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Measurements of lower carbonyls in Rome ambient air
Pergamon
Atmospheric Environment Vol. 30, No. 22, pp. 3757-3764, 1996 Copyright © 1996 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved P I I : $1352-2310(96)00110-0 1352-2310/96 $15.00 + 0.120
MEASUREMENTS OF LOWER CARBONYLS IN ROME AMBIENT AIR M. P O S S A N Z I N I , V. D I P A L O , M. P E T R I C C A , R. F R A T A R C A N G E L I a n d D. B R O C C O Istituto sull'Inquinamento Atmosferico del C.N.R., Area della Ricerca di Roma Via Salaria Km 29,300C.P. 10, 00016 Monterotondo Stazione (Roma), Italy (First received 7 August 1995 and in final form 13 March 1996)
Abstract--Ambient levels and diurnal profiles of lower carbonyls were measured in Rome during selected days of summer 1994 and winter 1995. The most abundant carbonyls were formaldehyde (up to 27 ppb) followed by ethanal ( < 17 ppb) and acetone ( < 9 ppb). Gas-phase concentrations of other seven carbonyls were in the 0-3 ppb range. The results were discussed with respect to direct emissions and photochemical production. Using carbonyl/CO concentration ratios mobil source emissions of carbonyls were estimated for the urban area. The secondary production of C~-C 3 aldehydes from reactions of alkenes with 0 3 and OH radicals during the early morning hours of summer days was also calculated. The daytime pattern of carbonyls was found to be similar to that of toluene in wintertime and close to that of ozone in summer periods conductive to photochemical pollution episodes. Copyright © 1996 Published by Elsevier Science Ltd Key word index: Carbonyls, alkenes, primary and secondary pollutants, diurnal profiles.
INTRODUCTION Carbonyls play a central role in the chemical reactions taking place in the troposphere because they are generally the first stable intermediates in the photooxidation mechanism of organic compounds (Carlier et al., 1986). It has been calculated that the secondary production of C1-C~. aldehydes in U.S. cities usually predominates over direct emissions of the same aldehydes from auto exhausts (Altshuller, 1993). Carbonyls are in turn di.rect precursors of peroxyacylnitrates and ozone through peroxyradicals (HO2, RO2 and RC(O)O2) generated by photolysis or OH radical attack. In spite of their great importance in urban air quality and health hazard assessment (NCR, 1981), lower carbonyls have received little attention in air pollution monitoring programs to date. Due, to a large extent, to analytical problems (Vairavamurthy et al., 1992) knowledge of ambient levels and diurnal profiles of carbonyls has been limited to a few urban areas in the last decade. Recently, thorough investigations on the presence of lower carbonyls have also been carried out in rural and marine environments (Shepson et al., 1991; Zhou and Mopper, 1993). Results of short-te rm urban measurements indicate that formaldehyde is the most abundant carbonyl pollutant followed by ethanal (Salas and Singh, 1986; Grosjean, 1982, 1988; Haszpra et al., 1991; Grosjean et al., 1993; Satsumabayashi et al., 1995). The reverse
has been observed in the major urban areas of Brazil, where the large scale use of ethanol as a vehicle fuel gives rise to prevailing ethanal emissions (Grosjean et al., 1990). The present paper reports data of measurements made in Rome, where ambient concentrations and daily variations of lower carbonyls have been obtained in different seasons. Rome can represent a typical urban area of the Mediterranean region, where atmospheric circulation exhibits marked diurnal cycles with the alternation of high stability conditions during the night and strong vertical mixing during the day. Ground level concentrations and temporal evolution of air pollutants are strongly influenced by mesometeorological conditions involving local landsea wind systems. Furthermore, a relatively intense solar radiation and high temperatures give rise to frequent photochemical pollution episodes (Cecinato et al., 1979). This investigation has been carried out during the gradual replacement (started on 1992) of tetra-alkyl lead with methyl tertiary butyl ether (MTBE) as a gasoline additive in catalyst equipped cars. The percentage of catalyst vehicles during the experiment period ranged from 15 to 20% of total number of circulating vehicles. A significant reduction in carbonyl emissions, but not in the carbonyl/hydrocarbon concentration ratios, is expected from the use of catalytic converters; the blending of MTBE with gasoline has been found indeed to produce an increase in aldehyde
3757
3758
M. POSSANZINI et al.
emissions with respect to gasoline, powered fuels (AItshuller, 1993). In addition, the effects of catalyst deterioration should be taken into account (Cabella et al., 1993). Relationships of carbonyls with other pollutants, such as ozone, carbon monoxide and specific hydrocarbons have also been examined in an attempt to get an idea of the relative contribution of direct emissions and photochemical processes to the measured ambient carbonyl levels.
2. EXPERIMENTAL
Sampling of volatile carbonyls and hydrocarbons together with measurements of ozone and carbon monoxide were made in the centre of Rome, near Piazza Venezia during the 0800-2000 h (local time) period of selected summer and winter days. Five sunny days following overnight periods of stable atmospheric conditions were chosen in June-July 1994 and January-March 1995, respectively. Sunrise and sunset occurred, on the average, at 0600 and 2000h and at 0700h and 1700h in the above periods, respectively. The sampling time was 1 h and a total of 113 samples were collected. The sampling inlet was positioned at a height of 3 m above ground level. Air samples were delivered via a Teflon tube with manifold to the sampling systems or the analysers. Carbonyls were collected at an air flow rate of 1.0 E min -1 on silica cartridges coated with 2,4-dinitrophenyl-hydrazine (Sep Pak DNPH, Waters). An ozone trap, consisting of a Kl-coated annular denuder (10 cm long, 10 and 13 mm in annulus diameters) was connected to the upstream ends of cartridges (Possanzini and Di Palo, 1995). The derivatized products were extracted with 3 ml acetonitrile and analyzed by HPLC with u.v. detection, according to a procedure described elsewhere (Possanzini and Di Palo, 1995). Separation was performed on a 250 x 4 mm Hypersil ODS, 5 #m column under the following conditions: mobile phase = methanol/water/acetonitrile (60:33 : 7 v/v/v isocratic); flow rate = 1.0 ml min- 1, injection volume = 20 #1 and detector wavelength, 2 = 365 nm. Typical HPLC analysis time was less than 18 min. The performance of the DNPH method was assessed by analysis of ten cartridges each spiked with 100 #1 of a carbonyl mixture containing 10 #g ml- 1 of formaldehyde, ethanal and acetone, respectively. Concentrations of spiked samples were all within + 10% of the calculated concentrations and the relative standard deviations were 8% for HCHO, 13% for CHaCHO and 16% for CHaCOCH 3. These values also included the blank variability (20-40 ng HCHO, 30-70 ng
CH3CHO and 5(~100 ng CHaCOCH 3 per cartridge). Identification and quantitation of individual carbonyls involved the use of external standards, i.e. direct injection of a calibration mixture prepared by dissolving known amount of freshly synthesized hydrazones in acetonitrile (1 #gml-1). Sampling and analysis of the hydrocarbons were performed with a VOC Air Analyser (Chrompack), which is an automatic gas chromatographic system making use of cryogenic adsorption (0.35 f air sample) and thermal desorption steps in the hourly measuring cycle. Detection time and peak response of replicate analyses of an air sample by this method did not deviate by more than 10% relative standard deviation. 0 3 and CO were monitored with a UV photometer (Environnement mod. 1003 AH) and a NDIR analyser (Environnement mod. CO 10M), respectively. The precision of such analysers was better than 5% in the respective 0-100 ppb 0 3 and 0-10 ppm CO concentration ranges.
