Variation in aerosol black carbon concentration and its emission estimates at the mega-city Delhi

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Variation in aerosol black carbonconcentration and its emissionestimates at the mega-city DelhiTarannum Bano a , Sachchidanand Singh a , N. C. Gupta b , KirtiSoni a , R. S. Tanwar a , S. Nath a , B. C. Arya a & B. S. Gera aa CSIR National Physical Laboratory, New Delhi, Indiab Guru Gobind Singh Indraprastha University, New Delhi, India

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To cite this article: Tarannum Bano, Sachchidanand Singh, N. C. Gupta, Kirti Soni, R. S.Tanwar, S. Nath, B. C. Arya & B. S. Gera (2011): Variation in aerosol black carbon concentrationand its emission estimates at the mega-city Delhi, International Journal of Remote Sensing,DOI:10.1080/01431161.2010.512943

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International Journal of Remote SensingiFirst, 2011, 1–16

Variation in aerosol black carbon concentration and its emissionestimates at the mega-city Delhi

TARANNUM BANO†, SACHCHIDANAND SINGH∗†, N. C. GUPTA‡, KIRTISONI†, R. S. TANWAR†, S. NATH†, B. C. ARYA† and B. S. GERA†

†CSIR National Physical Laboratory, New Delhi, India‡Guru Gobind Singh Indraprastha University, New Delhi, India

(Received 7 April 2010; in final form 27 July 2010)

Simultaneous measurements of aerosol black carbon (BC) mass concentrationusing an Aethalometer Model AE-42 and mixing layer height (MLH) using amonostatic sonic detection and ranging (SODAR) system were carried out fromJanuary 2006 to January 2007 at the mega-city Delhi. The BC concentration gener-ally had a typical diurnal variation with morning and late-afternoon/night peaks.The average BC concentration during the whole period of observation was fairlyhigh at 14.75 µg m−3. The BC concentration nearly doubled during cloudy-skyconditions compared to that during clear-sky conditions. The seasonal variationshowed a maximum average concentration during the winter (25.5 µg m−3) and aminimum during the monsoon season (7.7 µg m−3), with post- and pre-monsoonvalues at 13.7 and 9.4 µg m−3, respectively. The average BC concentrations werestrongly affected by the ventilation coefficient, a product of average wind speed(WS) and average MLH, and were found to be strongly anticorrelated. A simplemodel of BC concentration along with the MLH and WS was applied to estimatethe average BC emission, which was found to vary in the range 11 000–17 000 kg ofBC per day. The maximum emission during the day averaged every hour for differ-ent months lay in the range 1000–2100 kg h−1. The mean monthly emission variedin the range 0.35–0.52 Gg per month, giving rise to an annual estimated emissionof 4.86 Gg in the year 2006 over Delhi.

1. Introduction

Carbonaceous aerosols have received much attention recently because of their poten-tial role in regional climate change (Japar et al. 1986, Penner and Eddleman 1993,Liousse et al. 1996, Menon et al. 2002, Novakov et al. 2005, Varotsos 2005, Tzanisand Varotsos 2008), agriculture (Chameides et al. 1999), air quality and health effects(Jansen et al. 2005, Varotsos et al. 2005, Highwood and Kinnersley 2006). Blackcarbon (BC) is a light-absorbing aerosol, that is, the by-product of incomplete com-bustion of carbonaceous fuel. Because of its absorptive nature, BC accounts directlyfor the reduction in incoming short-wave solar radiation at the Earth’s surface, lead-ing to heating of the atmosphere (Horvath 1993, Jacobson 2001), and thus possiblychanging the temperature structure in the troposphere, which in turn affects the cloudmicrophysical properties and thereby rainfall mechanisms (Menon et al. 2002). BC

*Corresponding author. Email: ssingh@nplindia.org

http://www.tandf.co.uk/journalsDOI: 10.1080/01431161.2010.512943

2 T. Bano et al.

aerosols have also been reported to be acting as cloud condensation nuclei, once theybecome hydrophilic (Twomey 1977). Thus, the fraction of BC in the total aerosol loadin the atmosphere is crucial to the aerosol radiative forcing at the Earth’s surface andalso at different levels of the atmosphere (Varotsos et al. 2006). The increase in BCfraction can also change the surface albedo and the single scattering albedo of theatmosphere, leading to a change in sign of aerosol radiation forcing (Solomon et al.2007).

