A chemical element balance for the Pasadena aerosol

A Chemical Element Balance for the Pasadena Aerosol M. S. M I L L E R , S. K. F R I E D L A N D E R , A~D G. -~[. H I D Y W. M. Keelc Laboratory of Environmental Health Engineering, California Institute of Technology, Pasadena, California 91109

t~eceived July 1, 1971; revised November 29, 1971; accepted November 29, 1971 A method has been developed for calculating on a chemical element-by-element basis the contribution of various sources to the aerosol of a polluted atmosphere. Four maior sources have been considered and their relative contributions estimated by employing certain trace elements characteristic of each source; sodium, aluminum, lead, and vanadium were utilized for sea salt, soil, automobile emissions, a~ad fuel oil fly ash, respectively. Most of the data used in the calculations were taken from the results of other investigators. New data are reported for soil dust and fuel oil fly ash, including evidence for a fractionation effect in wind raised dust. Calculated compositions for the Pasadena aerosol are in fair agreement with measured values for many elements. Deviations can be explained in most eases in terms of other anthropogenic sources and atmospheric reactions. INTRODUCTION The chemical composition of the atmospheric aerosol together with its particle size distribution play key roles in determining air pollution effects. For example, the health hazard associated with airborne particles depends on (a) the site of deposition in the lungs, which is related to particle size, shape and density, and (b) the effect on biological tissue which depends on the chemical eomposition. In this connection, the ratio of sulfate ion and sulfuric acid in the particulate m a t t e r to sulfur dioxide in the gas phase is important in regions polluted by smoke from fossil fuel combustion. The presence of lead and other metals as well as carcinogenic material m a y also have an affect on human health. Visibility in urban regions is sensitive to the pollution aerosol size distribution, hence to the hygroseopicity (1). Here again the chemical nature, including the ratio of sulfate ion and sulfuric acid to sulfur dioxide, is important. The ehemieal eomposition also affects the refractive index

of the particulate matter. On a global scale, the pollution aerosol Copyright @ 1972 by Aeademlc Press, Inc.

transports lead, zinc, and other metals generated anthropogenically as far as the polar regions (2). There is also evidence t h a t D D T is transported globally on the tale aerosol with which it is introduced in agricultural applications. The condensation of water vapor in the atmosphere takes place on aerosol particles, and the nucleation of ice from water vapor or supercooled water droplets in clouds is an important factor in the formation of rain. Chemical modification of the condensation and ice mmlei by gaseous or particulate pollution could produce changes in the microscale processes occurring in clouds, and thereby affect climate; however, few cases of detectable cloud modification have been documented (3). In characterizing polluted atmospheres, it is important to recognize that they are dynamic systems in which phase changes, coagulation, and chemical reactions all take place simultaneously, along with diffusion and sedimentation. As a result, aerosol particles in a given volume element v a r y not only in size but also in chemical composition; moreover, particles in the Journal of Colloid and Interface Science, Vol. 39, No. 1, April 1972

165

166

MILLER, FRIEDLANDER, AND HIDY

same size range, in general, differ in chemical composition among themselves. Friedlander (4) has proposed a statistical theory which relates the microscopic characteristics of a complex aerosol to some aspects of its macroscopic behavior. In the discussion which follows, however, only the average chemical composition over all particle sizes is considered. Hidy and Friedlander (5) have estimated the relative importance of major natural and anthropogenic sources of the Los Angeles aerosol from limited data on their chemical composition. The fractional contribution of each source was estimated from data on emissions of gases and partieles, in combination with the use of tracer elements for some sources. Their results suggested that about two-thirds of the Los Angeles aerosol is anthropogenic. Of this, as much as one-third is secondary in origin, coming from chemical reactions or condensation of vapors in the atmosphere. An alternate method for constructing a breakdown of the contributions from major sources is proposed in this study. By making use of a more detailed elemental analysis, mainly of the inorganic fraction of the aerosol, a mass balance can be set up based on only four sources. These are airborne soil dust, sea salt, automobile emissions, and fuel oil combustion. It will be shown below that much of the average particle composition can be accounted for by applying this method to data for the Pasadena aerosol. Furthermore, information can be obtained from the analysis about the atmospheric chemistry of some of the constituents. Since the method is general, it can be applied, in principle, to the aerosol sampled anywhere with due consideration for the identifiable major sources. METHOD OF CALCULATION The chemical composition of atmospheric particulate matter is highly complex; several dozen different elements are known to be present in detectable quantities, and these may combine to form hundreds of possible chemical compounds. In this paper, emphasis is placed on a chemical element balance. Concerning the effects of parJ o u r n a l of Colloid a n d Interface Science, ¥ol. 39, No. 1, April 1972

ticulate matter discussed above, it will eventually be necessary to obtain more information on the nature of the chemical compounds present. In a given urban basin, there are many sources of particulate matter, natural and anthropogenie. These may be either primary or secondary depending on whether the particulate material is emitted as such from the source or formed in the atmosphere. If sampling is carried out at a fixed point, the material collected will represent contributions from a variety of sources depending on the nature of the winds. The sampling process leads to an averaging over time of contributions by separate sources. The percentage of any element i in the aerosol pl, is given by the expression

p~ = ~. a~pljC~, $

[1]

where p~j is the percentage of element i in the particulate matter from source j, Cj is the mass of material from source j per unit mass of aerosol in the volume of air sample, and a~j is the coeffÉcient of fractionation. From the definition of Cs, the continuity relation

