Study of air–soil exchange of polycyclic aromatic hydrocarbons (PAHs) in the north-central part of India – a semi arid region

Abstract

Soil is the major environmental reservoir of organic compounds and soil–air exchange is a key process in governing the environmental fate of these compounds on a regional and global scale. Samples of air and soil were collected to study the levels of PAHs in the air and soil of the Agra region. Concentrations of PAH measured at four locations in the city of Agra, covers industrial, residential, roadside and agricultural areas. Samples were extracted with hexane by ultrasonic agitation. Extracts were then fractioned on a silica-gel column and the aromatic fraction was analysed by GC-MS. The mean concentration of the total PAH (T-PAH) in the air of Agra was 24.95, 17.95 and 14.25 ng m⁻³, during winter, monsoon and summer respectively. The average concentration of T-PAH in the soil of Agra was 12.50, 8.25 and 6.44 μg g⁻¹ in winter, monsoon and summer seasons respectively. The aim of this study was to investigate the rate of approach to equilibrium partitioning of PAHs between air and soil compartments and to determine the direction of net flux of the studied PAH between air and soil. Calculated soil–air fugacity quotients indicate that the soil may now be a source of some lighter weight PAHs to the atmosphere, whereas it appears to be still acting as a long-term sink for the heavier weight PAHs to some extent in this region.

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Environmental impact

Air–soil exchange processes are crucial in maintaining residues of POPs in the atmosphere long after fresh use or release of a POP has been controlled. To the research scientist, there are fascinating questions to address, concerning the kinetics and thermodynamics of air–surface exchange of POPs which will influence the nature and extent of their regional or global distribution and their rate of supply to organisms. A full understanding of these processes is essential if we are to understand the dynamic nature of PAHs behaviour and to enable us to model regional, global or ecosystem transfers of PAHs. It has been hypothesised that air–soil exchange is the key process that controls the levels of many semivolatile organic compounds (SOCs) such as PAHs and PCBs present in soils and/or air.

1. Introduction

Polycyclic aromatic hydrocarbons (PAHs) are among the pollutants of concern for human health due to carcinogenic and mutagenic properties of certain compounds from the PAH class. Contributions from natural sources of PAHs is limited, being restricted to spontaneous forest fires and volcanic emissions. In cities, the sources of PAH are exclusively anthropogenic, i.e., they are formed during incomplete combustion and pyrolysis of organic matter, such as coal, oil, wood, and fuels like diesel and petrol. Automobile exhaust has been recognized as the major PAH contributor in urban areas.¹ They undergo thermal decomposition and react with a number of atmospheric chemicals producing derivatives, which can be more toxic than the original compounds.² Although some persistent organic pollutants (POPs) are released slowly into the atmosphere,³ these omnipresent compounds are subject to redistribution and transformation processes.⁴ During their atmospheric residence, PAHs are redistributed in the atmosphere between gas and particulate phase, transported over long distances.⁵ Atmospheric deposition constitutes the main input of semi-volatile organic compounds to soil.⁶ Once entered into soil, they accumulate in horizons rich in organic matter where they are likely to be retained for many years due to their persistence and hydrophobicity.⁷ Soil is the primary terrestrial reservoir of persistent organic pollutants such as PAH's⁸ and the atmosphere is their main transport vector⁹ so, it is important to determine the amounts of PAHs in soil as their concentration in soil correlates significantly with the corresponding levels in air^10,4 and is a good indicator of the surrounding air pollution and the proximity of sources. Soil–air exchange is therefore a key process governing the environmental fate of these compounds on a regional and global scale. Air to soil transport may occur through dry depositions of aerosols, wet deposition or sorption to soil constituents.¹¹ Soil to air diffusion is driven by the chemical potential gradient between soil and atmosphere. Consequently, soils are an important reservoir for these compounds¹² and exchanges between soils, water and the atmosphere is a widely studied process.^13,14

Air–surface exchange processes are crucial in maintaining residues of POPs in the atmosphere long, after fresh use or release of a POP has been controlled. To the research scientist, there are fascinating questions to address, concerning the kinetics and thermodynamics of air–surface exchange of POPs which will influence the nature and extent of their regional or global distribution and their rate of supply to organisms. A full understanding of these processes is essential if we are to understand the dynamic nature of PAHs behaviour and to enable us to model regional, global or ecosystem transfers of PAHs.¹⁵ It has been hypothesised that air–soil exchange is the key process that controls the levels of many semi-volatile organic compounds (SOCs) like PAHs, PCBs present in soils and/or air.¹⁶

