Environ. Sci. Technol. 1991, 25,782-788
7782-44-7; 3-chlorophenol, 108-43-0;4-chlorophenol, 106-48-9.
Literature Cited Porter, J. W. Chem. Eng. Prog. 1989, 85(4), 16-25. Barbeni, M.; Pelizzetti, E.; Borgarello, E.; Serpone, N. Chemosphere 1987, 16, 2225-2237. Bowers, A. R.; Eckenfelder, W. W.; Gaddipati, P.; Monsen, R. M. Water Sei. Technol. 1989, 21, 477-486. Merz, J. H.; Waters, W. A. J. Chem. SOC.1949, 2427-2433. Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J . Phys. Chem. Ref. Data 1988, 17, 513-886. Farhataziz; Ross, A. B. Selected Specific Rates of Reactions o f Transients from Water in Aqueous Solution. III. Hydroxyl Radical and Perhydroxyl Radical and Their Radical Zons; Report NSRDS-NBS 59; National Bureau
of Standards: Washington, DC, 1977. Dorfman, L. M.; Taub, I. A.; Buhler, R. E. J. Chem. Phys. 1962, 36, 3051-3061. Sudoh, M.; Kodera, T.; Sakai, K.; Zhang, J. Q.;Koide, K. J . Chem. Eng. Jpn. 1986, 6 , 513-518. Murphy, A. P.; Boegli, W. J.; Price, M. K.; Moody, C. D. Environ. Sei. Technol. 1989, 23, 166-169. Watts, R. J.; Miller, G. C.; Smith, B. R.; Rauch, P. A,; Tyre, B. W. Division of Environmental Chemistry, American Chemical Society, Miami Beach, Florida, September 1C-15, 1989; Extended Abstract, pp. 346-349. Walling, C. Acc. Chem. Res. 1975, 8, 125-131. Jefcoate, C. R. E.; Lindsay Smith, J. R.; Norman, R. 0. C. J . Chem. Soc. B 1969, 1013-1018. Collins, P. F.; Diehl, H.; Smith, G. F. Anal. Chem. 1959, 31, 1862-1867. Tulp, M. T. M.; Olie, K.; Hutzinger, 0. Biomed. Mass Spectrom. 1977, 4 , 310-316. Sedlak, D. L.; Andren, A. W. In preparation. Sehili, T.; Bonhomme, G.; Lemaire, J. Chemosphere 1988, 17, 2207-2218.
(17) Eberhardt, M. K.; Yoshida, M. J . Phys. Chem. 1973, 77, 589-597. (18) Walling, C.; Johnson, R. A. J . Am. Chem. Soc. 1975, 97, 363-367. (19) Kunai, A.; Hata, S.; Ito, S.; Sasaki, K. J . Am. Chem. Soc. 1986, 108, 6012-6016. (20) Ito, S.; Kunai, A.; Okada, H.; Sasaki, K. J . Org. Chem. 1988, 53, 296-300. (21) Groves, J. T.; Van Der Puy, M. J . Am. Chem. SOC.1974, 96, 5274-5275. (22) Fessenden, R. J.; Fessenden, J. S. Organic Chemistry; PWS Publishing: Boston, MA, 1982. (23) Schofield,P. J.; Ralph, B. J.; Green, J. H. J . Phys. Chem. 1964, 68, 472-476. (24) Stein G.; Weiss, J. J . Chem. Soc. 1951, 3265-3274. (25) Voudrias, E. A.; Larson, R. A,; Snoeyink, V. L. Enuiron. Sci. Technol. 1985, 19, 441-449. (26) Shindo, H.; Huang, P. M. Nature 1982, 298, 363-366. (27) Larson, R. A.; Hufnal, J. M. Limnol. Oceanogr. 1980,25, 505-5 12. (28) Eckschlager, K.; Veprek-Siska, J. Collect. Czech. Chem. Commun. 1972, 37, 1623-1634. (29) Eckschlager, K.; Horsak, I.; Veprek-Siska,J. Collect. Czech. Chem. Commun. 1974, 39, 2353-2362. (30) Tyson, C. A.; Martell, A. E. J . Am. Chem. Soc. 1972, 94, 939-945. (31) Martell, A. E.; Smith, R. M. Critical Stability Constants; Plenum: New York, 1977; Vol. 3. Received for review March 12,1990. Revised manuscript received October 8, 1990. Accepted December 4, 1990. This work was funded by the U S . Air Force Officeof Scientific Research, Grant AFOSR-88-0301. Funding was also obtained from the University of Wisconsin Sea Grant College Program under grants from the Office of Sea Grant, NOAA, U S . Department of Commerce,and the State of Wisconsin (Federal Grant NA84AA-D-00065).