I $2

= 1

[2]

must hold. The coefficient a~ represents the fraction of species i in source j which appears at the sampling site. There is considerable evidence in the literature for ion fractionation in sea spray. There also appears to be some elemental fractionation in blowing dust compared with the soil from which it originates. These phenomena are discussed further below. Fractionation also takes place in the atmosphere as a result of diffusion and sedimentation if particles of different sizes have different chemical compositions. No attempt has been made to take into account fractionation occurring in this way. If the chemical composition of the sources is known, Eq. [1] can be solved for the individual source contributions, Cj. This is the method of calculation which has been adopted in this paper.

AEROSOL CHEMICAL BALANCE

167

EMISSION SOURCES AND ATMOSPHERIC DATA

Bruyevich and Kulik (9) have demonstrated fractionation of sea water by distillation, and the production of spray by Natural Sources of Aerosols aeration with subsequent evaporation. DisThe Ocean. The movement of smog into tillation resulted in ion separation in agreethe northeastern part of the Los Angeles ment with field data for antarctic snow. basin around Pasadena arises from prevailAeration produced a less distinct pattern ing westerly to (locally) southwesterly of ion separation because water entered the winds. One would expect aerosols originating receiver used to catch particles. The ratios from sea spray to be potentially large conof SO4/C1, Ca/C1, and Mg/C1 increased tributors to the background entering the while N a / K and C1/Br were lowered comarea. Precise statements about the compared with sea water. Bruyevieh and Kulik position of this maritime aerosol cannot be cite other work demonstrating fractionamade because of inadequate aerometrie tion. studies of background aerosols to the west The surface of the sea normally has a and northwest of Southern California. thin layer of organic material. Dean (10) Data are available indicating significant has proposed a recipe for New Zealand fraetionation in the conversion of sea water rain consisting of 1 liter of distilled water, into the marine aerosol. Winchester and 0.5 ml of sea water and 4 mg dried plankton Duce (6, 7) have shown that the ratios of and algae which approximates the ionic iodide and bromide to chloride in Hawaiian content of rainfall in New Zealand. Based rains and western Alaskan snow are not on this observation, MaeIntyre (11) procompatible with the ratios found in sea poses that the organics in sea water include water. Rain water and snow in these areas surface active materials which may interact would be expected to contMn oceanic aerowith appropriately charged ions through sol, yet they are depleted in chloride relative the formation of electrical double layers. to the other two halogens. To explain this These surface active materials, he suggests, anomaly, Winchester and Duce proposed are compressed on bubble formation and photochemical processes, but initial fraetionproduce fraetionation when the bubbles ation at the sea-air interface also can exbreak, injecting particles into the atmosplain many of their observations. phere. Duce, Woodcock, and Moyers (8) have Despite the work reported so far on ion collected samples of sea salt aerosol using a fraetionation at the air-sea interface, no six-stage impaetor. Their analysis of the method has been proposed for estimating elemental composition in different size quantitatively the extent of fractionation fractions has demonstrated fraetionation of observed. halides among different sizes of the sea In the absence of theoretical or empirical salt particles. The ratio of iodide to chloride methods for calculating the degree of increased by a factor of 50 to 100 from the fractionation in the sea salt aerosol, it largest to the smallest particles analyzed; was decided, as a first approximation, that the smallest particle had a ratio 1000 times the percentages of the ions in oceanic sea that in sea water. The ratio of bromide to sMt would be used in this paper. The comchloride over the sea varied with particle position of sea water is shown in Table I. size passing through a minimum with most Soil. Windborne dust arising from soil values below the ratio in sea water. Over adds to the background aerosol from oceanic land or near points where surf was present, sources. To cheek locally for the characthe Br/C1 ratio was greater than found in teristics of wind driven dusts, soil samples sea water. Duee, et al., propose two types of were collected in and near Pasadena; these bromide containing particles, one consisting samples were analyzed by spectrographic of initially formed particles which lose Br through photochemical processes and a and X-ray fluorescent techniques. Results second consisting of particulate matter are reported in Table II (samples 1-3). from sources other than sea water. The data are consistent with the general Journal of Colloid and Interface Science, Vol. 39, No. I, April 1972

168

MILLER, FI~IEDLANDER, AND HIDY TABLE I

PERCENTAGE BY WEIGHT OF ELEMENTS IN SEA SALT, AUTOMOBILE PARTICULATE EMISSIONS~ A N D F U E L OIL F L Y ASH

Sea Salt (126, b)