In India, a few studies have reported atmospheric PAH concentrations in Delhi,¹⁷ Mumbai,¹⁸ Ahmedabad.¹⁹ To our knowledge there has been a shortage of air–soil exchange studies of PAHs in India particularly in this region of India. Therefore, the aim of the present study is to analyse the air–soil exchange process by calculating soil–air fugacity quotients using our analytical data of this region for priority PAHs.

2. Methods and materials

2.1. Regional site description

Agra, the city of Taj Mahal (27°10′N 78°02′E) is located in the north central part of India about 200 km South of Delhi in the Indian state of Uttar Pradesh. Agra is considered as a semi-arid zone as two third of its boundary are surrounded by the Thar desert of Rajasthan. Three highways cross the city. The climate during the summer is hot and dry with temperatures ranging from 20 °C to 48 °C. In the monsoon season it ranges from 14.5 °C to 38.2 °C, whereas in winters, the temperature ranges from 3.5 °C to 29.5 °C. The downward wind is south–southeast i.e. SSE 29% and northeast i.e. NE 6%, in the summer and it is west–north-west i.e. WNW 9.4% and north–northwest i.e. NNW 11.8% in winters. Table 1 illustrates meteorological conditions measured during the study period. The atmospheric pollution load is high because of the downward wind; pollutants may be mainly transported to the different areas from an oil refinery situated in Mathura (50 km from the centre of Agra city). Agra has 1 271 000 of population. 3 86 635 vehicles are registered and 32 030 generator sets are used. It has been indicated earlier that in Agra, 60% of the pollution is due to vehicles.²⁰ St. John's College, which is situated in the heart of Agra city, is considered as a roadside area. It lies by the side of a road that carries a maximum traffic density of about 10⁵ vehicles per day, which results in the production of smoke, and total suspended particulate matter by engine idling and gear changes. Towards the north is located Dayalbagh which is an exclusively rural (agricultural) area. Taj trapezium (the area surrounding the Taj Mahal 10 400 ∼ km²) located to south of the site, which is considered to be an urban (residential) area is totally a green belt. Fig. 1 shows the different locations on the map of Agra.

Table 1 Meteorological conditions measured during the study period

Meteorological parameter	Mean	S.D.	Range
a The dominant wind direction during summer, monsoon and winter seasons remained as west and north-northwest, east and south–southwest and west–northwest and north-northwest respectively.
Summer
Wind speed (m s^−1)^a	4.7	0.62	0.2–9.2
Air T (°C)	33.2	3.4	20–48
Relative humidity (%)	48.3	7.1	18.4–62.7
Monsoon
Wind speed (m s^−1)^a	3.7	0.66	0.2–6.9
Air T (°C)	31.8	2.4	14.5–38.2
Relative humidity (%)	89.3	7.7	35.6–96.2
Winter
Wind speed (m s^−1)^a	3.8	0.83	0.1–7.5
Air T (°C)	19.1	1.7	3.5–29.5
Relative humidity (%)	78.8	8.4	38.4–88.7

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Fig. 1 Map of Agra showing the different sites.

2.2. Sampling, extraction and analysis

Air sampling stations were set up in four locations representing industrial, residential roadside and agricultural areas. Each station was monitored for ambient air quality twice a week with small pumps (Envirotech Handy Sampler APM 821, Okhla, New Delhi, India) in a scheduled manner throughout the year. The pump was set at 2 L min⁻¹, and run continuously for 20 h aspirating air through XAD-2 resin (150 mg) retaining PAHs in the gaseous phase. Sample flow was measured before and after sampling using a calibrated rotameter with an accuracy of ±1%. After sampling the XAD-2 resins were placed separately in 4 mL screw-top vials. In each vial 2 mL methylene chloride was added and they were shaken for 2 min. These vials were allowed to settle for 30 min. From each vial containing XAD resin, 1 mL extract was transfer to an auto sampler vial for further analysis on GC-MS. For quality assurance and quality control (QA/QC) three sets of XAD-2 tubes, blank spike and blank spike duplicate (BS/BSDs) were spiked with a PAH spiking solution mixture of PAHs (Supelco 4-8902, U.S.A) which was custom made. Recoveries of each PAH from the XAD- 2 tubes ranged between 96% to 120% and 97% to 126% respectively and its relative percent difference (RPD) ranged from 0% to 12%.⁵ The limit of quantification for various PAHs ranged from 13.7 ng m⁻³ to 125 ng m⁻³ and 76% of the samples of the total PAHs exceeded the limit of quantification.

