to the ISRO-GBP, Department of Space for providing financial support to carry out the observations and for fellowships of two authors MPR and VK. Finally, thanks are also due to the BSNL (Department of Telecommunications, Government of India) authorities at Microwave Tower Station, Sinhagad for all the help during the field observations conducted at Sinhagad site. References Arnott, W.P., Moosmuller, H., Rogers, C.F., Jin, T., Bruch, R., 1999. Photoacoustic spectrometer for measuring light absorption by aerosols: instrument
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Atmospheric abundances of black carbon aerosols and their radiative impact over an urban and a rural site in SW India M.P. Raju a, P.D. Safai a, *, K. Vijayakumar a, P.C.S. Devara b, C.V. Naidu c, P.S.P. Rao a, G. Pandithurai a a b c
Indian Institute of Tropical Meteorology, Dr. Homi Bhabha Road, Pune 411008, India Amity Centre for Ocean-Atmospheric Science and Technology (ACOAST), Amity University Haryana, Gurgaon (Manesar) 122 413, India Department of Meteorology and Oceanography, Andhra University, Visakhapatnam, India
h i g h l i g h t s Long term seasonal BC climatology at two environmentally different sites in SW India. Three times more BC at Pune (urban site) than at Sinhagad (rural site). BC enhancement in winter in terms of concentration and mass fraction to TSP. Significant cooling at SUF and warming at ATM leading to high heating rates.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 February 2015 Received in revised form 1 September 2015 Accepted 2 September 2015 Available online xxx
Observations on black carbon (BC) aerosols over an urban site (Pune) and a rural, high altitude site (Sinhagad) during summer and winter seasons over the period of 2009e2013 are reported. Apart from the temporal variation of BC over both the sites, its mass fraction to total suspended particulates (TSP) is studied. Finally, using the chemical composition of TSP and BC in the OPAC model, season-wise optical properties of aerosols are obtained which are further used in the SBDART model to derive the aerosol radiative forcing (ARF) at surface and top of the atmosphere and thereby the atmospheric forcing and heating rates in each season over both the sites. BC mass concentration and its mass fraction to TSP (Mf BC) were higher at Pune than at Sinhagad, indicating impact of more anthropogenic sources. At both the sites winter season witnessed higher BC concentrations than summer as well as higher Mf BC which is due to the prevailing favorable meteorological conditions in winter. Diurnal variation of BC showed different patterns at Pune and Sinhagad in terms of strength and occurrence of high and low values that could be attributed to varying local boundary layer conditions and source activities at both the sites. Negative ARF indicated cooling at top of the atmosphere and at surface leading to warming of the atmosphere at both the sites. However, surface cooling and atmospheric warming was more dominant at Pune leading to higher atmospheric heating rates, underlining the impact of absorbing BC aerosols which were about three times more at Pune than Sinhagad. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Urban site Rural site Black carbon Temporal variations Aerosol radiative forcing Atmospheric heating rates
1. Introduction The spatial and temporal variability in aerosol physical and chemical characteristics results in large uncertainty in the assessment of their contribution towards aerosol radiative forcing and consequently in climate change (IPCC, 2013 and references therein).
* Corresponding author. E-mail address: [email protected] (P.D. Safai).