Physical Factors Influencing Winter Precipitation Chemistry Jeffrey L. Collett, Jr.,+ Andri S. H. Privet, Johannes Staehelin," and Albert Waldvogel Atmospheric Physics ETH, Honggerberg HPP, 8093 Zurich, Switzerland
Two case studies of winter precipitation events highlight the roles of transport and snow crystal riming (the capture of supercooled cloud droplets by snow crystals) in determining precipitation chemistry. In one case, passage of a cold front leads to a change in the air mass producing precipitation over the monitoring site. A simultaneous decrease in precipitation ion concentrations is observed. Correlations of the ion concentrations with the pseudoequivalent potential temperature, which serves as an air mass identifier, suggest that the decrease in ion concentrations is caused by the air mass change, rather than by washout of aerosols and gases from the atmosphere. In the second case, evidence is presented indicating that an increase in precipitation ion concentrations results from significant capture of polluted cloudwater droplets by the snow crystals. Influences on precipitation chemistry from both processes, transport and riming, can be large. In order to study other processes influencing precipitation chemistry that occur on similar time scales (minutes to hours), such as aerosol and gas scavenging or aqueousphase oxidation, it is important to evaluate possible confounding effects of tranmort and riming.
Introduction The chemical composition of precipitation is determined 'Present address: Institute for Environmental Studies, 1101 W. Peabody Dr., University of Illinois at Urbana-Champaign, Urbana, IL 61801. 782
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through the interaction of numerous chemical and physical processes. Some of the more commonly studied of these include nucleation and impaction scavenging of aerosol particles and scavenging of soluble gases within the cloud (1-41, below-cloud scavenging of aerosol particles and soluble gases by precipitation particles (4-a), and chemical reactions within the aqueous phase (9, IO). Our experience in studying aerosol scavenging in winter precipitation systems (4, 7) has revealed great variety between individual events. Processes primarily responsible for controlling precipitation chemistry in one case may play a subordinate role in another event. In this paper we will focus on two frequently ignored physical processes that we have observed playing an important role in determining the chemistry of winter precipitation in central Switzerland: an air mass change and the capture of supercooled cloud droplets by snow crystals (riming). Two case studies will be used to illustrate the influence these processes exert on precipitation chemistry. One of the weaknesses of ground-based precipitation studies is their limited ability to provide information about the temporal evolution of parameters of interest in a moving air mass. Because the air being sampled at a fixed station is changing with time, evaluating processes like aqueous-phase sulfur oxidation or aerosol scavenging can be greatly complicated by a change in background conditions that occurs on a similar time scale. This situation may be particularly problematic during frontal passage (4,
0013-936X/91/0925-0782$02.50/0
0 1991 American Chemical Society
5 ) ,when large gradients in pollutant concentrations, as well as meterological parameters, typically are observed over periods of several minutes to a few hours. We will illustrate the importance of this phenomenon with regard to precipitation chemistry in the first case study. Analysis of the second study will focus on the role of snow crystal riming in contributing to precipitation pollutant loadings in mixed-phase (water-ice) clouds. Snow crystals in such an environment can grow by two mechanisms: vapor deposition and accretion of supercooled cloud drops (riming). Ice crystals growing by vapor deposition tend to be rather clean (11,12),although recent laboratory work indicates that SO2 capture by ice during vapor deposition can be significant (13). Subsequent scavenging of interstitial aerosol particles by the snow crystals is thought to contribute little to their chemical composition (12). Much of the aerosol mass present in a cloud is located within the cloud droplets (14). When an ice crystal grows through the riming process, therefore, attachment of cloud droplets to the ice crystal surface also implies the incorporation of aerosol constituents into the resulting precipitation. Consequently, rimed crystals generally are expected to contain higher concentrations of most ions than unrimed crystals. Several authors (12, 15, 16) have observed this effect and commented on its importance with regard to precipitation chemistry and/or the removal of aerosol species from the atmosphere via wet deposition.