A1 Ba Br C (noncarbonate) Ca C1 Cu Fe

I K Mg Mn Na NH4+ NO3Pb Si SO~ V Zn

0.00046-0.0055 0.00014 0.19 0.0035-0.0087 1.16 55.04

Automobile

PS 400(17)

4 API (I7)

0.8 0.36

7.9 0.09

58.1 0.14

18.1 0.28 0.5

(18)

0.8 0.1

7.9 (13) 6.8 (13)

--

0.00005-0.0005 0.00014 1.1 3.69 0.0000025-0.000025 30.61 0.0000014-0.000014 0,000003-0.002 0.000012-0.000014 0.00014-0. 0094 7.68 0.0000009 0.000014--0.000040

Fuel Oil Particulate

0.4 (13)

40 (13)

0.14 (16)

analysis of earth crustal rock. It must be noted that crustal rock, itself, may enter the aerosol directly through grinding and digging by man. The composition of dust entrained in a laboratory air flow was determined and compared with the parent soil (samples 4-7 of Table II). The dust was obtained by taking two soil samples, one from a San Gabriel mountain site and one from a field near the laboratory, placing them in glass tubes 2.22 cm in diameter and 83.7 cm in length, and drawing filtered room air through the tubes at velocities of 1.6-2.7 mph. The airborne dust was collected on 0.45 tz millipore filters and analysed. It is interesting to note the differences and similarities of the results obtained from soil samples I, 2, and 3. The data for the construction site must be cautiously interpreted since the original source of the fill, which has been at its present site for approximately one year, is unknown. Results for Al, Na, Co, Zn, Ti, and Si are consistent for all three samples. Mountain soil was found to be less rich in Fe, Mg, Ba, Cu, Mn, Journal of Colloid and Interface Science, Vol. 39, No. 1, April 1972

1.3 .

0.008

0.20

2.16

2.59

0.2 6 ±

0.12 0.02 0.67

0.42 0.02 2.22

O.2 0.06 0.06 5 -4- 1

0.09 0.28 17.5 1.70

0.18 0.45 25.0 3.20 0.05

1

0.07 1 7 =h 2 0,02

and V than the valley soil. This difference m a y be explained in terms of anthropogenic introductions. Of particular interest, in light of recent moves to limit the use of tetraethyl lead in gasoline, is the order of magnitude difference in soil lead concentration at the street site compared to the more remotely located mountain site. F r o m an examination of samples 4-7

in Table II, it is apparent that fractionation of soil occurs during dust rise by winds. Table III lists the average percentages for the two dust and the two precursor soil samples. The ratio of aluminum to silicon changes from 0.27 to 0.41 during dust rise; this alteration in A1/Si ratios is surprising since aluminum and silicon are usually present as aluminosilicates. The mechanism of fractionation is not known. The percentage of elements in the soil dust is utilized in the solutions of Eq. [1].

Anthropogenic Sources of Aerosol Automobile Emissions. Aerosol emissions from auto exhaust are mostly lead-bearing material and relatively nonvolatile hydro-

AEt%OSOL CHEl%~[ICAL BALANCE

169

TABLE I I PERCENTAGE BY W E I G H T OF ELE1KENTS IN SOIL AND SOIL DUST

Sample

1 Construction Site Adjacent to Lab

2 Site Near Traveled Street

3 San Gabriel Foothill Soil I

4 Soil SDust [ 7 San Gabriel 6 Foothill Soil from Foothill] Field Near [ DUStFieldfrOmnear Soil II Laboratory Laboratory

ii A1 Ba Ca Co Cu Fe In K~ Mg Mn Na Ni Pb Si Ti V Zn

7.4 0.21 2.1 0.0009 0.0018 1.5

4- 1.0 4- 0.02 4- 0.3 4- 0.0003 4- 0.0004 4. 0.2 0 2.2 4. 0.2 1.0 4- 0.2 0.087 4- 0.015 2.7 4- 0.4 3.0019 4- 0.0003 3.0024 4- 0.0004 294-5 O.36 4- 0.06 D.0046 4- 0.0006 <0.01

I

7.4 4. 1.0 6.9 4- 1.0 6.4 4- 1.9 9.0 4- 1.17.4 4- I. 7.4 4- 1.1 0.17 -4- 0.02 0.071 4- 0.010 0.05 0.15 0.06 0.07 1.8 4- 0.3 0.5 4- 0.1 0.6 0.9 1.4 2.t 0. 001 0.0007 < 0.0005 0.001 0.00I 0.002 0.0028 4- 0.000~ 0.0010 4- 0.0002 0.001 0. 007 0. 003 0.008 1.9 4- 0.2 0.8 4- 0.2 1.6 2.7 2.3 3.6 0 0 0 0 0 0 2.2 4- 0.2 3.4 4- 0.2 34-1 14-0.5 24-1 24.1 i.i 4- 0.2 0.24 4- 0.06 0.2 0.9 1.1 1.9 0.0028 4- 0.0006 0.0010 4- 0.0002 0.05 0.09 0.08 0.13 2.1 4- 0.4fi.7 4- 0.3 2.8 4- 0.3 2 . 3 4- 0 . 3 2.6 4- 0.4 2.1 4- 0.4 0.0023 4- 0.0008 <0 0 0 0 4 0.003 0.003 O.006 0.025 4- 0.004 0.0036 4- 0.0005 0.005 0.01 0.007 0.02 304-5 334-5 27 4- 2.8( 20 4- 2 {26 ~ 2 194-2 0.36 4- 0.06 0.18 4- 0.06 0.5 0.3 ' 0.4 t 0.3 0.0043 4- 0.0001 0.0009 4- 0.0003 0.003 0.004 ~ 0.006 0.007 0.012 4- 0.0O2 0.011 4- 0.002 0.01 <0.01 I <0.01 0.01 I