RESULTS AND DISCUSSION Six carbonyls and two pairs of unresolved isomers from formaldehyde to benzaldehyde were identified and quantitated in Rome air. Their concentrations are listed in Table 1. Levels of formaldehyde and ethanal were comparable with those encountered in the centre of Paris (Kalabokas et al., 1988). As expected, peak concentrations were significantly higher in summer than in winter. This means that in situ formation of carbonyls from reactive hydrocarbons (i.e. alkenes) was competitive with their emissions from autovehicles under conditions of strong solar irradiation, high temperatures ( > 30°C) and poor ventilation ( < 2 m s - 1). Ambient concentration of formaldehyde in the ranges 9-27 ppb (summer) and 8-17 pbb (winter) characterized the two periods. Ethanal correspondingly reached maximun hourly values of 17 and 7 ppb. The third most abundant carbonyl was acetone with summer levels of up to 9 ppb. Propanal never exceeded 3 ppb, whereas the other carbonyls were all below 2 ppb. Figure 1 shows diurnal variations of the major carbonyls along with the corresponding averaged concentrations of ozone and toluene. We can distinguish two cases both referred to stable weather conditions. When it was cold, as on 25 January (Tmax < 7 ° 0 , the diurnal profile of carbonyls was close to those of
Table 1. Ambient air concentrations (ppb) of carbonyls in Rome. Time period: 0800-2000 h. Integration time: 1 h June--July 1 9 9 4 (n = 56)
Formaldehyde Ethanal Propanal Acrolein Acetone Methylvinylketone + methacrolein Butanal + 2-butanone Benzaldehyde
January-March 1995 (n = 57)
Range
Mean
Range
Mean
8.8-27.7 3.1-17.4 0.6-3.0 0.4-1.0 4.2-9.2 0.4-1.0 0.7-2.4 0.4-1.0
17.0 9.3 1.8 0.7 6.8 0.7 1.4 0.7
8.2-17.0 2.9-6.6 0.5-1.6 0.3-0.8 3.4-5.8 0.24).6 0.6-1.6 0.3-0.8
11.2 4.6 0.9 0.6 4.4 0.4 1.0 0.5
Lower carbonyls in Rome ambient air
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3759
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Jan 25,1995 Fig. 1. Diurnal profiles of lower carbonyls, ozone and toluene measured at Rome in summer (a) and winter (b) days.
3760
M. POSSANZINI et al.
hydrocarbons, such as toluene, benzene, acetylene, etc., suggesting that photochemical production was unimportant compared with the primary emission source (e.g. autoexhaust). Formaldehyde showed a more distinct pattern with respect to other carbonyls, which, anyhow, behaved like HCHO in the daytime. Carbonyls as well as hydrocarbons began to rise early in the morning and showed three maxima corresponding to 0900-1000, 1300-1400 and 1900-2000 h periods, whereas the afternoon minimum was attributed both to convective mixing and decreased traffic (Fig. lb). During a warm and sunny day, as 27 July, a very different profile was observed for carbonyls, which conformed to that of ozone with one maximum at the middle of the day (Fig. la). This indicated that photooxidation of reactive hydrocarbons was the preponderant source of carbonyls under summertime conditions. In fact, the absolute levels of carbonyls were almost 50% higher in spite of halved hydrocarbon concentrations. It is worth noting that each carbonyl showed its maximum centred about 1 h before that of ozone. This could be explained with the different removal mechanisms involving the two chemical species. The balance between the photochemical production rates and the loss rates resulted in a decrease of carbonyl levels at the time of the maximum ozone build-up. The short chemical lifetime of lower aldehydes in the troposphere (Carlier et al., 1986) supports this feature. Diurnal variations of C1-C 3 aldehydes and acetone were similar in all occasion and suggested common primary sources and analogous formation and decomposition pathways.. Daytime removal mechanisms include photolysis and reactions with OH radicals. The substantial photochemical formation of carbonyls from 03- and OH-hydrocarbon reactions clearly emerged from the aldehyde/acetylene plots of Fig. 2. Acetylene is an unreactive hydrocarbon emitted from the same sources which emit primary aldehydes. The ratio of aldehyde to acetylene aimed to normalize the carbonyl concentrations by counteracting the diffusional effects. This ratio should not change appreciably in the absence of in situ formation and removal processes. This is not the case of Fig. 2b, where diurnal variations of aldehydes with a plateau persisting from 11.00 to 17.00 h were consistent with the 03 behaviour and, thus, with the photochemical activity associated with morning emissions. By contrast, little variations in the aldehyde/acetylene ratio were observed in the winter day (Fig. 2a) when 03 exhibited its typical, although less marked profile. Formaldehyde/ethanal and ethanal/propanal concentration ratios were also calculated. The summerwinter differences were not statistically significant in this case and data averaged over all measurements were used. The above ratios did not show large variations (from 0.8 to 4.1 for C1/C2 and from 3.1 to 8.7 for
C2/C3) and the average value for C1/C 2 (2.1) was close