The use of traditional fuels and poor combustion technologies, especially in devel-oping countries, has resulted in significant BC emission. For example, the combustionof solid biofuels, such as wood, agricultural waste and dried animal manure in cookingstoves, is the largest source of BC emission in India (Venkataraman et al. 2005).However, in a mega-city like Delhi, the major sources of BC emission are vehicularpollution and thermal power plants (Prasad et al. 2006), which have considerableeffects on air pollution and radiation (Varotsos et al. 1995). Considering the key rolethat BC plays in atmospheric radiative as well as health problems, studies on BCaerosols have become an important topic in India. Many short-term studies on BCaerosols from Indian coastal sites and over the oceans are available in the literature(Babu et al. 2002, 2004, Moorthy and Babu 2006), focusing on the radiative impact ofBC. A few long-term BC studies are also available from the Indian mainland (Lathaand Badrinath 2005, Tripathi et al. 2005, Safai et al. 2007), generally characterizing theseasonal behaviour of BC emission and its relationship with meteorological parame-ters. In this study, we have examined the variations in BC characteristics with respectto the mixing layer height (MLH) measured with a sonic detection and ranging(SODAR) system throughout the year from an Indian location. We have estimatedBC emission over Delhi using a simple box model and studied the emission pattern.Important seasonal and other variations of BC throughout the year are also presented.

2. Observation site and instruments

The sampling site was Delhi (28.38◦N, 77.12◦E, 300 m a.s.l.), the capital city of India,which is not only one of the most densely populated areas (10 340 persons km−2) butalso one of the most highly polluted mega-cities in the world. Delhi experiences asemi-arid climate and extreme weather conditions. The summer temperatures peakat more than 45◦C and the very low temperatures in winter can be as low as 1◦C inthe nighttime. The rainy season in Delhi is generally during July and August whenhigher humidity levels occur. In the pre-monsoon period (April–June), frequent duststorms from western and northwestern desert regions cause large-scale loading ofdust aerosols over Delhi. This causes considerable reduction in visibility and in theradiation flux reaching the surface (Singh et al. 2005).

Details of the weather during the period of observation (January–December 2006)are presented in figure 1, which shows the variation in monthly averaged temperature,relative humidity, wind speed (WS), wind direction and total rainfall. The averagemonthly temperature is at a minimum during the winter month of January, gradu-ally increasing to a maximum in May. The pre-monsoon dust storms and infrequentrainfall decrease the temperature slightly during June, and a further decrease followsduring the monsoon and winter seasons. The average relative humidity is highestduring the monsoon months of July–September and at a minimum during the pre-monsoon months of April–June. The dry period of March–June generally shows alower relative humidity, with the monthly average remaining below 50%. The average

Aerosol black carbon at Delhi 3

320 (e)

Jul-06

Figure 1. (a) The monthly average temperature, (b) relative humidity, (c) wind speed, (d) winddirection measured with respect to the north and (e) total rainfall measured during 2006 atDelhi.

WS during the year was at a maximum during the pre-monsoon month of May, whenit was of the order of 3 m s−1. The wind direction was generally southeasterly exceptduring May and July–August when it was southwesterly. To see the correspondingaverage monthly BC concentration changes as a function of the weather conditions,we have divided the year-long observations into four seasons: summer or pre-monsoon(March–May), monsoon (June–September), post-monsoon (October–November) andwinter (December–February).

