Environ. Sci. Technol. 1991, 2 5 , 2012-2023
Kaiser, E. W.; Andino, J. M.; Siegl, W. 0.; Hammerle, R. H.; Butler, J. W. J. Air Waste Manage. Assoc. 1991, 41, 195. LoRusso, J. A,; Kaiser, E. W.; Lavoie, G. A. Combust. Sci. Technol. 1983, 33, 75. Adamczyk, A. A.; Kach, R. A. Combust. Sci. Technol. 1986, 47, 193. Namazian, M.; Heywood, J. B. SAE Tech. Pap. Ser. 1982, No. 820088.
(14) Atkinson, R. Chem. Rev. 1986, 86, 69. (15) Dryer, F. L.; Glassman, I. h o g . Astronaut. Aeronaut. 1979, 62, 255. (16) Lavoie, G. A,; Adamczyk, A. A.; Kaiser, E. W.; Cooper, J. W.; Rothschild,W. G. Combust. Sci. Technol. 1986,49,99. (17) Atkinson, R. Int. J . Chem. Kinet. 1986, 18, 555.
Received for review March 12,1991. Revised manuscript received June 13, 1991. Accepted June 24, 1991.
Dynamic Partitioning of Semivolatile Organics in Gas/Particle/Rain Phases during Rain Scavenging Wangteng Tsalt and Yoram Cohen" Department of Chemical Engineering, University of California, Los Angeles, California 90024
Hlroshl Sakugawa and Isaac R. Kaplan Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90024
A simple model for studying the below-cloud rain scavenging of semivolatile organics (RSSVO) is presented. The dynamic partitioning of semivolatile organics in gas/particle/rain phases during a rain event is discussed in relation to field data for rain scavenging of semivolatile organics. The RSSVO model considers the gas and particle scavenging, dry deposition, source emissions, atmospheric degradation of organics, and meteorological conditions such as height of cloud base, rain rate, wind direction, and temperature. Case studies for pyrene and fluoranthene rain scavenging are illustrated for the 11/9-10/82 and 3/11/82 Los Angeles rain events. It is shown, based on the above case studies, that (1)wet scavenging of semivolatile organics is affected by the variation of particle size distribution during rain events and (2) the chemical gaseous and particle-bound atmospheric concentrations vary significantly during rain events, and thus, the use of a time-invariant scavenging coefficient in multimedia mass balance calculations should be reexamined. ~~
~
~
~~
Introduction Precipitation scavenging is an important intermedia transport process responsible for the removal of particlebound air pollutants from the atmospheric via in-cloud and below-cloud scavenging. Numerous field studies and modeling studies of precipitation scavenging have been conducted since the early 1970s. Research activities on rain scavenging in the 1970s included studies on the collection efficiency of particles by raindrops with the goal of gaining basic understanding of scavenging mechanisms (1-8). In the 19809, a greater emphasis was placed on integrated modeling and field studies in order to understand acid deposition. For example, numerous comprehensive models that include in-cloud and below-cloud scavenging mechanisms and cloud microphysics and chemistry have been developed and applied to identify and to quantify the major scavenging mechanisms responsible for the scavenging of acidic compounds (9-19). The interested reader is referred to excellent reviews of the complexity of precipitation scavenging published by Hales (10, 20) and to a comprehensive proceedings on the subject by Pruppacher et al. (21). t Present address: Systech Engineering, Inc., 3744 Mt. Diablo Blvd., Ste 101, Lafayette, CA 94549.
2012
Environ. Sci. Technol., Vol. 25, No. 12, 1991
In addition to the important link between precipitation scavenging and acidic deposition, rain scavenging has also been identified as an important pathway for chemical exchange between the atmosphere and natural water surfaces. For example, semivolatile organics such as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) are removed from the atmosphere and enter water surfaces by dry deposition and by precipitation scavenging of gases and particles (22). The relative importance of gas and particle wet scavenging processes depends on the particle size distribution, the partitioning of organic compounds in the gas and particle phases, and the Henry's law constant (23). Given the complexity of rain scavenging, it has been common practice in most mass balance studies to apply empirical approaches that make use of an overall rain scavenging ratio (Woverall)for the calculation of wet deposition fluxes (22-30), as defined below: (1) Woverall = Wg(1 - 4) + Wp4 in which
w, = Cw(d)/C,(g) wp = C,(P)/C,(P)
(2) (3)
where Wg and W,, are the gas and particle scavenging ratios, respectively, and 4 is the fraction of organic compound adsorbed on the atmospheric particle phase. Cw(d) is the dissolved pollutant concentration in rainwater at ground level, C,(P) is the pollutant concentration, in the particle-bound form, in rainwater, and C,(g) and C,(P) are the atmospheric concentrations of the chemical in the gaseous and particle phases, respectively. It is important to note that often W, and Wp have been taken as timeinvariant parameters independent of the particle size distribution and the rate of rainfall. In this paper, we explore the process of below-cloud rain scavenging for semivolatile organics through the use of a simple rain scavenging model that accounts for the dynamic partitioning of semivolatile organics in the gas/ particle/rain phases, as well as the dynamic variation of particle size distribution during the rain event. Finally, a discussion is presented regarding the uncertainty in estimating the scavenging ratio in relation to the interpretation of field data for rain scavenging of semivolatile organics.
