The almost total anthropogenic control of the nitrogen cycle has led to wide ranging trans-national and national efforts to quantify the effects of reactive nitrogen on the environment. A number of monitoring techniques have been developed for the measurement of nitric acid and subsequent estimation of nitrogen deposition within large networks and for process studies on shorter measurement campaigns. We discuss the likelihood that many of these techniques are sensitive to another important gas-phase component of oxidized nitrogen: dinitrogen pentoxide (N_2O_5). We present measurements using a MARGA wet annular denuder device alongside measurements of N_2O_5 with a discussion of evidence from the laboratory and the field which suggests that alkali- and aqueous-denuder measurements are sensitive to the sum of HNO_3 + 2N_2O_5. Nocturnal data from these denuder devices should be treated with care before using HNO_3 concentrations derived from these data. This is a systematic error which is highly dependent on ambient conditions and is likely to cause systematic misinterpretation of datasets in periods where N_2O_5 is significant proportion of NO_y. It is also likely that deposition estimates of HNO_3 via data obtained with these methods is compromised to greater and lesser extents depending on the season and environment of the sampling location.
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