Absorbing carbonaceous aerosols i.e. black carbon (BC), especially from the south/south east Asian region have become one of the crucial entity for atmospheric warming and related changes in atmospheric thermodynamics as well as changes in precipitation patterns over this region (Ramanathan et al., 2001; Menon et al., 2002). Lack of systematic and continuous long term physical and chemical data of aerosols from regions representing different environments is a major limitation and source of uncertainty in climate models (IPCC, 2013; Bond et al., 2013). The uniqueness of BC aerosols lies in their inert chemical
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character, fine size and high absorbing potential. These properties together with the increasing sources for BC make it imperative to give proper and sincere attention to monitor these aerosols from different regions. So far, various studies on BC aerosols and related impacts on atmospheric radiation are reported from Indian region. However, continuous data for a longer period of time are still few (Safai et al., 2013 and references therein). In this paper, we present a most comprehensive data set on BC aerosols, collected for a period of 5 years covering both summer and winter seasons from an urban site (Pune) and a rural high altitude site (Sinhagad) located in western India. These observations have been in progress for the past decade under the ISRO-GBP-ARFI project. Even though both these sites are separated by aerial distance of about 20 km; the difference in their altitude as well as that in the surroundings and also in the presence of anthropogenic sources, makes it interesting to study the character of BC aerosols in varying seasons over these sites. Apart from results related with temporal variations of BC, their impact on major aerosol optical properties and atmospheric short wave radiative forcing along with atmospheric heating rates are presented for both the sites in summer as well as winter season during the period of 5 years from 2009 to 2013. 2. Details on sampling sites and methodology 2.1. Sampling sites at Pune and Sinhagad Observations on BC aerosols were conducted in summer (MarcheApril) and winter (DecembereJanuary) during 2009e2013 at two sites, namely, Pune (18.53 N, 73.8 E, 559 m AMSL), one of the rapidly expanding metropolises in India and Sinhagad (18.36 N, 73.75 E, 1450 m AMSL), a rural, high altitude site near Pune. Fig. 1 depicts the sampling locations of two sites at Pune and Sinhagad. The site at Sinhagad is located at about 45 km by road, to the south-west of Pune city. Sampling site, situated on top of the hill is a flat terrain with an area of about 0.5 km2 and is surrounded by other hill-tops of comparable heights. The sampling site is surrounded by a few scattered hamlets with very meager population. However being a historical fort, Sinhagad attracts tourist activity through out the year which is in fact increasing significantly during
the last few years. During winter through summer, the surrounding area is generally dry and production of major pollutants is from local sources such as wood burning carried out for cooking and also some grass burning or charcoal-making. In addition, sometimes agricultural burning activity also takes place in the nearby surrounding villages during certain period. However, there is no major anthropogenic activity carried out in the nearby vicinity of the sampling site. Overall, Sinhagad is considered as a rural background location. Observations were carried out in the campus of telecom departments (BSNL, Government of India) Microwave Tower which is a protected area and does not face human interference. Pune is a rapidly expanding metropolitan city situated at about 100 km inland from the west coast of India. The environment in the immediate vicinity of the station is urban, with several industries nearby. The sampling site lies to the north east of the main city of Pune at about 10 km from city center and has been experiencing tremendous growth in terms of urbanization as well as vehicular activity in the last few years. Observations were carried out on the terrace of the Institute's building at about 12 m above the surface. The weather at both the sites during summer season is very hot with mostly gusty surface winds. High temperature gives rise to more convective activity during this season. During this season, winds are mostly from west/north west, where Arabian Sea and parts of arid region (Thar Desert) are located. Fair-weather conditions, with clear sky and very low relative humidity exist during the winter season. Low-level inversions during morning and evening hours, and dust haze during morning hours, occur during this season. Winds in the winter season are observed mainly from Indian continent; mostly from north east/east sector (Pune falls in the up-wind region for Sinhagad during winter). More details on the environmental and meteorological conditions for these two sites have been mentioned elsewhere (Raju et al., 2011; Vijayakumar and Devara, 2013; Safai et al., 2013, 2014a). 2.2. Monitoring of BC aerosols Observations on BC mass concentration were made using an Aethalometer (Magee Sci. Inc. USA, Model AE-42) at Pune and Sinhagad during summer and winter season. Atmospheric air was sampled at the flow rate of 3 L/min and at the data was collected at 5 min interval. Details of the instrument and its operation have
Fig. 1. Location of sampling sites at Pune and Sinhagad in the map of India.