According to Wild and Jones²¹ and Ribes,⁸ it has been estimated that soil contains the vast majority (>90%) of the total environmental burden of polycyclic aromatic hydrocarbons, therefore, the concentrations of PAHs in soil were also measured simultaneously throughout the year. Soil samples were collected with the help of an auger from 0 to 10 cm of topsoil. A total of 281 soil samples were collected (70 from each location) for analysis. Collected samples were sieved through a 20-mesh sieve and stored in polybags in a cool and dark place till analysis. The analysis was carried out by gas chromatography/mass spectrometry (GC-MS) [Hewlett Packard (HP), 6890 GC/5972 MS] controlled by HP enviroquant software operating in selective ion monitoring (SIM) mode as described in detail in our earlier study (Masih et al., 2010b). Detailed procedure of analysis was followed from a response engineering and analytical contract (REAC) method, USEPA. The REAC method is based on a modified National Institute for Occupational Safety and Health (NIOSH) method 5515 for the analysis of PAHs in air samples.²²

3. Results and discussion

3.1. PAHs in air

Seasonal concentration of PAHs in air is shown in Table 2a. According to the Table, the total PAH (T-PAH) concentrations during the winter were 47.93, 29.86, 16.80 and 5.30 ng m⁻³ at the industrial, roadside, residential and agricultural sites respectively. In the monsoon season, the T-PAH were 39.64, 17.87, 10.98, 3.28 ng m⁻³ at the industrial, roadside, residential and agricultural sites respectively, whereas during the summer it was 30.24, 16.30, 8.14, 2.23 ng m⁻³ at the industrial, roadside, residential and agricultural sites respectively. Table 2a also illustrates that throughout all the seasons, industrial sites had the highest T-PAH concentration followed by roadside, residential and agricultural sites. High concentrations at the industrial site can be attributed to the presence of industries that strongly emit PAH on their gaseous effluents. Concentrations at the roadside may result from the proximity of the busy road, which has very intense automobile traffic, about 10⁵ vehicles per day. At the residential site, heavy diesel generators are used to generate electricity because of erratic electricity supply; moreover combustion activities also take place. Trapido²³ estimated PAH content at an agricultural site to be about 0.1 ng m⁻³ which is considered as a background value for PAHs. Observed values of PAHs at the agricultural site are higher than the background levels in all the seasons which can be due to atmospheric transport of PAHs from sources to remote sites. These results also indicate that PAH concentrations are strongly linked to the land use of the site. Trends of major PAHs found in the present study were benzo(g,h,i)perylene > benzo(b)fluoranthene > benzo(a)anthracene > chrysene > fluoranthene at the industrial site during all seasons might be due to industrial-oil burning, benzo(g,h,i)perylene > benzo(b)fluoranthene > chrysene > benzo(a)anthracene > fluoranthene at the roadside and agricultural in all seasons. This may be due to emissions coming from vehicles and wood/oil/coal combustion.²⁴ At residential sites different trends were found in every season i.e., benzo(g,h,i)perylene > benzo(b)fluoranthene > chrysene > benzo(a)anthracene > fluoranthene during winters, benzo(b)fluoranthene > benzo(g,h,i)perylene > benzo(a)anthracene > chrysene > phenanthrene in monsoon and benzo(g,h,i)perylene > benzo(a)anthracene > benzo(b)fluoranthene > chrysene > fluoranthene during summers might be generated from oil fumes during frying activities on gas while cooking food.^25–29 Frying and use of oil in large quantities is very common in Indian style of cooking and other combustion activities like emission from generator sets.⁵

Table 2 (a) and (b). Individual and mean concentrations of air (ng m⁻³) and soil (μg g⁻¹) PAHs during different seasons

