Aciditv of atmospheric aerosols A summary of data concerning their chemical nature and amounts of acid
Roger L. Tanner Brookhaven National Laboratory Upton, N . Y. I I973 Brian P. Leaderer John B. Pierce Foundation Laboratory New Haven, Conn. 0651 9
John D. Spengler Harvard University School of Public Health Boston, Mass. 021 15 Atmospheric sulfuric and nitric acids are atmospheric contaminants resulting in large part from oxidation of sulfur and nitrogen oxides emitted from stationary and (for NO,) mobile combustion sources. These acids are implicated in the acidification of precipitation over large areas of the eastern United States, southeastern Canada, northern Europe, and Japan. In addition, sulfuric acid aerosols have recently been found to elicit significant alterations in the mucociliary clearance rates in healthy nonsmoking humans ( 1 ) . Concern over the potential adverse health and welfare effects associated with strong acids in aerosols has intensified in recent years with the prospect of increased use of high-sulfur fuels (principally coal). Weak and strong acids exist in the atmosphere in both gaseous and particulate form. Organic acids, the concentrations of which are 0.3-10 pg/m3, have been reported in rural, urban, and marine atmospheres ( 2 ) . Nitric acid is present in the atmosphere in concentrations ranging from 15 pg/m3 of sulfuric acid have been observed for periods of 6 h or more. Recent experimental and theoretical work (18) has indicated that sulfate aerosol more acidic than NH4HS04 should occur only when SO2 is being oxidized rapidly, when the concentration ratios of SO2 to NH3 are high, or when the equilibriumvapor pressure of NH3 over the partially neutralized H2S04 droplet exceeds the ambient NH3 partial pressure. The situation is more complicated-in ambient aerosols, in which partially ammoniated sulfate is present in mixtures with nitrate, carbonaceous, and other aerosol components in solid or liquid form, since these may affect its neutralization rate. In particular, recent data (19) suggest that the degree of mixing in the “well-mixed’’ boundary layer is sometimes inadequate to prevent vertical stratification of strong acid levels, since ammonia is emitted (and nitric acid removed by dry deposition) mostly at the earth’s surface. We need further information on the vertical distribution of strong acid and related species before emission and neutral-
0013-936X/81/0915-1150$01.25/0
@ 1981 American Chemical Society
ization rates may be used to predict acid levels in ambient aerosols.
I
FIGURE 1
Meens and standard deviation ofH + ( d f l
More acid in rural areas Measurements of aerosols collected in the eastern U S . indicate that strong acids are present more frequently, and in larger amounts, in rural samples than in urban samples. Twelve-hour average concentrations of strong acid, expressed as HzSO4, as high as 17 pg8/m3in samples from the Allegheny mountains of Pennsylvania have been reported (13). It is likely that strong acid concentrations were substantially higher for periods within the 12-h sampling time. High acid concentrations associated with high levels of sulfate aerosol were reported in samples from theCreat Smoky Mountains during periods of regional haze in September 1978 (20). Continuous 6-h samples were collected and analyzed for strong acid at four locations in the New York subregion for a 15-day period during the summerof 1977 (16,17,21).Thelocation of the sites and the average values of a e r m l strong acid are shown in Figure 1. The average aerosol strong acid concentration was highest for the rural sampling site, High Point, located west of New York City. The suburban site on Long Island exhibited the next highest averageconcentration, and the lowest averages were found at the urban sites in New York City and New Haven, Conn. Aerosol acidity at all sites was higher during periods of higher ozone concentrations. The authors suggest that samples collected east of New York City may be a combination of “fresh” and “aged” aerosol. In urban atmospheres, sulfate anion is usually present as neutral ammonium sulfate or partly neutralized ammonium bisulfate (NH4+/S042molar ratios between 1 and 2) (12.16, 17). Presumably, this greater extent of neutralization of urban sulfate-containing aerosols is ascribable to additional ammonia sources in urban areas. However, it may also he attributable, in part, to analytical interferences from coarse, basic particles, such as resuspended cement dust which may be present at high mass loadings in urban aerosols. Urban aerosol acidity measurements should be made on samples from which coarse particles have been excluded by virtual impaction or by cyclone separation techniques. Acid in urban areas It has been demonstrated that acid aerosol episodes can occur in urban areas (8-10, 12). With the aid of a
Ish Polnt 49 t 4.w
continuous sulfate monitor to distinguish sulfuric acid from ammoniasulfate neutralized species (22). two episodes of three days or longer duration were recorded in St. Louis, Mo., occurring concurrently with regional hazes in July 1977and February 1978. Cobourn et al. (12) ascribe the occurrence of H2S04 in urban aerosols to conditions in which atmospheric ammonia concentrations are exceptionally low. (Ammonia is temporarily d e pleted from the atmosphere.) Similar temporal variations in the acid fraction of sulfate-containing aerosols have been observed in St. Louis (12). Chicago (23). and the New York area (21). All sites were found to exhibit drastic changes in aerosol acidity, often within only a few hours. Figure 2 illustrates the magnitude of changes observed in the acid content and their relation to sulfate and ammonium in aermls observed at Brookhaven National Laboratory during a summer sampling period.