Please cite this article in press as: Raju, M.P., et al., Atmospheric abundances of black carbon aerosols and their radiative impact over an urban and a rural site in SW India, Atmospheric Environment (2015), http://dx.doi.org/10.1016/j.atmosenv.2015.09.023
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been discussed elsewhere (Raju et al., 2011; Safai et al., 2013). An absorption efficiency value of 16.6 m2/g was used to determine the BC mass concentration. In the Aethalometer, BC concentration is calculated from the rate of change of attenuation (ATN). However, it is observed that as the filter tape advances, the BC concentrations apparently rise (Arnott et al., 1999; Hansen et al., 2007; Virkkula et al., 2007) and therefore, the relationship between ATN change and BC concentration is not linear (Bond et al., 1999; Weingartner et al., 2003). Therefore, as the ATN increases, the measured BC concentration becomes underestimated. In order to account for this loading effect, we have used the correction technique described by Virkkula et al. (2007) and also employed by Park et al. (2010). Also at Sinhagad, being a high altitude site, pressure-temperature related corrections were carried out (Moorthy et al., 2004). The BC obtained from the Aethalometer correlated very well with EC from OCEC Analyzer for the entire one year period of 2012e13 at Pune (Safai et al., 2014b) indicating a good comparison between optical and thermal techniques for BC monitoring. The uncertainty involved in the BC mass concentration measurements is about 10% (Ramachandran and Rajesh, 2007). 2.3. Model derived optical and radiative properties Along with the BC measurements, samples for total suspended particulates (TSP) were collected in summer and winter season at Pune and Sinhagad using High Volume Air Sampler at an average flow rate of 1.1 m3/min. Daily one to two samples were collected with the duration of about 6e8 h each. Whatman-41 filter papers of 800 1000 size were used for collection of TSP. Details on sampling and chemical analyses of TSP are mentioned elsewhere (Safai et al., 2010). The mass concentration of water soluble and water insoluble ions obtained from TSP samples was used along with BC mass concentration in the OPAC (Optical properties of Aerosol and Cloud) model to obtain optical properties such as aerosol optical depth (AOD), single scattering albedo (SSA) and asymmetric parameter (ASP) as detailed by Hess et al. (1998). The measured optical properties were reconstructed by constraining the number densities of water soluble (WS), water insoluble (WIS) and BC (soot) components which were derived from measured mass concentrations. The AOD and SSA are reconstructed in a way until the modeled and observed/satellite derived values matched within ±5% deviation. The optical parameters were estimated at different wavelengths in spectral range of 0.2e3.0 mm. Season wise mixing layer heights for both the stations were adjusted using Padmanabha Murthy (1984). The OPAC-derived AODs were validated with those measured using MICROTOPS observations which were carried out only on clear sky days. Observations were taken in the triplet-mode in order to eliminate cloud contamination to the data from spectral variation of AOD. Details on these measurements are reported earlier by Vijayakumar and Devara (2013). The absolute uncertainty in AOD measurement from MICROTOPS varies between ±0.01 and 0.02 (Smirnov et al., 2000). However, when ever sufficient data on AOD was not available from MICROTOPS due to technical problems, we have used the Moderate Resolution Imaging Spectroradiometer (MODIS) derived AOD values at 500 nm. MODIS is a remote sensor onboard the two Earth Observing System (EOS) Terra and Aqua satellites. Level 3 MODIS Collection V005 daily AOD at a 1 1 grid from Terra and Aqua at 550 nm were utilized in the present study. MODIS Terra-and Aqua-derived aerosol products over land and oceans are validated, and used extensively to investigate spatiotemporal variations in aerosol optical characteristics (Remer et al., 2005; Kaskaoutis et al., 2008). The predicted retrieval uncertainty of MODIS-derived AODs is found to be