to that (2.5) resulting from literature data listed by Grosjean (1990) and related to 17 urban areas of different countries. By contrast, these data show a considerable variability (from 0.3 to 14.3) as one would expect from differences in fuel use, use of catalytic converters, etc. from one urban area to the next. On the one side, C1/Cz values from Brazil are relatively low due to the substantial use of ethanol containing fuels in that country; on the other, the highest value of the range (14.3) appears not to be representative (Shepson, 1992). It should be pointed out that CJC2 ratios usually vary from 1-2 (urban area) to about 10 (forested area); therefore they could be used as a measure of a biogenic source of formaldehyde (Shepson et al., 1991). C2/C3 ratios should be used likewise as indicators of anthropogenic origin for ambient carbonyls, since propanal is believed to be associated only with anthropogenic emissions. Then, C1/C3 would be high in rural atmospheres and low in polluted urban air. A value of 5.2 was found for Rome. The very limited number of urban ratios available show C2/C3 varying from 1.7 to 6.0 (Grosjean, 1992; Zhang et al., 1994), without considering Brazil, where ratios in the 1 8 4 9 range were measured (Grosjean, 1992). Emission rates for vehicle-emitted carbonyls could be estimated from our data using CO as tracer for mobile source emissions. In the Rome metropolitan area ( ~ 230 km 2) vehicles account for at least 95% of the total CO emissions, which are reported to be (0.9 _+ 0.2)x 103 metric tons day-1 (Allegrini et al., 1990). Hourly CO concentrations in the winter measurement days were in the range 1.5-10.6 ppm for the 0800-2000 h interval, with a mean value of 4.6 ppm. The latter was associated to the average concentrations of 11.2 and 4.6 ppb obtained for formaldehyde and ethanal, respectively. If one assumes that the carbonyl/CO emission rate ratio approaches the carbonyl/CO ambient concentration ratio, these values correspond to mobile source emission rates of 2.2 _+ 0.5 and 0.9 _+ 0.3 metric tons d a y - 1 for formaldehyde and ethanal, respectively. Very higher emission rates (17 tons H C H O and 20 tons CH3CHO per day) were estimated for Sao Paulo (Grosjean et al., 1990). However, a comparison between Rome and Sao Paulo cannot be made because of the great differences in number and distribution of vehicles, fuel composition, percentage and type of catalyst equipped vehicles, etc. Table 2 shows concentration data of alkenes integrated over the 0600-0900 h period for the same sampling site of Rome. In this case no significant seasonal variations were found in absolute values as well as in alkene composition. By exploiting the data of Table 2, the potential production of C1~C3 saturated aldehydes in the 0600-0900 h period was calculated under summertime conditions, according to the Altshuller's procedure (1993). The 2100-0600 h alkene emissions and
Lower carbonyls in Rome ambient air
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M. POSSANZINI et al.
3762
Table 2. Concentrations of C2-C 5 alkenes (ppb) in samples collected between 0600 and 0900 h in Rome June-July 1994
Ethene Propene
trans-2-Butene 1-Butene 1-Methylpropene
cis-2-Butene 3-Methyl-l-butene
trans-2-Pentene 2-Methyl-2-butene 1-Pentene 2-Methyl-l-butene
cis-2-Pentene
Range
Mean
Range
Mean
5.5 14.0 4.6-7.7 1.3-3.7 1.2-1.4 3.2-8.7 1.2-1.5 0.44).5 0.7-1.0 1.5-1.8 0.64).8 0.24).4 0.4-0.8
9.0 5.8 2.2 1.3 5.9 1.4 0.4 0.9 1.6 0.7 0.3 0.7
9.3-13.5 5.1 10.0 1.5-2.9 0.9-2.1 6.3-8.6 0.9-1.9 0.4-0.6 0.9-1.6 1.9-3.3 0.7-1.1 0.24).6 0.7-1.2
10.8 6.9 2.0 1.4 7.0 1.4 0.5 1.1 2.3 0.9 0.5 1.0
distribution were assumed to be equal to those of the 0600-0900 h period. Calculations only concerned the reactions of alkenes with 03 and OH radical. The nighttime presence of NO3 radical at levels of 10 ppt, indeed, was found to scarcely affect aldehyde production in the early morning hours (Altshuller, 1993). Based on available diurnal profiles of urban ozone during the warmer months, an average concentration of 10 ppb Oa over the 2100-0900 h period was chosen for Rome, whereas an average OH radical concentration of 1.0 × 10 6 mol cm -3 was assumed to be expected in the 0600-0900 h period (Felton et al., 1988). The entire period between 2100 and 0900h was divided into 1 h segments and the hourly fractional conversion of individual alkenes was calculated as In (l/f) = tl/f (Ros + Roll)
(1)
wherefis the fraction of alkenes present at time t and Ro 3, Roll are the reaction rates of each alkene with Oa and OH obtained by multiplying the rate constants ko3 and koa by the selected O3 and OH concentrations, respectively. Average fractional conversions, Fo, of each alkene to produce a given aldehyde, for both 2100-0600 and 0600-0900h emissions, were then obtained and the production of each aldehyde was calculated as Paid
FRo Yo + RorxYoa-]/ x'.~CaFcL 3Ro3~3+ Roa _]
January-March 1995
(2)
where Ca is the concentration of each alkene listed in Table 2 (column 2) and Yo3, YOH are the maximum fractional aldehyde yields, assuming complete conversion of alkenes to aldehydes (Altshuller, 1993). The data on the secondary production of Cl-C3 aldehydes during the 060ff4)900 h period as a result of both 2100~600 h and 0600-0900 h emissions are reported in Table 3 (columns 1 and 2). Aldehyde amounts are expressed as percentage of total alkene concentration on a ppb basis. They can be compared with the direct primary production of aldehydes from vehicular emissions (column 3 of Table 3). Emission data were ob-
Table 3. Aldehydesproduced between 0600 and 0900 h from reactions of alkenes with 03 and OH a and emitted from vehicular exhaustb (percent of alkenes on a ppb basis) Aldehyde Formaldehyde Ethanal Propanal