Site Description and Experimental Methods Measurements were carried out at three stations located along the northwestern slope of Mt. Rigi in central Switzerland. Mt. Rigi stands out as in isolated precursor to the main chain of the Swiss Alps. Its steep northwestern face provides a convenient opportunity to obtain information about “vertical” gradients in aerosol, gas, and precipitation concentrations. Site elevations are 430,1030, and 1620 m asl. Most of the results discussed here were obtained a t the top station, Rigi Staffel. This station is frequently located within the precipitating clouds, although the cloud base sometimes lies a few hundred meters overhead. The other two stations are primarily used for obtaining information about below-cloud scavenging of aerosols and gases. A more complete description of the sites can be found elsewhere ( 4 ) . Precipitation samples were collected with thermostated 43-cm polyethylene funnels directly attached to polyethylene sample bottles. The funnel was placed outside a t the onset of precipitation. Temporal variations in the degree of snow crystal riming were characterized through observations of Formvar replicas (17) of the crystals generated during precipitation. These replicas are stable, allowing for later careful observation under a laboratory microscope. Field photographs of snow crystals collected on a black velvet surface also were made a t the time of collection. Snowfall intensity was monitored by a snow spectrograph designed in our laboratory. Cloudwater samples collected a t the site were obtained with a Caltech heated rod cloudwater collector (18) modified for operation a t 220 V. This is an active cloudwater collector; stainless steel rods serve as collection surfaces. These may be internally heated to melt off deposits of rime ice. Heating is only applied for a brief period when the fan is off, in order to minimize evaporative losses. The theoretical cut point (droplet diameter collected with 50% efficiency) of the bank of collector rods is approximately 8 km. Measurements of cloud liquid water content were made with a Gerber Scientific particulate volume monitor (PVM-1001, which measures light scattered in the near forward direction by cloud droplets passing between the
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light source and the detector (19). Cloud drop size spectra were obtained with a modified Climet CI-8060 optical particle counter (20). Meterological measurements made a t the site included temperature, pressure, relative humidity, wind direction, and wind speed. Measurements of cloudwater and precipitation pH were made directly after sample collection using an Orion p H meter equipped with a combination electrode calibrated with pH 4 and 7 buffers. Each sample was stored in a polyethylene sample bottle and refrigerated for transport to the laboratory. Major ion analysis (Na+,K+, Ca2+ Mg2+ , NH4+, C1-, NO3-, and S042-)was completed using ion chromatography with conductivity detection. Individual ion detection limits ranged from approximately 0.5 to 2 WN. 9
Results and Discussion Case Study: April 12-13, 1989. The passage of an active cold front with a strong temperature gradient produced heavy snowfall beginning about 1900 on April 12, 1989, and lasting for approximately 14 h. Concentrations of major ions in the precipitation fell rapidly over the first few hours and then remained quite low throughout the remainder of the storm. Concentrations of NH4+,NO3-, and SO:- are illustrated in Figure 1;temporal profiles of the other major ion concentrations were similar in appearance, although concentration levels were lower for these species. While these concentration profiles appear typical of a situation where the air mass is gradually cleansed by the precipitation, it is important to examine the possibility that a change in the air mass being sampled a t the site may have been the cause of the fall in concentrations. The strength of the cold front producing the precipitation suggests that some significant changes in the air mass being sampled a t Rigi Staffel were likely. Observations of temperature and wind direction a t the site confirm this suspicion. Temperatures fell several degrees as the front passed, while the wind direction changed dramatically. Shortly before the onset of precipitation, there was a change in wind direction from southwest to predominantly north, suggesting that a change in air mass probably did occur. If the air mass being sampled a t the site changed, it is likely that background aerosol and gas concentrations, which contribute to the precipitation chemistry, changed as well. Environ. Sci. Technol., Vol. 25, No. 4, 1991
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Rather than relying solely on observations of wind direction and temperature, we can make use of other meterological parameters to help ascertain more definitely whether a significant air mass change took place. By selecting a parameter that remains relatively constant for several hours as a given air mass undergoes transport, we can utilize observations of this parameter a t a fixed site in order to define periods of air mass change. Haltiner and Martin (21) have pointed out the utility of the pseudoequivalent potential temperature, along with the potential temperature and the specific humidity, for air mass identification purposes. Here we will rely on the pseudoequivalent potential temperature as an air mass identifier. This parameter, which we shall denote as de, represents the thermodynamic properties of an air parcel referenced to a standard state. Starting with the initial state of the parcel, the standard refererence state is achieved by dry adiabatic expansion to the lower condensation level, followed