Analyzed by X-ray fluorescence; all the other elements determined by spectrographic analysis. carbons. A n a l y t i c a l d a t a on lead emissions were o b t a i n e d b y Hirsehler et al. (13) who f o u n d PbBrC1, a n d a - a n d ~-NH4C12PbBrC1 a n d PbBrC1-NH4C1 i n t h e fine p a r t i c u l a t e m a t t e r . T h e p e r c e n t a g e of lead observed b y Hirschler et al., 58-74 %, was c o n s i d e r a b l y higher t h a n the 4 0 % l a t e r observed b y Mueller, et al. (14), M u e l l e r (15), a n d N i n o m i y a , et aI. (16). However, this m a y be o n l y associated ~4th the gasoline b l e n d used i n t h e i r studies. T h e d a t a of Hirsehler e t a l . (13) yielded C 1 / P b a n d B r / P b ratios e q u a l to the results of N i n o m i y a et al. (16). C o n s e q u e n t l y , the c o m p o u n d s observed b y ! t i r s c h l e r et al. i n t h e e x h a u s t a p p e a r c o n s i s t e n t with the d a t a of N i n o m i y a e t a l . T a b l e I c o n t a i n s the results of Hirschler et al., scaled to 40 % lead i n t h e p a r t i c u l a t e m a t t e r , a level considered to be a realistic f r a e t i o n of t h e a u t o mobile emission for 1969 fuels. Fuel Oil Emissions. T h e fuels m o s t c o m m o n l y used b y s t a t i o n a r y sources i n the Los Angeles area are n a t u r a l gas a n d fuel oil. T h e fuel oil, of course, produces m u c h larger q u a n t i t i e s of aerosols t h a n n a t u r a l gas. D u r i n g the s u m m e r a n d early

TABLE

IIl

AVERAGE COMPOSITION OF AIRBORNE SOIL D U S T AS COMPARED WITH PRECURSOR SOIL % in Dust b

% in Soil c

A1 Ba Ca

8.2 4- 1.1 0.06 4- 0.01 1.5 4- 0.6

6.9 ± 1.5 0.10 4- 0.05 1.0 -4- 0.4

Co

0.002 4- 0.0005

Cu

0.008 4- 0.0005

Fe In

3.2 4- 0.05 0

K~ Mg Mn

1.5 4- 0.8 1.4 4- 0.5 0.11 4- 0.02

0.0008 ±

0.0002

0.002 4- 0.001 2.0 ± 0

0.4

2.5 ± 1 0.7 4- 0.4 0.06 4- 0.02

Na

2.5 4- 0.3

2.4 4- 0.4

Ni Pb Si Ti V Zn

0.004 4- 0.001 0.02 4- 0.005 20 :t: 2 0.4 4- 0.05 0.006 4- 0.002 <0.01

0.002 4- 0.001 0.006 4- 0.001 26 4- 2 0.3 0.004 4- 0.002 <0.01

Analyzed by X-ray fluorescence; all other elements determined by spectrographic analysis. b Average of two soil dust samples. Average of two soil samples used as sources for dust samples. Journal of Colloid and Interface Science, VG1. 39, No. l, April 1972

170

MILLER, FRIEDLANDER, AND HIDY

fall in Los Angeles, the major users of fossil fuels are strongly encouraged to burn natural gas. Fuel oil is used only as a second priority fuel, and finds its application in major sources mostly during late fall, winter, and early spring. During the late summer of 1969, when the Pasadena experiment was undertaken; two major local users of fuel, the Pasadena Light and Power plant in Pasadena and the California Institute of Technology power plant, were operating almost entirely on natural gas. For this reason, it is expected t h a t the contribution of fuel oil fly ash to the Pasadena aerosol during this sampling period should be small, coming mainly from more distant sources within the basin. The composition of fly ash arising from fuel oil combustion is highly variable, as shown in Table I, which includes results for a fly ash sample obtained from the cyclone separator at the Pasadena Power and Electric Company. The three fuel oil fly ash analyses of Table I were averaged, and the average was used in the caleulations discussed below, Atmospheric Particulate Matter