0013-936X/91/0925-2012$02.50/0
0 1991 American Chemical Society
Rain Scavenging of Semivolatile Organics (RSSVO): Model Description The formulation of the present model for the rain scavenging of semivolatile organics (RSSVO) is based on a chemical mass balance that includes the gas, particle, and rain phases in the below-cloud region. We approximate the complex below-cloud rain scavenging process by taking the atmospheric air phase (below-cloud) to be uniform and where convective winds are neglected. Another major assumption, consistent with other studies (31, 32), is that the particle-bound pollutant is adsorbed onto the particle surface with an equal tendency to adsorb on various particle surfaces (e.g., coarse or fine particles). Clearly, these are oversimplifications of the physical phenomena; nonetheless, the current approach is sufficient for the purpose of demonstrating the macrodynamics of below-cloud rain scavenging processes. Accordingly, the overall chemical mass balance on the below-cloud air phase (including the gas and particle phases) can be expressed by
eq 4) one must first realize that the particle rain scavenging ratio A, is a function of the particle size distribution. The particle size distribution, however, changes during the course of a rain event since the rain scavenging removal efficiency is a function of particle size. Also, the fraction of adsorbed chemical (eq 6) is a function of the particle surface area. As the particle size distribution changes, during rain, the available particle surface area for adsorption will change and thus $ will also vary during the course of a rain event. Therefore, the change in the adsorbed fraction $ is also reflected in a variation in the vapor-phase concentration of the chemical during rain. In the following section, the various model parameters in eq 4 and the solution algorithm are described. 1. Gas Scavenging Ratio. The normalized gas scavenging ratio (Ag*) for chemicals that are nonreactive in the aqueous phase can be derived theoretically from a chemical mass balance on a raindrop as it falls from cloud base to ground level
d(VaCJ (1 - q ) -dt -Ca(g)HwaJrainAAg* - Ca(P)JrainAA,- kVaC, + Sa (4)
d(C,(d1(4/3)T&3) dr
m
KoL(C~(~)H,, - Cw‘d’)TRd2+ CKwj(C;(p)Hw - C,(d))S,; j=l
where
c, = Ca(g)+ Ca(P) C,(P)
= Ca$
(10)
(5) (6)
Ca(g)= C,(l - 4)
(7)
Ag* = Wg/Hwa
(8)
A, = C,f(P)/C,(P)
(9)
-
in which Ca(g)and Ca(P)are the concentrations of the gaseous and particle-bound chemical in the atmospheric phase (ng/m3 of air), respectively, and C, is the total chemical concentration in the atmosphere (ng/m3 of air). is the chemical concentration, in the particle-bound form, in rainwater (averaged over raindrop size) at ground level (ng/m3 of water). The fraction of the organic compound adsorbed onto the atmospheric particle phase is denoted by $, and q is the volume fraction of the atmosphere occupied by raindrops, which is taken to be time invariant during a rain event of a given intensity. The volume of the atmosphere is denoted by V, (m3),H,, is the dimensionless water to gas partition coefficient (i.e., it is also equivalent to the gas scavenging ratio at the equilibrium is the precipitation condition) for the given chemical, Jrain rate (m/s), and A is the interfacial area (m2)between the atmosphere and the land (or water) surface below, in the region under consideration. It is noted that the left-hand side of eq 4 represents the rate of accumulation of the chemical in the air phase. The first term on the right-hand side of eq 4 accounts for the rate of chemical mass scavenged by raindrops via gas absorption, where Ag* is the normalized gas scavenging ratio (eq €9, which varies between 0 and l. The second term represents the rate of removal of the particle-bound chemical through particle scavenging by raindrops where A, is the particle scavenging ratio (eq 9). The third term represents the degradation by chemical reaction, which for simplicity we approximate by a first-order reaction where the reaction rate constant is given by k (9-l). Finally, the last term, Sa,is the net input of the chemical into the atmosphere from source emissions (ng/s). In order to determine the variation in the total atmospheric concentration of the chemical, C,, during rain (using
Cwfo
in which Rd is the radius of raindrop (m), Cw(d)is the chemical concentration (ng/m3), in the dissolved form, within a given size raindrop, and C;(P) is the chemical concentration (ng/m3 of particle) in a particle in size interval j (with a total of m size intervals) within a given size raindrop. The total particle surface area (m2)contributed by the particles with diameter of a within a given size raindrop is denoted by S,. r is the time of travel (belowcloud) for a raindrop (s), where r = 0 designates the starting time at the cloud base. The water to particle equilibrium partition coefficient is denoted by Hw and KoL and K are the overall liquid-phase mass-transfer coefficient”tm/s) for the mass transfer of chemicals from air to water (or water to air) and from the solid phase of the scavenged particles to water, respectively. It is acknowledged that, although the drop size may change during its journey from the cloud base to the ground, in the current simple model we neglect such variations in the drop size. Such effects can in principle be incorporated into the model calculations (18). The left-hand side of eq 10 represents the rate of accumulation of chemical mass in the dissolved phase of a raindrop. The first term on the right-hand side of eq 10 accounts for the rain scavenging of pollutant via gas absorption. The second term in eq 10 represents the possible rate of increase in the dissolved chemical mass due to dissolution of the particle-bound chemical (scavenged by rain). The recent studies on the kinetics of hydrophobic organic compound adsorption to natural sediments and soils (32)and studies on regeneration kinetics of adsorbents (33) suggest that, for “semivolatile” chemicals with low solubility, the rate of dissolution from the particle phase is slow. In fact, an order of magnitude analysis suggests that, during the short residence time of the falling raindrops in the atmosphere, the rate of dissolution will be smaller than the rate of gas absorption by 2 orders of magnitude. Thus, the dissolution from the particle phase is neglected in the current work. The above simplification is consistent with other recent studies on rain scavenging of particle-bound organics (34). The effect of the above approximation is that the dissolved concentration in the raindrop will be somewhat underestimated and thus the driving force for gas absorption (and hence the rate of gas Environ. Sci. Technol., Vol. 25, No. 12, 1991 2013
