Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 1 2 X class copernicus.cls. E T A , and L. J. Carpenter 1,2 1 , J. D. Lee 3,4 , P. Di Carlo 1,2 , M. J. Evans 1

Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, National Centre for Atmospheric Science (NCAS), University of York, Heslington,Centre York, YO10 of 5DD, Excellence CEMTEPs, Universita’ degli studi di L’Aquila, ViaDipartmento Vetoio, 67010 di Coppito, Scienze Fisiche e Chimiche, Universita’ degli studi di L’Aquila, Via Vetoio, 67010 Interferences in photolytic NO C. Reed Correspondence to: J. D. Lee ([email protected]) missing oxidant? Heslington, York, YO10 5DD, UK 2 UK 3 L’Aquila, Italy 4 Coppito, L’Aquila, Italy Date: 30 March 2016 measurements: explanation for an apparent 1

with version 2015/04/24 7.83 Copernicus papers of the L Manuscript prepared for Atmos. Chem. Phys. Discuss. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , 2 2 NO NO that has , IO 2 within the 2 2 NO NO 2 NO O 3 CH , 5 ” that is able to convert O X 2 environments which have x ,N 2 NO NO 2 concentrations such as the Antarc- x HO ( NO 2 NO 2 etc. A range of explanations exist in the literature 2 NO j into NO followed by chemiluminescent detection of NO , 3 2 : NO ratio i.e. the Leighton relationship. We find that there O 2 NO NO concentrations than might be expected from steady state analysis of at low concentrations (10s ppts) is non-trivial. A variety of techniques 2 2 for our instrument) and expand the parametric analysis to other atmo- NO NO 4 s . 0 selectively. Here we explore in the laboratory the interference on the photolytic ∼

2 , Higher PANs). We apply these parameters to the output of a global atmospheric 2

NO measurements from the thermal decomposition of peroxyacetyl nitrate (PAN) within

NO 2

are significant interferences in cold regions with low in the future, thus simplifying interpretation of data from remote locations. is likely instrument specific, it appears that the thermal decomposition to tal designs which reduce the heating within the cell seem likely to minimize the interference measurements and (2) the tic, the remote Southern Hemisphere and the upper troposphere. Although thisinstrument’s interference photolysis cell may give anbeen explanation reported for in the remote anomalously regions. high Better instrument characterization, coupled to instrumen-

BrO model (GEOS-Chem) to investigate the global impact of this interference on (1) the timescale ( tion in terms of the temperature of the light source, the ambient temperature and a mixing strument providing a potentially significant interference. We parameterize the decomposi- spheric compounds that decompose readily to be robust there have been a range of observations in low the photolysis cell. We find that approximately 5 % of the PAN decomposes within the in- based on UV-LED irradiation of a reaction cell has been used. Although this appears to NO however this has been plagued with interferences. More recently, photolytic conversion being prevalent. Historically this conversion has used a catalytic approach (molybdenum); NO to

exist, with the conversion of Measurement of Abstract most of which focus on an unknown and unmeasured “compound measured higher simultaneously measured NO, Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | + (1) (R1) (R2) (R3) etc.) concentra- 2 ) and hydroxyl (OH) 3 NO . These perturbations 2 O NO plays a central role in the HO x , due to the rapid photolysis 2 which is predominantly NO NO and is lost by its reaction with x NO 2 NO , HONO, 5 HO O (Leighton, 1961), in Eq. (1). 2 φ (Kley et al., 1981). ,N 2 3 3 is of central importance to understanding the NO x ) with NO (Dalsøren and Isaksen, 2006; Lelieveld 2 NO to form NO 3 P) 3 O and RO 2 HO is a greenhouse gas (Wang et al., 1995), adversely impacts hu- 3 NO + O( ), carbon monoxide (CO) and volatile organic compounds (VOCs). O + M 4 3 → 2 O φ CH = → + NO ] ] 3 2 2 into steady state and assuming that these three reactions are the only chem- 410nm) O 2 itself also poses a public health risk (Stieb et al., 2002). Thus understanding < [NO ( 2 → P) + M 2 3 NO and the reaction NO and hv [NO][O NO 2 NO

1 but includes small contributions from j + . k

2 2 2 + O( + NO NO During the daytime there is fast cycling between NO and

2 3

been interpreted to signify missing (i.e. non ozone) oxidants of The quantities in the relationship are readily measured and deviations from unity have Accurate quantification of atmospheric nitrogen oxides ( 1 Introduction 1 = reaction of peroxy radicals ( such as methane ( (Ainsworth et al., 2012; Ashmore, 2005; Hollaway et al., 2012). It is produced1979; through Levy the II, 1972) as it controls the concentration of other key atmospheric constituents NO tions is crucial for manychemistry aspects of of the tropospheric troposphere, chemistry. radical mainly concentrations. through its impactman on health ozone (Mauzerall ( et al., 2005; Skalska et al., 2010) and leads toet ecosystem al., damage 2004). The OH radical is the primary oxidizing agentIt in is the both atmosphere producedNO (Crutzen, through the reactionthe of sources, NO sinks with andcomposition distribution of the of troposphere. of O O Placing

istry occurring leads to the Leighton relationship, NO Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | (R5) (R4) C (Bridier ◦ and peroxy-acetyl x NO (Reaction R6) which generates (Fisher et al., 2014; Moxim et al., 3 x O and PAN (Reactions R4 and R5) is 2 NO (parts per billion) next to roads (Carslaw, NO + M 2 4 O 2 100 ppb NO 2 on aerosol surfaces (Brown et al., 2004; Dentener > + H 5 • O 2 C(O)O ) molecule which decays to its ground state through the C (Kleindienst, 1994). 3 1 ◦ from polluted to remote regions is not efficient, because B x 2 26 CH C(O)OO ( varies from 3 − ∗ 2 NO x  species in the remote atmosphere have been made over the last at CH x NO NO → + M 2 NO 2 years + O • + NO • . Multiple in-situ techniques are available such as LIF - Laser Induced Fluorescence . This exploits the reaction between NO and

3 is removed from the atmosphere on a timescale of around a day by the reaction of with OH and the hydrolysis of N O

CHO + OH C(O)OO

x 2 3 3 years Measurements of The concentration of

and extensively used approach has been based on the chemiluminescent reaction between NO and QCL - Quantum Cascade Laser (Tuzson et al., 2013). However, probably the most 40 (Matsumoto and Kajii, 2003), CRDS - Cavity Ring Down Spectroscopy (Osthoff et al., 2006), The equilibrium between peroxy-acetylhighly radicals, temperature sensitive. Thus theet al., PAN lifetime 1991) changes to from 5.36 30 min at 25 a electronically excited

NO NO 2009). Direct transport of nitrate are made in polluted regions (where concentrations in both CH 2005; Pandey et al., 2008) to low ppt (parts per trillion) in the remote atmosphere (Lee et al., and Crutzen, 1993; Riemer et al., 2003). Instead, reservoir species such as peroxy-acetyl 1996; Roberts et al., 2007). CH Volz-Thomas et al., 2003). by direct measurement (Brown et al., 2005, Mannschreck et al., 2004, Trebs et al,. 2012, ides, and the nitrate radical in the atmosphere which have subsequently been confirmed have been used to infer the existence of oxidants such as peroxy radicals, halogen ox-

regions where they thermally breakdown to release precursors such as acetaldehyde are elevated) and are subsequently transported to remote Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ∗ 2 (R8) (R6) (R7) NO , and 2 H , if it is first He 2 excited state , 2 2 O NO , NO 4 CH , 2 to NO converter (of either CO , 2 CO NO , Ar , 2 reaction occurs and the decay of N is proportional to the NO present before 3 2 in the air to be measured. Then, the sample NO into NO photolytically (Ryerson et al., 2000), 2 5 2 to NO + O ∗ 2 NO to NO converter. Now, only NO present in the sample NO + NO 2 NO , the sample flows through the ), to yield a signal which is linearly proportional to the number 2 ∗ 2 600nm) NO NO + 3NO > 3 ( (Drummond et al., 1985). The photons emitted are detected by a cooled ∗ 2 3 hv MoO O + NO → 2 2 → NO 3 allows the concentration of