4 T. Bano et al.

The BC concentration was measured during the 1-year period from January toDecember 2006 with a Magee Scientific Model AE-42 Aethalometer (Magee Scientific,Berkeley, CA, USA), which uses light of wavelength 880 nm from a high-intensityLED lamp and measures the attenuation of the light transmitted through BC particlesthat accumulate on a quartz fibre filter strip. A vacuum pump draws air so that parti-cles accumulate continuously on the filter paper. The attenuation of the light beam islinearly proportional to the amount of BC deposited on the filter strip. The BC con-tent of the aerosols deposited was derived at 5 min integrating time intervals. The flowwas maintained at 2 l min−1. The instrument has been factory calibrated and reportederrors in BC estimates are approximately ±2%. Many studies have been reported inthe literature using this technique for the monitoring of BC particles (Pakkanen et al.2000, Derwent et al. 2001, Safai et al. 2007) and further details of the instrument canbe found elsewhere (Hansen et al. 1984).

We also measured the MLH using monostatic SODAR. Highly directional shortbursts of sound energy are radiated into the atmosphere, which, after scattering fromatmospheric fluctuations of eddy sizes within the inertial subrange (0.1–10 m), arereceived back by the receiving antenna. The signals are processed to produce an onlinefacsimile display of the dynamics of atmospheric boundary layer (ABL) thermal struc-tures. The echogram structural details are further used to derive the mixing heightof the ABL. The SODAR was operated at a frequency of 2.25 kHz with a 100-mspulse duration, a cycle time of 6 s and electrical power of 100 W. The antenna wasa parabolic dish 1.22 m in diameter and the beam width was 15◦. The mixing heightwas obtained using the empirical relationship given by Singal et al. (1994). Furtherdetails of the technique and instrument are described elsewhere (Singal et al. 1985,1994, Asimakopoulos et al. 1992). Supplementary data for the total suspended par-ticulate matter (SPM) and the respirable suspended particulate matter (RSPM) wereobtained from the website of the Central Pollution Control Board (CPCB) of India(http://www.cpcb.nic.in/Air_Quality_Delhi.php).

3. Observations and results

3.1 Diurnal changes in BC concentration

Figure 2(a) and (b) shows the typical diurnal variations in BC concentration forclear- and cloudy-sky conditions during winter (24 and 30 January 2006, respectively)and summer days (16 and 21 April 2006, respectively). The BC concentration on awinter day is usually more than that during a summer day. The diurnal BC variationson clear-sky days during both summer and winter are usually characterized by themorning and evening peaks, which generally occur between 06:00 and 09:00 h andbetween 19:00 and 22:00 h, respectively. On cloudy days, during summer as well aswinter, the BC concentration measured at the surface can be very high, particularly atnighttime, possibly due to the accumulation of BC in the boundary layer. For compar-ison, the MLH is also plotted along with the BC variation. The MLH during cloudydays is lower than during clear or cloud-free days. When the MLH is low and there isan absence of proper mixing within the boundary layer, the BC concentration gets veryhigh. The average BC concentration for 24 h during the typical clear days of 16 April2006 (summer) and 24 January 2006 (winter) had values of 11.8 and 16.4 µg m−3,respectively, indicating that there is an increase of about 50% in the BC concentra-tion (peak as well as average value) from summer to winter days. The increase in BC

BC (24 January 2006, clear)

MLH (24 January 2006, clear)

BC (30 January 2006, cloudy)

MLH (30 January 2006, cloudy)

concentr

ation (

Time (h)

BC (16 April 2006, clear)

MLH (16 April 2006, clear)

BC (21 April 2006, cloudy)

MLH (21 April 2006, cloudy)

concentr

ation (

Time (h)

Figure 2. Typical diurnal variation of BC concentration and MLH in Delhi (a) on a clear-sky day (24 January 2006) and a cloudy-sky day (30 January 2006) during winter and (b) on aclear-sky day (16 April 2006) and a cloudy-sky day (21 April 2006) during summer.

concentration due to cloudy-sky conditions is very high: the 24 h average BC was15.5 µg m−3 on 21 April 2006 (cloudy day in summer) and 40.5 µg m−3 on 30 January2006 (cloudy day in winter).

