REACTIVITY ADJUSTED VOC MEASUREMENTS BY AIRTRAK - A FEASIBILITY STUDY

Measurements of ozone precursors are essential to the understanding of ozone-precursor relationships in urban and rural areas. The feasibili ty of earning out reactivity-adjusted VOC measurements using the Airtr ak instrument has been investigated by modeling simulations and experi ments. To quantitatively relate VOC to an ambient measurement of SP (s mog produced), SP-VOC relationships have been derived by simulating fl ow-mode experiments performed using an Airtrak instrument. It has been found that, when the airstream contains only VOC and NO and the NO co ncentration is constant, the relationship between SP and VOC concentra tions can be represented by a quadratic polynomial. This has been demo nstrated both by flow-mode experiments and through model simulations. Because the response time in the photolytic chamber is approximately 3 0 min, any fine structure in the VOC concentrations is smoothed out, b ut time-averaged (e.g., 1 h average) concentrations agree well with in let concentrations. To apply this methodology to the measurements of r eactivity-adjusted VOC concentrations in ambient air samples, ozone an d NOE have to be removed from air sample streams before VOC measuremen ts are made, and the NO concentration in the Airtrak reactor has to be constant. NO concentration in the reactor can be maintained at a cons tant level by feedback control, and efficient scrubbers for ozone and NO2 are available.