2

→ ) (Clough and Thrush, 1967: Drummond et al., 1985; Zabielski et al., 1984). Quenching

∗ 2 O NO With NO chemiluminescence analysers it is also possible to analyse

2 To measure NO and type) to the reactionto chamber where the is detected in the chemiluminescence reaction. The NO signal is then subtracted from the

Mo + 3NO exploiting Reaction (R1). flow is switched to bypass the

to mitigate. on drift in sensitivity and necessitates correcting for, sample drying, or sample humidifying concentration of NO regularly. Changing ambient humidity in the sample has a great effect still occurs thus it is necessary to calibrate the detectors response (sensitivity) to a known is minimised by operating at high vacuum to reduce collision probability, however quenching (Villena et al., 2012), or by converting H converted to NO, either catalytically (typically heated molybdenum) as in Reaction (R8) NO

NO + O release of a photon (Reaction R7) (Clough and Thrush, 1967: Clyne et al., 1964). tons emitted by the decay of excited NO Fontijn et al., 1970; Kelly et al., 1980; Peterson and Honrath, 1999). The number of pho- cence lifetime of the This forms the basis of the chemiluminescence analysis of NO (Drummondreaction et with al., 1985; photomultiplier tube (PMT) with the sample under lowdensity pressure of (to NO in maximize the theoccurs sample due fluores- gas to (Fontijn a et range al., of 1970). atmospheric Quenching compounds of ( the Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2 by NO 2 con- NO to 2 to NO 2 . 3 NO O recognised NO NO y are measured locations using 2 in the photolysis x chemiluminescent 3 NO O NO NO and NO by peroxy radicals produced varies greatly or calibration in by the 2 3 x O NO NO if a stable nighttime value is not obtainable). offset must often be taken when sampling a offset are typically made by taking a stable within the photolysis cell (Villena et al., 2012) 6 2 NO NO is oxidised to NO NO signal. Both techniques for converting 2 NO NO have been made in a range of low 2 photolytic conversion efficiency due to and by oxidation of NO. Where NO 2 2 NO NO species such as nitrous and nitric acid as well as organic nitrates y and more problematic, by peroxy radicals produced by e.g. PAN decom- ) signal giving the 3 x O NO ( 2

Yang et al., (2004) identified a statistical source of error when determining NO and Measurements of NO and

source of zero air (whichCorrection may for be used changing for cell (see Sect. 2.2, Eq.ambient 2) might air also is be not appropriate possible when i.e. instead in zero airchemiluminescence. which Yang necessarily et contains al. no demonstrate that uncertainties in averaging signals which nighttime value (Lee et al., 2009), whilst mediately obvious. Small corrections for discrepancy in the Leighton ratio as owing to generally lower LED temperatures, thus the effect on the Leighton ratio is not im- by 1 channel by switching the UV elements onform and the off thermal periodically decomposition there ofverter. may PAN However, within be we the have a still-warm, shown greater there but not to be illuminated, less PAN decomposition in a switched system two ways; by release of position within the inlet. This oxidation acts to perturb the observed Leighton relationship in in the inlet by

to the limit of detection (LOD). Corrections may be necessary for oxidation of technique in these locations can be challenging (Yang et al., 2004), often operating close

Honrath, 1999; Zhang et al., 2008). Measurement of (McClenny et al., 2002). the chemiluminescence technique (Huntrieser et al., 2007; Lee et al., 2009; Peterson and

NO + NO and the possibility for positive interference from thermal decompostion of NO photolysis products of VOCs reacting with sion has been previously demonstrated to be affected by a negative interference from the (Dunlea et al., 2007; Grosjean and Harrison., 1985; Winer et al., 1974). Photolytic conver-

itive responses to NO can be subject to interference. Catalytic conversion has been well documented to have pos- Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2 2 NO NO J , 3 O : NO ratio , with 5. The outcome 2 results in the least ∼ 2 NO NO species) to interfere with y ) locations, such as Antarctica and in the to then calculate x x NO systems to investigate the interference on the 7 x NO conversion efficeincy etc. 2 NO converter/chemiluminescence systems. In Sect. 4 we anal- limit of detection result large errors in 2 σ NO measurements. In Sect. 2 we provide details of the experimental 2 from PAN. In Sect. 5 we evaluate the impact of this interference on 2 NO , uncertainties in NO 2 10 times the 1 NO ∼

concentrations are higher than would be expected from the observed NO, measurements and on the Leighton relationship through the use of a global model and

2 2 Here we explore the potential of PAN (as a probe for other NO Some measurements made in remote (low

et al., 2011). An alternative explanation would be an unknown interference on the measured in trace quantities (Cantrell et al., 2003; Frey et al., 2013, 2015; Hosaynali Beygi retical realms by theorizing high turnover of short-lived species which may only have been typically relying on a mystery unmeasured oxidant, or pushing known chemistry into theo- Various explanations have been posited in order to overcome the apparent oxidation gap, nal concentration profile. along with reasonable concentrations of other oxidants (peroxy radicals, halogen oxides). measurement increasing its apparent concentration, if this interference has a similar diur- chemiluminescence

concentrations of PAN into provide conclusions in Sect. 6. studies undertaken. In Sect. 3 we describe the resultsyse of the experiments potential introducing for errors differing withmeasurement different of NO averaging PMT counts to determine NO and NO being that using weighted geometric mean, rather than unweighted or arithmetic mean for fall below NO either NO or (Bauguitte et al., 2012; Frey et al., 2013, 2015; Hosaynali Beygi et al., 2011). Measured extreme bias exhibited when signals fall below a signal to noisebias ratio from measurment uncertainty. This error is in addition toopen any ocean artifact have signal at times biasing identified an unexplained imbalance in the Leighton relationship Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 3 2 O NO aver- inch PFA pptv 4 / 1 are measured whilst sampling analyser (Sect. 2.1.2) on which 2 x NO NO analyser (Sect. 2.1.1) – on which the majority 8 x NO analyser x analyser from Air Quality Design, Inc. (AQD) is a custom dual channel x ). The wetted surfaces of the instrument are constructed of x NO with the NO signal measured with the BLC lamps active and inactive as de- converter efficiency is by internal automatic gas phase titration of NO with 2 2 converters are described in Sect. 2.2. In Sect. 2.3 the LIF instrument used to 2

NO NO

NO

Both instruments are calibrated for NO by internal, automatic standard addition. Calibra- Both instruments feature independent mass flow controlled sample flows on each chan-

tion for

tubing, with the exception of 316 stainless steel unions/MFC internals. to form aged over 1 minute. The dual channel design means that there are effectively two separate scribed by Lee et al., (2009). Artifacts in both NO and nel (NO and NO 2.1.1 Laboratory NO only temperature controlled BLC experiments were performed. zero air.