Pronounced diurnal patterns, with highest acid levels in midafternoon and lowest values at night, have been observed in St. Louis (12) and in the New York subregion (17). The observed diurnal patterns for acid aerosol for a 15-day summer sampling period at three locations in the New York subregion, with a 6-h time discrimination, are shown in Figure 3. The diurnal acid aerosol pattern at all three sites followed the diurnal omne pattern observed at those sites. At this time, the relative contributions of SO2 oxidation chemistry, temporal variations in NH3 and SO2 emission rates, diurnal variations in turbulent mixing rates, and varying height of the mixing layer to the diurnal patterns are not known. Quantities vary In summary, atmospheric aerosols do contain varying quantities of both weak and strong acids. To date, measurements have demonstrated that Volume 15. Number 10, October I981
1151
strong acid concentrations equivalent to as much as about 20 pg/m3 of sulfuric acid may exist in the ambient atmosphere for periods of several hours. Aerosols are likely to be more acidic in rural regions and are more likely to be neutral in urban regions. However, acid episodes have been
measured in urban areas, and strong acid may frequently be present in low amounts in “fine” aerosol particles. The principal deleterious effects of strongly acidic atmospheric aerosols (mainly associated with sulfates and nitrates) are related to the potential health effects of acid sulfates, their role
in visibility reduction, corrosive effects on physical structures, and their possible role as input to acidic precipitation. The first three of these effects may be addressed by assessing the frequency, duration, and level of exposure through monitoring the strong acid content of the fine aerosol particles (as determined by extraction and titration of samples) from selected urban and rural areas of the U.S. Special emphasis should be on those areas for which high annual means of sulfate or nitrate have been observed. A deficiency in evaluating the potential of acidic aerosols to contribute to the levels of strong acid in precipitation is the paucity of measurements from aloft of strong acid in aerosols. Also, greater efforts are necessary to determine vertical profiles of gaseous ammonia and nitric acid in the ambient atmosphere by means of recently developed continuous analysis techniques. This is especially important in view of the dynamic interactions possible between these gases and acidic aerosols. Referenees ( I ) Lipp?ann, M., et al. “Effect of Sulfuric Bronchial ClearAcid Mist on Mu-iliary ance in Healthy Nonsmokinp. - Humans.” J . Aerosol Sci., in press. (2) Kelserides. G., et al. Almm. Emiron. 1576,
August 1977
10,603-610. (3) Spicer. C. W. In “Adv. Environ. Sci. Technol.”; Wiley and Sons:New York, 1977; Vol. 7. pp. 163-262. (4) Rahn. K. A., et 81. Ann. N.Y.Aeod. Sci. 1979,322,143-151.
‘IGURE 3
Hew York Summer Aerosol Study of 19TI Mumal paitems for strong acid (H+)
( 5 ) lunge. C.; Scheich, G. Armm. Emiron.
:F
0
High Point, N.J. (12 days)
1971,5,165-175. (6) Charlsnn, R. J.. et al. Atmos. Emiron. 1978,/2,39-53. (7) Brosset, C.; Andreasson, K.; Fern, M. Almos. Enuiron. 1975,9,631-642. (8) Brosset, C. Almos. Emiron. 1978, /2, 25-38, (9) Tanner, R. L., et al. Atmos. Emiron. 1977, , I a
