Interferences in photolytic NO 2 measurements: explanation for an apparent missing oxidant?

Abstract. Measurement of NO 2 at low concentrations (tens of ppts) is non-trivial. A variety of techniques exist, with the conversion of NO 2 into NO followed by chemiluminescent detection of NO being prevalent. Historically this conversion has used a catalytic approach (molybdenum); however, this has been plagued with interferences. More recently, photolytic conversion based on UV-LED irradiation of a reaction cell has been used. Although this appears to be robust there have been a range of observations in low-NO x environments which have measured higher NO 2 concentrations than might be expected from steady-state analysis of simultaneously measured NO, O 3 , j NO 2 , etc. A range of explanations exist in the literature, most of which focus on an unknown and unmeasured “compound X ” that is able to convert NO to NO 2 selectively. Here we explore in the laboratory the interference on the photolytic NO 2 measurements from the thermal decomposition of peroxyacetyl nitrate (PAN) within the photolysis cell. We find that approximately 5 % of the PAN decomposes within the instrument, providing a potentially significant interference. We parameterize the decomposition in terms of the temperature of the light source, the ambient temperature, and a mixing timescale ( ∼ 0.4 s for our instrument) and expand the parametric analysis to other atmospheric compounds that decompose readily to NO 2 (HO 2 NO 2 , N 2 O 5 , CH 3 O 2 NO 2 , IONO 2 , BrONO 2 , higher PANs). We apply these parameters to the output of a global atmospheric model (GEOS-Chem) to investigate the global impact of this interference on (1) the NO 2 measurements and (2) the NO 2 : NO ratio, i.e. the Leighton relationship. We find that there are significant interferences in cold regions with low NO x concentrations such as the Antarctic, the remote Southern Hemisphere, and the upper troposphere. Although this interference is likely instrument-specific, the thermal decomposition to NO 2 within the instrument's photolysis cell could give an at least partial explanation for the anomalously high NO 2 that has been reported in remote regions. The interference can be minimized by better instrument characterization, coupled to instrumental designs which reduce the heating within the cell, thus simplifying interpretation of data from remote locations.