Atmospheric acidity measurements on allegheny mountain and the origins of ambient acidity in the Northeastern United States

Abstract Atmospheric acidity as HNO3(g), SO2(g), and aerosol H+ was measured on Allegheny Mountain and Laurel Hill in southwest Pennsylvania in August 1983. The aerosol H+ appeared to represent the net after H2SO4 reaction with NH3(g). The resulting H + SO 4 2− ratio depended on SO42− concentration, approaching that of H2SO4 at the highest SO42− concentrations. The atmosphere was acidic; the average concentrations of HNO3 (78 nmole m−3) and aerosol H+ (205 nmole m−3), NH4+ (172 nmole m−3) and SO42− (201 nmole m−3), and the dearth of NH3(〈 15 nmolem−3), show that the proton acidity (HNO3, H2SO4) of the air exceeded the acid-neutralizing capacity of the air by a factor of > 2, with one 10-h period averaging 263 and 844 nmolem−3 for HNO3 and aerosol H+, respectively. SO2 added another 900 nmole m−3 (average) of potential H+ acidity. HNO3 and aerosol H+ episodes were concurrent, on 7–8 day cycles, unrelated to SO2 which existed more in short-lived bursts of apparently more local origin. NOx was sporadic like SO2. Laurel and Allegheny, separated by 35.5 km, were essentially identical in aerosol SO42−, and in aerosol H+, less so in HNO3 and especially less so in SO2; apparently, chemistry involving HNO3 and aerosol H+ or SO42− was slow compared to inter-site transport times (1–2 h). From growth of bscat and decline of SO2 during one instance of inter-site transport, daytime rate coefficients for SO2 oxidation and SO2 dry deposition were inferred to have been, respectively, ~ 0.05 and ⩽ 0.1 h−1. HNO3 declined at night. Aerosol H+ and SO42− showed no significant diurnal variation, and O3 showed very little; these observations, together with high PAN NO x ratios, indicate that regional transport rather than local chemistry is governing. The O3 concentration (average 56 ppb or 2178 nmolem−3) connotes an oxidizing atmosphere conducive to acid formation. Highest atmospheric acidity was associated with (1) slow westerly winds traversing westward SO2 source areas, (2) local stagnation, or (3) regional transport around to the back side of a high pressure system. Low acidity was associated with fast-moving air masses and with winds from the northerly directions; upwind precipitation also played a moderating role in air parcel acidity. Much of the SO2 and NOx, and ultimately of the HNO3 and aerosol H+, appeared to originate from coal-fired power plants. An automotive contribution to the NOx and HNO3 could not be discerned. Size distributions of aerosol H+ and SO42− were alike, with MMED ~ 0.7 μm, in the optimum range for efficient light scattering and inefficient wet/dry removal. Thus, light scattering and visual range degradation were attributable to the acidic SO42− aerosol, linking the issues of acid deposition and visual air quality in the Northeast. With inefficient removal of aerosol H+, and inefficient night-time removal of HNO3, strong acids may be capable of long-distance transport in the lower troposphere. We obtained an accounting of aerosol mass in terms of composition, including aerosol H2O which was shown to account for much of the light scattering.