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

A widely used method for measuring nitrogen dioxide (NO₂) in the atmosphere is the conversion of NO₂ to nitric oxide (NO) on the hot surface of a molybdenum oxide (MoO) catalyst followed by the chemiluminescence detection of NO. Although it has long been recognized that this type of conversion may suffer from the positive interference of other oxidized nitrogen compounds, evaluations of such interference in the atmosphere are scarce, thus rendering it difficult to make use of a large portion of the NO₂ or NO_x data obtained via this method (often denoted as NO₂* or NO_x*). In the present study, we compared the MoO converter with a selective, more accurate photolytic approach at four differently polluted sites in China. The converter worked well at the urban site, which was greatly affected by fresh emissions, but, on average, overestimated NO₂ by 30%-50% at the two suburban sites and by more than 130% at the mountain-top site during afternoon hours, with a much larger positive bias seen during the top 10% of ozone events. The degree of overestimation depended on both air-parcel age and the composition of the oxidation products/intermediates of NO_x (NO_z). We attempted to derive an empirical formula to correct for this overestimation using concurrently measured O₃, NO, and NO₂* at the two suburban sites. Although the formula worked well at each individual site, the different NO_z partitions at the sites made it difficult to obtain a universal formula. In view of the difficulty of assessing the uncertainties of the conventional conversion method, thus limiting the usability of data obtained via this method in atmospheric research, we suggest that, in areas away from fresh NO_x emission sources, either a more selective NO₂ measurement method or a NO_y (NO_x and its reaction products and intermediates) instrument should be adopted.