Spatial Variations of Acid Precipitation in Southern California Howard M. Liljestrand"? and James J. Morgan W. M. Keck Engineering Laboratories, California Institute of Technology, Pasadena, California 9 1125
Wet-precipitation-only samplers were used to collect acid precipitation at nine sites in the Los Angeles basin of southern California during the 1978-1979 hydrologic year. Concentrations of the major cations (H+,NH4+, Na+, K+, Ca2+,and Mg2+)and the major anions (Cl-, NO3-, and S042-)as well as trace species were determined. The relative importances of natural and anthropogenic sources were calculated by a chemical balance. Variations of sea salt, soil dust, NH4+, SO42-, and NO3- contributions agreed with source distributions, scavenging, and advection patterns. The nitrate to non-sea-salt sulfate equivalent ratio varied from 0.4 near coastal stationary sources to 2.8 in rural mountain areas with a precipitation-weighted average ratio of 1.1. Equilibrium models are used to relate the chemical composition of rain ~ , P N O and , P"03 during precipwater with Pso,, P N H PNO,, itation. Mass-balance calculations indicate that less than 2% of the anthropogenic emissions of NO, and SO, in southern California are locally scavenged on an annual basis. Introduction
The chemistry of precipitation is determined by the mixing of anthropogenic and natural sources, by transport processes from long-range and local sources, and by scavenging kinetics. Eriksson (1,2), Junge ( 3 , 4 ) , and Granat ( 5 ) recognized sea salt, soil dust, ammonia, sulfur oxides, and nitrogen oxides as sources contributing to the chemical composition of rainwater. In general, concentrations of constituents decrease from their points of highest emission. Chemical-kinetic and scavenging processes mitigate local source effects and source distributions (6, 7). The acidity of precipitation is the net sum of the acidity or alkalinity from each chemical component contribution. While background pH values in the West tend to show net alkalinity ( 8 ) ,anthropogenic emissions of acidic gases (S02, NO,, and HC1) have led to regional precipitation of net acidity in northern Europe (9) and the northeastern United States (10) as well as locally acidic rainfall in other regions (11-13). Urban areas in the western United States have shown more acidic precipitation than the neighboring rural areas (14-16), and the pH of rural Western sites is decreasing ( 17). Since the Los Angeles area appears to be one with high rainfall acidity (18), a precipitation sampling program was initiated a t Pasadena in 1976 to characterize the chemical composition of acid precipitation and to estimate the overall efficiency of local scavenging processes for the period 1976-1977. This paper presents results for precipitation chemistry at nine locations in southern California (see Figure 1)for the hydrologic year 1978-1979 and discusses these results in terms of chemical source balance and equilibrium models. ' Experimental Methods
Sample collection techniques were based on recommendations of Galloway and Likens (19,ZO) and others (21,22). Precipitation was collected by clean all-glass and all-conventional polyethylene samplers for organic and inorganic species, respectively. The collectors, shown in Figure I of the
Present address: Department of Civil Engineering, Environmental Health Engineering Group, University of Texas, Austin, Texas 18712. 0013-936X/81/0915-0333$01.25/0
supplementary material (see paragraph at end of text regarding supplementary material), opened automatically at the onset of precipitation through dissolution of a poly(viny1 alcohol) paper strip used to maintain tension on lids covering the funnels. Funnels were manually re-covered during periods of no precipitation within a storm. Sequential increments of precipitation were segregated by using an inverted siphon and upward sloping ramp of T's connected to a series of bottles with float check valves. Sample bottles were replaced during and/or immediately after each storm event. Chemical analysis of samples was performed as coon as possible after each storm by methods described elsewhere (18).Conductivity was determined with a conductivity meter; pH was determined with a pH/mV meter, glass and doublejunction reference electrodes calibrated with dilute acid solutions as primary standards; ammonium was determined with an ammonia-specific electrode. Anions were determined with a Dionex Model 10 ion chromatograph using standard eluent (0.003 M NaHC03 plus 0.0024 M Na2C03). Metals were determined by atomic absorption spectrometry using the procedures described in Standard Methods (23). Standard methods were also used to check ammonium, phosphate, and fluoride concentrations. Chloride determinations were checked with the mercury(I1) thiocyanate colorimetric method (24).Filterable residue and suspended solids were determined with washed Whatman GF/C filters which had been baked at 450 "C for 24 h to remove organic carbon. Constant filter weights were determined for filters dried at 103 "C. Total organic carbon, particulate organic carbon, and total carbon were determined with a Dohrmann Envirotech DC-50 carbon analyzer by the methods of Grosjean (25).Volatile organic acid (10 to include weak acids of pK, -9.3 (35).Most of the 334
Environmental Science & Technology
X 6
z
.c
0
al I Q
c
-Lu Legend
Yestmod LOW Beach
L A
Pasadena W t
Yilron A Z U M
Rivcrrlde Y i l q h t m o d B i q Bear
Figure 2. Source contributions to mean acidity. The composition is calculated as the linear combination of five sources, given in ascending order-sea salt (predominantlyNaCl and MgS04), soil dust (predominantly Ca) neutralized acids, ammonia-neutralized acids, nitric and sulfuric acids from mobile sources (no crosshatched area),and sulfuric and nitric acids from stationary sources (crosshatchedarea).Relative amounts of sulfuric and nitric acids given by relative widths, with predominant acid first. Coastal sites are given first. Sampling sites farthest inland are presented last.