The morning and evening peaks in BC concentration arise from the combinedeffects of the turbulent dispersion processes in the mixing layer and the build-up oflocal anthropogenic activities, particularly the traffic load. The minimum BC con-centration is found during the daytime (11:00–18:00 h) due to enhancements in theMLH and increased convective activity. The timings of the morning and evening peaksvaried from day to day and also depended on the timing of the sunrise and sunset onan annual basis. The winter-time enhancement is mainly due to the comparatively lowMLH, as shown in figure 2(a) and (b).

6 T. Bano et al.

anuary

pril 2006

ugust 2006

ction (

BC fraction

Figure 3. Daily average SPM, RSPM, BC concentration and BC fraction percentage at Delhiduring January–December 2006.

The BC constitutes a significant fraction of the RSPM (PM10) – particulate matterwith diameter ≤ 10 µm – aerosol concentration over Delhi. Figure 3 shows the dailyaveraged BC concentration as observed during the year 2006 (January–December) andthe fraction of BC in RSPM along with the total SPM and RSPM concentration. Itcan be seen that the SPM concentration is very high during the summer months, witha peak during May, and then gradually decreases during the monsoon season reachinga minimum in August, and then again increases during the post-monsoon and winterseasons. The RSPM shows a similar pattern although the variation is not as strongas the SPM. However, the BC concentration is high during winter and low during theother seasons and almost follows the RSPM pattern. During November, the CPCB aswell as the BC data are not available on a daily basis. However, the monthly averagedata are available and were used in the rest of the study. The fraction of BC in theRSPM mass also varies considerably from day to day and is generally found to belarge during winter, when it can be as much as 15%. For the whole year, on average,the BC contributes to about 6% of the RSPM mass over Delhi.

3.2 Monthly and seasonal variation in BC concentration and mixing height

The monthly average BC concentration over Delhi shows large variations during theyear. The average monthly BC concentrations along with the standard deviations arepresented in figure 4, which shows that in general the concentrations during the wintermonths were much higher than those in the summer months. The highest BC con-centrations during the winter months were followed by the BC concentration in thepost-monsoon month of October. In general, low BC concentrations were observed

DateJa

concentr

ation (

Figure 4. Monthly average BC concentrations measured over Delhi during January–December 2006. Vertical bars denote the standard deviations for the month.

in the summer and monsoon months. The annual average BC concentration at Delhiduring January–December 2006 was 14.75 µg m−3. January showed a maximum aver-age BC concentration of 30.34 µg m−3 whereas a minimum monthly average BCconcentration of 5.73 µg m−3 was seen during the monsoon month of August. Formost of the time the BC concentration over Delhi remained in the range 5–10 µg m−3,which is a reasonably high value when compared with other cities. Except for the rainyseason of July–August, the nighttime average BC concentration over Delhi was alwaysmore than 10 µg m−3 and it went beyond 25 µg m−3 during winter nights. The monthlyaverage BC concentration of the present observations may be compared with anotherobservation at Delhi made during 2001–2002 by Rai et al. (2002). They had deter-mined the BC concentration by a thermal method after collecting the aerosol sampleson glass fibre filters using high-volume samplers. The average annual BC concentrationthey obtained was 17.9 µg m−3, with a minimum concentration occurring during May(6.7 µg m−3) and a maximum during December (27.9 µg m−3). These average monthlydata, when compared with our data, show similar monthly variations, but the averagemonthly BC concentration generally showed a slight decrease. This decrease in BCfrom 2001–2002 to 2006 may be due to better vehicular emission norms and a generalreduction in coal-based thermal power generation at Delhi.