PAHs	WINTER					MONSOON					SUMMER
	IND	RDS	RES	AGR	MEAN	IND	RDS	RES	AGR	MEAN	IND	RDS	RES	AGR	MEAN
Naphthalene	0.31	0.14	0.27	0.11	0.25	0.20	0.09	0.14	0.08	0.15	0.11	0.07	0.1	0.05	0.05
Acenaphthylene	0.94	0.16	0.24	0.09	0.35	0.69	0.11	0.18	0.06	0.26	0.50	0.10	0.16	0.04	0.21
Phenanthrene	1.78	0.35	0.57	0.18	0.72	1.48	0.3	0.28	0.12	0.55	1.30	0.22	0.17	0.09	0.45
Anthracene	0.36	0.13	0.18	0.1	0.15	0.22	0.08	0.10	0.06	0.15	0.12	0.06	0.08	0.05	0.05
Fluoranthene	2.72	1.27	1.60	0.73	1.58	2.26	0.87	0.13	0.38	0.95	1.32	0.74	0.7	0.24	0.75
Benzo(a)anthracene	3.42	1.32	2.14	0.79	1.95	2.91	1.22	1.97	0.43	1.65	2.07	1.07	1.77	0.31	1.35
Chrysene	3.27	3.11	2.98	0.86	2.55	2.76	2.42	1.91	0.51	1.90	1.77	1.09	1.2	0.42	1.12
Benzo(b)fluoranthene	11.42	9.66	4.17	0.93	6.55	9.62	4.92	3.04	0.57	4.55	6.56	3.01	1.63	0.46	2.95
Benzo(g,h,i)perylene	23.71	13.72	4.65	1.51	10.85	19.50	7.86	3.23	1.07	7.85	16.49	9.94	2.33	0.57	7.35
TOTAL	47.93	29.86	16.8	5.3	24.95	39.64	17.87	10.98	3.28	17.95	30.24	16.30	8.14	2.23	14.25

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PAHs	WINTER					MONSOON					SUMMER
	IND	RDS	RES	AGR	MEAN	IND	RDS	RES	AGR	MEAN	IND	RDS	RES	AGR	MEAN
Naphthalene	1.72	1.37	1.26	0.98	1.35	1.14	0.84	0.79	0.63	0.85	0.98	0.75	0.43	0.37	0.65
Acenaphthylene	0.86	0.65	0.46	0.53	0.65	0.59	0.37	0.33	0.33	0.40	0.32	0.27	0.19	0.22	0.25
Phenanthrene	0.31	0.38	0.38	0.19	0.40	0.21	0.27	0.31	0.10	0.25	0.14	0.19	0.18	0.07	0.15
Anthracene	1.82	1.22	0.72	0.48	0.95	1.11	0.91	0.51	0.21	0.65	1.00	0.90	0.41	0.15	0.65
Fluoranthene	1.98	2.07	1.35	0.81	1.55	1.49	1.03	0.42	0.52	0.85	1.36	0.89	0.44	0.32	0.75
Benzo(a)anthracene	0.72	0.79	0.48	0.34	0.55	0.55	0.47	0.37	0.19	0.35	0.41	0.33	0.27	0.13	0.25
Chrysene	5.13	4.11	4.19	2.81	4.06	4.38	3.22	3.12	2.07	3.15	3.18	2.3	2.86	1.39	2.45
Benzo(b)fluoranthene	1.88	1.71	1.53	1.37	1.65	1.45	0.93	0.61	0.78	0.95	1.26	0.93	0.67	0.58	0.86
Benzo(g,h,i)perylene	1.29	1.18	0.39	0.68	0.85	0.97	0.82	0.52	0.43	0.85	0.62	0.52	0.37	0.36	0.45
TOTAL	15.71	13.48	10.76	8.19	12.50	11.89	8.86	6.98	5.26	8.25	9.27	7.08	5.82	3.59	6.44