The laboratory instrument designed for fast response and very low limit of detection (LOD) of 2.5 of the experiments were performed and the “aircraft”

ments were employed; the “laboratory” converters – commonly referred to as Blue Light Converters (BLCs). Two similar instru-

Inc. (Golden, Colorado, USA) instruments equipped with UV-LED based photolytic Chemiluminescent measurements were performed using dual channel Air Quality Design 2.1 Instrumentation Sect. 2.7 describes residency time tests. Then in Sect. 2.6 we describe the experimental methodology of PAN interference tests and The by acetone photolysis in Sect. 2.4. We provide details of the zero air generation in Sect. 2.5. In Sect. 2.1 we describe the twoprovide chemiluminescence a reference instruments analysis used is for described. the We describe analysis. our protocol for production of PAN 2 Experimental details Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ) 2 σ NO ) on chan- 5% . Photochemi- by connection 1 ± y − ( 1 NO − signal. ppt 2 orders of magnitude) uncertainty arises from 1 NO ∼ − 5% ± counts s , and a parallel mode with 100 % 1 − flows, but which during zero both the sample 3 9 with the other to provide a constant, fast mea- O x ) respectively. The x NO NO . Alternatively, a single channel can be used with the BLC 2 NO reaction time by diverting the sample flow to a pre–reactor where 3 converter with a total sample flow of 2 standard L min O 2 standard concentration, the error of the sample and standard mass flow + NO ) of NO and NO NO reactions are accounted for and subtracted from the . Hz flow. In this way, photon counts arising from slower ( 3 3 O O 5 pptv

. measurement – the other 60 % of the duty cycle is devoted to NO (40 %) and to mea- 2

2 The nominal sensitivity of the instrument is 3.5 and 4.0 ∼

in e.g. humidity, which affects chemiluminescent quenching, occurred. The 1 min LOD (2 igated by performing all experiments under an overflow of stable zero air, thus no changes nel 1 (NO) andthe channel error 2 in the (NO and controllers, and the reproducibility of the sensitivity determination. Sensitivity drift was mit- is

increasing the duty cycle of the cal zero is determined for 30 seconds every 5 minutes. A photochemical zero is acquired by alkene + the detection cell though which usually only greater than 99 % of NO is reacted. The pre–reactor is a Teflon volume immediately prior to gas and

converter and a total sample flow of 1 standard L min to replicate different instrument designs; a switching mode with 40 % duty cycle of the vatory GAW station (Lee et al., 2009). In these experiments both modes were used in order of a catalytic converter to the inlet as is the set-up at the Cape Verde Atmospheric Obser- suring zero (20 %). In this case the second channel might be used for NO in a switching mode so that it is active for only 40 % of the duty cycle to provide NO and surement (1

to analyse NO with one channel, and immediately in front of the MFC flow control/low pressure side of the system. It is possible share the same vacuum pump – an Edwards XDS 35i. One channel is equipped with a BLC paths and share identical duplicate equipment; ozonizers, MFCs, PMTs etc. Both channels NO chemiluminescence instruments working in parallel. Both channels have identical flow Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | is (2) . ) on . [Ox] atm 5% k (Ryerson ± , 1 ( 2 C 0 pptv ◦ 1 . analyser with analyser with 1 − x x ∼ ppt NO NO 1 ) is − converter are primar- σ fluxes on an airborne 2 x NO NO to NO over a narrow wave- counts s 2 NO measurement to that provided by molyb- 2 NO 10 i ) t ] Ox ) respectively. The 1 min LOD (2 [ x k − jt NO is the residence time within the photolysis cell. Here − ( t exp − 1 analyser h  x per channel at ambient temperature and pressure. t ] 1 analyser, also from AQD, operates similarly to the lab − x Ox [ jt k NO converters + converter were carried out at ambient temperature and pressure; 20 2 2 jt

 NO

= The nominal sensitivity of the instrument is 8.3 and 11.6 Photolytic converters employ Reaction (R1) to convert

CE 1 standard L min platform at reduced pressure. However, in this study the sample flow rate waschannel a 1 (NO) constant and channel 2 ( the concentration and rate constant of anyet oxidant al., that 2000). reacts with NO to form NO determined by Eq. (2) where denum catalysts (Ridley et al., 1988; Ryerson et al., 2000). The conversion efficiency is temperature close to ambient. The special requirements for this UV-diodes which require active Peltier/forced air-cooling in order to maintain an operating the exception of the BLC. This is of a non-standardily design because that of uses the six high more sample powerful flow rates needed to measure length band, thus providing a more selective ther with variations implementation, though similar operation and results. Experiments with ei- tems have also been developed subsequently (Pollack et al., 2011; Sadanaga et al., 2010) Design and manufactured according to their proprietary standards (Buhr, 2004, 2007). Sys- Photolytic converters for the two chemiluminescent systems were supplied by Air Quality use on the ground. It can therefore be considered analogous to the lab some alterations to make it suited to aircraft operation. These changes do not affect its 2.2 NO

2.1.2 Aircraft NO The aircraft Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , 1 ), C 1 − (3) ◦ − nm ) cavity 1 95 − . s . All exper- 0 value of the 2 C j ◦ − > , UV Hex, Norlux nm photons cm which, with a standard flow rate mL is the quantum yield (dimensionless) of φ ), 2 ) depending on the combination of lamp units 11 1 cm − ( ), and so, the rate of production of additional NO s j 2 ( is the spectral photon flux ( 2 F NO λ ...0.6 ), NO 2 1 . )d − is the temperature (Sander et al., 2011). The s = 0 T λ, T j ( φ ) , gives a sample residence time of 0.96 s. Additional lamp end units λ, T 1 ( − σ ) inch Swagelok fittings acting as an inlet and outlet for the sample gas. λ ( 4 / F 1 is the rate constant ( j max Z min λ λ .

photo-dissociation, and

) = 2

The conversion efficiency of the standard BLC with a sample flow of 1 standard L min The volume of this illuminated sample chamber is 16 is the absorption cross section of

T atm (

1 iments were carried out at with sample gas at ambient temperature and pressure; 20

used whilst the external temperature of the converter was typically 34 to 45 was between 22 and 42 % ( were also supplied by AQD. Standard BLCs consist of two ends housing the UV-LEDs (1 W, 395 2.2.1 Standard BLC Teflon-like material block is housed which serves as a highly UV reflective ( elements and how efficiently the power is used. section with rubber gaskets forming an air-tight seal. Within the centre section asection propriety are converter is practically determined by the irradiance photolysis power of the UV emitting Corp) within a heat sink to which is attached a cooling fan. The ends arethought bolted which to the a sample gas central flows (Buhr, 2007). On two of the opposing sides of the centre of 1 standard L min NO

σ In Eq. (3) j The rate constant of photolysis of beyond that in the original sample is given in Eq. (3). Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ANs, and (Day et al., P 2 , Nichia Corp] NO PNs, nm P , 2 NO determination to compare 2 concentrations directly (Dari- . It was therefore possible to 2 measurement, as opposed to which, with a standard flow rate C NO ◦ 2 NO mL NO after conversion into 3 ) and therefore lower residence time, to HNO 12 300 Torr ∼ ANs), and P ). 1 − 5s . , gives a sample residence time of 0.60 s resulting in a conversion ), lower pressure ( = 6 1 1 − j − converters is desirable in order to properly know the source of any “arti- 2 , and without Peltier cooling reached 77 C NO ◦ PNs), alkyl nitrates ( signal. P 2

. A detailed description of the TD-LIF instrument can be found in Di Carlo et al. (2013), 3 NO

The volume of this illuminated sample chamber is 10 The Thermal Dissociation Laser Induced Fluorescence (TD-LIF) system is a custom in-