by saturated adiabatic expansion until all moisture is condensed, followed by dry adiabatic compression to 1000 mbar. de is conservative during modification of the air parcel by adiabatic isobaric evaporation or condensation, dry adiabatic expansion, and saturated adiabatic expansion (21). Values of de for this study were calculated from measured values of the pressure, temperature, and dewpoint temperature according to the procedure of Bolton (22). Figure 2 depicts the strong change in pseudoequivalent potential temperature (0,) that took place during the event of April 1 2 and 13. Beginning around 1800, de dropped approximately 20 K over the next several hours before leveling out, suggesting that a gradual change in air mass was occurring a t the site. As the front progressed, the warm air mass preceding the front was replaced a t our monitoring station by a colder postfrontal air mass. Typically, the warmer air preceding a front has a different mix of pollutants than the colder air mass behind the front ( 4 , 5 ) . The difference in pollutant concentrations between the two air masses may be due to one or more of the following factors: (1)different source contributions to the two air masses, (2) different precipitation histories of the two air masses, (3) different mixing heights in the two air masses, and (4)a difference in temperature, humidity, and cloud cover in the two air masses. The existence of a pollutant gradient across a frontal boundary suggests that the change in air mass observed a t the site might reasonably be expected to be accompanied by a significant change in pollutant concentrations, consistent with the pattern we observed in precipitation ion concentrations on this date. 784
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Figure 3. Correlations of SO:-, NH,' and NO,- with 8, at Rigi Staffel on April 12 and 13, 1989. A least-squares regression line, calculated for the first 12 data points in each time series, is plotted on each figure. Correlation coefficients are 0.95 for SO,*-,0.99 for NH,' and 0.92 for NO,-.
In order to examine the relationship between the precipitation ion concentrations and the change in air mass, we have followed the suggestion of Schumann (23) and correlated the ion concentrations with the simultaneously observed values of de. Only the first 12 samples were used, because concentrations after this time were near the detection limit. The observed correlations are quite good, as illustrated for SO?-, NH4+,and NO3- in Figure 3. The strength of these correlations suggests that the change in air mass may have been the primary factor responsible for the decrease in precipitation concentrations, at least during the first several hours of the event. Two other factors may also have acted to affect the precipitation composition: a change in precipitation intensity and a change in the extent of riming of the ice crystals. Precipitation rates exhibited an overall increase from 0.1 t o 0.5 mm h-l during the first several hours of precipitation, suggesting that a dilution effect may have been responsible for the decreasing precipitation concentrations. A closer comparison of ion concentrations with precipitation rates, however, reveals only a weak correlation. Sample to sample variations in precipitation rates fail to correlate with changes in ion concentrations. Limited observations of ice crystal riming during this event reveal heavy riming of the crystals from 2330 to 0320 (20), a period during which low ion concentrations were observed. This observation supports our hypothesis that the air mass change reduced background pollutant concentrations, since heavily rimed snow formed in a dirty air mass would exhibit much higher ion concentrations, as illustrated by the case study below. While the temporal profiles of precipitation ion concentrations were consistent with what many might consider a typical "washout" situation, it appears that removal of aerosols and gases by precipitation scavenging actually had very little to do with the observed concentration decrease. Rather, the changes were associated with transport. While in this event the degree of dependence of the precipitation chemistry on meteorological transport was dramatic, our
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experience indicates that transport effects frequently play an important role. Certainly the ability of transport processes to dominate other processes, such as scavenging or chemical reaction, in determining the resultant chemical composition of wet deposition necessitates careful monitoring of air mass changes if a proper understanding of the mechanisms of interest is to be achieved in a field study environment. Case Study: J a n u a r y 18,1990. In contrast to the case study described above, January 18,1990, featured passage of a weak cold front, which produced moderate amounts of snowfall beginning a t approximately 0900 and lasting until approximately 1545. A modest decline in temperature was observed during the event, while winds were light and variable. Major ion concentrations in the precipitation samples are shown in Figure 4. Snowfall both early in the event and toward the end of the event was characterized by low concentrations of NH,+, NO,, and SO>-; during the middle of the event concentrations of all three ions were elevated dramatically. Precipitation pH dropped from approximately 4.6 early in the event to 4.4 in the middle of the event. As concentrations of NH,*, NO;, and SO>- decreased toward the end of the event, the pH climbed back to 4.6. Concentrations of CaZ+and CI- exhibited smaller increases during the middle of the event; Na+, K+,and Mg2+Concentrationswere near the detection limit for all samples. These concentration profiles clearly represent something other than a simple washout scenario, which would produce a pattern of decreasing concentrations over time. In addition, correlations of ion concentrations with 0, are poor, indicating that an air mass change probably was nut responsible for the observed changes in precipitation chemistry. 