Fresenius J Anal Chem (1991) 339:777-779 Fresenius' Journal of

@ Springer-Verlag 1991

Effects of temperature on the concentration of polycyclic aromatic hydrocarbons (PAHs) adsorbed onto airborne particulates

F. Valerio and M. Pala

Istituto Nazionale per la Ricerca sul Cancro, 1-16132 Genova, Italy

Summary. Daily samples of airborne particulates (143) were collected along the year in La Spezia (Italy). Seasonal variations of atmospheric PAH concentrations, with highest winter values, have been observed. The concentration of PAHs was found to correlate negatively with the mean ambient temperature during the sampling period. Volatiliza- tion, photodegradation and seasonal modifications of emissions from urban traffic were found to be a possible explanation of this phenomenon.

Introduction

Since the 50s, atmospheric PAH concentrations have been regularly monitored in many countries. Usually, the lower and greater PAH concentrations have been found in summer and winter, respectively [I], which is usually explained due to the immissions from domestic heating plants. Only a prevalent use of coal in domestic heating can produce a significant emission of PAHs to the urban atmosphere, the specific emissions of benzo(a)pyrene (BaP) from coal are reported to be from 50,000 to about 4 million times the BaP specific emission of light oil [1]. However, a seasonal variation of atmospheric PAH concentration was also found in Liguria, a region placed in the north of Italy, with a mild maritime climate and an extended use of natural gas for domestic heating, where combustion produces only negligible amounts of PAHs. The present study tries to identify the causes of this trend.

Methods

From January to December 1988, 143 daily samples of airborne particulate matter (APM) were collected in the centre of La Spezia, a town with 112 000 inhabitants. Heavy metals (particularly vanadium and lead) and 9 PAHs, i.e., pyrene (PY), cyclopenta(c, d)pyrene (CYC), benzo(a)anthra- cene (BaA), chrysene (CRY), benzo(b)fiuoranthene (BbF), benzo(a)pyrene (BaP), benzo(e)pyrene (BeP), indenopyrene (IP), benzo(ghi)perylene (BghiP), were analysed.

The mean temperature during each sampling period was also registered.

Offprint requests to: F. VaIerio

Sampling

The sampling system was placed in the centre of La Spezia, near a street junction regulated by traffic lights and at 130 cm above the street level. The system consisted of a high volume air sampler (IP-10 by General Metal Work) operating automatically at a constant flow rate of 1.13 +_ 0.03 m 3 rain-1.

Glass fiber filters (Gelman A/E) with a collection surface of 500 cm 2 were used to sample AMP, with a sampling time of 24 h. Four samples were collected each week, including also Saturdays and Sundays, alternating.

Analytical procedure

After collection the filters were dried in a dessiccator for 24 h, weighted and analysed for PAH contents.

The organic fraction of material retained by each filter was extracted by sonication with 150 ml of cyclohexane. All extracts were concentrated by rotary evaporation to 0.5 ml and further separated on a precoated TLC preparative plate (Silica gel 60 Merck). The developing mixture was hex- ane: benzene (1:1 in vol). Using a UV lamp the fluorescent band containing PAH was marked, scraped and eluted with 12 ml of toluene and the eluate dried under nitrogen.

The solute was resuspended in 600 ~tl of benzene and the PAHs analysed by capillary column GC on a 25 x 0.2 mm I. D. SE-54 column (Supelchem) using a Perkin Elmer, model Sigma 2, gas chromatograph. The injection of the extracts was made by the splittless mode. The injector and detector temperature were set at 300 ° C.

Column temperature: 50°C for 1 min, 10°C/rain to 145°C, 5° C/rain to 290°C, and 290°C for 15 min.

PAHs were identified according to their retention times. Two disks of 10.4 cm 2 of surface were cut from each filter

for the analysis of metals. These samples were treated in a teflon beaker with 65% HNO3 for 5 h at 165°C. The residue was dissolved in 10% HNO3 and analysed by Inductively Coupled Plasma (ICP).

Results

The atmospheric concentration of all analysed PAHs showed a seasonal variation with the lowest values in summer and the highest in winter (Fig. 1).

The same trend was also found when the amount of each PAH adsorbed on APM was calculated (Table 1).