(1.5 standard L min The high power BLC of the aircraft instrument is designed to operate at a higher flow rate 2.2.2 High power BLC

fact” strument developed for aircraft and ground-based observations of cooled to 47 flective Teflon with inlets at opposing ends. The high power BLC lamps are actively (Peltier) nitrates ( this reason a greater number (six) of morelamps powerful are UV-LEDs placed [2 W, evenly 395 along two sides of a cylindrical cavity of the same highly UVelements re- via the temperature controller. Laser induced fluorescence (LIF) provides a direct 2000) and, coupled with a thermal dissociation inlet system, allows measurement of peroxy that of the standard BLC to allow fastare time used resolution in measurements order from that an the conversion aircraft. efficiency For be acceptable under these conditions. The control the internal temperature of the BLC by varying theof power 1 supplied standard L to min theefficiency of Peltier 93 % ( 2.3 TD-LIF analyser chemiluminescence with conversion. Awith the direct BLC method of HNO with a short description givenSalisburgo here. et It al., 2009; uses Matsumoto LIF and to Kajii, detect 2003; Matsumoto et al., 2001; Thornton et al., 2002). The TD-LIF comprises four main parts: the laser source, the detection cells system, Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , ). s (1 Torr pptv , a repetition W respectively into 3 HNO , and the last three heated at 2 ANs, and NO with a power of 3.8 P . The common inlet system is split in 1 nm − PNs, from a cylinder (NIST traceable) diluted in cells, respectively (Di Carlo et al., 2013). P 2 being MFC controlled. The time resolution of 3 L min 13 2 NO NO HNO ANs, and and the detection limits are: 9.8, 18.4, 28.1, and 49.7 P pulse-width. The detection cells system comprises four identical Hz ns PNs, C, to thermally dissociate P ◦ , 2 and 20 NO kHz ) for

(Di Carlo et al., 2013). To minimize quenching due to atmospheric molecules, and = 2

2

show the reaction sequence by which PAN is formed by acetone photolysis (Singh et al., 1987) and later by (Warneck and Zerbach, 1992; Flocke et al., 2005). Reactions (R9)–(R13) it was prepared by the photolysis in air of acetone and NO as described by (Meyrahn et al., In order to test the sensitivity of the instrument to peroxy-acetyl nitrate (PAN) interferences 2.4 PAN preparation

S/N the measurements is 10

by standard addition of known amount of checked for background by an over-flow of zero air in the detection cells; and calibrated to further reduce the background (Dari-Salisburgo et al., 2009). The TD-LIF is routinely four channels: one at200, ambient 400 temperature to and measure 550 This increases the fluorescence lifetime and facilitates the time-gating of the photomultiplier zero air – the flow of both zero air and a rotary vane pump to maintain a flow of 6 2013; Dari-Salisburgo et al., 2009). The pump system includes a roots blower coupled to rate of 15 detection, minimizing the non-fluorescence light that reaches the detector (Di Carlo et al., NO therefore increase the sensitivity of the TD-LIF, each cell is kept at low pressure (3–4 that is a gated photomultiplier with lens and long pass filters to optimize the fluorescence in the centre of the cell. Perpendicular to both (laser beam and air flow) there is the detector formed by a cube and two arms where the laser beam passes through the sample air flow cells, one for each compound class, to allow simultaneous measurements. Each cell is

Physics, model Navigator I) that emits light at 532 the inlet system and the pumps. The laser source is a Nd:YAG pulse doubled laser (Spectra- Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | (R9) (R15) (R14) (R13) (R12) (R10) (R11) , which 2 NO 14 2 NO 2 • 3 2 + CH 2 • 2 C(O)O C) acetone permeation oven consisting of a reservoir of HPLC • 2 3 ◦ + NO CO CH • 3 ONO O O + HO C(O)O → 3 3 • 2 3 2 CH 2 O CH CH 3 → CH CH (Pen-Ray mercury lamp, UVP). The Pyrex functions to filter wavelengths within the photolysis cell thus minimizing PAN photolysis (Mills et al., 2007). reacts stoichiometrically with the peroxy acetyl radical to form PAN. In prac- → → CH hv → → + NO 2 nm 2 2 • 2 nm → 2 NO + O + NO + NO + O CO +

2 • • 2 • 2 • 2 ) + O

O O C(O)O O O 3

3 3 3 • 3 3 3 Methyl nitrate is also found in the atmosphere through oceanic emission (Moore and A minor product also found in the photolysis of acetone is methyl nitrate, MeO Here,

Nwankwoala, 1995; Roumelis and Glavas, 1992; Warneck and Zerbach, 1992). Blough, 2002) and as a product of the thermal decomposition of PAN (Fischer and CH CH Zerbach, 1992). is presumed to make up the balance is shown in (R15) (Singh et al., 1995: Warneck and methyl nitrate (Williams et al., 2014) is shown in Reaction (R14) and formaldehyde which is typically approximately 1 % of the total yield (Mills et al., 2007). The proposed origin of the below 290 tred at 285 through which zero air flows; and a Pyrex glass photolysis cell illuminated by UV light cen- grade acetone (ACS grade, Acros) with a silicone permeation tube placed in the headspace

flow; a thermo-stated (30 flow control elements for the NO standard gas, the acetone flow, and the zero air diluent of > 95 % pure PAN (Flocke et al., 2005; Mills et al, 2007). The generator consists of: generator”, as used by Whalley et al. (2004), was employed to produce a consistent source tice, an excess of acetone is used to ensure that NO reacts completely. A dedicated “PAN

CH CH CH CH

(CH 1995) Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | was , and 2 x NO NO LIF analyser. Ad- 2 NO for 24 h when necessary. C ◦ C) molybdenum catalyst (Thermo Environ- ◦ 15 ) compressed air by subsequent filtering through d 40 T − , Sigma Aldrich) to ensure a consistent humidity through- × content of the zero air sources a direct measurement of 2 NO produced directly in the PAN generator, which other authors (Flocke et al., is emitted directly. 2 2 content of any zero air used is critical (more so than NO) in this study. In order NO NO PAN. Linearity and mixing ratio of the PAN output was confirmed by PAN-GC 2 NO

1 % ppbv ). Thus the NO content of both sources of zero air was considered to be insignificant

≤

The All flow rates within the PAN generator were calibrated using a Gilian Gilibrator-2 Air Zero air from both PAG 003 and filter stack was sampled by the

in Sect. 2.2.3 was used to compare the zero air sources (Table 1). required in order to avoid biasing the experimental procedure. The LIF instrument described

to determine the and comparably low. pptv served between the two sources of zero air, both showing below the LOD levels of NO (< 2.5 NO chemiluminescence analyser. No difference in the counts of the NO analyser was ob- Physics AG PAG 003 pure air generator (the industry standard) were both sampled by the ular Products) and activated charcoal (Sigma Aldrich) in ordergenerated to remove from ozone, the compressed air and filter cartridges system and zero air from an Eco out all experiments andA was second filter regenerated cartridge by placed heating after to the 250 molecular sievevolatile was organic packed with compounds Sofnofil (VOC) (Molec- which may be present in the compressed air. Zero air ditionally, high grade bottled zero air (BTCA 178, BOC Specialty Gasses) was analyzed that Zero air was generated from dried ( 2005) have found to be the main impurity, with the results presented2.5 in Sect. 2.6 Zero showing air 20.0 Flow Calibrator (Sensidyne). The PAN generator is capable of continuously producing 0.1– plete reduction back to NO using a heated (325 to quantify equipped with an ECD detector as described bymental). Whalley The et Laser al. Induced (2004), Fluorescence and instrument, also described in by Sect. com- 2.2.3, was used a cartridge of molecular sieve (13 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 1 x − NO , thus a 4– 1 NO mixture , BOC spe- 2 / − N ppb fluorescence was 1 2 improvement in ac- − analyser with excess NO in NO x fluorescence and scat- ppt 2 NO ppm NO after the acetone 180 counts s analyser from sampling ambient ∼ x whilst sampling 1.5 standard L min 0 ppbv 1 NO − ∼ analyser was then shut off so that the analyser is reduced by high humidity in am- x x 16 channel was counts s 2 NO NO NO analyser. Next, the acetone photolysis lamp of the x analyser fell to NO x into the PAN generator, the acetone flow was then adjusted NO 1 − mL min and the diluent flow of zero air adjusted to achieve the desired output 1 − improvement in zero background equates to 22–25 ) and the sensitivity of the d 1 . The counts recorded are therefore the sum of any − T 1 analysers were first calibrated for sensitivity/converter efficiency whilst sampling − x