The seasonal variation in BC concentration reveals that the average concentrationof BC during the winter (December–February) was 25.5 µg m−3; during the pre-monsoon summer season (March–May) it was about 9.4 µg m−3 and during themonsoon months (July–September) it was 7.7 µg m−3, whereas the average value forthe post-monsoon season (October–November) was 13.7 µg m−3. Thus, it is foundthat, over Delhi, the average BC concentration in winter was about 75% more thanthe annual average value. If we compare these seasonal values with the 2001–2002BC data at Delhi from Rai et al. (2002), we find that the seasonal average values in2001–2002 were higher in all the seasons. The 2001–2002 values were 26.1, 13.9, 12.2and 23.3 µg m−3 during the winter, summer, monsoon and post-monsoon seasons,respectively. This is also comparable with the observations at another urban location,Pune (Safai et al. 2007), in India, where the winter BC concentration was reportedto have increased by about 80% compared with the annual average. The average sum-mer BC concentration over Delhi was about 32% less than the average annual value.

8 T. Bano et al.

During the monsoon season, the BC concentration decreased by about 58% of theannual average, then increased gradually during the post-monsoon season but stillwas lower than the annual average by 20%. This indicates the washout of BC aerosolsdue to precipitation and scavenging during the monsoon period, which again startsrebuilding during the post-monsoon period. In winter, this may be mainly due tothe formation of an inversion layer and a low mixing layer, which increases the BCconcentration, whereas in summer high convective activity and relatively high WSare responsible for the dispersal of aerosols and hence the comparatively low BCconcentration.

Figure 5 shows the monthly variation of the mass concentration of the particlesby plotting the total SPM and RSPM mass concentrations over Delhi. The averageSPM concentration throughout the year was found to be about 528 µg m−3 whereasthe RSPM concentration was 229 µg m−3. During the pre-monsoon period theconcentrations of both SPM and RSPM show an enhancement but the increases aremore pronounced in the SPM. The concentration of particles in both these modesdecreased with the occurrence of rainfall during June to September. Figure 4 alsoshows the variation in BC fraction with respect to RSPM during different months inthe year 2006 over Delhi. As the BC particle size is always less than 10 µm, we havepresented the BC fraction with respect to RSPM. We can see that the BC fraction is amaximum during winter (10.1% in January 2006) and then decreases gradually duringsummer to become a minimum during the monsoon season (3.8% in August 2006). Itagain starts to increase during the post-monsoon and winter periods.

To observe the BC characteristics during different seasons in detail, hourly averageconcentrations of BC during different seasons are shown in figure 6, along with theaverage variation in MLH and the ventilation coefficient (VC). Although the averagediurnal variation of BC concentration has similar characteristics during the fourseasons, the winter months show very sharp peaks around 08:30 and 22:30 h. Thepeaks during other seasons are generally broad. Moreover, the average winter BC con-centration far exceeds the average values during other seasons, such as summer (bothpre- and post-monsoon) and the rainy season (monsoon), at all the hours through-out the day and night. Very high BC concentration values of about 49 µg m−3 can

ction (

Avg monthly SPM Avg monthly RSPM BC fraction RSPM

Figure 5. Monthly average SPM, RSPM and BC fraction over Delhi during 2006.

m–3)

Ventila

Time (h)

Summer Monsoon Post-monsoon Winter

Figure 6. Hourly average BC concentrations, mixing heights and ventilation coefficientsduring four seasons (summer, monsoon, post-monsoon and winter) at Delhi.

be seen during winter nights around 22:30 h. Low mixing heights and low WS dur-ing winter are conducive to high BC concentrations as the dispersal of pollutants isminimal. Even during the daytime the minimum average BC concentration is as highas 7.5 µg m−3. During the pre-monsoon period, when several desert dust storms fromwestern and northwestern regions enhance the aerosol load over Delhi, the maximumaverage BC concentration was found to be 13.7 µg m−3 and the minimum concentra-tion was 4.5 µg m−3. These values were close to the values during the post-monsoonmonth of September, with a maximum at 14.9 µg m−3 and a minimum at 4.1 µg m−3.The lowest values were observed during the monsoon month of August when themaximum and minimum were at 9.7 and 2.7 µg m−3, respectively. The BC concen-tration was significantly reduced during the rainy season due to the scavenging effectof rainfall.