weak acids found have high pK, values and contribute little buffering to the free acidity. The results for southern California concur with the analysis of Galloway et al. (36)which identified possible weak acids in precipitation and concluded that strong acids control the pH of rainfall. Significance of pH Variations. The significance of areal variation in mean acidity for the 1978-1979 hydrologic year is assessed by a pairwise comparison (37) of results for the nine sampling sites. The differences among the acidities at Long Beach, Westwood, Central Los Angeles, and Pasadena shown in Figure 1 are not significant at the 95% confidence level. Azusa’s acidity is significantly different (99%confidence level) from Pasadena’s but not from the other Los Angeles County urban sites. Riverside and the mountain sites have mean acidities which are significantly different (95% confidence level) from the Los Angeles County urban sites. The wet flux of acidity is the product of annual mean acidity and annual precipitation. Since sites in and nearest the mountains receive the most precipitation, low mean acidities do not necessarily mean low fluxes of acidity. Mount Wilson and Long Beach have comparable depositions of wet acidity since Mount Wilson typically receives more than three times as much precipitation as Long Beach. Wet flux of acidity at
Big Bear is almost half that at Riverside because Big Bear received almost twice as much precipitation as did Riverside. The wet flux or loading of acidity may be an important factor in weathering processes and material damage, whereas the intensive quantity of acidity (mean or extreme pH values) is the important factor in biological toxicity. Thus lower mean acidities in the mountains does not preclude environmental insult. Determination of Chemical Sources. The results of a chemical source balance are shown in Figure 2. The chemical composition of the precipitation is assumed to be a linear combination of the compositions of the major sources-sea salt, soil dust, ammonia, mobile sources, and stationary sources of fossil-fuel combustion. The mass ratio of the ith chemical species to the total residue in rainwater (X,)is given by
where XiJ is the mass ratio of the ith chemical species for emissions from the j t h source and cr,j is the fractionation factor for the ith element from the j t h source. AJ is the source strength or the mass fraction of total residue from the j t h source to the rainwater. Figure 2 shows the source breakdown for the rainwater compositions given in Tables I and 11.Similar models have been used for aerosols (38)and precipitation (5, 10). Since there are more chemical species than sources, the least-squares best fit of the overdetermined system of equations is found to give the best estimate of each source contribution. There are still uncertainties in this type of mixingmodel calculation because of assumptions concerning the major sources and fractionation of species between the source and sink (31).Slinn (39) has reviewed the average fractionation factors for precipitation scavenging of a number of atmospheric species, both gases and aerosols. Several trends are evident from the acid-base source characterization. First, the net acidity can be viewed as the neutralization of strong acids (nitric and sulfuric) by bases (ammonia and soil dust) at all sampling sites. Second, sea-salt contribution is highest at the coastal sites, decreases inland, and is lowest in the mountains. Third, non-sea-salt sulfate decreases from the western coastal stations to the inland and mountain sites. This trend coincides with the locations of major sources of sulfur oxide emissions (40). Fourth, the nitrate to non-sea-salt sulfate ratio increases from coastal to inland sites. The ammonium and soil-dust trends are less obvious. The
Table II. Precipitation-Weighted Mean Concentrationsa total Fa, PN
Long Beach Westwood Central Los Angeles Pasadena Mt. Wilson Azusa Wrightwood Riverside Big Bear Catalina Is. (30)
AI, PN
Mn, PN
Si(OH)i, PM
total phosphate, Irgf L
total organic carbon, mg/L
particulate organlc carbon, mg/L
suspended solids, mg/ L
collection perlod
1.3
0.18
0.053
0.21
0.019
3.3
0.22
6.0
2/79-3179
0.49
0.15
0.082
0.44
0.29
4.2
0.51
4.3
9/78-9179
1.4
0.25
0.1 1
0.78
0.092
3.4
0.19
6.4
iina-9m
2.2
0.22
0.051
0.56
0.67
3.1
0.18
2.8
9/78-9179
0.50
0.88
0.035
0.55
0.26
6.3
0.37
9.5
10178-9/79
0.62
0.56
0.13
1.o
0.27
6.6
0.55
9.2
0.22
0.76
0.10
0.25
9.24
0.17
0.10
1.2
1.80
0.76
0.029
0.31
0.38
0.22
0.27
iina-9/79 12/78-9179
0.19
4.2
0.26
6.1
11178-31 79 1/79-3179 7/66-2167
(30) Additional precipitation-weighted mean concentrations for trace species at all nine sites sampled during 1978-1979 are as follows: H2P04F- < 1 pN,