21004)600 h emissions 7.1 7.8 2.2
06004)900h Vehicular emissions emissions 5.8 10.9 3.0
7.3 3.2 0.2
aAverage Oa concentration: 10ppb over 2100-0900h period. Average OH radical concentration: 1 x 106 mol cm-a between 0600 and 0900 h. b From Sigsby et al. (1987). tained from dynamoter/dilution tube testing of U.S. gasoline-powered vehicles tested over three different driving cycles (Sigsby et al., 1987). They were assumed to hold for Rome, where hydrocarbon compositions similar to those found at New York (Lonneman et al., 1986) were observed; that is, percent of paraffins = 42 _ 4, percent of olefins = 22 __+3, percent of aromatic = 27 ___4, percent of acetylene = 9 + 2 (Brocco, 1993). It can be first and foremost seen from Table 3 that the primary production of aldehydes is comparable to the secondary one from either 2100-0600 h or from 06004)900 h emissions. For ethanal and propanal production exceeds emission by factors of 2-3 and 10-15, respectively. Such factors are somewhat higher than those found by Altshuller (1993). In our calculations, however, only the C 2 - C 5 alkene fraction, accounting for about 90% of total alkenes, was used. The distribution of alkenes as given in Table 2 approaches that of original alkene emissions from vehicular sources (Possanzini, 1981), but the rates of reaction of alkenes are much lower between 0600-0900 h than later in the day, when higher concentrations of Oa and OH are available. Nevertheless, the experimental data of Table 4, where carbonyl/ alkene concentration ratios related to the early morning period are tabulated, show significant differences between summer and winter months. In summer the
Lower carbonyls in Rome ambient air Table 4. Concentrations ratios between individual CI-C3 carbonyls (0800-0900 h) and total C2-C5 alkenes (0600-0903 h) measured in Rome Percentage of alkenes June-July 1994 January-March 1995
Formaldehyde Ethanal Acetone Propanal
Range
Mean
Range
Mean
22-34 11-25 10-18 2-3
28 18 14 3
18-24 7-13 7-11 1-2
20 9 9 1.6
above ratios double for ethanal and propanal and increase substantially for formaldehyde and acetone. This indicates the presence of an important photochemical source of lower carbonyls just after sunrise as emphasized by the results of Table 3.
CONCLUSION The results show that the lower carbonyls measured in the centre of R o m e are photochemically produced at a large extent in summer, whereas direct vehicular emissions are their principal source in winter. Pollution problems associated to carbonyls are therefore not restricted to the urban area, but they involve downwind areas where the polluted air masses are transported within a distance such that reactive hydrocarbons are already photooxidized. In fact, peak concentrations of formaldehyde ethanal and acetone at the E M E P station of Montelibretti (30 km NorthEast of Rome) are usually observed 2-4 h after midday during late spring and summer conditions (Ciccioli et al., 1987). This temporal shift corresponds both to the mean transport time of urban pollutants and the mean residence time of the most reactive alkenes in the troposphere. The presence of carbonyls in this rural site mostly originates from this photochemical source, while both the primary and secondary sources contribute to their burden in the urban centre.
REFERENCES
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3763
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Zhang J., He Q. and Lloy P. J. (1994) Characteristics of aldehydes: concentrations, sources and exposures for indoor and outdoor residential microenvironments. Envir. Sci. Technol. 28, 146-152. Zhou X. and Mopper K. (1993) Carbonyl compounds in the lower marine atmosphere over the Caribbean Sea and Bahamas. J. geophys. Res. 98, 2385-2392.
Atmospheric Environment Vol. 30, No. 22, pp. 3757-3764, 1996 Copyright © 1996 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved P I I : $1352-2310(96)00110-0 1352-2310/96 $15.00 + 0.120
MEASUREMENTS OF LOWER CARBONYLS IN ROME AMBIENT AIR M. P O S S A N Z I N I , V. D I P A L O , M. P E T R I C C A , R. F R A T A R C A N G E L I a n d D. B R O C C O Istituto sull'Inquinamento Atmosferico del C.N.R., Area della Ricerca di Roma Via Salaria Km 29,300C.P. 10, 00016 Monterotondo Stazione (Roma), Italy (First received 7 August 1995 and in final form 13 March 1996)
Abstract--Ambient levels and diurnal profiles of lower carbonyls were measured in Rome during selected days of summer 1994 and winter 1995. The most abundant carbonyls were formaldehyde (up to 27 ppb) followed by ethanal ( < 17 ppb) and acetone ( < 9 ppb). Gas-phase concentrations of other seven carbonyls were in the 0-3 ppb range. The results were discussed with respect to direct emissions and photochemical production. Using carbonyl/CO concentration ratios mobil source emissions of carbonyls were estimated for the urban area. The secondary production of C~-C 3 aldehydes from reactions of alkenes with 0 3 and OH radicals during the early morning hours of summer days was also calculated. The daytime pattern of carbonyls was found to be similar to that of toluene in wintertime and close to that of ozone in summer periods conductive to photochemical pollution episodes. Copyright © 1996 Published by Elsevier Science Ltd Key word index: Carbonyls, alkenes, primary and secondary pollutants, diurnal profiles.