. Table 1 shows the photomultiplier 2

10 mL min NO

counts s NO ∼

counts s NO signal measured by the NO value was recorded on the form PAN. Complete NO conversion to PAN was indicated by the fact that in all cases the flow vented to the atmosphere. The system was then allowedPAN generator to was stabilize switched until on a so consistent that acetone was photolysed in the presence ofwas NO illuminated to by the photolysis lamp; > 99.5 % conversion was reported by Mills et al., The 2.6 Experimental procedure point (- 40 filter system. 2 h. This is because the PAN from the generator is in zero air which hasreaction a cell very to low decrease. dew After establishing an NO flow (4.78 PAN generator always exceeded the sample requirements of the 4.5 curacy. Consequently, all dilution, zeroing, and PAN generation utilized the Sofnofil/Carbon zero air by overflowing the inlet from the internal source prior to the experimentbient for at sample least gas. This(humid) air means to that zero switching air the causes the sensitivitycialty to gases) of rise 0.5 slowlyto and the humidity insidemixing of ratio. the The internalanalyser zero was air sampling of zero the air from the PAN generator. Note that the total flow from the 3 the PAG 003 and BTCA 178 zero air sources in that a lower signal for of zero air. The dark counts of the PMTtered in laser the light. absence It of is laser clear that light the are Sofnofil/Carbon typically filter less systemobserved. than has Typical an sensitivity advantage of over the both LIF for Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2 NO signal 2 analyser x NO analyser was which can be NO x 2 was then added NO NO NO . signal observed while 2 NO NO 4000) of the ∼ . The corresponding directly (i.e. rather than as a con- 2 ppbv and a production of NO was observed indicating that peroxy radicals analyser was allowed to sample acetone that analyser inlet. In this way, any peroxy radicals 2 x signal relative to zero air was observed during . 17 within the BLC), a direct measurement of x channel when sampling various mixing ratios of 2 2 2 NO NO NO NO NO NO NO or production of NO gas. No additional NO

In order to investigate any interference from unreacted acetone, the To investigate any interference from unreacted peroxy radicals left over from photolysis It should be noted that sampling acetone does cause a small increase in the chemilu- To test whether the PAN generator produced any NO

this case each point represents a 10 min average as does the zero measurement. With sampling PAN from the generator lies within the noise of the zero signal measurement. In had been photolysed within the PAN generator in the absence of PAN from the generator as shown in Fig. 1. It is evident that the

lamps/assemblies and analyser operation modes. of acetone and pure zero air was measured in the LIF was recorded once stable. This procedure was repeated for various combinations of BLC allowed to sample the output of the PAN generatorthese with experiments at the any photolysis mixing lamp ratio off of i.e. a flow of excess acetone, produced in (R6), the downstream of the photolysis cell atexiting the the PAN generator will cause a loss of was employed using the TD-LIF described in Sect. 2.2.3. The measured signal relative to to achieve PAN mixing ratios of between 0.2 to 1.3 (2007) with the same system. The diluent flow from the PAN generator was then varied minescent zero count (a few hundred counts on a signal of (Clemitshaw et al., 1997) like chemistry within the BLC. every 5 minutes described in Sect. 2.1.1. reaction or surface loss) before entering the BLC, to cause an interference or ‘PERCA’ et al., 2007) and is accounted for in the measurements by the photochemical zero taken quantified. No loss of from acetone photolysis within the PAN generator do not have a long enough lifetime (self as acetone does react with ozone; this chemiluminescence interference is known (Dunlea sequence of decomposition of PAN to Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2 x ppt NO NO at 1000 2 ppt NO for 1 and 2 channel operation 1 analyser channels (which share is the number of points averaged − s x n NO where n √ 18 PFA inlet linking the PAN generator to the , and averaging 6000 points (i.e. 10 min) the preci- m pptv measurements 2 LOD of 9.8 Hz and subsequently NO within the photolytic convertor. 2 NO

– taking a 10

pptv It was therefore determined that only PAN could be an interfering species in the BLC

has an active cooling of the convertor. In Sect. 3.5 we investigate whether photolytic de- in Sect. 3.3, and in Sect. 3.4 we explore the interference in the aircraft instrument which tory instrument with a range of BLC convertors, eliminating any possibility for inlet effects instruments measurements. In Sect. 3.1 and 3.2 we explore the interference in the labora- The residence time of PAN in the 2.7 2.7 Residence time nitrate to discussion we address the possibility of photolytic and thermal dissociation of PAN or methyl yield from PAN synthesis by acetone photolysis i.e. 1 % (Mills et al., 2007). In the following interference observed is significantly greater than any expected or reported methyl nitrate discounted due to it being less thermally labile that PAN itself. Additionally, the percentage from the PAN generator. The small percentage of methyl nitrate which may be produced is In this section we describe experiments investigating thel impact of PAN on the two photolysis cell with the same generator. 3 Impact of PAN on NO et al., 2007), albeit at higher PAN mixing ratios and lower residence times within the acetone altering the sample flows through the each of the PAN (< 1 %) is produced by the PAN generator. This is less than previously estimated (Mills analysers (shown in Table 2) was varied bya varying common inlet). the Residence flowrespectively. times rate. were This 2.10 was and achieved 1.05 by (Lee et al., 2009). It is therefore conservatively estimated that < 10 0.1 an averaging time of 10 min the theoretical limit of detection is estimated to be less than sion improves by a factor of approximately 1/ Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ; an average of 5.8 %. 2 NO is on average highest at the lowest converter analyser, equipped with a BLC as described 19 2 analyser in constant mode analyser in switching mode x x x NO NO signal from PAN to be negligible (Ryerson et al., 2000). 2 signal resulting from PAN using the same three BLC units 2 NO produced obscured any trend, however it is clear that there is NO 2 NO signal is proportional to increasing PAN mixing ratios. An artificial signal 2 NO

The percentage conversion of PAN to

found the contribution to the 2010) have also reported a small PAN interference with photolytic converters, while some 3.1 Standard BLC and laboratory NO et al., 1987). Previous studies (Fehsenfeld et al., 1990; Ridley et al., 1988; Sadanaga et al., thermal decomposition could be the source. exclusively or within the inlet of the system, as has been claimed previously (Fehsenfeld composition of PAN could lead to the interferences and in Sect. 3.6 we investigate whether 3.2 Standard BLC and laboratory NO It is not clear from either Fig. 2 or 3 whether the PAN decomposition occurs within the BLC efficiency and vice versa. Potential reasons for this effect are addressed in Sect. 3.6. still a significant proportion of PAN measured as 3.3 Inlet residence time effects the lower amounts of corresponding to 8 to 25 % of the initial PAN mixing ratio was generated. mode operation (Fig. 2). It is possible that the greater variation in the measurement due to between conversion efficiency and signal are not as clearly evident here as for constant the artifact lower lamp temperature as a result of operating only 40 % of the time. The relationship ratio recorded by the analyser is presented in the following sections. Figure 2 shows that is lower in all cases than in the corresponding constant mode. This is likely due to the PAN was introduced toin the Sect. laboratory 2.2.1, diluted in zero air through a range of mixing ratios. The resulting mixing Figure 3 shows the artifact operated in switching mode (40 % duty cycle). The percentage PAN conversion observed Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | . In other C ◦ measured was signif- 2 NO signal is observed when sam- 2 converter 2 NO 20 signal between the cooled and uncooled high pow- 2 NO inch PFA tubing shielded from light and held at 20 4 / ) positive bias. The photolytic converter designs in these studies vary m 1 pptv 2.7 ∼ NO. → 2

concentrations measured whilst sampling a range of PAN mixing ratios (Fig. 4). It is

