The Measurement of Peroxy Radicals in Remote Atmosphere: The Perca Technique

Peroxy radicals act as chain carriers in trace gas oxidation reactions. Total peroxy radicals are HO2 + RO2. HO2 is the hydro peroxy radical and RO2 is the family of alkyl peroxy radicals, where R is an alkyl group. The oxidation chemistry driven by peroxy radicals is vital for controlling tropospheric ozone production/loss rates. The thesis details the University of Leicester PEroxy Radical Chemical Amplifier (PERCA (Version 3.5)) instrument. Peroxy radical measurements using a single- and dualchannel PERCA instrument were taken in two relatively clean sites: the tropical rainforest boundary layer in Borneo for two campaigns in 2008 and the tropical marine boundary layer at Cape Verde for three campaigns in 2009. In the tropical rainforest boundary layer in Borneo, the diurnal cycle of peroxy radicals’ concentrations showed an asymmetric shape that was considerably shifted towards the afternoon owing to the high [HO2+RO2] measured for midday, these can persist during the afternoon under low NOx conditions. Night-time [HO2+RO2] were measured up to 29 pptv (parts per trillion by volume). The calculation of production and loss terms in the ozone budget indicated that there was net ozone production in the rainforest. Ozone production rates were greatest when [NOx] were high, while increasing [isoprene] decreased ozone production. The peroxy radical measurements for the Seasonal Oxidant Study (SOS) campaigns at Cape Verde show a diurnal cycle that reaches a maximum at midday and a minimum during the night. Net ozone production was negative (i.e. an ozone loss) and was maximised for summer months. An analysis of the Cape Verde ozone loss rate calculated with and without including halogen species is presented, and concludes that whilst ozone production was mainly NOx controlled, halogen chemistry could have a major contribution to the ozone loss.