INTRODUCTION Carbonyls play a central role in the chemical reactions taking place in the troposphere because they are generally the first stable intermediates in the photooxidation mechanism of organic compounds (Carlier et al., 1986). It has been calculated that the secondary production of C1-C~. aldehydes in U.S. cities usually predominates over direct emissions of the same aldehydes from auto exhausts (Altshuller, 1993). Carbonyls are in turn di.rect precursors of peroxyacylnitrates and ozone through peroxyradicals (HO2, RO2 and RC(O)O2) generated by photolysis or OH radical attack. In spite of their great importance in urban air quality and health hazard assessment (NCR, 1981), lower carbonyls have received little attention in air pollution monitoring programs to date. Due, to a large extent, to analytical problems (Vairavamurthy et al., 1992) knowledge of ambient levels and diurnal profiles of carbonyls has been limited to a few urban areas in the last decade. Recently, thorough investigations on the presence of lower carbonyls have also been carried out in rural and marine environments (Shepson et al., 1991; Zhou and Mopper, 1993). Results of short-te rm urban measurements indicate that formaldehyde is the most abundant carbonyl pollutant followed by ethanal (Salas and Singh, 1986; Grosjean, 1982, 1988; Haszpra et al., 1991; Grosjean et al., 1993; Satsumabayashi et al., 1995). The reverse
has been observed in the major urban areas of Brazil, where the large scale use of ethanol as a vehicle fuel gives rise to prevailing ethanal emissions (Grosjean et al., 1990). The present paper reports data of measurements made in Rome, where ambient concentrations and daily variations of lower carbonyls have been obtained in different seasons. Rome can represent a typical urban area of the Mediterranean region, where atmospheric circulation exhibits marked diurnal cycles with the alternation of high stability conditions during the night and strong vertical mixing during the day. Ground level concentrations and temporal evolution of air pollutants are strongly influenced by mesometeorological conditions involving local landsea wind systems. Furthermore, a relatively intense solar radiation and high temperatures give rise to frequent photochemical pollution episodes (Cecinato et al., 1979). This investigation has been carried out during the gradual replacement (started on 1992) of tetra-alkyl lead with methyl tertiary butyl ether (MTBE) as a gasoline additive in catalyst equipped cars. The percentage of catalyst vehicles during the experiment period ranged from 15 to 20% of total number of circulating vehicles. A significant reduction in carbonyl emissions, but not in the carbonyl/hydrocarbon concentration ratios, is expected from the use of catalytic converters; the blending of MTBE with gasoline has been found indeed to produce an increase in aldehyde
3757
3758
M. POSSANZINI et al.
emissions with respect to gasoline, powered fuels (AItshuller, 1993). In addition, the effects of catalyst deterioration should be taken into account (Cabella et al., 1993). Relationships of carbonyls with other pollutants, such as ozone, carbon monoxide and specific hydrocarbons have also been examined in an attempt to get an idea of the relative contribution of direct emissions and photochemical processes to the measured ambient carbonyl levels.
2. EXPERIMENTAL
Sampling of volatile carbonyls and hydrocarbons together with measurements of ozone and carbon monoxide were made in the centre of Rome, near Piazza Venezia during the 0800-2000 h (local time) period of selected summer and winter days. Five sunny days following overnight periods of stable atmospheric conditions were chosen in June-July 1994 and January-March 1995, respectively. Sunrise and sunset occurred, on the average, at 0600 and 2000h and at 0700h and 1700h in the above periods, respectively. The sampling time was 1 h and a total of 113 samples were collected. The sampling inlet was positioned at a height of 3 m above ground level. Air samples were delivered via a Teflon tube with manifold to the sampling systems or the analysers. Carbonyls were collected at an air flow rate of 1.0 E min -1 on silica cartridges coated with 2,4-dinitrophenyl-hydrazine (Sep Pak DNPH, Waters). An ozone trap, consisting of a Kl-coated annular denuder (10 cm long, 10 and 13 mm in annulus diameters) was connected to the upstream ends of cartridges (Possanzini and Di Palo, 1995). The derivatized products were extracted with 3 ml acetonitrile and analyzed by HPLC with u.v. detection, according to a procedure described elsewhere (Possanzini and Di Palo, 1995). Separation was performed on a 250 x 4 mm Hypersil ODS, 5 #m column under the following conditions: mobile phase = methanol/water/acetonitrile (60:33 : 7 v/v/v isocratic); flow rate = 1.0 ml min- 1, injection volume = 20 #1 and detector wavelength, 2 = 365 nm. Typical HPLC analysis time was less than 18 min. The performance of the DNPH method was assessed by analysis of ten cartridges each spiked with 100 #1 of a carbonyl mixture containing 10 #g ml- 1 of formaldehyde, ethanal and acetone, respectively. Concentrations of spiked samples were all within + 10% of the calculated concentrations and the relative standard deviations were 8% for HCHO, 13% for CHaCHO and 16% for CHaCOCH 3. These values also included the blank variability (20-40 ng HCHO, 30-70 ng
CH3CHO and 5(~100 ng CHaCOCH 3 per cartridge). Identification and quantitation of individual carbonyls involved the use of external standards, i.e. direct injection of a calibration mixture prepared by dissolving known amount of freshly synthesized hydrazones in acetonitrile (1 #gml-1). Sampling and analysis of the hydrocarbons were performed with a VOC Air Analyser (Chrompack), which is an automatic gas chromatographic system making use of cryogenic adsorption (0.35 f air sample) and thermal desorption steps in the hourly measuring cycle. Detection time and peak response of replicate analyses of an air sample by this method did not deviate by more than 10% relative standard deviation. 0 3 and CO were monitored with a UV photometer (Environnement mod. 1003 AH) and a NDIR analyser (Environnement mod. CO 10M), respectively. The precision of such analysers was better than 5% in the respective 0-100 ppb 0 3 and 0-10 ppm CO concentration ranges.