2 NO The effect of actively cooling the BLC lamps is significant as apparent in the much lower From these experiments it is evident that a significant Table 2 demonstrates that the residence time of PAN within the inlet does not affect

mitigate any signal arising from PAN. therefore evident that there is a significant effect of cooling the UV-LEDs which acts to for NO recorded when sampling zero air) in the uncooled case. The conversion efficiency was 93 % icantly lower than in the uncooled case; this accounts for any increased artifact (the signal 3.4 High powered and actively cooled photolytic NO tion within the inlet was ruled out. time, it is quite possible that significant PAN decomposition occurs. supplied which represents a significant interference. The possibility of thermal decomposi- applications, for example if the inlet is heated, contaminated, or has a very longpling residence PAN diluted in zero air. The signal seen corresponds to around 8 to 25 % of the PAN Figure 4 shows theered difference BLC in described in Sect. 2.2.2. In all of the cooled cases the

consists of contribution from PAN decomposition in the inlet to any artifact signal. The inlet in this case a small (2 to 4 the signal arising from PAN decomposition in our system. This rules out any significant Others (Val Martin et al., 2008) acknowledge the possibility for interference andwithin estimate the GAW network (Penkett et al., 2011). greatly in their implementation and do not have the same ubiquity as BLCs used here, i.e. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | de- HONO quartz collector. The spectrom- π 21 . Calibration was carried out in a light sealed chamber. cm conversion efficiency of the complete whole BLC. Rather, the 2 NO

Spectral radiograms of the UV-LED elements of standard BLCs were obtained using an Figure 5 shows the spectral emission of six different BLC UV-LED units. These units

Ocean Optics QE65000 spectral radiometer coupled to a 2 atmophseric compounds. the spectral output of a range of LED units against the absorption cross-sections of various pending on their spectral overlap. Here we investigate this possibility with BLCs by taking considerably. This is because Teflon-like block is a bulk reflector, that is, the UV penetrates with solvent and mild abrasion of the surface will however recover the conversion efficiency will not lead to a recovery of the conversion. Scrupulous cleaning of the reflective cavity ments of a converter whose conversion efficiency has dropped below 30 % with new lamps There exists potential for photolytic interferences in photolytic converters e.g. efficiency by half, but by a much smaller percentage. Additionally, replacing the UV-LED ele- 3.5 Possible photolytic interferences of BLCs ity. For example, disabling one of the two lamps in a BLC does not reduce the conversion conversion efficiency is strongly dictated by the condition of the reflective Teflon-like cav- rectly proportional to the operation. It should be pointed out however, that the light intensity of the UV-LEDs is not di- overall output, or failure of individual array elements determined by visual inspection during collector distance fixed at 50 intensity of their outputs declines, this decrease in intensity can be due to dimming of the irradiance standard F-1128. The lamp was operated at 8 Amps DC (125 V), with the lamp- of the whole BLC unit had fallen below acceptable limits. As the LED units age, the relative light source is a 1000 W quartz-halogen tungsten coiled-coil filament lamp with spectral ranged in age from new to nearing the end of their service life i.e. the conversion efficiency Gooch and Housego) and ultra-linear power supply (OL 83A, Gooch and Housego). The same distance between the lamp and collector. eter and collector optics were calibrated using an NIST traceable light source (OL FEL-A, Spectra of the BLC UV-LED lamps were taken within the same light-proof chamber with the Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ab- 2 radicals NO 3 NO quantum yield (Gardner, 1987; Koepke 2 NO 22 radicals may consititute a photolytic interference. 3 NO quantum yield and is therefore photon efficient. It is also shown 2 , and 2 exchange with the sample stream. NO y signal as new material off-gasses (as can other surfaces e.g. LED die). NO 2 BrONO , and NO into the material. It is this reflective property that makes most efficient use , more so in the case of PLC optics than for UV-LEDs. It is evident that PAN is x

2 NO

1cm HONO NO ∼

Figure 6 depicts the absorption cross sections of atmospheric nitrogen compounds

HONO which the UV-LEDs do not. Both systems suffer some overlap with PAN, methyl ethyl, and isopropyl nitrate. PLC optics exhibit a great deal of overlap with rity in PAN synthesis. There is only very minor overlap in the PLC optics spectrum with overlap either with methyl, ethyl or isopropyl nitrate – methyl nitrate being a minor impu- such as It has been shown (Carbajo and Orr-Ewing, 2010; Talukdar et al., 1997) that there is no spectral output of UV not having overlap with the PAN absorption spectrum. Other species sorption band and the that there in minimal overlap with HONO and no overlap in the spectrum at all with PAN. and BrO unable to be photolysed in a BLC, nor other types of photolytic converter, due to the narrow et al., 2010). It can be discerned that the UV-LED output overlaps fully with the

to the artifact only be removed by removing the layer of Teflon contaminated. The porosity also gives rise solvents can damage the block), however those which have moved below the surface can at least below the surface). Solvent may remove most of the contaminants (though strong of varying running hours. Also shown is the the Teflon block is somewhat porous and so contaminants may penetrate into the bulk (or PLC 760, and the averaged measured spectral output of six individual BLC UV-LED arrays any UV absorbing material on the surface reduces the reflectivity dramatically. Additionally, effect as (Schott, 1997) used in lamp-type photolytic converter (PLC) optics e.g. Eco Physics AG of the light and achieves high conversion efficiency at low residence times. Adsorption of Sample gas may also diffuse into, and out of, the bulk very slowly(Sander giving et a al, small 2011) memory against the measured spectral output of UG5 UV-passing filter glass up to Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | and (R16) x ; Reac- 2 NO NO occurs above ) between con- 2 43 . NO produced thermally ) the model predicts 2 = 0 C ◦ 2 converter is significantly NO 2 R . C NO ◦ convertor efficiencies when as- ) and indeed with output intensity 96 NO . → = 0 2 2 R NO 23 2 conversion cavity is heated by the lamps leading to 2 + NO NO • . 2 NO C(O)OO 3 . C CH ◦

2 . At the maximum LED surface temperature recorded (80

C ◦

C(O)OONO

3 Table 3 describes the power draw and surface temperature of three BLC UV-LED lamp The model output indicates that measurable PAN decomposition to It is known that the major product from thermal decomposition of PAN is 50

positively with the power drawn by each lamp ( sembled as a complete BLC. The surface temperature of the individual lamps correlate pairs measured during tests, along with their from IUPAC Evaluated Kinetic Data (Atkinson et al., 2006). that the temperature experienced by the sample gas within the ducted in FACSIMILE kinetic modelling software (MCPA Software Ltd.) using rate constants nation of the light output, heat dissipation, and power to the cooling fan. It is clear however within the BLC with a residence time of 0.96 s is shown in Fig. 7. The model run∼ was con- (Fig. 2), but withverter each efficiency lamp and temperature. pair It there is is worth noting only that weak the correlation power consumption ( is a combi- above ambient. In fact, theexternal entire temperatures of the converter of between 34 and 45 tion (R16) (Roumelis and Glavas, 1992; Tuazon et al., 1991). The A model of the gas phase thermal decomposition of PAN over a range of temperatures experiments was 20 – representative of using a BLC in constant mode. The ambient temperature during the was recorded once a stable maximum had been reached and maintained for at least 10 min surface temperature and power draw of the light emitting element. The surface temperature tests and are summarized in Table 3. Each lamp was run constantly whilst recording the The thermal and electronic characteristics of the standard BLC lamps were found in bench 3.6 Possible thermal interferences in BLCs within the converter mayattributed then to atmospheric be photolysed to NOCH and thus be measured as Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | C would cause a 4.6 % ◦ was found to be, on average, 2 NO measured during experiments with the 2 NO 24 . However, as only the two UV-LED lamps are at 2 concentration is derived, which is an inverse function 2 NO NO to NO conversion efficiency but instead a constant value, then 2 NO

