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Aerosol mass concentration determination using piezoelectric crystal sensors was investigated. Five areas of influence were studied: temperature, humidity, particle collection characteristics, response linearity, and mass sensitivity. A theoretical review is included. Neither air stream temperature nor humidity fluctuations were compensated by the use of a reference crystal. The temperature induced error was satisfactorily reduced by minimizing the inlet temperature change rate. The humidity induced error resulted principally from moisture absorption and desorption by the aerosol deposit. The observed linear response limits ranged from 0.2 to 6 mug/mm2 for various aerosols and instrument designs. No relationship between the linearity limit and the point of complete saturation was apparent. The mass sensitivity was a function of the deposit size and location. Energy trapping theory helped to predict the mass sensitivity distribution. The mass sensing ability decreased for particle sizes beginning at approximately 2 mum diameter, reaching essentially zero at 20 mum. The use of viscous crystal coatings appeared to improve the sensing ability in the 2 to 20 mum size range.

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does not necessarily represent the position of the Puget Sound Air Pollution Control Agency.108(e) of the Clean Air Act. Diffusion modeling need not be dropped under such a program, but should be continued for informational and advisory purposes. The legal test for approval of a source would be that it met the requirements of the code of practices, not that it would meet all air quality standards. This would eliminate an open invitation in most present indirect source regulations for either the proponents or opponents of projects to litigate modeling studies. ConclusionsIndirect Source Regulations present problems to the reviewing authority. The effectiveness of these regulations is reduced since the authority lacks control over tailpipe emissions and its sole means of control is provided through diffusion modeling of alternative source configurations. The modeling process lacks sufficient reliability to be the only criterion for approval since it is subject to inherent modeling error and to error involved in estimating future worst case, short term traffic, and atmospheric conditions. This problem, and the problems of equity posed by ISR, lead to enforcement problems which further reduce effectiveness.Enforceability of indirect source regulations might be improved by requiring new indirect sources to conform to a code of practices of measures which would reduce air pollutant concentrations, rather than to require that they model so as not to predict standard violations. This would prevent litigation based on modeling studies and thereby improve enforcement, as well as tie the ISR process to Section 108(e) planning.

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