RESULTS AND DISCUSSION Six carbonyls and two pairs of unresolved isomers from formaldehyde to benzaldehyde were identified and quantitated in Rome air. Their concentrations are listed in Table 1. Levels of formaldehyde and ethanal were comparable with those encountered in the centre of Paris (Kalabokas et al., 1988). As expected, peak concentrations were significantly higher in summer than in winter. This means that in situ formation of carbonyls from reactive hydrocarbons (i.e. alkenes) was competitive with their emissions from autovehicles under conditions of strong solar irradiation, high temperatures ( > 30°C) and poor ventilation ( < 2 m s - 1). Ambient concentration of formaldehyde in the ranges 9-27 ppb (summer) and 8-17 pbb (winter) characterized the two periods. Ethanal correspondingly reached maximun hourly values of 17 and 7 ppb. The third most abundant carbonyl was acetone with summer levels of up to 9 ppb. Propanal never exceeded 3 ppb, whereas the other carbonyls were all below 2 ppb. Figure 1 shows diurnal variations of the major carbonyls along with the corresponding averaged concentrations of ozone and toluene. We can distinguish two cases both referred to stable weather conditions. When it was cold, as on 25 January (Tmax < 7 ° 0 , the diurnal profile of carbonyls was close to those of
Table 1. Ambient air concentrations (ppb) of carbonyls in Rome. Time period: 0800-2000 h. Integration time: 1 h June--July 1 9 9 4 (n = 56)
Formaldehyde Ethanal Propanal Acrolein Acetone Methylvinylketone + methacrolein Butanal + 2-butanone Benzaldehyde
January-March 1995 (n = 57)
Range
Mean
Range
Mean
8.8-27.7 3.1-17.4 0.6-3.0 0.4-1.0 4.2-9.2 0.4-1.0 0.7-2.4 0.4-1.0
17.0 9.3 1.8 0.7 6.8 0.7 1.4 0.7
8.2-17.0 2.9-6.6 0.5-1.6 0.3-0.8 3.4-5.8 0.24).6 0.6-1.6 0.3-0.8
11.2 4.6 0.9 0.6 4.4 0.4 1.0 0.5
Lower carbonyls in Rome ambient air
30
3759
~CH20
-=-CH3CH
+ CH3COCH3
• C2H5CHO
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July 27,1994 20 - -
•
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=- CH3CHO
4, CH3COCH3
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+03
"1( . . . . . . . . . . . . . . . . . . . . .
~
.................................
60
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==
5
20
oo 08:30
12:30
16:30
19:30
Jan 25,1995 Fig. 1. Diurnal profiles of lower carbonyls, ozone and toluene measured at Rome in summer (a) and winter (b) days.
3760
M. POSSANZINI et al.
hydrocarbons, such as toluene, benzene, acetylene, etc., suggesting that photochemical production was unimportant compared with the primary emission source (e.g. autoexhaust). Formaldehyde showed a more distinct pattern with respect to other carbonyls, which, anyhow, behaved like HCHO in the daytime. Carbonyls as well as hydrocarbons began to rise early in the morning and showed three maxima corresponding to 0900-1000, 1300-1400 and 1900-2000 h periods, whereas the afternoon minimum was attributed both to convective mixing and decreased traffic (Fig. lb). During a warm and sunny day, as 27 July, a very different profile was observed for carbonyls, which conformed to that of ozone with one maximum at the middle of the day (Fig. la). This indicated that photooxidation of reactive hydrocarbons was the preponderant source of carbonyls under summertime conditions. In fact, the absolute levels of carbonyls were almost 50% higher in spite of halved hydrocarbon concentrations. It is worth noting that each carbonyl showed its maximum centred about 1 h before that of ozone. This could be explained with the different removal mechanisms involving the two chemical species. The balance between the photochemical production rates and the loss rates resulted in a decrease of carbonyl levels at the time of the maximum ozone build-up. The short chemical lifetime of lower aldehydes in the troposphere (Carlier et al., 1986) supports this feature. Diurnal variations of C1-C 3 aldehydes and acetone were similar in all occasion and suggested common primary sources and analogous formation and decomposition pathways.. Daytime removal mechanisms include photolysis and reactions with OH radicals. The substantial photochemical formation of carbonyls from 03- and OH-hydrocarbon reactions clearly emerged from the aldehyde/acetylene plots of Fig. 2. Acetylene is an unreactive hydrocarbon emitted from the same sources which emit primary aldehydes. The ratio of aldehyde to acetylene aimed to normalize the carbonyl concentrations by counteracting the diffusional effects. This ratio should not change appreciably in the absence of in situ formation and removal processes. This is not the case of Fig. 2b, where diurnal variations of aldehydes with a plateau persisting from 11.00 to 17.00 h were consistent with the 03 behaviour and, thus, with the photochemical activity associated with morning emissions. By contrast, little variations in the aldehyde/acetylene ratio were observed in the winter day (Fig. 2a) when 03 exhibited its typical, although less marked profile. Formaldehyde/ethanal and ethanal/propanal concentration ratios were also calculated. The summerwinter differences were not statistically significant in this case and data averaged over all measurements were used. The above ratios did not show large variations (from 0.8 to 4.1 for C1/C2 and from 3.1 to 8.7 for
C2/C3) and the average value for C1/C 2 (2.1) was close
to that (2.5) resulting from literature data listed by Grosjean (1990) and related to 17 urban areas of different countries. By contrast, these data show a considerable variability (from 0.3 to 14.3) as one would expect from differences in fuel use, use of catalytic converters, etc. from one urban area to the next. On the one side, C1/Cz values from Brazil are relatively low due to the substantial use of ethanol containing fuels in that country; on the other, the highest value of the range (14.3) appears not to be representative (Shepson, 1992). It should be pointed out that CJC2 ratios usually vary from 1-2 (urban area) to about 10 (forested area); therefore they could be used as a measure of a biogenic source of formaldehyde (Shepson et al., 1991). C2/C3 ratios should be used likewise as indicators of anthropogenic origin for ambient carbonyls, since propanal is believed to be associated only with anthropogenic emissions. Then, C1/C3 would be high in rural atmospheres and low in polluted urban air. A value of 5.2 was found for Rome. The very limited number of urban ratios available show C2/C3 varying from 1.7 to 6.0 (Grosjean, 1992; Zhang et al., 1994), without considering Brazil, where ratios in the 1 8 4 9 range were measured (Grosjean, 1992). Emission rates for vehicle-emitted carbonyls could be estimated from our data using CO as tracer for mobile source emissions. In the Rome metropolitan area ( ~ 230 km 2) vehicles account for at least 95% of the total CO emissions, which are reported to be (0.9 _+ 0.2)x 103 metric tons day-1 (Allegrini et al., 1990). Hourly CO concentrations in the winter measurement days were in the range 1.5-10.6 ppm for the 0800-2000 h interval, with a mean value of 4.6 ppm. The latter was associated to the average concentrations of 11.2 and 4.6 ppb obtained for formaldehyde and ethanal, respectively. If one assumes that the carbonyl/CO emission rate ratio approaches the carbonyl/CO ambient concentration ratio, these values correspond to mobile source emission rates of 2.2 _+ 0.5 and 0.9 _+ 0.3 metric tons d a y - 1 for formaldehyde and ethanal, respectively. Very higher emission rates (17 tons H C H O and 20 tons CH3CHO per day) were estimated for Sao Paulo (Grosjean et al., 1990). However, a comparison between Rome and Sao Paulo cannot be made because of the great differences in number and distribution of vehicles, fuel composition, percentage and type of catalyst equipped vehicles, etc. Table 2 shows concentration data of alkenes integrated over the 0600-0900 h period for the same sampling site of Rome. In this case no significant seasonal variations were found in absolute values as well as in alkene composition. By exploiting the data of Table 2, the potential production of C1~C3 saturated aldehydes in the 0600-0900 h period was calculated under summertime conditions, according to the Altshuller's procedure (1993). The 2100-0600 h alkene emissions and
Lower carbonyls in Rome ambient air
1,4
-=- CH20/C2H2
~C
• C)3
-~C
3761
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CH3COCH3/C2H2
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e~ e~
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Jan 25,1995 Fig. 2. Diurnal variations of ratios of carbonyls to acetylene at Rome during summer (a) and winter (b) days.