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3.2. PAHs in soil

Soil PAH concentrations in different seasons are presented in Table 2b. During winters, total PAH (T-PAH) concentrations were 15.71, 13.48, 10.76 and 8.91 μg g⁻¹ at industrial, roadside, residential and agricultural sites respectively. In monsoon, T-PAH concentrations were 11.89, 8.86, 6.98 and 5.26 μg g⁻¹ at industrial, roadside, residential and agricultural sites respectively, whereas during summers it was 9.27, 7.08, 5.82, 3.59 μg g⁻¹ at industrial, roadside, residential and agricultural sites respectively. In soils also, industrial sites had the highest T-PAH concentration followed by roadside, residential and agricultural site in all the seasons.²⁰ Trends of major soil PAH found in present study were chrysene > fluoranthene> benzo(b)fluoranthene > anthracene at industrial site during winters and summer, whereas in monsoon it was chrysene> fluoranthene > benzo(b)fluoranthene > naphthalene. At the roadside the PAH pattern was chrysene > fluoranthene > benzo(b)fluoranthene > anthracene in winter and monsoon seasons, whereas during summer it was chrysene > benzo(b)fluoranthene > fluoranthene > naphthalene. In the case of the residential site, the PAH trend was chrysene > benzo(b)fluoranthene > fluoranthene > naphthalene during winter and summer, whereas in the monsoon season it was chrysene > naphthalene > benzo(b)fluoranthene > benzo(g,h,i)perylene. At the agricultural site these trends were chrysene > benzo(b)fluoranthene > naphthalene > fluoranthene in winter and monsoon seasons, whereas during summer it was chrysene > benzo(b)fluoranthene > naphthalene > benzo(g,h,i)perylene. At almost all the sites, chrysene, benzo(b)fluoranthene, fluoranthene and naphthalene were the predominant compounds.

3.3. Air–soil exchange of PAHs

In order to gain knowledge about air–soil exchange processes, soil–air fugacity quotients were calculated using air and soil concentrations obtained in this study. The aim was to investigate the rate of approach to equilibrium portioning of PAHs between air and soil compartments and to determine the direction of net flux of the studied PAHs between air and soil. One approach to determine whether or not compartments are in equilibrium with each other is to look at a snap-shot by comparing the fugacities of the compounds in the compartments. Fugacities can be calculated from the concentrations and fugacity capacities of compounds in the compartments. If the fugacities are more or less equal then the compartments are close to equilibrium, but if there is a large disparity then there will be a tendency for a chemical to move from one compartment to the other in order to establish equilibrium conditions. By dividing the fugacity of one compartment with another a fugacity quotient is calculated. Fugacity quotients are a convenient way of expressing the relative fugacities of two environmental compartments and have been used previously in the literature.^30–32 All the necessary physical-chemical data were taken from ATSDR,³³ and Finizio.³⁴ The fugacity of a compound in a particular phase can be calculated from the concentration, C (g m⁻³), using:

f = C / z M,

where M is the molecular mass (g mol⁻¹) and z is the fugacity capacity of the phase for the compound (mol m⁻³ Pa ⁻¹). Fugacity capacity for air is defined by:

Z_A = 1 / RT

where R is the gas constant (8.314 J mol⁻¹ K⁻¹) and T is the absolute temperature. Fugacity capacity of soils can be estimated using:

Z_S = f_ocρ_sK_ocZ_w

where f_oc is the fraction of organic carbon (2.26% for the Agra region soil), ρ_s is the soil density (1.36 g cm⁻³ for all calculations) and K_oc is the soil organic carbon–water partition coefficient, defined according to Karickhoff³⁵ using:

K_oc = 0.41 K_ow

Z_w, the fugacity capacity of water, is calculated from:

Z_w = 1 / H

where H is the Henry's Law constant (Pa m³ mol⁻¹).

The two terms in the equation for calculating fugacity that are likely to be temperature dependent are soil organic carbon–water partition coefficient (K_oc) and calculated Henry's Law constant H (Pa m³ mol⁻¹) for PAHs, are illustrated in Table 3. All K_oc data used in calculations were calculated from K_ow data at 25 °C, since the enthalpy data required for correcting K_ow for temperature are not available in the literature. However, temperature corrected values of H were calculated. Temperature correction of H can be achieved by using the integrated van't Hoff equation:³⁶

ln(H₁/ H₂) = −ΔH_AW(l / T₁ − T₂) / R

Where, H₁ and H₂ are Henry's Law constants at two temperatures, T₁ and T₂ are temperatures in K, R is the gas constant (8.314 J mol⁻¹ K⁻¹) and ΔH_AW is the enthalpy of air–water exchange (J mol⁻¹). Enthalpies of air–water exchange, ΔH_AW, are ideally derived from measurements of Henry's Law constants made at different temperatures, but can also be derived when both temperature dependent solubility and vapour pressure data exist. In this study, ΔH_AW for different PAHs were derived using the approach suggested by De Maagd.³⁷ For fluorene and phenanthrene, experimental values of ΔH_AW are available and are 57 kJ mol⁻¹³⁶ and 29 kJ mol⁻¹³⁸ respectively. For other PAHs an average value measured for six PAHs of 41 kJ mol⁻¹ was used.³⁶

