Delhi, the Capital of India is situated between latitudes 28˚24'17" and 28˚53'00N and longitudes 76˚50'24" and 77˚20'37"E at 216 meters above the mean sea level (MSL). It is spread over an area of approximately 1500 sq. km. The site on NH-2, a “kerbside”, is in proximity of one of the busiest traffic intersections (Ashram Chowk) in Delhi. It is surrounded by ring roads connecting to Sarai Kale Khan, Mathura road, Lajpat nagar and Nizamuddin area, in different directions with a flyover on outer ring road (Sarai Kale Khan?Lajpat Nagar). The junction connects two States, namely UP (through Noida Toll Bridge) and Haryana (through Mathura road leading to Faridabad and to Gurgaon via Dhaula Kuan). Nizamuddin railway station and Sarai Kale Khan bus stand are about 2 km away from the Ashram Chowk. Overall, the study zone can be defined as high activity zone with very high traffic activity throughout day and night. There are several small scale industries, Okhla Industrial Area, a waste management plant and slumps nearby.

Sampling has been conducted for 24-hour duration for a sampling cycle of 12 - 12 hrs (i.e., morning sampling 8 am to 8 pm and night 8 pm to 8 am) using PM_2.5 Sampler (Ecotech Instruments Pvt. Ltd., APM 550) which works on WINS impactor and runs at a constant flow rate of 16.67 L/min during December 2013?January 2014 for alternate weeks. The sampling details and average flow rate have been recorded and carefully maintained throughout the study. Field blank samples are collected by mounting blank filters onto the sampler for about 10 min without pumping any air. The polytetrafluoroethylene (PTFE) filters are pre-conditioned for 2 days in a controlled room (temperature: 20˚C ± 1˚C, relative humidity: 50% ± 5%) before and after the sampling and then weighed using an analytical balance (Sartorius, CPA2PF). After sampling, the sampled filters are sealed in aluminum foil bags and stored in a freezer (−20˚C) prior to analysis.

Elemental compositions of the TSP samples have been determined by Energy Dispersive X-Ray Fluorescence (ED-XRF) spectrometry (Epsilon 5 ED-XRF, PANalytical B. V.). The X-ray source is a side window X-ray tube with a gadolinium anode and operated at an accelerating voltage of 25 - 100 kV and a current of 0.5 - 24 mA (maximum power: 600 W). The characteristic X-ray radiation is detected by a germanium detector (PAN 32). Each sample is analyzed for 30 min to obtain a spectrum of X-ray counts versus photon energy with the individual peak energies matching specific elements and peak areas corresponding to elemental concentrations. In this study, 25 elements (i.e., Si, P, S, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Br, Rb, Sr, Y, Cd, In, Sn, Sb, Ba, Pb) have been determined. Blank correction is performed for QA/QC purposes.

Excess cancer risks (ECRs) have been calculated using the unit risk and the PM-bound concentration of the metals which represents the total ambient concentration of the metals. ECRs can be calculated simply by using the following formula [8]-[10]:

ECR = CA x IUR (1)

where CA is the concentration of the pollutant (μg/m³) and IUR is the inhalation unit risk (μg/m³)⁻¹. The information on the carcinogenic types and the unit risks of the metals is obtained from the US EPA database for IRIS (Integrated Risk Information System) [11].

The daily mean PM_2.5 concentration at the sampling site has been observed to be 286.23 (±41.1) μg/m³. Observed concentrations are three to four times higher as compared to the air quality standard for PM_2.5 of 60 μg/m³ [12] and 35 μg/m³ [13]. This is because the selected sampling site is stop-and-go site at a national highway which has high traffic volume of approximately 170,000 vehicles per day [14].

Concentrations were found to be higher during night time as compared to daytime (Figure 1). This can be attributed to the movement of inter-state diesel-fueled heavy duty vehicles whose entry is allowed in the city from the national highway during the nighttime (after 8 pm). Also, due to the inversion conditions that prevail during the night in winters, fine PM particles remain suspended in the atmosphere for a very long time. During weekends, the concentrations of PM_2.5 are again increasing when compared to weekdays. It may be due to additional weekend trips.

Si, being the crustal element, is found to be maximum i.e. 33%, followed by K, 21% and S, 20%. The concentration trend shows Si > K > S > Ca = Fe > Zn = Pb > Br (Figure 2). However, it is observed that Si has high co-relation with Ca, Fe and K, which may be due to crustal origin of all three elements; while S, Br and Pb may be from vehicular exhaust emissions and/or abrasions due to brake and tyre wear.

The high concentrations of Zn may have its origin from automotive sources i.e. lubricating oil, wear and tear of vulcanized rubber tyres and corrosion of galvanized vehicular parts but it is considered to be a good marker for tyre wear [15] [16]. Although direct emissions of Pb from vehicular exhausts ceased as leaded gasoline was banned in Delhi but lead is still persistent in dust from earlier emissions because of its long residence time in theenvironment [17]. Copper is observed to be coming from brake wear, tyre wear and wheel bearing [18]. Copper can be attributed to solid waste dumping and incineration also [19]. Apart from road dust Mn is mostly emitted by automobile exhaust [20] [21]. It is used as an additive in unleaded gasoline and Manganese tricarbonyl compound in unleaded petrol is used to enhance automobile performance but they have toxicological significance [22].

The elements of As, Cd, Cr, Ni and Pb are the known carcinogenic metals among the twenty-five metals investigated in this study and are introduced by exposure through the inhalation pathway. The IARC [23] [24] has classified Cd and Ni are classified as Class 1 carcinogenic elements and Pb as Class 2B carcinogenic element. The chromium identified in this study included Cr (III), however, it is known that the concentration ratio of carcinogenic Cr (VI) to non-carcinogenic Cr(III) in ambient air is about 1:6. Therefore, the concentration of Cr (VI) used for the carcinogenic risk assessment was calculated as one seventh of the total concentration [9] [10] [25]. There are different forms of nickel including nickel refinery dust, nickel subsulfide (Ni₃S₂), nickel carbonyl and nickel-soluble salts (nickel chloride, nickel sulfate). The US EPA has classified nickel refinery dust and nickel subsulfide as group A materials, known human carcinogens. Lead is classified as Group B2, but human evidence is inadequate in IRIS thus, values for IUR were selected from California Environmental Protection Agency.

Table 1 shows the estimated ECR of PM-bound carcinogenic elements for the average values of As, Cd, Cr, Ni and Pb. As has the highest ECR followed by Cd. Cr(VI) has high ECR than Ni and Pb even though its concentration in the PM is lowest and Pb has the highest, because of the much higher unit risk of Cr (VI), ranging from 7 to 50 times, compared to the unit risk of the Ni or Pb forms. The total ECRs based on the average values of As, Cd, Cr, Ni and Pb in PM_2.5 is 4.34 × 10⁻⁵. These results indicate that 4 or 5 people out of 100,000 could get cancer after exposure to the toxic trace metals in ambient PM_2.5 residing or working near to a national highway.