Interferences in Photolytic NO2 Measurements: Explanation for an Apparent Missing Oxidant?
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Atmos. Chem. Phys., 16, 4707–4724, 2016 www.atmos-chem-phys.net/16/4707/2016/ doi:10.5194/acp-16-4707-2016 © Author(s) 2016. CC Attribution 3.0 License. Interferences in photolytic NO2 measurements: explanation for an apparent missing oxidant? Chris Reed1, Mathew J. Evans1,2, Piero Di Carlo3,4, James D. Lee1,2, and Lucy J. Carpenter1 1Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, Heslington, York YO10 5DD, UK 2National Centre for Atmospheric Science (NCAS), University of York, Heslington, York YO10 5DD, UK 3Centre of Excellence CEMTEPs, Universita’ degli studi di L’Aquila, Via Vetoio, 67010 Coppito, L’Aquila, Italy 4Dipartmento di Scienze Fisiche e Chimiche, Universita’ degli studi di L’Aquila, Via Vetoio, 67010 Coppito, L’Aquila, Italy Correspondence to: Chris Reed ([email protected]) Received: 6 October 2015 – Published in Atmos. Chem. Phys. Discuss.: 23 October 2015 Revised: 1 April 2016 – Accepted: 6 April 2016 – Published: 15 April 2016 Abstract. Measurement of NO2 at low concentrations (tens ship. We find that there are significant interferences in cold of ppts) is non-trivial. A variety of techniques exist, with regions with low NOx concentrations such as the Antarctic, the conversion of NO2 into NO followed by chemilumines- the remote Southern Hemisphere, and the upper troposphere. cent detection of NO being prevalent. Historically this con- Although this interference is likely instrument-specific, the version has used a catalytic approach (molybdenum); how- thermal decomposition to NO2 within the instrument’s pho- ever, this has been plagued with interferences. More recently, tolysis cell could give an at least partial explanation for the photolytic conversion based on UV-LED irradiation of a re- anomalously high NO2 that has been reported in remote re- action cell has been used. Although this appears to be ro- gions. The interference can be minimized by better instru- bust there have been a range of observations in low-NOx ment characterization, coupled to instrumental designs which environments which have measured higher NO2 concentra- reduce the heating within the cell, thus simplifying interpre- tions than might be expected from steady-state analysis of tation of data from remote locations. simultaneously measured NO, O3, jNO2, 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 NO2 selectively. Here we explore in the 1 Introduction laboratory the interference on the photolytic NO2 measure- ments from the thermal decomposition of peroxyacetyl ni- Accurate quantification of atmospheric nitrogen oxide (NOx, trate (PAN) within the photolysis cell. We find that approx- which is predominantly NO C NO2 but includes small con- imately 5 % of the PAN decomposes within the instrument, tributions from NO3,N2O5, HONO, HO2NO2, etc.) concen- providing a potentially significant interference. We param- trations is crucial for many aspects of tropospheric chemistry. eterize the decomposition in terms of the temperature of the NOx plays a central role in the chemistry of the troposphere, light source, the ambient temperature, and a mixing timescale mainly through its impact on ozone (O3) and hydroxyl (OH) (∼ 0:4 s for our instrument) and expand the parametric anal- radical concentrations. O3 is a greenhouse gas (Wang et al., ysis to other atmospheric compounds that decompose read- 1995) that adversely impacts human health (Mauzerall et al., ily to NO2 (HO2NO2,N2O5, CH3O2NO2, IONO2, BrONO2, 2005; Skalska et al., 2010) and leads to ecosystem damage higher PANs). We apply these parameters to the output of (Ainsworth et al., 2012; Ashmore, 2005; Hollaway et al., a global atmospheric model (GEOS-Chem) to investigate the 2012). It is produced through the reaction of peroxy radi- global impact of this interference on (1) the NO2 measure- cals (HO2 and RO2) with NO (Dalsøren and Isaksen, 2006; ments and (2) the NO2 : NO ratio, i.e. the Leighton relation- Lelieveld et al., 2004). The OH radical is the primary oxidiz- ing agent in the atmosphere (Crutzen, 1979; Levy, 1972) as Published by Copernicus Publications on behalf of the European Geosciences Union. 