0, in fact changed little over the event, suggesting that no significant air mass change occurred during this period. Ion concentrations also failed to exhibit any correlation with precipitation rates, which varied from 0.01 to 0.5 mm h-'. As an alternative hypothesis, let us consider the possibility of a change in cloud microphysical conditions, resulting in a change in the predominant mode of snow crystal growth, as an explanation for the unusual concentration profiles. As discussed earlier, snow crystal chemistry can be influenced by the mechanism of crystal growth. Several researchers have reported significantly higher ion concentrations in cloudwater than in simultaneously sampled precipitation. We have observed the same phenomenon
Figure 5. Photographs of heavily rimed and essentially unrimed snow crystals (dendmes and needles) collected a! Riii Staffel on January 18, 1990. The diameter uf the iliuminaled area in each photograph is approximately 3 mm.
in samples collected a t Mt. Rigi. The difference can be particularly strong between cloudwater and snow crystals grown by the vapor deposition process (12). Therefore, when microphysical conditions favor snow crystal growth by riming, the attachment of numerous cloud droplets to the ice crystal surface usually leads to a significant enrichment in snow crystal ion concentrations. Observations of snow crystal photographs made in the field and snow crystal replicas collected during the event reveal significant differences in the extent of riming occurring during different periods (24). Photographs and replicas were made on average once every 5 min. Figure 5 illustrates the difference in the degree of riming observed on several crystals. While there was generally some variation in the degree of riming observed for different crystals collected simultaneously, it was nevertheless clear that a period of heavy riming of most crystals occurred during the middle of the event, while little riming was observed during the early and late periods of precipitation. The period of heavy riming of the snow crystale collected a t Rigi Staffel coincided quite closely with the period of elevated ion concentrations. A coincidence of both the onset and decline in ion concentrations with the beginning and end of the period of heavy riming suggests that the higher concentrations during the middle of the event were most likely due to the riming. In order to explore this relationship further, we can attempt to correlate the major ion concentrations in the crystals with the average degree of riming observed during each precipitation sampling period. The degree of riming was calculated by estimating the percent volume of each Envlron. Sci.
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snow crystal contributed by the attached cloud drops, based on observations of the Formvar replicas under a microscope (24). Because of the difficulty of this process, the results obtained should be considered as semiquantitative. Since many crystal replicas were created during collection of each precipitation sample, however, averaging of the individual crystal results (weighted by the intensity of snowfall recorded at the time each replica was created) over each precipitation sampling period should reduce random errors somewhat. Figure 6 illustrates the result of this process for the two predominant snow crystal shapes observed: dendrites and columns (or needles). Two peaks in the degree of riming are apparent, with the second, significantly larger than the first, corresponding to the period of heavy riming mentioned above. The temporal evolution in the extent of riming appears very similar to the major ion concentration profiles shown in Figure 4. Both riming maxima are clearly reflected in the precipitation ion concentrations. The strongest correlation exists between precipitation NO; concentrations and the extent of riming (see Figure 7); however, both NH4+and SO:- concentrations are also clearly correlated with the extent of riming (the extent of dendrite riming was used for these correlations because most of the precipitation mass was associated with the dendritic snow crystals). This evidence suggests that contributions of attached cloudwater droplets to precipitation ion concentrations were indeed significant. Cloud base during snowfall on this date was usually located above the sampling station, although from time to time the site was briefly enveloped by clouds. Unfortunately these periods were too brief to enable sampling of cloudwater for chemical analysis during the precipitation. Although cloudwater concentrations of major ions are typically much higher than those of precipitation, it would be useful to know what the relationship was during this particular case. During a 20-min period of cloud interception beginning 15 min after the precipitation stopped, we were able to obtain one cloudwater sample. It contained major ion concentrations several times higher than those in the final precipitation sample collected several minutes earlier. It seems reasonable to assume, therefore, that cloudwater ion concentrations during the preceding period of precipitation also substantially ex786
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Figure 7. Correlations between precipitation concentrations of SO,*-, NH,' and NO3- and the extent of snow crystal riming (for dendrites) for samples collected at Rigi Staffel on January 18, 1990.
