Reports

Acid Precipitation in the Western United States

Abstract. Precipitation chemistry data from a rural area at an elevation of 2900meters near the Continental Divide in Colorado show unexpectedly low pH valuesforbulk precipitation. A significant downward trend in pH over the last 3 years is associ-ated with increasing amounts of nitric acid in precipitation.

High acidity of precipitation was firstdocumented in northern Europe (1). Theextended record of precipitation chemis-try and pH taken at the Hubbard Brookwatershed in New Hampshire has alsoshown that extremely acidic rainfall, pre-sumably of anthropogenic origin, occursin the eastern United States (2). The datafrom northern Europe and the easternUnited States suggest that substantial re-gional changes in the chemistry of pre-cipitation have been caused by the addi-tion of acidic substances to the atmo-sphere in connection with fossil-fuelcombustion. Although there has beensome controversy about the exact causeof the increasing acidity (3), it seemsmost likely at present that the greatchanges in pH can be largely accountedfor in terms of sulfuric and nitric acidsderived from the terrestrial release ofsulfur dioxide and nitrogen oxides.Although it is now obvious that indus-

trialization and high population densitycan change the precipitation chemistryover large areas, it is not presently clearhow much of the continent or the bio-sphere has been affected. Our data giveevidence of surprisingly low precipi-tation pH in the Colorado Rockies,suggesting that changes in the pre-cipitation chemistry may be more wide-spread than presently realized and thatthe acid rain phenomenon first noticed inthe industrial portions of the easternUnited States may be typical of signifi-cant portions of the western UnitedStates as well.The study site is located in the Como

Creek watershed, Boulder County, Col-orado, at an elevation of 2900 m and 6km east of the Continental Divide. Thearea in a 850-km radius around the wa-tershed has an extremely low populationdensity except for a strip along theRocky Mountain front range (Fort Col-

lins-Boulder-Denver), 20 to 50 km eastand 1300 m lower in elevation than thestation (4). The next closest major urbanarea is Salt Lake City, 600 km west, andthen Phoenix, 900 km southeast. A thor-ough analysis of weather patterns influ-encing the Como Creek area has beenconducted by Barry (5), whose datashow that air movement is predomi-nantly from the northwest over a re-gion of very low population density.Although sporadic upslope air move-ment from the east does occur, it is com-mon only in the spring and early summermonths and even then is interrupted by

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Fig. 1. (A) The pH of bulk precipitation and(B) the loading rate of N03--N in bulk precipi-tation over a period of 150 weeks in the ComoCreek watershed, Colorado. Significant lineartrends are shown by the lines.

long periods of southeasterly air flowover the Continental Divide. We wouldtherefore expect the air to receive onlysporadic and limited contamination fromnearby urban sources of pollution.We collected the bulk precipitation

(that is, the composite of wet plus dryprecipitation) at two stations that were Ikm apart and whose elevations differedby 50 m, over weekly intervals for 150consecutive weeks (6). The data from thetwo stations do not differ significantly(P > .05) and are averaged for the pres-ent purposes. The pH measurementswere converted to H+ concentrationsprior to averaging and then reconvertedto pH. The pH measurements andchemical analyses were made on the daythe sample was collected (7).

Figure IA shows the pH of the bulkprecipitation samples through the 3-yearcollection period. Many of thepH valuesare very low. Moreover, there is a cleardecline over the 3-year period. Thedownward trend is highly significant, asindicated by the linear regression ofpHagainst time (pH = -0.0053 * (weeknumber) + 5.43; P<.01). The analysisthus indicates that the pH was 5.43 witha standard error of 0.11 at the beginningof the study (week 1, June 1975) and de-clined at an average rate of 0.0053 unitper week with a standard error of 0.0013over the succeeding 3-year period. Atthe end of 3 years, the average pH haddeclined 0.80 unit, from 5.43 to 4.63, oralmost an order of magnitude in H+ con-centration.The downward trend inpH appears to

cut across seasons and across years withvery different weather patterns. For ex-ample, the winter and spring of 1976-77were extremely dry and produced verylow snowpack at the station. There is noevidence for the exclusive association ofdeclining pH with upslope weather, aswould be expected if the pH declinewere caused exclusively by the directmovement of pollutants from Denverover the station. Corroborating datafrom other locations are unavailable. Ap-parently no systematic, extended studyhas ever been done of precipitation pHin Colorado or in fact of any location inthe western United States, althoughseveral government agencies are now in-terested in beginning such studies.The increase in acidity can also be

expressed in terms of the H+ input (inmilligrams per square meter per week)to the watershed, which in a biogeo-chemical sense is more important thanpH. The results are very similar, how-ever. The regression line based on log-transformed values is given by log(H+ input) = 0.00455 * (week number) =

SCIENCE, VOL. 207, 11 JANUARY 1980

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1.43; P = .001. The average loading ratethus increased from 0.037 to 0.179 mgm-2 week-l over the 150 weeks of thestudy.