A limited amount of data for the Los Angeles smog aerosol is available (19-21). Recent data of Mueller et al. (21) obtained from 0900 hours to 2000 hours on 9/3/69 are shown ia T a b l e IV together with older d a t a from two other sources. The concentrations reported b y Mueller and his coworkers are for air sampled at Caltech's Keck Laboratory in Pasadena. Samples were collected through a vertical pipe 67 ft long with its inlet 22 ft above the roof of the laboratory. Background information on the site is given b y Hidy and Friedlander (22). Two local sources which m a y have contributed to the measured concentrations were a building under construction at a site adjacent to the sampling area, and hoods from metallurgical laboratories at Caltech vented on the roof. CALCULATION OF SOURCE COEFFICIENTS, C# T o identify the origins of all inorganic material analyzed b y Mueller et al. (21), Journal of Colloid and Interface Science, Vol. 39, No. 1, April 1972

TABLE IV PERCENTAGES OF CHEMICAL ]~LEMENTS IN PASADENA PARTICULATE ~V~ATTER COMPARED -WITH OTHER DATA FOR LOS ANGELES Mueller et al. (21)a NASN (19) Magill el al. (20)

Al Ba Br C (nonearbonate) Ca C1 (dry analysis) Cu Fe I K Mg Mn Na NH4~ NO~Pb Si S04~v Zn

0.8 0.04 0.6 19

2.5-6.0 0.1-0.24

0.99 0.07 0.03 3.2 0.006 0.32 1.1 0.03 1.0

2.5 0.07b 1.1 ~

0.1-0.24 2.5-6.0 1.1-2.5

0.03 b

3.3

0.6 c 11.3 ~ 1.3 b

0.01 0.18

12.1¢ 0.006 b 0.16b

Trace-0.24

0.8-6.0 2.5-6.0

a Total mass loading = 101.5 mierogram/m3, measured at 50% relative humidity. b Measured in Burbank. Total mass loading -123 microgram/mS. c Measured in L.A. Total mass loading = 119 microgram/mS. it was necessary to choose tracer elements for each source. The tracers selected were major constituents for each individual source, including lead for automobile emissions, sodium for sea salt, aluminum for soil and vanadium for fuel oil. The use of this method leads to a result equivalent to the maximum possible contribution b y each source. Lead in the Los Angeles area has one major source, automobile emissions. Lead in the divalent oxidation state is a stable species, making it ideal as a monitoring d e m e n t or tracer. F r o m the data of Mueller et al. (14) and Mueller (15), lead accounts for 40 % of automobile particulate matter. The recent data of Mueller et al. (21) show an ambient concentration of 3.3% lead

AEROSOL CHEMICAL BALANCE in atmospheric particulate m a t t e r yielding a value for C~uto of 0.082. Aluminum, a major constituent of soil, was used to obtain the contribution of soil dust to the aerosol. F r o m the data of Mueller et al. (21) and the data in Table I I I for soil dust, C~oi~ was calculated to be 0.098. The ambient vanadium concentration in atmospheric particulate m a t t e r has been used to monitor fuel oil consumption. This m a y be suitable for a single source, but as shown in Table I, fuel oil fly ash varies significantly from source to source. However, as an approximation for the contribution of fly ash to the aerosol, the average vanadium concentration was obtained from the data in Table I and utilized with the data of Table IV, correcting for a soil contribution, to obtain Cf~oIoll (Cf.o.) equal to 0.0024. Lemke et al. (23) give data for the contributions of anthropogenic sources to particulate loading in the Los Angeles atmosphere. T h e y find the ratio of automobile to fuel oil emissions of particulate m a t t e r to be 3:1, implying C~.o. should be about 0.04. The difference between these C~.o. values m a y arise in part because less fuel oil was burned during the late summer sampling period t h a n the annual average given b y Lemke et al. Moreover, as pointed out above, fuel oil was not burned at the Pasadena power plant during the sampling period. The sea salt source coefficient was determined using sodium as a tracer. Chloride is not suitable because of the reaction of NaC1 with NO2, as will be discussed later. The contributions of soil and fuel oil fly ash were considered in the analysis, and the solution of Eq. [3] yielded a C~, equal to 0.025. 30.6

Cs~+

2.5 C~oil + 2.6 C~.o. -- 1.0.

[3]

Data from Tables I and III were substituted into Eq. (4) to obtain detailed estimates of the elemental concentrations resulting from the contributions of the four sources.

17I

pi = 0 . 0 2 5 p i . . . . -~- 0 . 0 9 8 p l , soli + 0.082 pl, ~uto + 0.0024 pi, ~....

