Strong and Weak Acidity of Aerosols Collected over the Northeastern United States Ronald J. Ferek,*+Allan L. Lamus,$ Phillip L. Haagenson,' and John W. Wlnchesiert Department of Oceanography, Florida State University, Tallahassee, Florida 32306, and National Center for Atrnospherlc Research, Boulder, Colorado 80307
Total sulfate, acidity, and principal cations were measured in aerosol samples collected from an aircraft over the northeastern United States and at ground level on filters protected by a denuder tube for NHS. Aerosol acidity was determined by aqueous extraction of the filters and titration with NaOH. Strong and weak acid components were resolved by using Gran plots, and for 30 fall and summer samples the weak acidity comprised 9 f s = 26 f 12% of the total acidity. The strong acidity averaged 25 f 18% of the total sulfate equivalents. Hydrogen ion plus ammonium ion balanced closely the sulfate ion concentration with the ratio of equivalents of [(H+) + (NH4+)]/(S042-)= 1.03 f 0.14 for 36 samples. A phase diagram for the H20-H2S04-(NH4)2S04system was used to determine that the aerosol particles were liquid at the ambient relative humidity in 24 of 47 cases. For the 24 liquid samples the calculated pH averaged 0.11 f 0.59.
Introduction The existence of acid aerosols has been known for some time. Early measurements by collection on filters and subsequent titration were made during polluted conditions in Los Angeles (I) and London (2), where sulfuric acid levels on the order of 15-80 pg of H2S04/m3and 7-18 pg/m3 were found, respectively. Brosset (3) and Brosset and Ferm (4) used a similar technique to measure acid aerosols in Sweden and found elevated levels of strong acidity and sulfate in air masses transported from the south. Under these conditions the strong acidity accounted for up to one-third of the total sulfate, which was usually in the range 10-20 pg/m3. Charlson et al. (5) developed an in situ method that utilized the relation between light scattering by aerosols and relative humidity to detect the presence of acid sulfates. They found that frequently in the St. Louis region up to half or more of the submicrometer aerosols contained strong acid. Stevens et al. (6) collected aerosols at a site in North Carolina using a denuder for gaseous NH, before filtration. Over a 4-day period the H+ concentration varied from near zero to more than 50% of the S042-anionic equivalent concentration, and the sum of the H+ and NH4+invariably equaled the S042-equivalents in the samples. Later (7) at a rural site in Tennessee over the course of 6 days all the aerosol samples were markedly acidic, and the average H+ con+ Florida
State University.
* National Center for Atmospheric Research. 0013-936Xl83/0917-03 15$01.5010
centration was equivalent to 5.15 f 2.79 pg of H2S04/m3. Again the H+plus NH4+ equivalents equaled the S042-, and H+ averaged 0.47 equivalent fraction of the total sulfate. NO,- composed only a small fraction of the sulfate. On the basis of these observations it was considered likely that a large fraction of the aerosol over the northeastern United States is composed of strong acid sulfates. These previous studies, however, were limited to a few ground-level sites where the influence of local and soilderived emissions may have altered the aerosols such that they were not generally representative of the entire boundary layer. In addition, limited data on weak acids have been reported. Since acid aerosols may play an important role in such environmental problems as the formation of acid rain, inhalation of toxic aerosols, and deposition of corrosive acids on surfaces, it was decided to undertake a study of the acidity of aerosols as part of the Acid Precipitation Experiment (APEX) conducted by the National Center for Atmospheric Research (NCAR) in cooperation with the US. Environmental Protection Agency (EPA). In order to gain an understanding of the regional character of the aerosol throughout the boundary layer, samples were collected from an aircraft over a wide range of locations and meteorological conditions at different times of the year (spring and fall, 1979, and summer, 1980).
Experimental Section Aerosols were sampled principally from aircraft and also at ground level as part of three month-long field programs: APEX 11,Mar 31-Apr 22,1979; APEX 111, Nov 1-24,1979; APEX IV, July 22-Aug 15,1980. The region of study was the northeastern United States from Indiana to the Atlantic coast with a large portion of the sampling taking place in the Ohio River Valley and the Adirondack region of upstate New York. Most samples were collected in somewhat polluted air masses that were generally near precipitating frontal systems or well-defined convective showers. The approach was to sample air within the inflow to storms prior to interaction with precipitation or clouds. Efforts were made to avoid major industrial and urban plumes, and the aircraft flew routes that covered a few tens of kilometers or more during most sampling periods. Therefore, the samples were generally representative of regional air mass characteristics. Aerosol samples were collected through an isokinetic inlet mounted on the top of the cabin of an NCAR
0 1983 American Chemlcal Society
Environ. Sci. Technol., Vol. 17, No. 6, 1983 315
Table I. Aerosol Acidity Results APEX 11, Mar 31-Apr 22, 1979 acidity filter measurementsC
date (1979)
locat ion
Mar 3 1 Apr 4 Apr 8 Apr 8 Apr 12 Apr 16 Apr 17 Apr 17 Apr20 Apr20 Apr 22
Eastern NE over Lake Erie NE of Columbus, OH SE of Columbus, OH Albany to Ithaca, NY N of Albany, NY 35 mi E of NYC 35 mi E of NYC SEOH SEOH Central IL
altitude: f t msl
RH, %b
1700 1600
69 54
1800
88
2000 2200 2000
50 24 86 56 72 27 23 84
1000 4000 2000 2000 1100
3 S
total acidity (H,S04 total acidity, equiv), nequivl p g of m3 sO4'-/m3 4 46 46 17