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±(0.05 þ 0.15AOD) over land and ±(0.03 þ 0.05AOD) over ocean respectively (Remer et al., 2005). The OPAC derived SSA at 500 nm was validated with SSA derived from the AURA-OMI satellite (http://gdata1.sci.gsfc.nasa.gov/daac-bin/G3/gui.cgi?instance_ id¼omi). The optical parameters deduced from the OPAC model were then fed into the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model (Ricchiazzi et al., 1998) for the forcing estimations. This algorithm includes multiple scattering in a vertically inhomogeneous, non isothermal plane-parallel media, and is shown to be computationally efficient in reliably resolving the radiative transfer equation. This radiative transfer computer code characterizes atmospheric aerosol radiative effects using as input the solar zenith angle, the spectral AOD, the spectral SSA, and the spectral ASP. Columnar water vapor (CWV) from MODIS and columnar ozone from OMI were used to derive net fluxes in the spectral range 0.2e3.0 mm. The surface albedo used in the present study is a combination of ocean, sand and vegetation. The aerosol radiative forcing for 33 atmospheric layers calculated at 10 zenith interval were used to determine the diurnal averages. The average aerosol radiative forcing for surface (SUF) and top of the atmosphere (TOA) were determined and atmospheric forcing (ATM) was calculated as the difference between forcing at TOA and that at SUF. Valenzuela et al. (2012) have reported a good agreement over a common spectral interval between the simulated SBDART global irradiances at surface and those provided by AERONET and CM-11 pyranometer measurements. Panicker et al. (2010) have reported overall uncertainty in aerosol radiative forcing calculations from SBDART within the range of 20%, considering uncertainties in BC measurement, OPAC simulations, meteorology and spectral albedo used in SBDART. 3. Results and discussion 3.1. Temporal variation of BC and its mass fraction to TSP at Pune and Sinhagad The hourly variation of mean BC mass concentration in summer and winter at Pune and Sinhagad for the entire observational period during 2009e2013 is shown in Fig. 1. BC mass concentration was enhanced by about 2.5e3 times in winter than in summer at both the sites. However, the range of variation in BC concentration was comparatively more at Pune than at Sinhagad indicating towards the impact of varying anthropogenic emissions at Pune. The major reason behind the observed enhancement of BC in winter is the prevailing regional meteorological conditions that are conducive for not only BC but most of the other aerosol components during this season (Safai et al., 2010, 2013; Raju et al., 2011). Overall the BC mass concentration at Pune was about 3 times more than that at Sinhagad during the period of study across both summer and winter seasons. It is also seen from Fig. 2 that BC concentrations at Pune exhibited a dominant and significant peak during morning (around 09.00) and a weaker plateau around evening hours (20.00e22.00), implying the influence of anthropogenic emissions (mainly high traffic density) as well as changes in the local boundary layer. The pollutants that are accumulated since previous night hours that are residing at the surface, start rising in the morning hours as the boundary layer is lifted up which enhances their concentration near the surface. This is called “Fumigation effect” (Stull, 1988). Similar results have been reported by Babu and Moorthy (2002). The diurnal variation of BC at Sinhagad was quite different in terms of the times of occurrence of BC peaks and lows as well as in terms of the strength of these peaks. There was a peak around noon hours (12.00e13.00) and that around night hours (20.00e23.00).
Please cite this article in press as: Raju, M.P., et al., Atmospheric abundances of black carbon aerosols and their radiative impact over an urban and a rural site in SW India, Atmospheric Environment (2015), http://dx.doi.org/10.1016/j.atmosenv.2015.09.023
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Fig. 2. Hourly variation of BC during summer and winter at Pune and Sinhagad during 2009e2013 (please note the difference in scale for both a and b).