4708 C. Reed et al.: Interferences in photolytic NO2 measurements it controls the concentration of other key atmospheric con- Matsumoto and Kajii, 2003), CRDS (cavity ring-down spec- stituents such as methane (CH4), carbon monoxide (CO), troscopy; Osthoff et al., 2006), and QCL (quantum cascade and volatile organic compounds (VOCs). It is both produced laser; Tuzson et al., 2013). However, probably the most ex- through the reaction of NO with HO2 and is lost by its re- tensively used approach has been based on the chemilumi- action with NO2. NO2 itself also poses a public health risk nescent reaction between NO and O3. This exploits the re- (Stieb et al., 2002). Thus, understanding the sources, sinks, action between NO and O3 (Reaction R6), which generates ∗ 2 and distribution of NOx is of central importance to under- a electronically excited NO2 ( B1) molecule which decays to standing the composition of the troposphere. its ground state through the release of a photon (Reaction R7) During the daytime there is fast cycling between NO and (Clough and Thrush, 1967; Clyne et al., 1964). NO2, due to the rapid photolysis of NO2 and the reaction of NO C O ! NO∗ (R6) NO and O3 to form NO2 (Kley et al., 1981). 3 2 ∗ 3 NO2 ! NO2 C hv .> 600nm/ (R7) NO2 C hv .< 410nm/ ! NO C O. P/ (R1) 3 This forms the basis of the chemiluminescence analysis of O2 C O. P/ C M ! O3 C M (R2) NO (Drummond et al., 1985; Fontijn et al., 1970; Kelly et al., O C NO ! O C NO (R3) 3 2 2 1980; Peterson and Honrath, 1999). The number of photons ∗ Placing NO2 into steady state and assuming that these three emitted by the decay of excited NO2 to NO2 is proportional reactions are the only chemistry occurring leads to the to the NO present before reaction with O3 (Drummond et al., Leighton relationship, φ (Leighton, 1961), in Eq. (1). 1985). The photons emitted are detected by a cooled photo- multiplier tube (PMT) with the sample under low pressure k [NO][O ] 1 D 1 3 D φ (1) (to maximize the fluorescence lifetime of the NO∗) in order j [NO ] 2 NO2 2 to yield a signal which is linearly proportional to the num- The quantities in the relationship are readily measured and ber density of NO in the sample gas (Fontijn et al., 1970). deviations from unity have been interpreted to signify miss- Quenching of the NO2 excited state occurs due to a range ing (i.e. non-ozone) oxidants of NO. These perturbations of atmospheric compounds (N2, Ar, CO, CO2, CH4,O2, He, have been used to infer the existence of oxidants such as per- H2, and H2O) (Clough and Thrush, 1967; Drummond et al., oxy radicals, halogen oxides, and the nitrate radical in the 1985; Zabielski et al., 1984). Quenching is minimized by op- atmosphere, which have subsequently been confirmed by di- erating at high vacuum to reduce collision probability. How- rect measurement (Brown et al., 2005; Mannschreck et al., ever, quenching still occurs; thus, it is necessary to calibrate 2004; Trebs et al., 2012; Volz-Thomas et al., 2003). the detectors response (sensitivity) to a known concentration The concentration of NOx varies from > 100 ppb (parts of NO regularly. Changing ambient humidity in the sample per billion) next to roads (Carslaw, 2005; Pandey et al., 2008) has a marked effect on drift in sensitivity and necessitates to low ppt (parts per trillion) in the remote atmosphere (Lee being corrected for, sample drying, or sample humidifying to et al., 2009). Direct transport of NOx from polluted to re- mitigate. mote regions is not efficient, because NOx is removed from With NO chemiluminescence analysers it is also possible the atmosphere on a timescale of around a day by the reac- to analyse NO2 regarding whether it is first converted to NO, tion of NO2 with OH and the hydrolysis of N2O5 on aerosol either catalytically (typically heated molybdenum) as in Re- surfaces (Brown et al., 2004; Dentener and Crutzen, 1993; action (R8) (Villena et al., 2012) or by converting NO2 into Riemer et al., 2003). Instead, reservoir species such as per- NO photolytically (Ryerson et al., 2000), exploiting Reac- oxyacetyl nitrate are made in polluted regions (where con- tion (R1). centrations in both NO and peroxyacetyl precursors such as x C ! C acetaldehyde are elevated) and are subsequently transported Mo 3NO2 MoO3 3NO (R8) to remote regions, where they thermally break down to re- To measure NO and NO2, the sample flows through the NO2 lease NOx (Fischer et al., 2014; Moxim et al., 1996; Roberts to NO converter (of either type) to the reaction chamber, et al., 2007). C ∗ where the NO O3 reaction occurs and the decay of NO2 • • C CH3CHO C OH C O2 ! CH3C.O/OO C H2O (R4) to NO2 allows the concentration of NO NO2 in the air to • be measured. Then, the sample flow is switched to bypass the CH3C.O/OO C NO2 C MCH3C.O/O2NO2 C M (R5) NO2 to NO converter. Now, only NO present in the sample is The equilibrium between peroxyacetyl radicals, NO2, and detected in the chemiluminescence reaction. The NO signal PAN (Reactions R4 and R5) is highly temperature-sensitive. is then subtracted from the NO C NO2 (NOx) signal, giving ◦ Thus, the PAN lifetime changes from 30 min at 25 C (Bri- the NO2 signal. Both techniques for converting NO2 to NO dier et al., 1991) to 5.36 years at −26 ◦C (Kleindienst, 1994).