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I J a n u a r y F e b r u a r y M a r c h

[ 1 I I I l Apri l May J u n e J u l y Aug. O c t . November December

F i g . 1. Seasonal variations of atmospheric concentrations of PAHs in La Spezia (Italy). Palls include the 9 components reported in the experimental section

T a b l e 1. Correlation between PAH and Pb adsorbed on APM versus temperature

Linear Confidence correlation level coefficient

lnPy = 47.23 - 0.15 T -0.8929 <0.001 lnCYC = 39.1l - 0.12T -0.6871 <0.001 In BaA = 43.54 - 0.14 T -0.8401 <0.001 In Cry = 30.23 - 0.10 T -0.4395 <0.001 In BbF = 36.37 - 0.12T -0.6859 <0.001 In BaP = 35.95 - 0.12 T -0.6873 <0.001 In BeP = 27.51 - 0.09 T -0.6833 <0.001 l n I P = 14.66 - 0.04T -0.3033 <0.01 In BghiP = 25.50 - 0.07 T -0.5649 <0.001 lnPy = 33.2 - 0.11T -0 .59 <0.001 c lnPy = 3 1 . 6 3 - 0.10T -0.8737 <0.001 ~

Pb = 168.411-529 T -0.4918 <0.001

° 33 samples collected in Genova, near a street-junction, from October to January. d elaboration of data from [8] - Concentration of PAHs and Pb :mg/kg - T = K

P A H sources

A stat is t ical ly s ignif icant l inear cor re la t ion was found be- tween all ana lysed P A H s and lead concen t ra t ions (Table 2). This result indicates u rban traffic as the ma in P A H source. A c o n f i r m a t i o n c a m e also f r o m the re la t ive a b u n d a n c e o f specific P A H s . In fact, the ra t io I N D / B g h i P in samples col lected dur ing the hea t ing per iod (mean air t empera tu re < 1 5 ° C ) was found to be 0.39, typical o f a u t o m o b i l e emissions [2].

F u r t h e r m o r e , no statist ical differences were found when the m e a n v a n a d i u m concen t r a t i on in A P M col lected in cold days (79.3 + 45.2 mg/kg) was c o m p a r e d wi th tha t f ound in w a r m e r per iod (70.4 +_ 44.5 mg/kg) . V a n a d i u m is a specific t racer o f emiss ions o f oil bu rn ing furnaces [3], therefore this resul t suppor t s the negligible effect o f hea t ing plants on P A H po l lu t ion in La Spezia.

T a b l e 2. Correlation between PAHs versus lead concentrations

Linear Confidence correlation level coefficient

Py = -1631.6 + 3.7 [Pb] 0.5540 < 0.001 CYC = - 206.8 + 2.7 [Pb] 0.4351 <0.001 BaA = - 667.1 + 1.4 [Pb] 0.5822 <0.001 Cry = - 1471.6 + 2.1 [Pb] 0.5965 <0.001 BbF = -2322.8 + 3.7 [Pb] 0.6443 <0.001 BaP = - 1031.9 + 1.7 [Pb] 0.6026 < 0.001 BeP = 512.1 + 1.2 [Pb] 0.4979 <0.001 IP = - 143.7 + 1.0 [Pb] 0.4965 <0.001 BghiP = - 229.0 + 2.2 [Pb] 0.5570 <0.001

- Concentration of PAHs: ng/1000 m 3 - Concentration o fPb: gg/1000 m 3

Correlation with temperature

The da ta set ob ta ined in La Spezia pe rmi t t ed to carry ou t an adequa t e statist ical analysis and invest igate the effect o f t empera tu re on the a m o u n t o f P A H s adso rbed on A P M tha t resul ted to be very remarkable . As it is shown in Table 1, stat ist ically s ignif icant cor re la t ions were found be tween the loga r i thm of P A H concen t ra t ions and m e a n air t empera- tures measu red dur ing the sampl ing per iod. This corre la t ion , as well as the slopes o f the respect ive cor re la t ion curves, were par t icu lar ly h igh for Py and BaA, the slopes decrease wi th P A H r ing-number .

Therefore , a decrease o f ambien t t empera tu re f r o m 25° C to 5°C is expected to increase the concen t r a t i on o f Py and BaP adsorbed on A P M by a fac tor o f 20 and 11, respectively. As discussed before, the role o f hea t ing plants in this phe- n o m e n o n is qui te negligible so that o ther causes such as traffic intensity, p h o t o d e g r a d a t i o n or vola t i l i sa t ion mus t be taken in to cons idera t ion .

Traffic

A seasonal va r i a t ion was observed in lead concen t r a t ion and a negat ive l inear cor re la t ion be tween this meta l and ambien t t empera tu re was also found (Table 1). These results were in

agreement with those found by other authers [4] who explain this correlation as an effect of increase of traffic density in colder days and of higher motor speeds in idle conditions when temperature is lower.

According to our results it is possible to estimate that concentrations of lead emitted by cars increase by about two times when temperature decreases from 25 ° to 5 ° C. Due to the high correlation between lead and PAH concentrations (Table 2) a similar increase is expected for PAHs.

Volatilisation

It is well known that PAH undergo a relatively easy volatilisation also at ambient temperature [5, 6]. This effect is relevant for PAHs up to four rings, but higher molecular weight species are preferentially found adsorbed on APM in equilibrium with a negligible concentration of their vapours.