These observed variations in the strength and time of occurrence of peaks and lows in BC are attributed to the surrounding topography and associated boundary layer evolution; and also to the different source activities i.e. more fossil fuel burning at Pune and more biomass/bio-fuel burning at Sinhagad. It is interesting to note that the fumigation effect was not observed at Sinhagad where no morning peak was observed. Results from other high altitude sites in India (Nainital-Dumka et al., 2010 and Darjeeling-Sarkar et al., 2015) also report the similar feature with no morning peak. The diurnal variation of BC at Sinhagad is also different than that reported at other high altitude sites in India, e.g. Nainital (Pant et al., 2006; Dumka et al., 2010) at 1950 m AMSL in western Himalayan foothills and Darjeeling (Sarkar et al., 2015) at 2250 m AMSL in eastern Himalayan foothills. This suggests the importance of changes in local BC sources (biomass/bio-fuel burning for domestic
and agricultural purposes) apart from the changes in local boundary layer parameters. The mass fraction of BC to TSP (Mf BC) was significantly higher during winter at both the sites. The mean Mf BC at Pune was 1.9% and 9.0% during summer and winter, respectively whilst Mf BC was 0.8% and 4.3% during summer and winter respectively, at Sinhagad. This is not only due to the high BC concentrations in winter but also the TSP concentration was comparatively less in winter (78.6 ± 18.2 mg/m3 at Pune and 65.0 ± 19.4 mg/m3 at Sinhagad) than in summer (160.3 ± 15.2 mg/m3 at Pune and 130.1 ± 25.5 mg/m3 at Sinhagad). Mf BC values of 10e15% have been reported particularly in winter from South Indian region (Babu and Moorthy, 2002; Latha and Badrinath, 2003) which is also due to low TSP concentration with substantial BC mass concentration. Mf BC values of about 5% are reported in the suburban regions of Europe and North America,
Please cite this article in press as: Raju, M.P., et al., Atmospheric abundances of black carbon aerosols and their radiative impact over an urban and a rural site in SW India, Atmospheric Environment (2015), http://dx.doi.org/10.1016/j.atmosenv.2015.09.023
M.P. Raju et al. / Atmospheric Environment xxx (2015) 1e8
(Ramanathan and Crutzen, 2003). 3.2. Aerosol radiative forcing and heating rates at Pune and Sinhagad The aerosol optical properties such as AOD, SSA, ASP, each at 500 nm, and Angstrom Exponent were computed using the TSP chemical composition and BC from Aethalometer in the OPAC model as described in section 2.3. These parameters along with CWV from MODIS and columnar ozone from OMI were used to compute the ARF over the spectral range of 0.25e4.0 mm at SUF, TOA and ATM for each season at Pune and Sinhagad during the period of 2009e2013. The OPAC derived AOD and SSA values were found to match reasonably well with those obtained from MICROTOPS and OMI, respectively at mid-visible wavelength of 500 nm (Fig. 3). The mean difference in AOD values was 0.02 whereas in SSA it was 0.03, which are well within the retrieval uncertainties. The mean OPAC derived AOD during summer season (0.48 and 0.36 at Pune and Sinhagad, respectively) was found to be more than that in winter (0.32 and 0.25 at Pune and Sinhagad, respectively). This corroborates well with the observed TSP values which were 161 and 130 mg/m3, respectively at Pune and Sinhagad during summer and, 78.6 and 65 mg/m3, respectively at Pune and Sinhagad during winter. BC showed a very good anti-correlation with SSA at both the sites (R2 ¼ 0.52, p ¼ 0.002) indicating its impact on this vital climate variable. Mean ARF for the entire period of 2009e2013 was more in the summer season over Pune (52.0, 9.7 and 42.3 W/m2 at SUF, TOA and ATM, respectively) than that in winter (35.4, 6.3 and 29.3 W/m2 at SUF, TOA and ATM, respectively). Similarly over Sinhagad, mean ARF was more in summer (29.4, 11.8 and 17.5 W/ m2 at SUF, TOA and ATM, respectively) than that in winter (22.5, 8.0 and 14.5 W/m2 at SUF, TOA and ATM, respectively). More atmospheric forcing in summer may be due to the observed higher TSP and AOD values during summer for both the sites. As observed from Fig. 4, negative forcing values observed at SUF and TOA which implies a net cooling effect at surface as well as at the top of the atmosphere. The difference between the ARF at TOA and the SUF gives the atmospheric forcing (ATM). The higher forcing at ATM over Pune in summer indicates more absorbing dust as BC values are moderate in this season. However at Sinhagad, there was no significant difference between ARF at ATM in summer and that in winter which shows nearly equal impact from absorbing