The variation in BC concentration measured at the surface also depends upon thechange in MLH. It is lowest during the night, allowing pollutants to build up, andhighest at midday when convective heating is maximal, and emissions are effectivelymixed into a larger volume. In figure 6, the average MLH measured using monostaticSODAR simultaneously is shown along with the BC concentration. In general, itshows an anticorrelation behaviour with the BC variation. As the average WS alsokeeps changing and affects the BC concentration, it is found that the VC is a bet-ter parameter for characterizing the average BC concentration in the MLH. The VCis obtained as the product of the average WS and the MLH. The variation of the

10 T. Bano et al.

m–3)

In(VC) (m2 s–1)

00 1 2 3 4 5 6 7 8 9

Figure 7. Variation of ln(BC concentration) versus ln(VC).

average BC concentration with the average VC clearly shows that, when the VC is ata maximum, the BC concentration is at a minimum. When the VC is close to 0 (gen-erally during winter), the BC concentration is very high, and when the VC is high(during summer due to high WS and high MLH), the BC concentration is low. TheBC concentration is generally found to decrease exponentially with the increase inVC. To visualize this relationship we have plotted the log natural of BC concentration(in µg m−3) versus log natural of the VC in figure 7. The slope of this plot shows thatthe two are strongly anticorrelated, with a correlation coefficient of –0.75. This givesrise to the relationship ln(VC) = –0.24 ln(BC) + 3.32, indicating an exponential decaywith exponent –0.24. The R2 value for the plot is 0.57.

Excluding any transported component, the BC concentration simply depends onthe emission strength, removal and horizontal and vertical dispersion (Husain et al.2007). As the vehicular traffic in the city remains heavy throughout the day, the lowconcentration of BC during midday appears to be strongly impacted by the increasedhorizontal and vertical dispersion at this time. The combination of decreased mixingheight and increased evening commuter traffic produces the very high concentrations.Thus, the BC concentrations depended significantly on the VC.

3.3 Estimation of BC emission over Delhi

The measurements of BC concentration, MLH and WS have been used in a simplebox model to estimate the average emission of BC over Delhi. The model is based onthe assumptions that (1) the volume of the box is considered as the effective area of thecity multiplied by the MLH so that the average mass of the BC emitted is the mean BCconcentration multiplied by the effective volume; (2) the mean volume over the cityis filled once every 24 h; and (3) the average diurnal BC concentration is consistentover the city and the observation site is representative of the whole city. Under theseassumptions the mass of BC emission may be given as

[BC]emission = [BC]concentration A[MV + MH] (1)

where [BC]emission is the mass of BC emission, [BC]concentration is the measured averageBC concentration, A is the area of the city and MV and MH are the rates of vertical

18 000

16 000

14 000

12 000

10 000

DateJa

Feb-06

Apr-06

Aug-06

Sep-06

Oct-06

Nov-06

Dec-06

Figure 8. Monthly mean BC emission estimated over Delhi during 2006.

and horizontal mixing, respectively. As the mean volume over the city is assumed tobe filled once every 24 h, MV can be written as

MV = MLH24h

where MLH is the average MLH or mixing height measured directly using the SODARdescribed earlier. Thus, the average rate of vertical mixing MV can be estimated. As ithas been assumed in this box model that the box of air (that is defined by the productof MLH and the area) is mixed, this mixing will depend on the horizontal WS. AsVC is the product of MLH and WS, when VC is divided by the average diameter ofthe city it gives the rate at which horizontal mixing takes place. In this way the rateof horizontal mixing, MH, can be approximated using the average VC and the averagediameter of the city (De) taken according to the first assumption described above.Thus,