absorption) will be accordingly overestimated. Given the above simplifications, the average concentrationof the dissolved chemical (in rainwater) at ground level, CWcd),is obtained by integrating eq 10 (for a single raindrop of size R d )between the limits r = 0 to T = L,/ut [ L , is the height of cloud base (m) and ut is the terminal velocity of a given size raindrop (m/s)] and subsequently averaging the chemical concentration over the spectrum of raindrop sizes (35, 36). Accordingly
Ut
= 81.1Rd Rd 5 5 x m =130(2Rd)1/2 Rd > 5 x m
The raindrop size distribution can be conveniently approximated by the Marshall-Palmer distribution (43): N R =~ N o eXp(-CRd)
Note that V I is related to q , the volume fraction of the atmosphere occupied by rain drops [i.e., q = V , / ( l + VJ]. In order to calculate A,*, it is necessary to specify KOL, ut, and NRd. The overall mass-transfer coefficient (KOL) can be estimated by use of the two-film resistance theory (37),i.e. 1 - + -1 -= KOL kl
Hwa
(13)
k,
where kl and lz, are the liquid-side and gas-side masstransfer coefficients, respectively. The gas-side masstransfer cbefficient (k,) can be calculated from (38)
k, = (D,/2Rd)(2+ 0 . 6 S ~ ’ / ~ R e l / ~ )
(14)
in which D, is the chemical diffusivity in the gas phase estimated by the Chapman-Enskog method (39). Sc is the gas-phase Schmidt number [Sc = vair/Dg,in which vair is the air kinematic viscosity (m2/s)]and Re is the raindrop’s Reynolds number (Re = Rdut/vair).. The liquid-side mass-transfer coefficient ( k l )was estimated from a parameterization of the numerical model of Walcek et al. (40-42) for mass transfer of trace gases into raindrops. Accordingly klRd/DI = 14.5 (15) in which D1/Do = 1 Rd < 5 X m = 21.34(Rd/Rdo)- 20.34 = 18.07
Rd
>9x
5x
m
< Rd
1 pm) and attachment to host particles (for a < 1 pm). The dependence of collection efficiency on the particle size and raindrop size is also treated in the Slinn model (19, 20, 55). Specifically, in Figure 1, the collection efficiency labeled t = 0 was obtained for dry particles and 0.5 mm radius raindrops. A higher collection efficiency is expected when the effect of water vapor is considered. For example, the collection efficiency labeled t = 2000 s, at a < 1 pm, was obtained following the approximate relation proposed by Slinn (55), E = [l - exp(-at) + E(0) exp(-at)], where a is the attachment rate for Brownian coagulation with droplets, t is the time scale for particle growth processes, and E(0) is the collection efficiency for dry aerosols. The collection efficiency labeled t = 2000 s, at a > 1 pm, was obtained following Slinn (55)by assuming that during rainfall the particles grew by water vapor condensation to the size (and correspondingly increased collection efficiency) given by a ( t ) = [a: t / 1 0 0 ] 1 / 2in , which a, is the diameter of dry aerosols. From Figure 1,it is seen that the effective collection efficiency of Slinn increases by about 3-4 order of magnitudes (for a < 4 pm) as the time scale for particle growth processes increases from 0 to 2000 s. This implies that the particle growth processes may significantly influence the collection efficiency. The comparison of the performance of the models of Slinn (55) and Ryan and Cohen (36) on the prediction of pollutant rain concentration is discussed under Case Studies. Given the gas and particle scavenging ratios, the instantaneous total pollutant concentration in rainwater (i.e., dissolved plus particle bound), C, (ng/L), at ground level, is given by
+
C,
= C,(g)Hw,A,*
+ Cip)A,
(29)
When model results are compared with field data it is important to note that pollutant rain concentrations re2016
Environ. Sci. Technol., Vol. 25, No. 12, 1991
where torepresents the beginning of the sampling period (i.e., Vs = 0), C, is the instantaneous rainwater concentration obtained from eq 29, and the variables on the right-hand side of eq 29 are obtained from the solution of the RSSVO model (eqs 4-29), consistent with the measured rain concentration It is convenient to quantify the degree of rain scavenging by using the following overall rain scavenging ratios w = c,/c, (32)
G.
and
- --
w = c,/c,
(33)
w
where W and are the instantaneous and average overall scavenging ratios, respectively, and is the average airphase concentration of the chemical (during each sampling period of rain event) defined by (34) Equations 4-34, subject to the appropriate initial conditions, constitute the governing equations of the RSSVO model. In the present work the RSSVO model equations were solved simultaneously by the predictor and corrector method (9). The algorithm of the RSSVO model is shown schematically in Figure 2. The basic model results include the instantaneous concentrations of the dissolved and particle-bound chemical in rainwater (at ground level), the instantaneous gaseous and particle-bound concentrations in the atmospheric phase, and the variations in particle size distribution during rainfall. By use of these results, the various averaged concentrations can be determined as described above. Case Studies
The RSSVO model was applied in a number of test studies to rain scavenging OS pyrene and fluoranthene in Los Angeles. Sequential sampling data of rain rates and PAH concentrations in rainwater were previously reported by Kawamura and Kaplan (56) for rain events on 11/ 9-10/82 and 3/11/82 monitored in Los Angeles. In particular, the most complete field data sets for pyrene and fluoranthene in those two events were selected for this case study. The various model parameters for the two case studies are described below. Model Input Parameters. The meteorological data required for the rain scavenging model include rain rate, height of cloud base, wind direction, and temperature.
Table I. Meterological Data Applied in the Simulation of the 3/11/82 and 11/9-10/82 (Los Angeles) Rain Events
samples
duration
precip rate: cloud temp,* mm/h base! m OC
wind directnb
A B C D E
11:OO-13:35 13:35-15:05 15:05-16:25 16:25-19:OO 19:00-20:00
3/11/82 1.8 3000 2.4 3000 3000 1.5 670 1.4 2.2 850
16.7 15.0 15.0 15.6 15.0
SW-W W-N NE E-SE SE
A break B C break D E
1:30-4:30 4:3O-9:30 9:3O-12:45 12:45-16:OO 16:OO-19:45 19:45-23:25 23:25-9:40
11/9-10/82 1.3 1830 2134 2.0 400 1.1 850 1520 0.75 1220 1.4 1160
13.3 11.1 13.3 14.4 11.7 11.1 10.6
SE-NE NE-E E-SE-SW SW-W W-NW N-NE SW-NE-E
"From Kawamura and Kaplan (56). bFrom the observations (on a 3-h interval) at Los Angeles International Airport, 10 km south to UCLA (59).