vT/vt ¼ g/Cp*DF/DP where vT/vt is the heating rate (K/day), Cp is the specific heat capacity of air at constant pressure, g is the acceleration due to gravity
1.0
0.9
0.9 0.8
AOD MICROTPS AOD OPAC SSA Satellite SSA OPAC
0.6 0.5
0.5 0.4
0.3
0.3 0.2 0.1
Sinhagad Su m 09 W in 09 Su m 10 W in 10 Su m 11 W in 11 Su m 12 W in 12 Su m 13 W in 13
Pune Su m 09 W in 09 Su m 10 W in 10 Su m 11 W in 11 Su m 12 W in 12 Su m 13 W in 13
0.1
SSA Satellite SSA OPAC
0.6
0.4
0.2
AOD MICROTOPS AOD OPAC
0.7 AOD/SSA
0.7 AOD/SSA
dust and BC. The absorbing dust at both the sites could have its origin either in the local soil or in long range transport of desert dust from West African/Arabian region, especially during the summer when winds are generally westerly/north westerly. Pandithurai et al. (2004) have reported 0.0, 33.0 and 33.0 W m2 ARF, respectively for TOA, SUF and ATM at Pune during November 2001 to April 2002. Similarly, Kumar and Devara (2012) have reported 6.8, 40.0 and 32.4 W m2 ARF, respectively at TOA, SUF and ATM in the summer and 6.0, 36.5 and 30.8 W m2 ARF, respectively at TOA, SUF and ATM in the winter during 2004e2009. The atmospheric ARF observed in the present study at Pune is more during the summer and almost similar during the winter as compared to that reported earlier by Kumar and Devara (2012). The ATM forcing at Pune during summer (42.3 W/m2) and winter (29.3 W/m2) was comparable with that reported at Ahmedabad (48.0 W/m2 in summer and 28.0 W/m2 in winter, Ganguly and Jayaraman, 2006). However, it was comparatively more during summer and less during winter than that reported at Dibrugarh (35.7 W/m2 in summer and 33.3 W/m2 in winter, Pathak et al., 2010), Thiruvananthapuram (37.6e32.8 W/m2 in summer and 52.9 to 46.6 W/m2 in winter, Babu et al., 2007) and Visakhapatnam (20.8 W/m2 in summer and 44.2 W/m2 in winter, Sreekanth et al., 2007). The ATM forcing at Sinhagad in summer (17.5 W/m2) and winter (14.5 W/m2) was comparatively higher than that reported at another high altitude location in western India, Gurushikhar (9.0 W/m2 in summer and 4.3 W/m2 in winter, Ramachandran and Kedia, 2011). However, it was less in summer and high in winter as compared with that at Nainital, a high altitude location in Western Himalayan foothills (28.4 W/m2 in summer and 6.0 in W/m2 in winter, Srivastava et al., 2015). The ARF at ATM was used to compute atmospheric heating rate which is direct indicator of warming potential of aerosols at a given location. Heating rate (K/day) has direct implications for climate change and subsequent impacts on atmospheric circulation as well as hydrological cycle (Ramanathan et al., 2001). High forcing at ATM indicates towards excess energy trapped in the atmosphere that reflects in more heating rates. The average heating rate of the atmosphere due to aerosols is defined as
1.0
0.8
5
Fig. 3. Comparison between OPAC derived AOD and SSA with MICROTOPS derived AOD and OMI derived SSA for Pune and Sinhagad.
Please cite this article in press as: Raju, M.P., et al., Atmospheric abundances of black carbon aerosols and their radiative impact over an urban and a rural site in SW India, Atmospheric Environment (2015), http://dx.doi.org/10.1016/j.atmosenv.2015.09.023
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M.P. Raju et al. / Atmospheric Environment xxx (2015) 1e8
60 50 40
Pune
Sum m er W inter
30 20
W/m
2
10 0 -10
TOA
SUF
ATM
-20 -30 -40 -50 -60
60
Sum m er W in te r
50 40 30
S in h a g a d
W/m
2
20 10 0 -1 0
TO A
SUF
AT M
-2 0 -3 0 -4 0 -5 0 -6 0 Fig. 4. Seasonal mean aerosol radiative forcing at TOA, SUF and ATM during summer and winter over Pune and Sinhagad during 2009e2013.