Table 3 Soil organic carbon–water partition coefficient (K_oc) at 25 °C and calculated Henry's Law constant (Pa m³ mol⁻¹) for PAHs

PAHs	K oc ^a	H ^b
a ATSDR ^33 b Finizio^34
Naphthalene	1.38	43
Acenaphthylene	1.40	8.4
Phenanthrene	4.15	3.24
Anthracene	4.15	3.96
Fluoranthene	4.58	1.04
Benzo(a)anthracene	5.30	0.012
Chrysene	5.30	0.581
Benzo(b)fluoranthene	5.74	0.014
Benzo(g,h,i)perylene	6.20	0.075

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Soil–air fugacity quotients were calculated by dividing the calculated soil fugacities by air fugacities. Fugacity quotient values near to one show equilibrium between air and soil, as when two phases are in equilibrium their fugacities are equal. A fugacity quotient greater than one indicates a tendency for a compound to volatilize from soil, whereas a value of less than one indicates a tendency to remain in soil and a capacity for air, to supply soil with more compound. The soil and air data in Table 2a and 2b for winter, monsoon and summer seasons respectively were used to calculate soil–air fugacity quotients in this semi-arid region of India. The aim of this study was to gain an understanding of relative fugacities of different PAHs in soils and air in the environment of the Agra region. The mean soil organic carbon (OC) contents used in the fugacity calculations was 2.26% for the soil of the Agra region while the mean soil density (ρ_s) was 1.36 g cm⁻³ for all calculations. The calculated seasonal soil–air fugacity quotients for the selected PAHs are shown in Fig. 2.

Fig. 2 Seasonally calculated soil–air fugacity quotients at different sites in Agra.

All the low molecular weight (LMW) PAHs and middle molecular weight (MMW) PAHs have soil–air fugacity quotients much greater than one, indicating a high tendency to move from soil to air in the environment of the Agra region. The high molecular weight (HMW) PAHs like fluoranthene and chrysene, also show a high tendency to move from soil to air as soil–air fugacity quotients are greater than one throughout all the seasons, but benzo[a]anthracene, benzo[b]fluoranthene and benzo[ghi]perylene do not show a positive tendency to move from soil to air as the soil–air fugacity quotients are lower than one during the winter. There is also the possibility, that lighter compounds are being formed in the soil (e.g. by partial biodegradation of heavier compounds). Naphthalene, a LMW-PAH has a very high fugacity quotient followed by the MMW-PAHs (acenaphthylene, phenanthrene and anthracene) and HMW-PAHs (fluoranthene, chrysene, benzo[a]anthracene, benzo[b]fluoranthene and benzo[ghi]perylene). HMW-PAHs have soil–air fugacity quotients lower in comparison to LMW and MMW-PAHs, indicating a slow but positive tendency to gradually move from soil to air except in winter.