MH = VC × 1De

As the area of Delhi is considered to be 1483 km2, the equivalent diameterDe = 43.45 km. Using equations (1)–(3), the BC mass emission was estimated overDelhi. Figure 8 shows the monthly mean BC emission estimated in units of kg day−1.The emissions estimated were found to be more or less consistent throughout the year,varying in the range of 11 000–17 000 kg of BC per day. The maximum emission duringthe day averaged every hour for different months during the year 2006 varied in therange 1000–2100 kg h−1. The mean monthly emission varied in the range 0.35–0.52 Ggper month giving rise to an annual emission of 4.86 Gg in the year 2006 over Delhi.

4. Discussion and conclusions

Very limited BC concentration measurements are available for the mega-cities in theAsian region. Table 1 shows a list of some of the urban centres where BC con-centrations have been measured and these are compared with our data over Delhi.

12 T. Bano et al.

Table 1. Average BC concentrations at different urban locations.

Location Study Type PeriodBC concentration

(µg m−3)

Kanpur Tripathi et al.(2005)

Urban December 2004 6–20

Hyderabad Latha andBadrinath(2005)

Urban January–December 2003 ∼10.0 (November–April),∼4.0 (June–October)

Bangalore Babu et al.(2002)

Urban November 2001 4.2

Pune Safai et al.(2007)

Urban January–December 2005 4.1

Lahore Husain et al.(2007)

Urban 22 November 2005–31 January 2006

Mumbai Venkataramanet al. (2005)

Urban January–March 1999 12.5

Trivandrum Babu andMoorthy(2002)

Urbancoastal

August 2000–October2001

1.5–5

Ahmedabad Ganguly et al.(2006)

Urban 2003–2005 1.5–7.3

Delhi Rai et al.(2002)

Urban 2001–2002 6.7–26.9

Delhi Present study Urban January–December 2006 5.7–30.3

The annual average BC concentration at Delhi during January–December 2006 was14.75 µg m−3, which is a very high value compared to other urban regions. However,the Lahore concentration is significantly higher after comparing the winter periodvalues of Delhi (25.75 µg m−3), showing that there is little difference in BC concentra-tion during the winter months. The higher concentration over Delhi may be attributedto the greater influence of anthropogenic sources such as vehicular and industrialemissions in the region. From the previous observations of average monthly BC dataobtained during the period 2001–2002 by Rai et al. (2002), we find that the presentBC shows similar monthly variations, although the average monthly BC concentra-tions generally showed a slight decrease. This decrease in BC from 2001–2002 to 2006may be due to better vehicular emission norms and a general reduction in coal-basedthermal power generation at Delhi.

The diurnal profiles observed over Delhi are very similar to those observed atother cities such as Hyderabad (Latha and Badrinath 2005), Kanpur (Tripathi et al.2005), Pune (Safai et al. 2007) and Ahmedabad (Ganguly et al. 2006). The increasingtendency of BC towards the morning and evening may be due to decreased mixingheight and increasing vehicular emission. In addition, the BC concentrations in theevening were much lower than those in the morning. This was probably due to thehigher ambient air temperature and thus higher mixing height in the evening than inthe morning.

Figures 3–5 also show that BC concentrations are comparatively very high dur-ing the winter and then decrease gradually during the spring, monsoon and summerseasons. There are several reasons for the higher BC concentration over Delhi duringthe winter. Both the ABL and the WS are low during the winter (figure 1), whereas

the wood, waste and coal burning increases (Khillare et al. 2004). The concentra-tion of soot also increases during the winter due to transport of pollutants fromthermal power plants by prevailing northwesterlies. There are three coal-based ther-mal power plants in Delhi, which are estimated to emit about 6000 tonnes of fly ashper day (Srivastava and Jain 2007). In addition, the air masses reaching the areaduring this period have to travel longer over land than during the monsoon season.Furthermore, the occurrence of rain is infrequent as compared with the monsoonseason, and a higher level of aerosol content is maintained in the atmosphere. Thus,the concentration of BC increases considerably during the winter. On the contrary, theconcentration of BC during the summer and monsoon is generally low. The summerseason is characterized by dry, continental westerlies and, in this period, a thermallow is located at the surface in northwest India and adjoining Pakistan. As a result,aerosols of local and land origin are continuously lifted up resulting in a relatively lowBC concentration at the surface.