These meteorological data shown in Table I were obtained from the monitoring data of Kawamura and Kaplan (56) and observations at Los Angeles International Airport, 10 km south to UCLA (59). The compartmental data, emission rates of particles and PAHs, and physicochemical properties of PAHs are summarized in the Table 11. Specifically, the surface area of Los Angeles County was determined to be approximately 1.04 X lolom2 (60). The emission rate of particles from Los Angeles County was estimated to be 415 tons/day based on the emission inventory for the South Coast Air Basin of California (61). The mass concentration of total suspended particles in Los Angeles was estimated to be 80 pg/m3 based on the monitoring study of Gray et al. (62). The emission rates of PAHs (Sa, ng/h) in Los Angeles were estimated from mobile source emission (EpAH, pg/vehicle-km) and information on vehicle miles traveled in Los Angeles (VMT, miles/day). Accordingly, the emission rate for PAH was calculated from Sa = (VMT)(EPAH) (35)
The daily average vehicle miles traveled for the Los Angeles County was reported to be 1.54 X lo8 miles/day by the California Department of Transportation (63), which is the total VMT by various types of vehicles such as trucks and passenger cars. Although emission data for PAHs in Los Angeles are lacking, estimates can be made from the mobile source emission data recently reported by Benner et al. (64) for the Baltimore Harbor Tunnel and by Handa et al. (65) for two roadway tunnels in Japan. The average emission rates reported by Handa et al. (65) for p3;rene (i.e., Nihonzaka Tunnel-15 and 32 pg/vehicle-km estimated for diesel-fueled and gasoline-fueled vehicles, respectively; Tsuburano Tunnel-47 and 91 pg/vehicle-km estimated for diesel-fueled and gasoline-fueledvehicles, respectively) are significantly higher than the pyrene emission values reported by Benner et al. (64) (Le,, 8 f 3 pg of pyrene/ vehicle-km). Therefore, the emission rates reported by Benner et al. and Handa et al. can be considered as the lower and upper limits, respectively. In the current work, average pyrene emission of 30 rglvehicle-km, which is within the reported limits, was found to be a reasonable choice. It is important to note that, while Handa et al. (65) did not provide fluoranthene emission data, Benner et al. (64) reported fluoranthene emission to be nearly identical with pyrene emission. Thus, an average emission value of 30 pglvehicle-km was also taken to be a reasonable approximation for the emission of fluoranthene. Subsequently, by use of the daily average vehicle miles traveled for Los Angeles County and eq 35, the daily average emission rates of pyrene and fluoranthene were estimated to be 3.1 X 10" ng/h (Table 11). Since the actual diurnal variations in emission rates were not available, the potential effect of such emission variations, between morning and afternoon rush hours, were studied by estimating the likely range of these variations based on the diurnal change of traffic volume (66). Accordingly, the emission rates of particles and PAHs during the morning and afternoon rush hours were assumed to be a factor of 2.25 and 3.0, respectively, higher than the daily average emission rates. An additional simulation was carried out where the emission rates for the morning and afternoon rush hour periods were taken to be 50% lower than the above base values. Moreover, the emissions of
Table 11. Compartmental Data for the Los Angeles County and Physicochemical Properties of Pyrene and Fluoranthene
(A) Compartmental Data parameter
value
land and water surface areas, m2 daily av emissn rate of particle matter, tons/day daily av emissn rates of pyrene and fluoranthene, ng/h initial ambient air concn, ng/m3 pyrene fluoranthene Initial mass concn of total suspended particles, rg/m3
1.04 X 10'O 415" 3.1 X IO""
California Almanac (60) SCAQMD (61) eq 35 of this work
ref
0.7' (0.7)d 1.9 (1.2)
Gordon and Bryan ( 6 3 , Grosjean (68)*
80
Gray et al. (62)
(B) Physicochemical Properties parameter
PPene
fluoranthene
ref
mol wt degradatn rate const in atmosphere, h-' saturation vapor pressure, mmHg Henry's law const, Pa m3 mol-' molal vol, cm3/mol bp, " C
202 0.29 2.5 x 104 0.125 213.8 388
202 0.19 5.0 X lo6 0.134 217.3 377
EPA (69) Dragoescu and Friedlander (46) EPA (69) Baker and Eisenreich (22) Reid et al. (39) Pearlman et al. (70)
OThe diurnal variations of the emission rates of both particles and PAHs were taken into account in the model based on the diurnal change of traffic volume reported by the 1976 SCAG Urban/Rural Travel Survey (66). *The 1-year average (1971-1972) ambient air concentrations of pyrene and fluoranthene in Los Angeles reported by Gordon and Bryan (67)are 1.9 and 2.0 ng/m3, respectively. The mean concentrations of pyrene and fluoranthene (for summer and winter 1981) in Los Angeles reported by Grosjean (68)are 1.62 (range 0.48-3.64) and 0.94 (range 0.24-1.98) ng/m3, respectively. '3/11/82 rain event. 11/9-10/82 rain event. Environ. Sci. Technol., Vol. 25, No. 12, 1991 2017
1
I
1
Physicochemical Properties
Meteorological Data
of Selected Chemical
+ Initial Conditions
Total pollutant air concentration Initial particle size distribution Emission rates .....................................................................................................................................................................................................................................................
Determine Vapor-partide partitioning (0) Partidescavenging -+ ratio (4) Gas scavenging ratio
v
Mass balance on pollutant air concentration Mass balance on pollutant rain concenbation Balance on particle number concentration
@g*) I
I
I
1
Model Results at timet Total chemical concentration in the atmosphere (Ca) Concentratlon of the particlebound chemical in the atmosphericphase (Ca(P) ) Concentration of the gaseous chemical in the atmospheric phase (C,(g)) Instantaneous total pollutant rain concentration (C w) Particle size distribution (Nj )
t=t find ?
Figure 2.
Schematic description of the RSSVO model.
particles and PAHs during the period of 11 p . m . 4 a.m. were assumed to be negligible. The initial ambient air concentrations of PAHs for the simulations of rain events on 3/11/82 and 11/9-10/82 in Los Angeles were estimated from reported ambient air concentrations of PAHs in Los Agenels (67, 68). Monitoring data were not available to establish the concentration of PAHs in the dissolved and particulate phases at the cloud base (Cwoj(P) and Cwid)).Thus, in the simulations described below, both Cwoj(p) and Cwo(d) were taken to be zero. This assumption results in a lower limit estimate of the concentration of scavenged particles in rainwater at ground level. Finally, the pertinent physicochemical properties (e.p., degradation rate constant in atmosphere, solute sature ion vapor pressure, Henry's law constant, molal volume, and boiling temperature) for pyrene and fluoranthene are given in Table 11. Simulation Results. Simulations of rain scavenging of pyrene and fluoranthene during the 3/11/82 and 11/ 9-10182 rain events in Los Angeles were performed, given the above compartmental data, meteorological conditions, 2018
t-t+At
Environ. Sci. Technol., Vol. 25, No.