!4]

Results of calculations are shown in Table VI. DISCUSSION OF RESULTS The calculation of source coefficients suggests that sea salt and soil contribute about 10.9% to the aerosol after corrections are made for the loss of chloride from particulate m a t t e r in the atmosphere (see below). Automobiles and combustion of fuel oil contribute 8.2 % and 0.24 %, respectively, to the total Pasadena particulate matter. Table V compares the above results with the estimates of Hidy and Friedlander (5} based primarily on National Air Sampling Network (NASN) data (19); agreement is surprisingly good, considering that the estimates are based on two different sets of data. The main deviation in the comparison is the contribution of fuel oil, which is to be expected since the Pasadena power plant, and probably others, were operating on natural gas when the measurements w e r e made. The combination of ~4nd-raised dust TABLE V COMPARISON OF THE PERCENTAGE CONTRIBUTIONS OF SOURCES OBTAINED FROM SOURCE COEFFICIENTS wiT~ ESTIMATESor HI.Y AND FRIEDL A N . E ~ (5) This Work

Sea Salt Aerosol Soil Dust Primary Automobile Emissions Fuel Oil Fly Ash SO~-a. b

2.5 9.8 8.2 0.24

NOa-a, b

C (noncarbonatep. ~ Fe~Oaa, ~

Hidy and Friedlamder

5 6.8-16.8 12.5 4.2 12.1 11.3

17 4.5

Data from Table V. b Species generated partially from gas phase reactions. Corrected for approximately 2% carbon originating in primary automobile emissions. Hence this includes aircraft emissions, secondary formation and natural contributions. Iron oxide arising from anthropogenie sources. Jou*'nal of Colloid and Interface Science, VoL 39, No. I, April 1972

172

MILLER, FItIEDLANDEIt, AND HIDY

5L

i

two between calculated and analytical results does not appear significant in view of the variability suggested by the data in Table IV. Magnesium, calcium, and barium show the presence of anthropogenie sources in addition to rock dispersion. Barium is added to diesel fuel as a smoke suppressant. Heavy Metals

Lack of accord between calculated percentages and experimental quantities for Cu, Fe, and Zn can probably be ascribed V Cu Mn eo i Zn K AI Co i No Mg Fe Pb f f . g / l O 0 m 3 I /.zg/5 m 3 I /.zg/m3 to anthropogenic sources not included in this analysis, particularly metal working FIG. i. Comparison of ealcuIated background facilities. Zinc oxide, a component of rubber concentrations of several elements with experimental data (21). Based on 100 ~/ma particle tire treads, may have contributed to the zinc concentrations. loading: [] natural background; [] measured. Manganese is unique among the heavy and marine aerosol together with secondary metals in that the calculated and experiproducts formed from organic vapors given mental atmospheric concentrations differ off by plants and other gases constitute the only by a factor of three. Soil dust appears aerosol natural background. It should be to be the major source of manganese. noted that the soil component of Table V Halides includes an anthropogenie contribution resulting from dust rise produced by dirt Each of the three halides examined in moving operations and other activities of this paper shows different behavior. Chloman. As a first approximation, for purposes ride in the atmosphere is present in less of comparison, the natural background can than its calculated percentage, bromide be taken to be the sum of the soil and sea is about the same, while iodine is greater salt. than calculated. A histogram of the calculated natural The lower percentage of chloride than background contributions and the percent- expected can be explained by atmospheric ages obtained from the data of Mueller fraetionation. Although automobile emiset al. (21) for metallic elements is shown in sions contain some chloride, most of this Fig. 1. This figure shows the impact of element arises from the maritime aerosol. man's activity on the elements in his en- As the sea aerosol is advected into the Los vironment. Some of the anthropogenic Angeles basin, it interacts with anthropointroductions, for example the heavy metals, genie pollutants of primary and secondary are of particular interest as possible health nature. This results in a loss of chloride perhazards. haps as shown in Eqs. [5] and [6] for reactions in the presence of HNOs (from the oxiAlkali and Alkaline Earths dation of NO2) and H2SO4 (from the oxidaAmong the elements in these two groups tion of SO@: of the periodic table, best agreement was HNOa q- NaC1 --* NaNOa q- HC1 [5] obtained between calculated and analytical data for potassium (Table VI) ; most of this II~SO4 -t- N a C 1 - + N a H S O 4 q- H C 1 [6] d e m e n t in the aerosol particulate matter originates from background oceanic and The discrepancy between calculated and exterrestrial aerosols and from anthropogenic perimental data can be explained in terms contributions associated with the disper- of this loss. It is interesting to note that if sion of rock. The deviation by a factor of this is the explanation, and sodium is Journal of Colloid and Interface Science, Vol. 39, No. 1, April 1972

173

AEROSOL C H E M I C A L B A L A N C E T A B L E VI

CALCULATED ATMOSPHERIC LOADINGS OF ELEMENTS IN PASADENA PARTICULATE MATTER COMPARED TO EXPERIMENTAL RESULTS (PERCENTAGES) Element

Sea Salt

Soil

Automobile

Fuel Oil

Experimental (21)