and DP is the atmospheric pressure difference between top and bottom boundary of each layer (Liou, 2002). In this study, we calculated the heating rate for the whole atmospheric column, which is the difference in heating rates between an aerosol-laden and an aerosol-free atmosphere. Large differences between forcing at TOA and SUF during summer and winter indicate the trapping of large energy in the atmosphere over Pune and Sinhagad which is responsible for the increase in atmospheric heating at Pune and Sinhagad (Fig. 5). Atmospheric heating rates over Pune were 1.2 and 0.82 K/day in summer and winter, respectively whereas those at Sinhagad were 0.49 and 0.41 K/day, respectively. Similar results have been reported for Pune earlier by Panicker et al. (2010) and Kumar and Devara (2012). This indicates the considerable influence of absorbing BC aerosols, particularly in winter as well as that of absorbing dust, particularly in summer over both Pune and Sinhagad. Such strong atmospheric absorption of incident solar
radiation plays an important role in altering the atmospheric thermodynamic conditions and thereby affects atmospheric circulation considerably (Chou et al., 2002; Lau and Kim, 2006). Atmospheric heating rates during different seasons are reported earlier over Ahmedabad (0.6e1.13 K/day), Trivandrum (0.62e1.51 K/day), Visakhapatnam (0.09e1.23 K/day), and Kanpur (~1 K/day) by Ganguly and Jayaraman (2006), Babu et al. (2007), Sreekanth et al. (2007), and Dey and Tripathi (2008), respectively. Over Ranchi, Latha et al. (2014) reported seasonal mean aerosol heating rate was maximum in summer (~1.15 K/day) and minimum in winter (~0.45 K/day). Srivastava et al. (2015) have reported heating rates for Nainital, a high altitude location in Western Himalayan foothills (~2000 m AMSL) as 0.17 K/day and 0.80 K/day during winter and summer, respectively. The seasonal heating rate differs from place to place depending on abundance of aerosols, seasons, location, underlying surface, etc.
Please cite this article in press as: Raju, M.P., et al., Atmospheric abundances of black carbon aerosols and their radiative impact over an urban and a rural site in SW India, Atmospheric Environment (2015), http://dx.doi.org/10.1016/j.atmosenv.2015.09.023
M.P. Raju et al. / Atmospheric Environment xxx (2015) 1e8 1 .6
Pune S in h a g a d
Heating Rate (K/Day)
1 .4 1 .2 1 .0 0 .8 0 .6 0 .4 0 .2
Wi n2 013
Su m2 013
Wi n2 012
Su m2 012
Wi n2 011
Su m2 011
Wi n2 010
Su m2 010
Wi n2 009
Su m2 009
0 .0
Fig. 5. Atmospheric heating rates computed for Pune and Sinhagad during 2009e2013.
4. Conclusions The five-year seasonal study on BC aerosols at an urban (Pune) and a rural high altitude (Sinhagad) site during 2009e2013 under the ISRO-GBP/ARFI project revealed the following results: BC mass concentration at Pune was predominantly more than that at Sinhagad during the period of study across both summer and winter seasons. Enhancement in the BC mass concentration was about 2.5e3 times in winter than in summer at both sites mainly due to conducive meteorological conditions. The diurnal variation of BC at Pune differed from that at Sinhagad in terms of time of occurrence of peaks and lows, and their strengths due to difference in topography and surrounding source activities. Mf BC was significantly higher during winter at both the sites due to comparatively high BC and low TSP concentrations in winter. During both the seasons and at both the sites, negative forcing at SUF and TOA indicated cooling effect and positive forcing at ATM indicated warming of the atmosphere. Higher cooling at SUF and warming at ATM, particularly in more summer led to higher heating rates at Pune mainly due to higher BC mass concentrations at this site. Increasing concentrations of BC aerosols, especially related with fossil fuel burning at Pune and bio-fuel burning at Sinhagad; along with high incursion of dust aerosols in summer, have shown their impact on atmospheric heating over both the sites. Acknowledgments Authors are thankful to the Director, IITM for all the encouragement and support to undertake this work. Thanks are also due to the ISRO-GBP, Department of Space for providing financial support to carry out the observations and for fellowships of two authors MPR and VK. Finally, thanks are also due to the BSNL (Department of Telecommunications, Government of India) authorities at Microwave Tower Station, Sinhagad for all the help during the field observations conducted at Sinhagad site. References Arnott, W.P., Moosmuller, H., Rogers, C.F., Jin, T., Bruch, R., 1999. Photoacoustic spectrometer for measuring light absorption by aerosols: instrument
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Please cite this article in press as: Raju, M.P., et al., Atmospheric abundances of black carbon aerosols and their radiative impact over an urban and a rural site in SW India, Atmospheric Environment (2015), http://dx.doi.org/10.1016/j.atmosenv.2015.09.023