Determination of Aerosol Strong Acidity Losses Due to Interactions of Collected Particles: Results from Laboratory and Field Studies

-Existing methods of measuring atmospheric aerosol strong acidity adequately prevent neutralization of fine-particle acidity by removing coarse alkaline particles and gaseous ammonia from air samples. However, these techniques do not consider particle interactions on the collection medium; therefore, they may still underestimate the actual aerosol acidity. Assessment of acid neutralization due to such interactions is made possible using annular denuder technology in conjunction with a newly designed filter pack. The amount of sulfate-related acidity neutralized by the collected ammonium nitrate (and possibly ammonium chloride and organic acid ammonium salts) is determined. Laboratory data suggest that large fractions of sulfate-related aerosol acidity are neutralized by ammonium nitrate particles during collection on filter media. Field data from the Harvard Acid Aerosol Health Effects Study also suggest that ammonium nitrate and possibly other ammonium salts, such as ammonium chloride, neutralize collected acid aerosols. For low-acid aerosol concentrations, the correction factor is significant; whereas, for high-acid concentrations, correction is negligible. Key word index: Aerosol acidity, sulfuric acid, ammonium nitrate, acid neutralization, annular denuder, filter pack particle interactions.