ceeded the ion loadings measured in the snow crystals, reinforcing our hypothesis that a fluctuation in the extent of riming was the primary factor controlling precipitation concentrations at the site. It is natural to wonder what factors led to the fluctuations in the extent of riming. It is known that riming will not occur unless both the ice crystals and the cloud drops exceed a minimum size. Most of the snow crystals observed during this event were dendrites, columns, and needles. The minimum crystal diameters required for riming to occur are 300 and 50 pm for dendrites and columns, respectively (25). Dendrites observed on January 18 typically were considerably larger than these minimum sizes for all periods. Some of the columns had diameters of approximately 50 pm, although it is not clear that unrimed columns had smaller diameters than did rimed columns. These observations suggest that crystal size probably was not the critical factor affecting the degree of riming. Previous work has found that dendritic crystals typically collect cloud droplets with a diameter of approximately 20 pm, while columns tend to capture drops with diameters around 40 pm (25). Attached cloud drop diameters on dendrites and columns observed in the present study averaged approximately 30 and 45 pm, respectively (24). Borys et al. (12) observed a strong dependence of crystal riming on the presence of cloud droplets larger than 10 pm in diameter. It is worth exploring whether changes in the cloud droplet size spectrum may have been responsible for fluctuations in the extent of riming in the present case. Cloud droplet size spectra and values of cloud liquid water content are available for the periods when the site was immersed in clouds. As indicated above, however, the cloud base was almost always some distance above the site. The representativess of the data collected during the brief periods when cloud base reached down to Rigi Staffel, for conditions within the cloud as a whole, is questionable. Nevertheless, it is interesting to note that the droplet size distribution observed a t the site during the period of heaviest riming contained many more droplets with di-
ameter larger than 20 pm than were observed during most of the other brief interception periods. Without more information, however, it is impossible to conclude with certainty that the increase in crystal riming was due to a shift in the cloud drop spectrum to larger sizes. Aside from the issue of what factor or factors led to the observed fluctuations in the extent of riming, it seems likely that the riming process was largely responsible for increases in precipitation ion concentrations. Simultaneous observations of precipitation concentrations made a t the lower two research stations revealed similar, but somewhat smaller, increases in major ion concentrations during the period of heavy riming documented a t Rigi Staffel. The fact that the increases in ion concentrations were smaller a t the lower two stations suggests that spatial inhomogeneities in the extent of riming may have been present. Orographic effects associated with flow over Mt. Rigi, for example, could result in a shift in the cloud droplet size spectrum to larger sizes, possibly resulting in heavier riming of precipitation falling a t the top station, close to the summit, than might be observed a t the other two stations. Since riming can have an important influence on precipitation chemistry in mixed-phase clouds, it is important to evaluate its possible influence when attempting to examine the role of other processes affecting precipitation chemistry, such as aerosol scavenging or aqueous-phase oxidation. Compared to the effect of an air mass change, however, the effect of riming can be extremely difficult to detect. In cases where precipitation falls as snow, the task is laborious. Once the snow crystals drop through the melting zone and form raindrops, the task may be impossible without additional information from observations made a t higher elevations. Conclusions