o
M. POSSANZINI et al.
3762
Table 2. Concentrations of C2-C 5 alkenes (ppb) in samples collected between 0600 and 0900 h in Rome June-July 1994
Ethene Propene
trans-2-Butene 1-Butene 1-Methylpropene
cis-2-Butene 3-Methyl-l-butene
trans-2-Pentene 2-Methyl-2-butene 1-Pentene 2-Methyl-l-butene
cis-2-Pentene
Range
Mean
Range
Mean
5.5 14.0 4.6-7.7 1.3-3.7 1.2-1.4 3.2-8.7 1.2-1.5 0.44).5 0.7-1.0 1.5-1.8 0.64).8 0.24).4 0.4-0.8
9.0 5.8 2.2 1.3 5.9 1.4 0.4 0.9 1.6 0.7 0.3 0.7
9.3-13.5 5.1 10.0 1.5-2.9 0.9-2.1 6.3-8.6 0.9-1.9 0.4-0.6 0.9-1.6 1.9-3.3 0.7-1.1 0.24).6 0.7-1.2
10.8 6.9 2.0 1.4 7.0 1.4 0.5 1.1 2.3 0.9 0.5 1.0
distribution were assumed to be equal to those of the 0600-0900 h period. Calculations only concerned the reactions of alkenes with 03 and OH radical. The nighttime presence of NO3 radical at levels of 10 ppt, indeed, was found to scarcely affect aldehyde production in the early morning hours (Altshuller, 1993). Based on available diurnal profiles of urban ozone during the warmer months, an average concentration of 10 ppb Oa over the 2100-0900 h period was chosen for Rome, whereas an average OH radical concentration of 1.0 × 10 6 mol cm -3 was assumed to be expected in the 0600-0900 h period (Felton et al., 1988). The entire period between 2100 and 0900h was divided into 1 h segments and the hourly fractional conversion of individual alkenes was calculated as In (l/f) = tl/f (Ros + Roll)
(1)
wherefis the fraction of alkenes present at time t and Ro 3, Roll are the reaction rates of each alkene with Oa and OH obtained by multiplying the rate constants ko3 and koa by the selected O3 and OH concentrations, respectively. Average fractional conversions, Fo, of each alkene to produce a given aldehyde, for both 2100-0600 and 0600-0900h emissions, were then obtained and the production of each aldehyde was calculated as Paid
FRo Yo + RorxYoa-]/ x'.~CaFcL 3Ro3~3+ Roa _]
January-March 1995
(2)
where Ca is the concentration of each alkene listed in Table 2 (column 2) and Yo3, YOH are the maximum fractional aldehyde yields, assuming complete conversion of alkenes to aldehydes (Altshuller, 1993). The data on the secondary production of Cl-C3 aldehydes during the 060ff4)900 h period as a result of both 2100~600 h and 0600-0900 h emissions are reported in Table 3 (columns 1 and 2). Aldehyde amounts are expressed as percentage of total alkene concentration on a ppb basis. They can be compared with the direct primary production of aldehydes from vehicular emissions (column 3 of Table 3). Emission data were ob-
Table 3. Aldehydesproduced between 0600 and 0900 h from reactions of alkenes with 03 and OH a and emitted from vehicular exhaustb (percent of alkenes on a ppb basis) Aldehyde Formaldehyde Ethanal Propanal
21004)600 h emissions 7.1 7.8 2.2
06004)900h Vehicular emissions emissions 5.8 10.9 3.0
7.3 3.2 0.2
aAverage Oa concentration: 10ppb over 2100-0900h period. Average OH radical concentration: 1 x 106 mol cm-a between 0600 and 0900 h. b From Sigsby et al. (1987). tained from dynamoter/dilution tube testing of U.S. gasoline-powered vehicles tested over three different driving cycles (Sigsby et al., 1987). They were assumed to hold for Rome, where hydrocarbon compositions similar to those found at New York (Lonneman et al., 1986) were observed; that is, percent of paraffins = 42 _ 4, percent of olefins = 22 __+3, percent of aromatic = 27 ___4, percent of acetylene = 9 + 2 (Brocco, 1993). It can be first and foremost seen from Table 3 that the primary production of aldehydes is comparable to the secondary one from either 2100-0600 h or from 06004)900 h emissions. For ethanal and propanal production exceeds emission by factors of 2-3 and 10-15, respectively. Such factors are somewhat higher than those found by Altshuller (1993). In our calculations, however, only the C 2 - C 5 alkene fraction, accounting for about 90% of total alkenes, was used. The distribution of alkenes as given in Table 2 approaches that of original alkene emissions from vehicular sources (Possanzini, 1981), but the rates of reaction of alkenes are much lower between 0600-0900 h than later in the day, when higher concentrations of Oa and OH are available. Nevertheless, the experimental data of Table 4, where carbonyl/ alkene concentration ratios related to the early morning period are tabulated, show significant differences between summer and winter months. In summer the
Lower carbonyls in Rome ambient air Table 4. Concentrations ratios between individual CI-C3 carbonyls (0800-0900 h) and total C2-C5 alkenes (0600-0903 h) measured in Rome Percentage of alkenes June-July 1994 January-March 1995
Formaldehyde Ethanal Acetone Propanal
Range
Mean
Range
Mean
22-34 11-25 10-18 2-3
28 18 14 3
18-24 7-13 7-11 1-2
20 9 9 1.6
above ratios double for ethanal and propanal and increase substantially for formaldehyde and acetone. This indicates the presence of an important photochemical source of lower carbonyls just after sunrise as emphasized by the results of Table 3.
