Laboratory studies on the retention of nitric acid, hydrochloric acid and ammonia on aerosol filters

Retention efficiencies of nitric acid, hydrochloric acid and ammonia were measured for different filters, with particular emphasis on cellulose (CE) and cellulose acetate-nitrate (CA) materials. Gases were produced either by nebulising aqueous solutions or by a novel technique based on the desorption from ammonium salts deposited on quartz fibre (QF) filters. Efficiencies for pure acidic gases and ammonia on CE and CA ranged from very low (⩽3.6%) to low (∼10% for HNO3 on CE). In contrast, if acidic gases and ammonia were supplied in equimolar concentrations, they were retained (almost) completely on CE, with high efficiency on CA (60–80% for NH3+HNO3; 20–45% for NH3+HCl), also with high efficiency on glass fibre filter, but with very low efficiency on QF and Teflon (Tf) filters (<1%). For CA, retention efficiencies were found to increase with increasing relative humidity and to decrease with decreasing mean pressure at which the filters were exposed to the gases. Once retained on CA filters, the retained gases may be lost again during subsequent exposure to clean air.