2~

N&

K Mg Ca Ba V Mn Fe Cu Zn A1 C (nonearbonate) Pb C1 Br I NH~+ NOaSO4

9.2 2.9 3.5 2.2 3.5 7.0

X X X X X X

10-2 10- 2 10-6 10-s 10-7 10-6

6.8 X 10-7 7.5 X 10-5 1.5 X 10-4 3.2 X 1.4 4.8 )< 3.5 X 1.9 X 2.5 X 0.19

10-7 10-a 10-6 10-7 10-5

0.14 0.15 5.9 X 5.9 X 1.1 X 0.31 7.8 X 9.8 X 0.80

10-a 10-4 10-= 3.3 ;K 10-= 10-4 10- 4

2.0 X 10-a

1.1 :X 10-a

3.3 0.56 0.65

solely m a r i t i m e in origin, t h e c o n v e r s i o n of NaC1 to o t h e r salts in t h e Los Angeles a t m o s p h e r e is q u i t e r a p i d a n d c o m p l e t e , t a k i n g as l i t t l e as a few h o u r s to achieve significant conversion. T h e m e a s u r e d i o d i d e c o n c e n t r a t i o n is s e v e r a l o r d e r s of m a g n i t u d e h i g h e r t h a n c a n be a c c o u n t e d for f r o m its n o m i n a l conc e n t r a t i o n in sea w a t e r . I o d i d e m a y b e introduced into the atmosphere by man t h r o u g h t h e v e n t i n g of organic iodides used in i n d u s t r i a l processes. I o d i d e e n r i c h m e n t also m a y arise f r o m n a t u r a l origins. I t h a s b e e n shown t h a t I / C l r a t i o s in p r e c i p i t a t i o n collected in m a r i t i m e air h a v e v a l u e s 100-1,000 t i m e s g r e a t e r t h a n t h e I / C 1 r a t i o in sea w a t e r (10, 24-26). M i y a k e a n d T s u n o g a i (26) h a v e p r e s e n t e d d a t a for t h e p h o t o c h e m i c a l release of i o d i n e f r o m t h e surface of t h e sea. T h i s i o d i n e m a y be c a p t u r e d b y p a r t i c l e s a b o v e t h e ocean t h r o u g h c h e m i c a l processes. D u e e , W o o d c o c k , a n d M o y e r s (8) h a v e d e m o n s t r a t e d t h a t sea s a l t i o d i d e is concent r a t e d in s m a l l aerosol p a r t i c l e s p e r m i t t i n g long r e s i d e n c e t i m e s in t h e a t m o s p h e r e ;

4.8 1.4 4.4 9.5 8.0 8.6 3.3 8.4 7.7 9.1

X ;< X X X N X X X X

10-4 10-a 10-4 10-a 10-5 10-a 10-4 10-5 10-3 10-~

2.7 X 10-a 1.2 X 10-3

5.1 X 10-2

0.23 0.18 6.3 ;< 1.0 X 1.1 X 0.35 1.1 }( 2.1 X 0.80 9.1 X

10-a 10-2 10-2 10-3 10-3 10- 2

3.3 2.0 0.65 3.5 X 10-e 1.9 × 10-r 2.5 X 10-~ 0.24

1.1 0.99 4 f 1 X 3 X 3.2 3 X 0.18 0.80 19

10-= 10-= 10-2 10-~

3.3 7 X 10-: 0.6 6 X 10-~

t h i s will p e r m i t e n r i c h m e n t of i o d i d e r e l a t i v e to chloride in t h e aerosol. Of course, if t h e i o d i d e e n r i c h m e n t is due to n a t u r a l causes, t h e effect should n o t be l i m i t e d t o u r b a n basins. B r o m i d e ion is m o r e n e a r l y c o n s e r v e d in t h e a t m o s p h e r e t h a n chloride; t h e calc u l a t e d a n d a n a l y t i c a l results for b r o m i d e agree r e m a r k a b l y well. T a b l e V shows t h a t a u t o m o b i l e emissions c o n t r i b u t e m o s t of t h e b r o m i d e t o t h e aerosol. I t is i n t e r e s t i n g to n o t e t h a t t h e r a t i o of b r o m i d e t o l e a d o b t a i n e d b y M u e l l e r et al. (21) in P a s a d e n a is 0.18, in excellent a g r e e m e n t w i t h d a t a f r o m H i r s e h l e r et al. (13) who d e t e r m i n e d a r a t i o of 0.21. T h e a g r e e m e n t of t h e d a t a of H i r s e h l e r et al., which i n v o l v e d p a r t i c u l a t e m a t t e r obtained directly from automobiles, and t h e d a t a of M u e l l e r et al. (21) i m p l i e s t h a t l e a d a n d b r o m i d e a r e p r e s e n t in t h e s a m e particles. Recent electron microprobe analyses of t h e a t m o s p h e r i c aerosol (26) h a v e i n d e e d s h o w n t h e a s s o c i a t i o n of l e a d a n d b r o m i d e in t h e s a m e particles. (The locat i o n for t h e a t m o s p h e r i c s a m p l i n g was n o t Journal of Colloid and Interface Science, Vol. 39, No. 1, April 1972