Figure lB shows the dissolved nitratenitrogen (N03--N) delivered weekly tothe watershed in bulk precipitation. Thedata have been log-transformed to re-duce the scatter of points. The increasein NO3--N (in milligrams per square me-ter per week) is statistically significant:log (NO3--N) = 0.0023 * (week num-ber) + 0.211; P = .003). The equationimplies an average NO3--N loading rateof 1.63 mg m-2 week-' at the beginningof the study and 3.60 mg m-2 week-1 150weeks later. This is a difference of 0.141mmole of NO3- per square meter perweek.A mole of NO3- derived from gaseous

NO, is presumably associated with amole of H+, as gaseous NO, compoundscombine with water to produce HNO2and HNO3 (8). Thus the increase inNO3--N over the 150-week period wouldbe associated with an increase in H+ of0.141 mmole m-2 week-1. This is exactlysufficient to account for the observed in-crease in the H+ loading rates (0.142mmole m-2 week-'). The amounts of nit-rite nitrogen (NO2--N) in our precipi-tation samples were consistently below 5percent of the NO3--N and can thereforebe omitted from consideration here.The average loading rate for sulfate is

18.0 mg m-2 week-1 over the 150 weeks.There is a suggestive but not statisticallysignificant (P > .05) trend toward in-creasing sulfate loading rates with time(9). Thus, even though sulfuric acid maybe a major contributor to total acidity,nitric acid appears to be the componentresponsible for the observed increase inprecipitation acidity.Our data indicate that the mean acidity

of bulk precipitation near the Continen-tal Divide is unexpectedly high and thatthe acidity of precipitation has increasedat a surprisingly fast pace over the past 3years. The downward trend inpH cannotbe extrapolated and may even reverse,but it is suggestive enough to deservecareful observation in the future. Thelow mean pH is noteworthy even if thedownward trend does not continue, how-ever. No local, sizable sources of pollu-tants are properly situated to account forthis trend easily. Two possibilities arethat complex and unsuspected mecha-nisms are moving pollutants substantialdistances from the front-range urban cor-ridor contrary to the prevailing patternsof air mass movement or that very wide-spread changes in precipitation cheipis-try are presently occurring in the wptt-em United States because of significantSCIENCE, VOL. 207, 11 JANUARY 1980

increases in the release of nitrogen oxidesfrom multiple sources throughout theWest. If the former possibility proves tobe correct, our results imply the existenceof powerful atmospheric dispersingmechanisms which can move acidiccomponents against strong prevailingweather patterns. If the second possi-bility proves to be correct, our resultsimply a very widespread change in pre-cipitation chemistry.

WILLIAM M. LEWIS, JR.MICHAEL C. GRANT

Department ofEnvironmental,Population, and Organismic Biology,University of Colorado, Boulder 80309

References and Notes

1. S. Oden, Water Air Soil Pollut. 6, 137 (1976).2. G. E. Likens and F. H. Bormann, Science 184,

1176 (1974); , J. S. Eaton, R. S. Pierce, N.M. Johnson, WaterAir Soil Pollut. 6, 435 (1976).

3. G. E. Likens, N. M. Johnson, J. N. Galloway,F. H. Bormann, Science 194, 643 (1976).

4. The density of roads and the amount of trafficnear the station are extremely low. The nearesthard-surface road (a two-lane highway) to thesampling station is 5 km away. Seasonal surgesin traffic do not appear to be related to H+ input.

