A CALIBRATED PEROXY RADICAL CHEMICAL AMPLIFIER FOR GROUND-BASED TROPOSPHERIC MEASUREMENTS

A calibrated peroxy radical chemical amplifier (PERCA) instrument has been developed at the University of East Anglia (UEA) and deployed in several recent field measurement campaigns. The UEA PERCA utilizes mod ulated chemical amplification of NO2 by HO2 and RO2 radicals in the pr esence of 3.0 ppmv NO and 7.0% vol/vol CO, with the amplification fact or or chain length approximately equal to Delta NO2/([HO2] + [RO2]). N O2 measurements and calibrations are made in the lineal response regim e of an improved Scintrex LMA-3 NO2-luminol chemiluminescence detector with an estimated systematic uncertainty of +/- 10% and a precision o f 5%. A novel calibration source of CH3O2 radicals has been developed. It is used routinely during field campaigns to measure chain lengths of 175 +/- 15 for a new inlet system. The source is based upon 253.7 n m photolysis of CH3I in air and is capable of producing between 25 and 100 pptv CH3O2 radicals. The detection limit of the PERCA varies from 0.2 to 2 pptv for a 30 min averaging time and is largely dependent up on the stability of the sum of the concentrations of ambient NO2 and O -3 (O-x) which contribute to a background signal. The estimated system atic uncertainty in the measurements of HO2 and RO2 is +/- 30%. Observ ations of peroxy radicals made during the recent WAOWE'94 and WAOSE'95 field measurement campaign on the north Norfolk coast of England are presented to demonstrate the performance of the PERCA and are discusse d briefly in terms of nighttime radical chemistry and net photochemica l production of O-3 and peroxyacetyl nitrate (PAN).