12. 1991
emission rates, and the physicochemical properties of the selected pollutants. Figures 3 and 4 illustrate the comparison of actual and predicted pyrene and fluoranthene rain concentrations for each of the sequential samples taken during the 3/11/82 and 11/9-10/82 rain events. From Figures 3 and 4, it is seen that the predicted pyrene and fluoranthene rain concentrations, based on the collection efficiencies of Slinn (55) with t = 2000 s and of Ryan and Cohen (36), are in good agreement with the available field data. Although the time scale in the Slinn model could be optimized as an empirical parameter to provide better fit with the data, this was not attempted. Within the scope of this work it suffices to conclude that the Slinn model for the collection efficiency appears to be adequate in describing the data. It is noted that both the field data and model prediction illustrate that pyrene and fluoranthene rain concentrations for the fourth and fifth samples of the 3/11/82 rain event increased drastically relative to the first three samples.
(q)
-
Actual data [Kawamura and Kaplan (56)] Prediction: with collection efficiency of Ryan and Cohen (36)
I___ Prediction: with collection efficiency of Slinn (55) - t=2000 s 80
70 60 50 40
x)
20 10
A
B
C
0 D
E
B
A
D
C
E
Rain Duration
Rain Duration
Flgure 3. Predicted and measured pyrene and fluoranthene sampler rain concentrations (-CJ at ground level for the 3/11/82 Los Angeles rain event. Time duration for each sequential rain sample: A, 11:OO-13:35; B, 13:35-1505; C, 1505-16:25; D, 16:25-19:OO; E, 19:OO-2O:OO.
Actual data [Kawamura and Kaplan (56)J Prediction: with collection efficiency of Ryan and Cohen (36) Prediction: with collection efficiency of Slinn (55)
- t=2000 S
60
11/9-10/82Rnin Event
11/9-10/82Rain Event
50 $ 4 v
0
C 0 *-
F
x)
Y
fi
2
0)
20
8 10
0 A
B
C
D
E
Rain Duration
A
8
C
D
E
Rain Duration
Flgure 4. Predicted and measured pyrene and fluoranthene sampler rain concentrations (C,) at ground level for the 11/9-10/82 Los Angeles rain event. Tlme duration for each sequential rain sample: A, 1:30-4:30; B, 9:30-12:45; C, 12:45-16:OO; D, 19:45-23:25; E, 23:25-9:40.
The above trend is consistent with increased emissions during later afternoon rush hour traffic, the decrease in cloud base height, and easterly wind direction (e.g., wind blowing from downtown Los Angeles toward the UCLA sampling site), which transported the highly polluted air from downtown Los Angeles to the sampling site at UCLA (see Table I). It is also noted that, for the 11/9-10/82 rain event, the highest pyrene and fluoranthene rain concentrations were obtained for the second sample, which is consistent with the morning rush hour traffic, the low cloud base, and easterly wind direction (Tables I and 11). Although the rush hour traffic in the late afternoon (e.g., during the break between the fourth and fifth sampling periods) could have allowed the atmospheric concentration of the pollutants to increase (Figure 6), the westerly wind
(Table I) that carried the clean air from the coastal areas to the sampling site, as well as the higher cloud base relative to that in the period of the second sample, led to a lower pollutant rate concentrations for the fourth sample relative to the second sample. Clearly, the wind direction and height of cloud base are significant in determining the levels of pollutants in the collected rainwater. Specifically, the influence of the 50% decrease of rush hour emissions of particles and PAHs on the predicted pyrene rain concentrations for the 3/11/82 and 11/9-10/82 events is shown in Figure 5 [using the collection efficiency of Ryan and Cohen (36)]. From Figure 5, it is seen that the decrease of rush hour emissions significantly influences the agreement between the predicted and measured pyrene rain concentrations (e.g., samples D and E of the 3/11/82 Environ. Sci. Technol., Vol. 25, No. 12, 1991 2019
-
Actual data [Kawamura and Kaplan ( 5 6 ) ] Prediction for the base test case where the morning and afternoon rush hour emission rates are a factor of 2.25 and 3.0, respectively, higher than the daily average emission rate. Prediction based on emission rates during rush hours, which are SO percent lower than the base test case. 60 Pyrene 11/9.10/82 Rain Event .
A
B
C
D
E
B
A
Flgure Influence of rush hour emissions on predicted ground-level pyrene rain concentrations (C,) rain events.
event and sample B of the 11/9-10/82 event). Although one could attribute part of the deviation of the predicted concentrations from the reported data to the neglect of the effect of convective winds in the simulations, it is difficult at present to evaluate the precise effect of convective winds since data on PAH emissions in the Los Angeles area are lacking. The availabilty of such data will undoubtedly improve the ability to better quantify the wet scavenging of PAHs. It is important to note that in most field studies the reported rain scavenging ratio (eq 1)is determined on the basis of a constant atmospheric concentration (CJ. Due to sampling difficulties, reported field measurements of the rain scavenging ratio are generally values averaged over different sampling periods. In reality, pollutant concentrations in both air and rainwater are time dependent during rain events. Thus, the rain scavenging ratio is expected to vary with time during a given rain event. In the present model, the temporal variations in the atmospheric and rainwater concentrations are determined and thus one can evaluate the corresponding change in the rain scavenging ratio (eqs 32 and 33) during rain events. As an illustration, the predicted instantaneous total pyrene concentrations in rainwater (C,) and in the air phase (C,) and the instantaneous concentrations of the particle-bound pyrene in the atmosphere (C,(p)) and rainwater (C$')) at ground level for the 11/9-10/82 event (using the collection efficiency of Ryan and Cohen) are given in Figures 6 and 7 , respectively. As Figure 7 illustrates, although less than 30% of the atmospheric pyrene exists in the particle-bound form (Le., C,(P)/C, I0.3), particle scavenging accounts for more than -50% of the resulting total pyrene rainwater concentration when C,(p)/C, 1 0.05. It is important to recognize that the concentration of pyrene in rainwater decreases drastically as the rainfall continues, especially during the initial periods of rainfall or at the initial period after the rainfall restarts following a break period (Figure 6). The pyrene air concentration during rainfall is sensitive to the variations of cloud base height and source emissions. 2020
Environ. Sci. Technol., Vol. 25, No. 12, 1991
E
D
C
Rain Duration
Rain Duration
for the 3111/82 and 11/9-10182
-3s
Angeles
h
0
gm 2.0 v
Time (hr) Figure 6. Predicted instantaneous pyrene concentration in rainwater (C,) and air phase (C,) for the 11/9-10/82 Los Angeles rain event. I "
\
O 0 b
4
Q
I2
16
20
24
i'"
8
12
Time ( h r )
Figure 7. Predicted ratio ofparticle-bound pyrene to total pyrene concentrations in rainwater (Cwf(p)/Cw) and the ratio of particle-bound pyrene to total pyrene concentrations in the atmospheric phase (C,'P)IC,) for the 11/9-10/82 Los Angeles rain event.
