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 modulated chemical amplification of NO2 by HO2 and RO2 radicals in the presence of 3.0 ppmv NO and 7.0% vol/vol CO, with the amplification factor or chain length approximately equal to ΔNO2/(2>+2>). NO2 measurements and calibrations are made in the linear response regime of an improved Scintrex LMA-3 NO2-luminol chemiluminescence detector with an estimated systematic uncertainty of ¿10% and a precision of 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 nm 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 upon the stability of the sum of the concentrations of ambient NO2 and O3 (Ox) which contribute to a background signal. The estimated systematic uncertainty in the measurements of HO2 and RO2 is ¿30%. Observations 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 discussed briefly in terms of nighttime radical chemistry and net photochemical production of O3 and peroxyacetyl nitrate (PAN). ¿ 1997 American Geophysical Union