3.4. Statistical analysis (Univariate Pearson's correlation matrix)

Correlation matrix is a common way of hypothesizing potential precursors of species in the environmental samples. Correlation between any species suggests the likely sources of pollutants. Correlation analysis was performed by using Univariate Pearson correlation coefficient for all pairs of PAH compounds along with meteorological parameters to determine relationships between themselves and to hypothesize probable sources on the assumption that two or more components may correlate either due to a common origin or atmospheric behaviour. Table 4a, 4b and 4c illustrates the correlation pyramid between the fugacity quotients of different PAHs with meteorological parameters like temperature, relative humidity and wind direction in winter, monsoon and summer seasons respectively. According to these results, significant correlations among most of the PAHs and with meteorological parameters were observed. During the winter season, BAA is well correlated with PHE (r = 0.95), CHR with ACE (r = 0.87), BBF with ACE, CHR (r = 0.97, 0.96), BGP with PHE, BAA (r = 0.96, 0.84), NAP is also dependent on temperature and relative humidity along with BAA and CHR. Anti-correlations among some of the PAHs and metrological parameters were also seen i.e.NAP was anti correlated with FLT, BAA and CHR (r = −0.96, −0.85 and −0.91), RH with PHE and BGP (r = −0.84 and −0.87), TEMP with FLT (r = −0.88). In the monsoon season, FLT is correlated with ACE, (r = 0.86), BAA with ACE (r = 0.82), CHR with ACE, FLT (r = 0.87, 0.99), BBF with ACE, FLT, BAA, CHR (r = 0.98, 0.76, 0.91, 0.78), BGP with ACE, BAA, BBF (r = 0.93, 0.94, 0.98), again, NAP is also dependent on temperature and relative humidity whereas PHE was dependent on wind. The anti-correlated species were, PHE with NAP (r = −0.97), PHE with TEMP and RH (r = −0.79 and −0.97), WS with NAP and RH (r = −0.95 and −0.99). During summers, ACE is correlated with NAP (r = 0.75), PHE with NAP (r = 0.91), BAA with NAP, ACE (r = 0.79, 0.99), CHR with ACE, BAA (r = 0.90, 0.84), BBF with ACE, BAA, CHR (r = 0.95, 0.90, 0.99), BGP with ANT, FLT, (r = 0.91, 0.82), ANT and BGP were dependent on wind. Anti-correlating variables were CHR with ANT (r = −0.86), BBF with ANT (r = −0.80), TEMP with FLT (r = −0.91), RH with PHE and BGP (r = −0.79 and −0.78), WS with BAA, CHR and BBF (r = −0.77 and −0.95 and −0.93). These results show that temperature, relative humidity and wind direction play a trivial role in the volatization process of PAHs in this semi-arid region of India.

Table 4 (a), (b) and (c). Correlation pyramid between fugacity quotients of different PAHs and meteorological parameters of winter, monsoon and summer seasons^a

	NAP	ACE	PHE	ANT	FLT	BAA	CHR	BBF	BGP	TEMP	RH	WS
a Correlation values >0.7 are in bold.
NAP	1.0
ACE	−0.64	1.0
PHE	−0.65	0.31	1.0
ANT	−0.41	−0.27	−0.05	1.0
FLT	−0.96	0.54	0.45	0.61	1.0
BAA	−0.85	0.51	0.95	0.07	0.69	1.0
CHR	−0.91	0.87	−0.03	−0.59	0.09	0.11	1.0
BBF	−0.44	0.97	0.14	−0.44	0.33	0.33	0.96	1.0
BGP	−0.46	0.04	0.96	−0.05	0.27	0.84	−0.25	0.10	1.0
TEMP	0.73	−0.46	0.02	−0.71	−0.88	−0.27	−0.14	−0.32	0.20	1.0
RH	0.63	0.07	−0.84	−0.43	−0.55	0.80	0.50	0.28	−0.87	0.16	1.0
WS	0.25	−0.84	0.24	0.29	0.26	−0.02	−0.92	−0.92	0.48	0.43	−0.56	1.0

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	NAP	ACE	PHE	ANT	FLT	BAA	CHR	BBF	BGP	TEMP	RH	WS
NAP	1.0
ACE	−0.05	1.0
PHE	−0.97	0.28	1.0
ANT	0.11	−0.04	−0.20	1.0
FLT	0.15	0.86	0.02	0.46	1.0
BAA	0.03	0.82	0.21	−0.59	0.45	1.0
CHR	0.22	0.87	−0.04	0.39	0.99	0.51	1.0
BBF	−0.06	0.98	0.31	−0.22	0.76	0.91	0.78	1.0
BGP	−0.22	0.93	0.46	−0.39	0.61	0.94	0.63	0.98	1.0
TEMP	0.92	0.23	−0.79	−0.20	0.23	0.42	0.33	0.26	0.14	1.0
RH	0.95	−0.18	−0.97	0.39	0.15	−0.23	0.21	−0.24	−0.42	0.75	1.0
WS	−0.95	0.04	0.95	−0.40	−0.28	0.13	−0.34	0.11	0.31	−0.79	−0.99	1.0