174

MILLER, Ft~IEDLANDER, AND HIDY

reported in (27)). The investigations of Pierrard (28), and Winchester and Duce .(6) imply that bromide should be photochemically active; yet, the loss of bromide from automobile particulate matter to other particles is not supported by the microprobe study. The question arises as to why bromide does not behave analogously to chloride on interaction with NO2 and S02 as might be expected from Eqs. [5] and [6]. The amount of water present in smog particles varies with humidity but it is probable that an aqueous solution is often associated with the individuM particles. Examination of thermodynamic data (29) shows that the partial pressures of HC1 above its aqueous solutions are larger than those of HBr in solutions of similar molar concentrations. Thus ttBr is less likely to be lost from a partide containing PbC1Br than HC1 when the particle interacts with NO~ or SO2. Carbon

As shown in Table IV, about 19 % of the Pasadena sample was carbon. Primary sources of carbon containing compounds include tarry material present in automobile exhaust emissions, tire dust, diesel exhaust, and soot from jet aircraft. Soot comprises about 95 % of the particulate matter emitted by jet turbine engines (30). Secondary formation processes involving hydrocarbons in the air probably produce significant quantities of carbon containing material in the particulate phase. A detailed breakdown by source of the 19 % of the Pasadena sample which is carbon is not possible a t this time. Several of the sources of the carbon, including diesel and aircraft exhausts, do not carry with them other trace elements in well defined proportions. The rates of the atmospheric conversion processes are not known. Hence the carbon balance problem remains one of active research at present. Materials Generated in the Atmosphere

Several aerosol constituents are produced by reactions in the atmosphere. Examples are sulfate, nitrate, and ammonium ions, Journal of Colloid and Interface Science, Vol. 39, No. 1, April 1972

and the hydrocarbon-derived materials discussed above. Although much effort has been expended in the study of the oxidation of SO2 to sulfate in the atmosphere, the detailed mechanism is complicated and not entirely understood (31); for example, a study by Scott and Hobbs (32) has demonstrated that aqueous ammonia is important for the production of atmospheric sulfate. At any rate, several routes are available for atmospherically generated sulfuric acid including reactions with sea salt (Eq. [6]) and absorption of ammonia to form ammonium sulfate. Thus the aerosol is expected to be enriched in sulfate by secondary reactions beyond the calculated value in Table VI; data from NASN (19) for Los Angeles are shown in Table IV and are in accord with this expectation. Atmospheric nitrate is believed to arise chiefly from the oxidation of nitrogen oxides via photochemical processes; one proposed process (33) is the following: 3NO2 q- H20 -* 2HNO3 + NO.

[7]

The interaction of gases containing NO2 with sea salt aerosol leads to an increase in nitrate composition concurrent with a decrease in chloride concentration (Eq. [5]). Reactions of NO2 with gas phase hydrocarbons also lead to fixation of nitrate. Thus, calculated nitrate concentration is less than that reported by NASN (19) (Table IV). Atmospheric aerosols are em'iched in ammonium ion by ammonia absorption from the gas phase. Animal and plant metabolic processes are natural sources of ammonia. Man perturbs the ammonium ion concentration through the manufacture and use of fertilizers and through the petroleum industry. Substances Not Included in the Calculation of Elemental Percentages

The analytical procedure of Mueller et al. (21) did not yield the concentration of water in the particulate matter equilibrated at a relative humidity of 50 % before weighing. Lack of data on the chemical compounds present in the aerosol precluded a

AEROSOL CHEMICAL BALANCE calculation of water concentrations based on relative humidities and thermodynamic

data. The heavy metals probably exist as oxides. A calculation of the percentage of oxygen in the particulate matter ori~nating from soil, based on the ratio of oxygen to aluminum in soil, gives 4.5 %. This is a minimum value which does not include water and the oxygenated anions. The presence of anions besides chloride, bromide, and iodide was not reported in the Pasadena data. A probable constituent is phosphate ion, which would be expected to originate from fertilizers and soil. SUMMARY AND CONCLUSIONS The choice of suitable tracer elements makes it possible to estimate the contributions of various types of sources to the concentrations of elements in the Pasadena air. I t would be interesting to a p p l y the method to other locations with different mixtures of source types, such as Chicago (35). Calculations of this kind also give an indication of interesting factors in the atmospheric chemistry of some pollution components. For example, our method reveals significant differences among the halides in the Los Angeles atmosphere. The calculation further suggests t h a t improvements can be made b y more detailed consideration of the metal processing industry in the Los Angeles area. The origins and nature of the noncarboRate carbon fraction of the smog aerosols are still not understood. Further effort will be required to obtain a better understanding of the chemistry of such sub-

stances. ACKNOWLEDGMENTS The authors wish to acknowledge the aid of Mrs. Elisabeth Bingham who performed elemental analyses of the soil samples and Dr. Peter K. Mueller and coworkers who provided analytical data for the composition of aerosol particulate matter obtained in Pasadena. This work was supported by the U. S. Public Health Service under Grants TO] ES00080 and AP00680.

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MILLER, F R I E D L A N D E R , AND t t I D Y

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