Ambient ozone, as a secondary air pollutant in the troposphere, is a major threat to human health, plants and the environment. In order to develop effective air quality policy to minimise ozone pollution, it is important to gain a quantitative understanding of the chemical factors that drive tropospheric ozone production. There are a number of limitations and uncertainties in the current models and indirect methods used to estimate chemical ozone production rates. Here, an Ozone Production Rate (OPR) instrument is developed to fulfil the demand of accurately measuring ambient ozone production rates in the atmosphere. This prototype system aims to directly measure the in situ oxidant (Ox: O3 + NO2) production rate p(Ox) in ambient air. This thesis describes the OPR experimental methodology, instrument properties and system characteristics. Two field deployments (London and India) are comprehensively discussed, and correction approaches are implemented to improve measurement accuracy. The field measurement results indicated the measured p(Ox) levels could be used to interpret modelled Ox production rates, and changes in ambient oxidant level. The OPR system could be a useful tool to determine the balance between advection and chemical production in controlling local ozone levels, and hence support ozone control policy.