Both an air mass change, associated with passage of a front, and a change in cloud microphysical conditions, reflected by a change in the degree of riming of snow crystals, have been observed to play an important role in determining the chemistry of winter precipitation. Gradients in pollutant concentrations across frontal boundaries can lead to large and rapid changes in precipitation chemistry a t a fixed location. Increases in the extent of snow crystal riming are reflected in increased precipitation ion concentrations, due to the attachment of cloudwater droplets, which tend to contain much higher ion concentrations than do snow crystals grown through the vapor deposition process. The magnitude of the effects these two processes can have on precipitaton chemistry is large, warranting a careful assessment of their role in any study attempting to interpret processes contributing to the chemical composition of wet deposition. Use of a meteorological parameter like the pseudoequivalent potential temperature as an air mass identifier can be a simple yet effective technique for screening those periods when a significant change in air mass has occurred. In some cases, where the effects of an air mass change are small, it may be possible to identify and separate these effects, permitting the influence of other processes to be evaluated independently. During the passage of strong frontal systems, however, frontal dynamics are likely to dominate the air mass chemistry to the extent that such a separation is not practical. Differences between rimed and unrimed snow crystals can be detected through field observations of the crystals and laboratory observations of snow crystals replicas under a microscope. Evaluating the extent of riming is, however, a tedious task producing results that are a t best semiquantitative. Nevertheless, because this process can play
such an important role in defining precipitation chemistry in mixed-phase clouds, considerable effort is warranted to estimate its influence in any given situation. Ideally, observations of the snow crystals and their chemistry should be accompanied by measurements of cloud drop size spectra and cloudwater chemistry. Since only relatively large cloud drops are captured by the snow crystals, it also would be useful to know how the cloudwater composition varies with drop size (26-28). Determining how precipitation chemistry is affected by an individual process such as transport, riming, aerosol and gas scavenging, or aqueous-phase chemical reaction can be difficult in a field environment. These processes can act simultaneously, and on similar time scales, producing a compound effect that may be impossible to interpret without a comprehensive set of field measurements that enable the simultaneous evaluation of all of the processes. Here we have dealt with methods to screen individual cases for the effects of an air mass change or a change in the degree of snow crystal riming. In a future paper we hope to expand this approach in order to classify a series of case studies conducted over the past several years according to the dominant process or processes responsible for determining the precipitation chemistry. Acknowledgments
We acknowledge our colleagues R. Heimgartner, A. Hering, D. Hogl, R. Luthi, L. Mosimann, B. Oberholzer, W. Schmid, T. Schumann, M. Steiner, and N. Syed for their advice and assistance with technical matters and for carrying out field measurements under adverse conditions. Literature Cited Hegg, D. A,; Hobbs, P. V. Atmos. Enuiron. 1986, 20, 901-909. ten Brink, H. M.; Schwartz, S. E.; Daum, P. H. Atmos. Enuiron. 1987, 21, 2035-2052. Leaitch, W. R.; Strapp, J. W.; Isaac, G. A. Tellus 1986,38B, 328-344. Schumann, T.; Zinder, B.; Waldvogel, A. Atmos. Enuiron. 1988,22, 1443-1459. Sheih, C. M.; Johnson, C. A,; DePaul, F. T. Atmos. Environ. 1983,17, 1299-1306. Knutson, E. 0.;Sood, S. K.; Stockham, J. D. Atmos. Enuiron. 1976, 10, 395-410. Zinder, B.; Schumann, T.; Waldvogel, A. Atmos. Enuiron. 1988, 22, 2741-2750. Levine, S. Z.; Schwartz, S. E. Atmos. Enuiron. 1982, 16, 1725-1734. Hegg, D. A.; Hobbs, P. V. Atmos. Enuiron. 1982, 16, 2663-2668. Chandler, A. S.; Choularton, T. W.; Dollard, G. J.;Eggleton,
A. E. J.; Gay, M. J.; Hill, T. A.; Jones, B. M. R.; Tyler, B. J.; Bandy, B. J.; Penkett, S. A. Nature 1988,336,562-565. Parungo, F.; Nagamoto, C.; Madel, R. J. Atmos. Sci. 1987, 44, 3162-3174.
Borys, R. D.; Hindman, E. E.; DeMott, P. J. J . Atmos. Chem. 1988, 7, 213-239.
Valdez, M. P.; Dawson, G. A.; Bales, R. C. J . Geophys. Res. 1989,940,1095-1103.
Jensen, J. B.; Charlson, R. J. Tellus 1984, 36B, 367-375. Scott, B. C. J . Appl. Meteor. 1981, 20, 619-625. Mitchell, D. L.; Lamb, D. J . Geophys. Res. 1989, 9 4 0 , 14831-14840.
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Received for review J u l y 23, 1990. Revised manuscript received November 27, 1990. Accepted December 3, 1990. T h i s project is a contribution to t h e E U R O T R A C subproject A L P T R A C . Partial funding was provided through a n interdisciplinary program of the Swiss Federal Institute of Technology known as W a B o L u (“ Wasser, Boden, Luft”, meaning Water, Soil, Air).