% decomposition of PAN to

In Sect. 3.1 the percentage conversion of PAN to 30

(Fig. 2) lies in the way that the verse relationship between percentage conversion of PAN to NO and conversion efficiency PAN, leaving thermal decomposition, modelled in Fig. 7, the remaining explanation. percentage of PAN thermally dissociated in each case is similar. Explanation for the in- lated to the measured malized to conversion efficiency, the percentage for the three BLCs is remarkably similar it seems highly unlikely that the source of the artifact signal is through direct photolysis of peratures are very similar at different convertor efficiencies (Table 3) suggests that the fractionally. If in fact the conversion efficiency of PAN to NO inis the consistent convertor with was the not fact re- that when the average conversion percentage in Fig. 2 is nor- decomposition of PAN andstandard BLCs. account Therefore, for together the with the spectral measurements reported in Sect. 3.5, highest at the lowest converter efficiency and vice versa. The fact that the convertor tem- of the assumed conversion efficiency as in Eq. (2) where converter efficiency is expressed the apparent relationship between CE and % conversion would disappear. This explanation shown more clearly in the inset that an average temperature of 60 was lower in switching mode than in constant mode for the same conversion efficiency. It is out when using the external surface temperature of the BLC as a proxy – the temperature 40 % duty cycle of the lamps, their surface temperature would be lower also. This is borne on the quartz surface of their converter. It is expected that in a switching mode with only an extent) it is a possibility. Ryerson et al. (2000) found no measurable PAN decomposition rubber, conformal coating etc. contacting the sample gas (some of which maybe heated to surface enhanced heterogeneous process, however as the BLC has; Teflon, stainless steel, somewhat lower. We assume that only gas phase decomposition occurs, rather than any BLC so that the average temperature seen by the sample gas over the 0.96 s residence is such an elevated temperature we can expect a temperature gradient/heating rate within the ∼ Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ) to 0 T by thermal decomposition of 2 in normal operation, with the sur- NO C ◦ ) of the instrument, a number of more C formed in Reaction (R16) may be vibra- ◦ 2 is then more readily photolysed to NO within NO ∗ 2 25 % at 60 , the 5 C ◦ of 8 to 25 % of the PAN supplied. The UV-LED light ∼ 2 conversion efficiency of a BLC, the greater the positive s) for temperature from the ambient temperature ( NO ) as it passes through the instrument. This can be param- NO ) τ t . The discussion above suggests that a similar proportion of )) through the dissipating of internal energy from the parent ( 2 1 → T A 2 measurements by NO chemiluminescence using BLCs has im- 2 NO ( 2 2 NO species. Whereas only a small fraction of PAN is found to convert y et al., 1995), this NO NO x NO y NO NO when PAN is present. 2 evolved from thermal dissociation of PAN is converted to NO within the BLC lead-

NO at the operating temperatures ( 2 2

NO NO The degree of gas phase thermal decomposition within the instrument will depend upon The positive bias in Above a threshold temperature of 25

the thermal profile of the air (

thermally labile compounds exist. to

of many common plications for remote background sites. Figure 8 shows the thermal decomposition profiles eterized as a mixing timescale (

temperature of the UV-LEDs causes a positive bias in face temperature correlating positively with power draw and outputPAN. intensity. The elevated

source was found to reach a temperature of 56 to 80 a spurious increase in measured mal operating conditions of a BLC equipped NO chemiluminescence instrument leading to We have shown that a significant proportion of PAN can be decomposed under the nor- 4 Atmospheric implications

error in tionally excited (likely the BLC than ground state the Consequently, the lower ing to the apparent inverse correlation between conversion efficiency and PAN “artifact”. PAN molecule (Ma

signal observed. (Table 4) with the average PAN decomposition of 4.6 % needed to produce the spurious Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | s y (4) (5) NO NO and → 2 NO with converters → concentrations and are , is calculated as 0.42 ∗ 2 x τ NO NO . Thermal decomposition infor- 2 from the decomposition of 2 ambient temperature, a BLC tem- NO C may be over-reported by many hun- 2 ◦ NO 2 HO , NO 2 NO 2 26 O 3 resolution, plus updates described in Sherwen et al. CH compounds. ; ] ◦ 5 y 5 . O  2 2 PAN NO , and the parameterized temperature within the instrument t τ N × )[ )) ; t − ◦ 2 ( 2  k residence time. This allows calculation of the potential interfer- s − NO B/T − )exp ] = 0 ) as in Eq. (4). T ; BrO exp( 2 − BLC PAN A T )[ NO C and a 1 BLC ◦ T ( ) = T k T ( − + ( k 0 = T ]

), the fraction of the compounds that will have decomposed can be found by numerical

)

t ) = t d t ( Figure 9 uses output from the GEOS-Chem model (version 9.2, http://www.geos-chem. Given a 1st order loss of the PAN like compound and a temperature profile within in the Figure 9 shows that in extreme circumstances

PAN

(

T d[

instrument as described in 4 we have 5: from the observed PAN decomposition ofperature 4.6 of %, 75 at 20

ence from other thermally labile value shown. The estimate assumes a BLC conversion efficiency of 100 % NO Interferences are calculated for each month of a one year simulation and the maximum mation are taken from IUPAC evaluated kinetic data (Atkinson et al., 2003, 2006, 2007). where conversion is less than unity – in this casedreds a of multiplying percent. factor These exists. are in regions that typically have low (2016), to provide an estimate of the interference on thus does not include the extra signal from the photolysis of org, Bey et al., 2001) run at integration. (

(typically MPAN; PPN; IO The rate at which thermal equilibrium is reached within the cell, Given the laboratory observations of the temperature dependence of the rate constant species within a BLC photolytic converter. The species used for this analysis are: PAN;

that of the BLC ( T Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2 ). s NO concentrations could 2 NO and a residency time of 1 s ” or, (b) theoretical mechanisms abundance, which has previously X 2 NO of 0.42 2 over a range of BLC lamp temperatures τ measurement by LIF and chemilumines- O y 3 2 CH NO and it is not surprising that thermal decompo- 27 x NO , BrO and IO. The model values are in general close 2 which may be accounted for, in part, by a previously 2 , RO 2 HO , T, , which we find due to 3 C are shown (described by ◦ O are high relative to , converters which operate above ambient temperature. This is especially pptv 2 2 2 environments. x NO NO NO j 2 bias shown in Fig. 9 impacts the modelled Leighton ratio. In Fig. 10 the model , NO 2 2 HO NO NO

The As shown in Fig. 10 the Leighton ratio can be extremely perturbed from what would be Unusual Leighton relationships have been seen in a range of previous studies (Bauguitte

sition can have an impact. Upper tropospheric over estimates of Leighton relationship and complicating interpretation. predicted for an unknown, unmeasured, selective oxidant “compound where we predict a significant interference (indicated on Fig. 9), impacting on the observed Beygi et al., 2011; Kanaya et al., 2007; Yang et al., 2004) leading to either hypothesizing (a) of Bauguitte, Cantrell, Frey, Griffin, Hosaynali Beygi, Kanaya, and Yang were all in locations channel. The instrumental decomposition ofbetween NO 20 and 95 Here there are significant interferences. true in low et al., 2012; Cantrell et al., 2003; Frey et al., 2013,by 2015; which Griffin NO et isunaccounted al., for converted instrument 2007; to bias Hosaynali NO our study has suggested. It is noteworthy that the studies