IE4
2
IE3
= - IE2
Y .
5
IEl IEO
:E-1 IE-2' IE-2
IE I
IEO
IE
a (pmJ Figure 9. Change in the distribution of particle number concentration [dNld (log a)] during the first sampling period Of the 11/9-10182 rain event (sample A in Figure 4).
A
8
C
D
E
Rain Duration Flgure 8. Comparison of predicted and measued pyrene sampler rain concentrations (C,) illustrating the effect of the variation of particle size distribution [based on the 1119-10182 rain event and using the collection efficiency of Ryan and Cohen (%)I. Time duration for ea& sequential rain sample: A. 1:30-430 6, 9:30-1245: C, 1245-1600; D. 19:45-2325: E, 2325-9:40.
For example, the decrease in cloud base from 2134 to 400 m a t the end of morning rush hour (Table I) led to an increase of pyrene air concentration in the initial period of the second sampling interval (i.e,, 930-11:OO a.m.) for the 11/9-10/82 rain event. It is worth noting that during the breaks in the rain event (Table I), which cover the morning and afternoon rush hours, the atmospheric pyrene concentration increases due to automobile emissions. Consequently, a high pyrene rain concentration results at the initial period after the rainfall restarts following a break period. The influence of the variation in the particle size distrihution during rainfall on the concentration of the scavenged pollutant in rain is demonstrated for pyrene in Figure 8. From Figure 8, it can he seen that the neglect of the variation of particle size distribution (i.e., when a constant particle size distribution is assumed) results in the overprediction of pyrene rain concentration for the 11/9-10/82 event. The use of a constant particle size distribution will lead to an overestimate of the amount of particles remaining in the atmosphere, during the rainfall, and thus a significant overprediction of the pollutant rain concentration will result. An illustration of the change in the particle size distribution [distribution of particle number concentration, i.e., dN/d (log a ) ] during rainfall is depicted in Figure 9 for the first sampling period of the 11/9-10/82 event (sample A in Figure 4). The above behavior is expected since particle rain scavenging, as can be concluded from the dependence of the collection efficiency on particle size (Figure l ) , is sensitive to particle size. Specifically, the coarse particles (a > 2 pm) and fine particles in the size range of a < 0.4 pm are removed efficiently by raindrops, whereas the wet removal of particles in the size range of a = 0.4-2 prn is less significant. Given the chemical concentration in rainwater and in t h e atmospheric phase, it is possible to determine the temporal variations of the overall instantaneous and average scavenging ratios (defined by eqs 32 and 33, re-
,E4
10
4
8
12
15
20
24
4
8
Time lhr)
Figure IO. Predicted overall scavenging ratio of pyrene for the 1I / 9-10182 rain event [using the collection efficiency of Ryan and Men (36)l.
spectively) as illustrated in Figure 10 for the rain scavenging of pyrene during the 11/9-10/82 rain event (using the collection efficiency of Ryan and Cohen). The predicted overall scavenging ratios, W and vary in the range of 2.03 X lo4-1.53 X lo5 and 2.72 X lO"1.53 X lo5, respectively. The nearly order of magnitude variation in the scavenging ratios during rainfall suggests that the application of average experimental values of rain scavenging ratios for mass balance calculations is questionable. It is also important to note that the reporting basis for field-measured scavenging ratios is not uniform. Some studies report the scavenging ratios, as calculated from eq 1, based on atmospheric concentration measured before, during, or even after the specific rain event (see Table 111). Yet, in some studies the reported scavenging ratios are values representing time periods that may he as long as several months in duration. The inconsistency in the reporting basis for rain scavenging ratios ultimately results in an uncertainty as to the extrapolation of reported values to other regions, different rain conditions, or atmospheric concentrations. Despite the uncertainty regarding the calculation basis (i.e., the pollutant air concentration) for field-measured scavenging ratios, the present model calculations for Los Angeles are compared with those reported hy Ligocki et al. (24, 251, Farmer and Wade (71),and McVeety and Hites (721, as shown in Table 111. The present study suggests that the rain scavenging ratio is not a universal constant that is characteristic of solely
w,
Environ. Sci. Technol.. VoI. 25, No. 12, 1991 2021
Table 111. Rain Scavenging Ratio of Pyrene and Fluoranthene
w (xio-3) pyrene
fluoranthene
rain rate, mm/h
duration, h
basis of chem air concn
1. Predicted in This Worka
(i) 11/9-10/82
20.3-153.0* 12.1-153.0' 27.2-153.0' (ii) 3/11/82 16.3-264.0b 16.0-145.0' 35.4-145.0'
18.2-98.7' 9.8-98.7' 21.5-98.7'
1.4-2.4
9.0
instantaneous initial cumulative av during each sampling period
14.9-174.0b 8.6-93.5' 26.0-93.5'
0.8-2.0
22.2
instantaneous initial cumulative av during each sampling period
7.8d
8.gd 10.4e 4.81
9.1e 3.3f
2. Reported from Other Field Studies not reported 5.8-41.5 not clearly specified 1.1 not clearly specified av during rain event not reported not reported summer arithmetic av
Collection efficiency of Ryan and Cohen (36) is used. Using the instantaneous pyrene or fluoranthene rain concentration. Using the cumulative average pyrene or fluoranthene rain concentration. dMean scavenging ratio reported by Farmer and Wade (71) for seven rain events (1982-1983) at an urban site in Norfolk, VA. eCalculate on the basis of data of Ligocki et al. (24, 25) on 3/16-20/84 Portland, OR, rain event. Note that rain duration was not specifically reported; however, the sample date (e.g., 3/16-20/84) and total rainfall amount (2.1 cm) were reported (24,25). fMean scavenging ratio reported by McVeety and Hites (72) for measurements of air and rain concentrations of PAHs at Siskiwit Lake. Note that the rain samples were collected on a weekly basis and thus the results are averages for the total number of discrete rain events.