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	NAP	ACE	PHE	ANT	FLT	BAA	CHR	BBF	BGP	TEMP	RH	WS
NAP	1.0
ACE	0.75	1.0
PHE	0.91	0.43	1.0
ANT	−0.02	−0.65	0.38	1.0
FLT	0.13	−0.17	0.31	0.59	1.0
BAA	0.79	0.99	0.49	−0.62	−0.27	1.0
CHR	0.42	0.90	0.02	−0.86	−0.19	0.84	1.0
BBF	0.54	0.95	0.16	−0.80	−0.16	0.90	0.99	1.0
BGP	0.29	−0.32	0.61	0.91	0.82	−0.32	−0.55	−0.47	1.0
TEMP	0.02	0.04	−0.02	−0.24	−0.91	0.18	−0.11	−0.08	−0.51	1.0
RH	−0.71	−0.32	−0.79	−0.44	−0.78	−0.28	−0.08	−0.18	−0.78	0.62	1.0
WS	−0.24	−0.81	0.16	0.97	0.44	−0.77	−0.95	−0.93	0.76	−0.12	−0.20	1.0

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Although soil–air fugacity quotients provide useful information, it is not always expected that the fugacities in soil and air will obtain unity (i.e. equilibrium conditions may not be reached). Local contamination sources, degradation processes and non-diffusive processes such as deposition in association with particles, tend to cause non-equilibrium conditions. During periods of reduced emissions, concentrations in various media will fall, but at different rates and this may cause a further departure from equilibrium. It is, however expected that equilibrium will be approached most closely for persistent and widely dispersed compounds like PAHs, which survive long enough in the environment to permit diffusion to occur in competition with other processes such as degradation. Fugacity quotients are thus likely to be closer to unity and values may be relatively constant spatially. Fine distribution of compounds within surface soils may influence the magnitude of calculated soil–air fugacity quotients. The soil concentration of a compound near the soil–air interface may be very different from concentration in the underlying soil.

Conclusion

Samples of air and soil were collected to study the levels of PAHs in aerosols and in soil of Agra region. Concentrations of PAHs were measured at four locations in the city of Agra, which covers industrial, residential, roadside and agricultural areas. In air, T-PAH concentrations during the winter were 47.93, 29.86, 16.80 and 5.30 ng m⁻³ at industrial, roadside, residential and agricultural sites respectively. In the monsoon season they were, 39.64, 17.87, 10.98, 3.28 ng m⁻³, whereas during the summer they were 30.24, 16.30, 8.14, 2.23 ng m⁻³ at industrial, roadside, residential and agricultural sites respectively. In soils, during the winter the T-PAH concentrations were 15.71, 13.48, 10.76 and 8.91 μg g⁻¹ at the industrial, roadside, residential and agricultural sites respectively. In the monsoon season they were, 11.89, 8.86, 6.98 and 5.26 μg g⁻¹, whereas during the summer they were 9.27, 7.08, 5.82, 3.59 μg g⁻¹ at the industrial, roadside, residential and agricultural sites respectively. In both the matrices i.e. air and soil, the industrial site had the highest total PAH concentration followed by the roadside, residential and agricultural sites. In order to gain knowledge about air–soil exchange processes, soil–air fugacity quotients were calculated using air and soil concentrations obtained in this study. Naphthalene, a LMW-PAH has a very high fugacity quotient followed by the MMW-PAHs (acenaphthylene, phenanthrene and anthracene) and HMW-PAHs (fluoranthene and chrysene). Benzo[a]anthracene, benzo[b]fluoranthene and benzo[ghi]perylene) do not show a positive tendency to move from soil to air as the soil–air fugacity quotients are found to be lower than one. A correlation matrix was performed between the fugacity quotients of different PAHs with meteorological parameters like temperature, relative humidity and wind direction during the winter, monsoon and summer seasons, which shows significant correlations among most of the PAHs and with meteorological parameters as these play a trivial role in the volatization process of PAHs. Fugacity quotients are a convenient way of expressing the relative fugacities of two environmental compartments. Knowledge of equilibrium position provides information about the tendency of a chemical to move from one compartment to another, but if a transport process is very slow it is possible that a non-equilibrium position could be maintained for a long period of time.

Acknowledgements

The authors are grateful to Revd. Dr J. K. Lal, Principal, St. Andrew's College, Gorakhpur for providing the necessary facilities. They also thank Dr B. N. Tripathi, (Head) Department of Chemistry, St. Andrew's College, Gorakhpur for his encouragement.

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