CONCLUSION The results show that the lower carbonyls measured in the centre of R o m e are photochemically produced at a large extent in summer, whereas direct vehicular emissions are their principal source in winter. Pollution problems associated to carbonyls are therefore not restricted to the urban area, but they involve downwind areas where the polluted air masses are transported within a distance such that reactive hydrocarbons are already photooxidized. In fact, peak concentrations of formaldehyde ethanal and acetone at the E M E P station of Montelibretti (30 km NorthEast of Rome) are usually observed 2-4 h after midday during late spring and summer conditions (Ciccioli et al., 1987). This temporal shift corresponds both to the mean transport time of urban pollutants and the mean residence time of the most reactive alkenes in the troposphere. The presence of carbonyls in this rural site mostly originates from this photochemical source, while both the primary and secondary sources contribute to their burden in the urban centre.
REFERENCES
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3763
ing of the Atmosphere (edited by Ciccioli P.), pp. 104-111. Societ/t Chimica Italiana, Rome. Carlier P., Hannachi H. and Mouvier G. (1986) The chemistry of carbonyl compounds in the atmosphere. Atmospheric Environment 20, 2079-2099. Cecinato A., Possanzini M., Liberti A. and Brocco D. (1979) Photochemical oxidants in the Rome metropolitan area. Sci. Total Envir. 13, 1-8. Ciccioli P., Draisci R., Cecinato A. and Liberti A. (1987) Sampling of aldehydes and carbonyl compounds in air and their determination by liquid chromatographic techniques (1987). In Physico-Chemical Behaviour of Atmospheric Pollutants. Proc. Fourth Eur. Symp. 1986 (edited by Angeletti G. and Restelli G.), pp. 133-141. D. Reidel, Dordrecht. Felton C. C., Sheppard J. C. and Campbell M. J. (1988) Measurement of the diurnal OH cycle by a 14C-tracer method. Nature 335, 53-55. Grosjean D. (1982) Formaldehyde and other carbonyls in Los Angeles ambient air. Envir. Sci. Technol. 16, 254-262. Grosjean D. (1988) Aldehydes, carboxylic acids and inorganic nitrate during NSMCS. Atmospheric Environment 22, 1637-1648. Grosjean D., Miguel A. H. and Tavares T. M. (1990) Urban air pollution in Brazil: acetaldehyde and other carbonyls. Atmospheric Environment 24B, 101-106. Grosjean D. (1992) Discussion: atmospheric concentrations and temporal variations of C1-C 3 carbonyl compounds at two rural sites in central Ontario. Atmospheric Environment 26A, 349-351. Grosjean E., Williams E.L. II and Grosjean D. (1993) Ambient levels of formaldehyde and acetaldehyde in Atlanta. Georgia. Air Waste 43, 469-474. Haszpra L., Szilagyi I., Demeter A., Turanyi T. and Berces T. (1991) Non-methane hydrocarbon and aldehyde measurements in Budapest, Hungary. Atmospheric Environment 25A, 2103-2110. Kalabokas P., Carlier P., Fresnet P., Mouvier G. and Toupance G. (1988) Field studies of aldehyde chemistry in the Paris area. Atmospheric Environment 22, 147-155. Lonneman W. A., Seila R. L. and Meeks S. A. (1986) Nonmethanol organic composition in the Lincoln Tunnel. Envir. Sci. Technol. 20, 790-796. NRC (National Research Council) Committee on Toxicology and Environmental Health Hazards (1981) Formaldehyde and Other Aldehydes. National Academy Press, Washington, D.C. Possanzini M. (1981) La valutazione dell'inquinamento fotochimico nelrarea di Roma. CNR Publ. AQ/3/16 (edited by Colacino M.), pp. 1-75. Consiglio Nazionale delle Ricerche, Rome. Possanzini M. and Di Palo V. (1995) Determination of olefinic aldehydes and other volatile carbonyls in air samples by DNPH-coated cartridges and HPLC. Chromatographia 40, 134-138. Salas L. J. and Singh H. B. (1986) Measurements of formaldehyde and acetaldehyde in the urban ambient air. Atmospheric Environment 20, 1301-1304. Satsumabayashi H., Kurita H., Chang Y. S. and Carmichael G. R. (1995) Photochemical formations of lower aldehydes and lower fatty acids under long-range transport in central Japan. Atmospheric Environment 29, 255-266. Shepson P. B. (1992) Discussion: atmospheric concentrations and temporal variations of C ~ C 3 carbonyl compounds at two rural sites in central Ontario. Atmospheric Environment 26A, 349-351. Shepson P. B., Hastie D. R., Schiff H. I., Polizzi M., Bottenheim J. W., Anlauf K., Mackay G. I. and Karecki D. R. (1991) Atmospheric concentrations and temporal variations of Ct-C a carbonyl compounds at two rural sites in central Ontario. Atmospheric Environment 25A, 2001-2015.
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Sigsby J. E. Jr., Tajada S., Ray W., Lang J. M. and Duncan J. W. (1987) Volatile organic compound emissions from 46 in-use passenger cars. Envir. Sci. Technol. 21,466-475. Vairavamurthy A., Roberts J. M. and Newman L. (1992) Methods for determination of low molecular weight carbonyl compounds in the atmosphere: a review. Atmospheric Environment 26A, 1965-1993.
Zhang J., He Q. and Lloy P. J. (1994) Characteristics of aldehydes: concentrations, sources and exposures for indoor and outdoor residential microenvironments. Envir. Sci. Technol. 28, 146-152. Zhou X. and Mopper K. (1993) Carbonyl compounds in the lower marine atmosphere over the Caribbean Sea and Bahamas. J. geophys. Res. 98, 2385-2392.