the chemical being scavenged by rain. The rain scavenging ratio is a function of numerous variables including the chemical type, rate and duration of rainfall, height of cloud base, source emissions during rainfall, particle size distribution, and temperature. Consequently, in order to properly interpret rain scavenging data, field studies should at the minimum report the above information. Also, since the rain scavenging ratio can vary significantly during rainfall, it is important that a consistent basis is used for reporting the concentrations in the atmospheric and rainwater phases. Simultaneous measurements of the atmospheric and rainwater concentrations would best serve to validate and improve existing rain scavenging models. Admittedly, such studies are difficult and represent an experimental challenge. Finally, it should be recognized that pollutant concentrations in the atmospheric phase can vary with vertical distance above ground level; in such cases the concept of a rain scavenging ratio is inappropriate and more definitive detailed models should be used. Summary and Conclusion
Below-cloud rain scavenging of semivoltaile organis was investigated by a mass balance model that accounts for the dynamic partitioning of atmospheric organics in gas/ particle/rain phases during a rain event. Results of case studies for pyrene and fluoranthene illustrated that (1)the variation of particle size distribuition during rainfall affects the prediction of rain scavenging of particle-bound organics and (2) the clarification of the basis of ambient air concentration of the selected chemical that is utilized in the calculation of scavenging ratio is essential for reducing the uncertainty associated with the use of scavenging ratios reported from field studies. The current model was rewritten in a flexible modular form. Thus, the code (written in Fortran and executable on the IBM PC/XT/AT-type computers) can be easily modified to accommodate different collection efficiencies, initial partial size distributions, raindrop size distribution, chemical sorption model, etc. Since the model is a dynamic box-type model, it is especially suited for dynamic multimedia mass balance studies of particle-bound organics. It is noted that, if spatial resolution of concentrations in the atmosphere (e.g., vertical concentration profiles) is of interest, then the present approach will have to be mod2022
Environ. Scl. Technol., Vol. 25,No. 12, 1991
ified to account for vertical concentration profiles as well as the effect of convective winds. Work is currently underway to include the detailed mechanisms of rain scavenging of semivolatile organics into a comprehensive precipitation scavenging model. Copies of the RSSVO model software can be obtained upon request by writing to Professor Yoram Cohen at the National Center for Intermedia Transport Research. Acknowledgments
We thank J. M. Hales for his helpful comments. Registry No. Pyrene, 129-00-0; fluoranthene, 206-44-0.
Literature Cited Grover, S. N. Pugeoph 1976, 114, 509-520. Grover, S. N.; Pruppacher, H. R.; Hamielec, A. E. J. Atmos. Sei. 1977, 34, 1655-1663. Wang, P. K.; Pruppacher, H. R. J . Atmos. Sci. 1977, 34, 1664-1669. Beard, K. V. In Precipitation Scavenging (1974); ERDA Symposium Series; (Champaign, IL, Oct 14-18, 1974, Semonin, R. G.; Beadle, R. W., Coordinators) COW-741003, NTIS; 1977; pp 183-194. Slinn, W. G. N. Water, Air, Soil Pollut. 1977, 7, 513-543. Williams, A. L. In Precipitation Scavenging (1974);ERDA SvmDosium Series: (ChamDaim. IL. Oct 14-18. 1974, S e m k n , R. G.; Beake, R. W:, Ckrdinators) COW-741003; NTIS; 1977; pp 258-275. Davenport, H. M.; Peters, L. K. Atmos. Enuiron. 1978,12, 997-1008. Radke, L. F.; Hobbs, P. V.; Eltgroth, M. W. J . Appl. Meteorol. 1980, 19, 715-722. Easter, R. C.; Hales, J. M. PLUVIUS: A Generalized One-Dimensional Model of Reactive Pollutant Behavior, Including Dry Deposition, Precipitation Formation, and Wet Removal. Report PNL-4046 ED2; Batelle Pacific Northwest Laboratories: Richland, WA, 1984. Hales, J. M. Atmos. Enuiron. 1989, 23, 2017-2031. Lee, 1,-Y.; Shannon, J. D. Atrnos. Enuiron. 1985,19,143-149. Carmichael, G. R.; Peters, L. K.; Kitada, T. Atmos. Enuiron. 1986,20, 173-188. Hong, M.-S. Carmichael, G. R. Atmos. Enuiron. 1986,20, 1989l1986. Rutledge, S.A.; Hegg, D. A.; Hobbs, P. V. J. Geophys. Res. 1986, 910, 385-402. Hegg, D. A.; Rutledge, S. A.; Hobbs, P. V.J . Geophys. Res. 1986,91D, 403-416.
(16) Seigneur, C.; Saxena, P. Atmos. Enuiron. 1984, 18, 2109-2124. (17) Seigneur, C.; Saxena, P. Atmos. Environ. 1988,22,101-115. (18) Tsai, W.; Altwicker, E. R. Atmos. Environ. 1990, 24A, 2473-2483. (19) Tsai, W.; Altwicker, E. R.; Asman, W. A. H. Atmos. Enuiron. 1990,24A, 2485-2498. (20) Hales, J. M. In Air Pollutants and Their Effects on the Terrestrial Ecosystem: Legge, A. H., Krupa, S. V., Eds.; Advances in Environmental Science and Technology; J. Wiley & Sons: New York, 1986; Vol. 18, Part 4, pp 211-251. (21) Pruppacher, H. R., Semonin, R. G., Slinn, W. G. N., Eds.