5. R. Barry, Arct. Alp. Res. Occas. Pap. 3 (1972);Arct. Alp. Res. 5, 89 (1973).

6. Wet precipitation includes snow and rain withits dissolved and particulate chemical com-ponents. Dry precipitation includes all materialsaccumulating on the collectors during periodswhen there is no wet precipitation. Large Plexi-glas collectors (0.25 m2) were used so that accu-rate analysis could be made even for weeks withvery limited amounts of wet precipitation. Thecollectors were mounted 4 m off the ground in

clearings shielded by trees and were heated toprevent freezing in winter. The collectors wereconnected to borosilicate glass bottles with sur-gical rubber tubing incorporating a vapor bar-rier. Exposure of distilled water to the collectorfor extended periods indicated no significantcontamination of the sample by the collector.The collecting surface was rinsed weekly andthe bottles and tubes were replaced weekly withcleaned, sterilized bottles and tubes. Additionaldetails are given in W. M. Lewis, Jr., and M. C.Grant, Water Resour. Res. 14, 1098 (1978).

7. ThepH was measured with a Radiometer model26 meter equipped with a combination electrodeand standardized against pH 4.0 and 6.0 buffersjust prior to use. The sample was then filteredthrough glass-fiber paper (mean pore size, 2Am), and the particulate and dissolved com-ponents were analyzed separately. Most impor-tant to the present discussion are the nitrate andsulfate analyses, but all major cations and anionswere included in the analysis (total measuredcation input, 1.08 meq mi2 week-'; total mea-sured anion input, 1.17 meq mi2 week-'; num-ber of samples = 150). Nitrate analysis wasdone by reduction of nitrate to nitrite and forma-tion of an azo dye [K. Bendschneider and R. J.Robinson, J. Mar. Res. 11, 87 (1952); E. D.Wood, F. Armstrong, F. Richards, J. Mar. Biol.Assoc. U.K. 47, 23 (1967)]. The sulfate analysiswas done by a barium chloride precipitationmethod [H. L. Golterman, Methods for Chem-ical Analysis ofFresh Waters (International Bio-logical Program Handbook No. 8, BlackwellScientific, Oxford, 1969)].

8. H. Georgii, J. Geophys. Res. 68, 3963 (1963).9. This amount of sulfate is equivalent to 0.376 meq

mi2 week-'. The standard error for weekly nmea-surements is 0.041 meq in2 week-', and the co-efficient of variation is 121 percent (number ofsamples = 129).

10. This research was supported in part by the For-est Service, U.S. Department of Agriculture,through the Eisenhower Consortium for West-ern Environmental Forestry Research (pub-lished as Eisenhower Consortium Joumal SeriesPaper No. 29) and by the University of Coloradothrough Biomedical Research Support grant153-2281.

11 December 1978; revised 18 April 1979

Direct Measurement of Solar Luminosity Variation

Abstract. Two rocketflights ofan absolute pyrheliometer, separated by 30 months,indicate an increase in solar luminosity (solar constant) of 0.4 percent. The signifi-cance of this result is considered in light of the instrument performance during therocket flights and ofpre- and postflight intercomparisons with independently main-tained pyrheliometers. There is a high probability that the measured difference isreal. Additional observations are required to determine whether the difference re-

sults from random fluctuations in solar luminosity, a nonrandom change of shortduration, or a sustained change that has climatological significance.

The energy that determines the earth'sclimate is derived from solar optical radi-ation through interactions with the ter-restrial ocean, landmass, and atmo-sphere. Recent models of climate in-dicate that small, persistent changes insolar luminosity have substantive Cdima-tological impact. The magnitude of theeffect depends on the model used, butmost predict pronounced sensitivity ofthe climate to changes in solar luminosi-ty (1-3).

Evidence from many sources (4-7) de-picts the earth's climate during the pastbillion years as nominally tropical, withnumerous cyclic excursions to colderperiods, some of which were severeenough to cause extensive glaciationfrom the poles to the middle latitudes.

Models predict that such climatologicalchanges can be produced by systematicvariation of the earth's insolation by aslittle as 0.5 percent per century (8).The high sensitivity of the earth's cli-

mate to insolation and the probable rela-tions between solar magnetic activityand solar luminosity (9) are compellingarguments for long-term monitoring ofsolar luminosity. This task requires ex-periment platforms outside the atmo-sphere and pyrheliometric instrumenta-tion capable of measuring solar flux ac-curately [that is, with uncertainties inlong-term precision (if not accuracy) ofno more than 0.1 percent]. The platformshave been available for some time, butthe required instrumentatiqn is a recentdevelopment (10). The experimental re-

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