Exposure to Carbon Monoxide, Respirable Suspended Particulates, and Volatile Organic Compounds While Commuting by Bicycle Michael A. J. Bevan,” Christopher J. Proctor, Joanna Baker-Rogers, and Nigel D. Warren British-American Tobacco Company Ltd., Fundamental Research Centre, Southampton, SO9
A portable air sampling system has been used to assess exposures to various substances while commuting by bicycle in an urban area. The major source of pollutants in this situation is motor vehicle exhaust emissions. Carbon monoxide, measured by electrochemical detection, was found a t peak concentrations in excess of 62 ppm, with mean values over 16 individual 35-min journeys being 10.5 ppm. Respirable suspended particulates, averaged over each journey period, were found a t higher concentrations (mean 130 pg m-3) than would be expected in indoor situations. Mean exposure to benzene (at 56 yg m-3) and other aromatic volatile organic compounds was also relatively high. The influence of wind conditions on exposure was found to be significant. Commuting exposures to carbon monoxide, respirable suspended particulates, and aromatic VOCs were found to be higher than exposures in a busy high street and on common parkland.
W
Introduction It has long been recognized that motor vehicle exhaust emissions are a major source of pollution in urban areas ( I ) . Such emissions may impact both on gross climatic effects, such as the formation of photochemical smogs (2), and on personal exposures to a range of substances ( 3 ) . Data from the EPA’s Total Exposure Assessment Methodology (TEAM) studies ( 4 ) suggests that an individual’s exposure to airborne substances is influenced far more by personal activities than by general levels of air pollution. Wallace et al. ( 3 , 5 )recently described 25 common activities thought to increase personal exposure to volatile organic compounds (VOCs). Clearly, with motor vehicles being a major source of airborne pollution, any personal activities involving close contact with vehicular exhausts may be highly influential on daily exposures. Chan et al. (6) measured car drivers’ personal exposures to VOCs while commuting either on urban roadways, on interstate highway, or in rural areas. These researchers found that drivers were exposed to significantly higher concentrations of VOCs than were found at roadside or on pedestrians. Hence proximity to the exhaust is likely to be important. Cortese and Spengler (7) compared fixed monitoring station data with personal carbon monoxide exposure. They found fixed-site monitoring underestimated personal exposures and that commuting by car resulted in mean personal carbon monoxide exposures that were twice that 788
Environ. Sci. Technol., Vol. 25, No. 4, 1991
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for commuting by rail mass transit. Persons who use bicycles as a mode of transport for getting to and from work have a high potential for exposure to vehicular emissions (8). On urban roadways in the United Kingdom, traffic is often slow moving or stationary a t peak periods and the cycling commuter will often be weaving around cars or stopping close to vehicles’s exhaust pipes. This study provides information on the extent of exposure on urban roadways in the city of Southampton, southern England.
Sample Collection and Analytical Procedures In order to collect the specific components of interest, a portable air sampling system (PASS)was used to house the various sampling devices (9). The PASS briefcase was seated on a pad of foam rubber to cushion the effects from cycling over uneven road surfaces and was strapped to a bicycle rack situated a t the rear of the cycle. The distances from the ground to an “average” exhaust pipe (saloon car), the “briefcase” sampling orifices, and a cyclist’s breathing zone were approximately 0.3, 1.0, and 1.5 m, respectively. Two routes were chosen, both of approximately 6 miles, between the districts of Eastleigh and Shirley, Southhampton. Cycling time over this distance was usually 35 min, though dependent on the weather and traffic conditions, a t a speed not normally exceeding 1 2 mph. Route 1 was chosen to represent a typical “urban” environment and route 2 as a “suburban” area with more open ground. Sampling times were either between 0800 and 0900 h or 1630 and 1730 h daily. A total of 16 runs were completed, 8 on each route evenly distributed by morning and evening. Additionally, two measurements were made cycling back and forth down a busy road through a shopping area and a further measurement was taken cycling around common parkland in the city. Finally, one measurement was made positioning the cycle and sampling apparatus 2 m from a stationary car with engine idling. The three main sampling ports on the PASS briefcase, namely, CO, respirable suspended particulates (RSPs), and the VOC inlets, were all located on different sides of the case. It was therefore necessary to change its orientation when the case was placed on the cycle rack to enable RSP samples to be collected from either the “nearside” or “offside” positions, i.e., when the RSP sample inlet was
0013-936X/91/0925-0788$02.50/0
0 1991 American Chemical Society