For this reason new sampling methods were developed to collect and analyse PAHs in both particulate and vapour phases, using adsorbing materials, for example polyurethane foam [7] for the latter.

Using this sampling method, Yamasaki [8] demonstrated that at ambient temperature, substantial amounts of three- to five-ring PAHs are in vapour phase, depending upon temperature, while the six ring PAHs were all found in the particulate phase. This author assumed that equilibrium of each PAH in vapor phase and the same PAH adsorbed on APM may be described by the following equation:

log (eAHxvap) = - A / T + A (1) (PAHxpar0/APM

where: A and B are constants for the individual PAHs. APM = Concentration of airborne particulate matter (ng/ m3); (PAHx,~p) = concentration of PAH as vapor (ng/m3); (PAHxpart) = concentration of PAH as particulate (rig/m3); T = temperature (K).

If Eq. (1) is applied to PY, it is possible to calculate that a temperature increase from 5 ° to 25 ° C [the range used in our experiment and applied also in Eq. (2)] induces an increase of PY concentration in the vapor phase of 10 times.

Experimental results of Yamasaki [8] demonstrated that the increase of PY in the vapor phase corresponds to an equivalent decrease of the amount of PY adsorbed on APM. Accordingly, to the meteorological conditions found in La Spezia during the cold season, the combination of traffic and temperature effects, is expected to produce an increase of PY concentration, adsorbed on APM, of 20 times, a value that agrees very well with our experimental results. In summary, according to this model, seasonal variations of PAH concentrations, both as vapor and condensed on APM, may depend on ambient temperature, volatility of each PAH and adsorption properties of APM.

This may be a general rule for the more volatile PAHs, which is practically the same found by Yamasaki in Osaka [8], a coastal town with similar marine climate to La Spezia, and in our previous research carried out in Genova, the capital of the Liguria region (Table 1). Otherwise, in this model the fact that PAHs possibly undergo also chemical transformation during their presence in the atmosphere is ignored.

Pho todegradation

It is well known that PAHs adsorbed on solid substrates and exposed to sun light undergo photodegradation, depending

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mainly on mean sun light intensity and the characteristics of the adsorbing substrate [9] but also on temperature and humidity [10]. According to Kamens' results [10] the constant decay (k) of PAHs adsorbed on residential wood smoke and exposed in a 25 m 3 outdoor Teflon film chamber, depends on temperature (T), mean sun light intensity (I) and water vapor concentration c(H20) according to the following equation:

lnk = constant + a ( ~ ) + blnI + lne(H20) (2)

where a, b, are constants specific for each PAH. Applying this equation to the mean values of T, I,

c(H20), found in Liguria in winter and summer, it is possible to calculate that the degradation rate of BaP in summer is about 6 times greater than in winter.

Therefore taking into account that urban traffic may account for a twofold increase of BaP in winter aerosol samples, the cumulative effects of traffic and meteorological conditions [Eq. (2)] may explain a winter increase of BaP concentration of 12 times, in good agreement with the ex- perimental results (11 times).

Conclusion

These results suggest that the concentration of more volatile PAHs on APM depends strongly on ambient temperature and on the characteristics of specific adsorption isotherms that control the equilibrium between the vapor and solid phases, while concentration of less volatile PAHs is much more affected by photodegradation reactions, whose rate depends directly or indirectly on ambient temperature.

In conclusion, in studies on atmospheric PAH exposure, it is very important to identify the main sources of these compounds and acquire informations about their seasonal immision variations. It is also necessary to measure mean air temperature, sun light intensity and humidity, during the sampling period, in order to determine the contribution of these meteorological conditions to PAH seasonal variations.

Acknowledgements. This study was supported by Municipality, Pro- vince and Sanitary district of La Spezia.

References

1. Grimmer G (1983) Handbook of polycyclic aromatic hydro- carbons. Dekker, New York Basel

2. Jaklin J, Krenmayr P (1986) Fate of hydrocarbons in the en- vironment. An analytical approach. Gordon and Breach, New York, p. 73- 82

3. EPA (1977) Scientific and technical report on vanadium. EPA/ 600/6-77/002

4. Hallez S, Thiessen L, Derouane A, Verduyn G (1989) Sci Tot Environ 86:265- 271

5. Pupp C, Lao CR (1974) Atmos Environ 8:915-925 6. Rondia D (]965) Int J Air Wat Poll 9:113-121 7. Thrane KE, Mikalsen A (1981) Atmos Environ 15:909- 918 8. Yamasaki H, Kuwata K, Miyamoto H (1982) Environ Sci Tech-

nol 16:189-194 9. Valerio F, Lazzarotto L (1985) Int J Environ Anal Chem

23:135-151 10. Kamens RM, Guo Z, Fulcher JN, Bell DA (1988) Environ Sci

Techno122:103-108

Received October 10, 1990