Precipitation Scavenging, Dry Deposition, and Resuspension; Proceedings of the Fourth International Confer(22) (23)
(24) (25) (26) (27)
ence, Santa Monica, CA, 29 November-3 December, 1982; Elsevier: New York, 1983. Baker, J. E.; Eisenreich, S. J. Environ. Sci. Technol. 1990, 24, 342-352. Eisenreich, S. J. In Sources and Fates o f Aquatic Pollutants; Hites, R. A., Eisenreich, S. J., Eds.; Advances in Chemistry 216; American Chemical Society: Washington, DC, 1987; pp 393-469. Ligocki, M. P.; Leuenberger, C.; Pankow, J. F. Atmos. Environ. 1985, 19, 1609-1617. Ligocki, M. P.; Leuenberger, C.; Pankow, J. F. Atmos. Enuiron. 1985, 19, 1619-1626. Schroeder, W. H.; Lane, D. A. Enuiron. Sci. Technol. 1988, 22, 240-246. Eisenreich, S. J.; Willford, W. A.; Strachan, W. M. J. In
Intermedia Pollutant Transport: Modeling and Field Measurements;Allen, D. T., Cohen, Y., Kaplan, I. R., Eds.; Plenum: New York, 1989; pp 19-40. (28) Bidleman, T. F. Environ. Sci. Technol. 1988,22,361-367. (29) Czuczwa, J.; Leuenberger, C.; Giger, W. Atmos. Environ. 1988,22, 907-916. (30) Leuenberger, C.; Czuczwa, J.; Heyerdahl, E.; Giger, W. Atmos. Environ. 1988, 22, 695-705. (31) Rounds, S. A,; Pankow, J. F. Environ. Sci. Technol. 1990, 24, 1378-1386. (32) Wu, S. C.; Gschwend, P. M. Environ. Sci. Technol. 1986, 20, 717-725. (33) Costa, C.; Rodrigues, A. Chem. Eng. Sci. 1985,40,707-713. (34) Mackay, D. M.; Paterson, S.; Schroeder, W. H. Environ. Sci. Technol. 1986, 20, 810-816. (35) Cohen, Y. In Pollutants in a Multimedia Environment; Cohen, Y., Ed.; Plenum: New York, 1986; p p 117-131. (36) Ryan, P. A,; Cohen, Y. Chemosphere 1986, 15, 21-47. (37) Lewis, W. K.; Whitman, W. G. Ind. Eng. Chem. 1924, 16, 1215. (38) Bird, R. B.; Stewart, M. E.; Lightfoot, E. M. Transport Phenomena; Wiley: New York, 1960; p 363. (39) Reid, R.; Prausnitz, J.; Sherwood, T. The Properties of Gases and Liquids, 3rd ed.; McGraw Hill: New York, 1979; pp 57-58. (40) Walcek, C. J.; Pruppacher, H. R. J. Atmos. Chem. 1984, I , 269-289. (41) Walcek, C. J.; Pruppacher, H. R.; Topalian, J. H.; Mitra, S. K. J. Atmos. Chem. 1984, I , 291-306. (42) Walcek, C. J.; Pruppacher, H. R. J. Atmos. Chem. 1984, 1, 307-324. (43) Marshall, J. S.; Palmer, W. M. J.Meteorol. 1848,5,165-166. (44) Scott, B. C. Atmos. Enuiron. 1982, 16, 1753-1762. (45) Miguel, A. H.; Friedlander, S. K. Atmos. Environ. 1978,12, 2407-2413.
(46) Dragoescu, C.; Friedlander, S. Aerosol Sci. Technol. 1989, 10, 249-257. (47) Whitby, K. T. Atmos. Enuiron. 1978, 12, 135-159. (48) Seigneur, C.; Hudischewskyj, A. B.; Seinfeld, J. H.; Whitby, K. T.; Whitby, E. R.; Brock, J. R.; Barnes, H. M. Aerosol Sci. Technol. 1986, 5, 205-222. (49) Junge, C. E. In Fate of Pollutants in the Air and Water Environments; Suffett, I. H., Eds.; Advances in Environmental Science and Technology; J. Wiley & Sons: New York, 1977; pp 7-25. (50) Pankow, J. F. Atmos. Environ. 1987, 21, 2275-2283. (51) Bidleman, T. F.; Foreman, W. T. Enuiron. Sci. Technol. 1987, 21, 869-875. (52) Bidleman, T. F.; Billing, W. N.; Foreman, W. T. Enuiron. Sci. Technol. 1986, 20, 1038-1043. (53) Yamasaki, H.; Kuwata, K.; Miyamoto, H. Environ. Sci. Technol. 1982, 16, 189-194. (54) Pankow, J. F. Atmos. Environ. 1988, 22, 1405-1409. (55) Slinn, W. G. N. In Atmospheric Science and Power Production; DOE/TIC-27601; NTIS DE 84005177; U. Department of Energy: Springfield, VA, 1984; pp 466-532. (56) Kawamura, K.; Kaplan, I. R. Atmos. Enuiron. 1986, 20, 527-535. (57) Dawson, G. A. Atmos. Enuiron. 1978,12, 1991-1999. (58) Van Noort, P. C. M.; Wondergem, E. Environ. Sci. Technol. 1985,19, 1044-1048. (59) National Climatic Data Center, Climatological Data (California), 1982. (60) California Almanac, 1984-1985 Edition; Fay, J. S., Lipow, A. G., Fay, S. W., Eds.; Presidio Press and Pacific Data Resources: California, 1984; Section 7. (61) Air Quality Management Plan: 1988 revision; South Coast Air Quality Management District, 1988; Chapter 2. (62) Gray, H. A.; Cass, G. R.; Huntzicker, J. J.; Heyerdahl, E. K.; Rau, J. A. Environ. Sci. Technol. 1986, 20, 580-589. (63) Draft SCAG Projections Burden 87B; California Department of Transport, Southern California Association of Governments, 1987. (64) Benner, B. A., Jr.; Gordon, G. E.; Wise, S. A. Enuiron. Sci. Technol. 1989,23, 1269-1278. (65) Handa, T.; Yamauchi, T.; Sawai, K.; Yamamura, T.; Koseki, Y.; Ishii, T. Environ. Sci. Technol. 1984, 18, 895-902. (66) 1976 Urban and Rural Travel Survey; California Department of Transport, Southern California Association of Governments, 1976; Vol. 4, Chapter 7, pp 109-119. (67) Gordon, R. J.; Bryan, R. J. Environ. Sci. Technol. 1973, 7, 1050-1053. (68) Grosjean, D. Atmos. Environ. 1983, 17, 2565-2573. (69) USEPA, Superfund Public Health Evaluation Manual; U.S. EPA, 1986. (70) Pearlman, R. S.; Yalkowsky, S. H.; Banerjee, S. J. Phys. Chem. Ref. Data 1984, 13, 555-562. (71) Farmer, C. T.; Wade, T. L. Water,Air, Soil Pollut. 1986, 29, 439-452. (72) McVeety, B. D.; Hites, R. A. Atmos. Environ. 1988, 22, 511-536.
s.
Received for review February 14, 1991. Revised manuscript received May 22,1991. Accepted June 18,1991. This work was funded in part by the United States Environmental Protection Agency Grant CR-812271-03 to the National Center for Interrnedia Transport Research at UCLA and the University of California Toxic Substances Research and Training Program.
Environ. Sci. Technol., Vol. 25, No. 12, 1991
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