The average BC concentration over Delhi during the winter (25.75 µg m−3),pre-monsoon (10 µg m−3), monsoon (6.22 µg m−3) and post-monsoon seasons(11.71 µg m−3) can be compared with the long-term average values from anothersemi-arid station, Ahmedabad (Ganguly et al. 2006), which had a maximum BC massconcentration during the post-monsoon season (7.3 µg m−3) that decreased graduallyduring the dry season (5.5 µg m−3), the pre-monsoon season (2.2 µg m−3) and themonsoon season (1.5 ± 0.8 µg m−3). Comparing the results of the present study ofBC measurements from other urban locations in India, we find that the BC mass con-centration over Delhi is the highest among all the urban cities, coastal or otherwise.This may be mainly because the vehicular density in Delhi is many times more thanin any other city in India. Moreover, heavy-duty diesel engine trucks also contributesignificantly to the increased BC concentration (Latha et al. 2004). In addition, a largenumber of thermal power plants in the Indo-Gangetic Plains contribute significantlytowards the BC emission in the region (Prasad et al. 2006).

The estimation of BC emission based upon the simple box model using observationsfrom a single point can be questioned as representative of the mean BC concen-tration over Delhi. However, the averaged diurnal profile obtained over a month isexpected to be fairly consistent over Delhi, more so because it is not located closeto any major source of BC. Thus, the estimated values are thought to give a fairlygood estimate of the average BC emission trend over Delhi. A similar approach toBC emission estimates from Karachi has been reported by Dutkiewicz et al. (2009),who found mean emissions of 15 000–22 000 kg day−1 over Karachi, which amountto 6.7 kilometric tons of BC per year. In another study, Yuan and Shao (2007) esti-mated the BC emission from Beijing City in China based on energy consumptionand the corresponding emission factor for the year 2000 and found it to be 7.77 Gg.They also projected the BC emission for the year 2008 to be at 2.97 Gg, excludingnon-commercial energy and open biomass burning, as the city was supposed to takevarious measures to reduce BC emissions during the Beijing Olympics. Our estimatesshow a BC emission of the order of 4.86 Gg over Delhi during 2006.

From this study of year-long measurements of BC concentration, MLH andmeteorological parameters over Delhi, it can be summarized that the BC concen-tration exhibited a strong diurnal variation with a morning (06:30–09:30 h) and alate evening/night (20:00–24:00 h) peak and with a minimum concentration duringthe daytime. The average annual BC concentration over Delhi was fairly high(∼14.75 µg m−3 during the observation period) compared to other urban centres

14 T. Bano et al.

in India (where measurements are available), varying from 6.22 µg m−3 during themonsoon season to 25.75 µg m−3 during winter. The BC concentration was basicallycontrolled by the VC, which was low during winter and nighttimes, leading to highsurface BC concentration. The BC emission estimated over Delhi using the simplebox model showed that it varied in the range 11 000–17 000 kg of BC per day. Theaverage maximum daily emission varied in the range 1000–2100 kg h−1. The meanmonthly emission varied in the range 0.35–0.52 Gg month−1 giving rise to an annualemission of 4.86 Gg in the year 2006 over Delhi.

AcknowledgementsPart of this work was sponsored by the ISRO-GBP (Indian Space ResearchOrganisation – Geosphere Biosphere Programme) during the ICARB (IntegratedCampaign for Aerosol trace gases and Radiation Budget) [G6] campaign. T.B. thanksCSIR for the Senior Research Fellowship provided during this work. We thank theanonymous referees for their suggestions in improving the manuscript.

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