Evaluating the uncertainties of thermal catalytic conversion in measuring atmospheric nitrogen dioxide at four differently polluted sites in China

► MoO and photolytic converters for measuring NO2 were compared at four field sites. ► Overestimation of NO2 by the MoO converter ranged from 6% to 280%. ► The overestimation depended on both air-parcel age and composition of NOz. ► A 'true' NO2 or a NOy system is recommended for areas away from fresh NOx emission sources.