New Instrumentation for Airborne Acid Rain Research

Abstract

Abstract Instrumental methods have been developed for continuously measuring sulfur dioxide, aerosol sulfur, nitrogen oxides, nitric acid, and ammonia from an aircraft sampling platform. Emphasis has been on modifications to reduce limits of detection to allow sampling at ambient concentrations in the vicinity of clouds. Characterizations based on ambient sampling data, in conjunction with data on cloud water composition and knowledge of the interphase behavior of nitrogen and sulfur oxides and oxyacids, will be used for quantitative evaluation of species contributions to the ionic content of precipitation. A flame photometric instrument with enhanced sensitivity for airborne measurements of both gaseous and aerosol sulfur compounds has been designed and constructed. Ambient airborne levels of less than 1 ppb may be detected using sensitivity enhancement, and effects of pressure changes on instrumental response have been eliminated. Sensitive, real-time measurements of NOx and HNO3 have been achieved by modified commercial ozone-chemiluminescence detectors. These modifications include: (a) operation at low pressure and large throughput; (b) use of an enlarged, more efficient reaction chamber; and (c) use of a pre-reactor to obtain more stable background. The detection limit for a dual channel instrument for aircraft use is < 0.2 ppb (5 second time constant). Nitric acid is determined continuously by difference using a nylon filter in one stream. An instrument has been constructed for airborne measurements of atmospheric ammonia based on a continuous, fluorescence-derivatization technique. Ammonia is scrubbed into aqueous acid, derivatized and measured in a small filter fluorimeter (HPLC detector). Current detection limit is about 0.25 ppb NH3 with response time of ∼2.5 min. Data acquired from instrument test flights and preliminary characterization flights with this instrument package are discussed.