PAN and cold (polar), or in the upper troposphere. Here, the concentration of compounds such as of NO, calculation shown in red is the Leighton ratio calculated from the modelled concentrations be as high as 150 been identified as a possiblecence interference (Browne in et al., 2011; Nault et al., 2015). is sampled every daylight hour for every surface grid boxto 1. for In the blue month the same of calculation March. is The performed but including the interferences on the Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ratio x NO to , however this y 2 species. NO NO y environments. Thermal decom- species. A convenient method for x y NO conversion efficiency, i.e. unity, to min- determination. 2 mixing ratio by thermally dissociating PAN NO NO 2 → 28 2 NO NO converter produces spuriously high readings; this is ” which selectively oxidizes NO to 2 has been shown to lead to apparent gaps in oxidation X 2 NO and to remove any multiplying effect of more easily photolysable . It would also seem prudent that the sample gas should contact 2 2 collected using some types of photolytic converter and chemilumi- 2 NO NO The authors would like to express their gratitude to Stephane Bauguitte of NO species within the NO y NO

In order to mitigate overestimation of the Investigators wishing to determine Leighton ratios, especially with the BLC/NO chemilu-

Acknowledgements. processes or memory effect; quartz being an ideal material for photolysis cells. tral radiometer/calibration equipment. The financial support of NCAS, the National Centre for At- imise uncertainty in vibrationally excited only chemically inert, and non-porous materials in order to mitigate anyFAAM heterogeneous for their scientific discussion, and Lisa Whalley of Leeds for the PAN generator and spec-

is desirable to have the highest possible by operating at reduced inlet pressure, with the additional benefit of faster response time. It an alternative i.e. where the ambient temperature is unfeasibly low. This can be achieved instrument where it may not be possible to maintain ambient temperature of the sample is and by cooling the photolysis cell. Reducing the residence time of the sample gas within an can be achieved by separating the gas flow from contact with the UV emitting elements, and other compounds it is imperative to avoid heating of the sample above ambient. This this would be with a pure PAN source, or any other readily available NO Measurements of 5 Conclusions nescence systems may be significantly biased in low

orization of an unknown “compound respect to interference from thermally decomposing NO position of especially true in pristine environmentsmay and be at high. high Over-reporting elevations of wherechemistry the which cannot be explained with any available measurements. This has ledis to likely the- anomalous and simply due to errors in minescence technique, would be advised to characterise their instruments thoroughly with Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | + 2 C(O)O 3 CH measurements x ⇔ + M 2 + NO species, Atmos. Chem. Phys., 4, 2 x C(O)O 3 and SO x , NO 29 x , HO x and between 30 and 760 torr, J. Phys. Chem., 95, 3594–3600, K

1461–1738, doi:10.5194/acp-4-1461-2004, 2004. Rossi, M. J., Troe,atmospheric J., chemistry: and Volume II IUPAC – Subcommittee: gas6, Evaluated phase 3625–4055, kinetic reactions doi:10.5194/acp-6-3625-2006, of and 2006. organic photochemical species, Atmos. data Chem. for Phys., E., Rossi, M. J.,istry: and Volume Troe, III J.: – Evaluated gasdoi:10.5194/acp-7-981-2007, kinetic phase 2007. reactions and of photochemical inorganic data halogens, Atmos. for Chem. atmospheric Phys., chem- 7, 981–1191, Saiz-Lopez, A., Roscoe, H. K., Wolff,during E. W., the and CHABLIS Plane, campaign: J. M. can C.:Atmos. source Summertime Chem. and NO Phys., sink 12, estimates 989–1002, unravel doi:10.5194/acp-12-989-2012, observed 2012. diurnal cycles?, Kinetic and theoretical studies of the reactions CH 28, 949–964, doi:10.1111/j.1365-3040.2005.01341.x, 2005. Rossi, M. J., andVolume Troe, J.: I Evaluated – kinetic gas and photochemical phase data reactions for of atmospheric O chemistry: spheric ozone on netBiol., primary 63, productivity 637–661, and doi:10.1146/annurev-arplant-042110-103829, implications 2012. for climate change, Annu. Rev. Plant doi:10.1021/j100162a031, 1991. Spencer, K. M., St. Clair, J. M., Weinheimer, A. J., Wisthaler, A., and Cohen, R. C.: Global and re- M between 248 and 393

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M.: The impact of the Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | molecular sieve × , normalized to the lowest reading, pptv 88.10 88.66 83.95 39 PAG 003 BTCA 178 Sofnofil/Carbon ) 23.06 26.17 - pptv observed from measurement of three zero air sources. (Eco 2 NO ) 1 − as well as the standard deviation of those averages. 1 − counts s Normalized concentration ( Signal ( Standard Deviation 1.64 1.56 1.81 Comparison of the Table 1. Physics AG PAG 003, BOC Specialtyfilters) Gasses by BTCA LIF. The 178, average and ofin Sofnofil/Carbon/13 ten raw 1 PMT min counts averages s is shown in Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | PFA inlet on measured m ) 1 − ) within a 2.7 ppb 40 Inlet residence time (s 0.84 1.05 1.40 2.10 (ppt) 60.2(%) 61.3 5.4 60.6 61.5 5.4 5.2 5.3 2 2 NO NO Effect of varying the residence time of PAN (1.0

concentrations. 2

Table 2. NO Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | C ◦ 0005 . 0 ± 05 C) Draw (A) . ◦ 0 ± 41 1 ± (%) 56 22 76.2 56.4 0.916 0.567 Lamp ConverterNo. BLC Lamp Surface efficiency1 Current Temperature2 ( 34 41 35 79.8 75.3 0.969 77.6 0.953 74.0 0.933 0.931 Peak surface temperature and current drawn by BLC lamps in a bench test at 20.0 showing converter efficiency, current, and surface temperature. Table 3. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 1 measured, and normalized to the ± 2 NO Converter 41 35 22 efficiency (%) 42 Measured %Normalized % 10.8 4.4 15.9 19.6 5.2 4.3 The average percentage conversion of PAN to Table 4. converter efficiency of each BLC. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 43 (left) and pptv (right) recorded by a LIF instrument when 1 − acp-2015-789-discussions-f01.png The average raw counts s the zero signal. signal is also shown (dashed black). The average signal while sampling PAN falls within the noise of sampling various mixing ratios of PAN (in red) and zero air (black). The variance of the zero air Figure 1. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | con- 2 NO %. = 22 44 %, Blue = 35 artifact signal (left panel) of the supplied PAN mixing ratio, and as %, Red 2 NO = 41 The measured acp-2015-789-discussions-f02.png Figure 2. a percentage (right panel), for three BLC units operating in constant mode separated by version efficiency: Green Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | %. = 22 %, Blue = 35 45 %, Red = 41 artifact signal as a function of the supplied PAN mixing ratio (left 2 NO The measured acp-2015-789-discussions-f03.png conversion efficiency: Green 2 NO by panel), and as a percentage (right panel), for three BLC units operating in switching mode separated Figure 3. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 46 artifact signal as a function of the supplied PAN mixing ratio (left 2 NO The measured acp-2015-789-discussions-f04.png BLC. panel), and as a percentage (right panel), for the cooled (blue) and uncooled (red) high powered Figure 4. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 47 acp-2015-789-discussions-f05.png Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 48 quantum yield is shown in black. 2 NO Shown is the spectral output vs. wavelength of two new, previously unused BLC lamps 6 (dashed) in blue. The within acceptable conversion efficiency, and two which fall below acceptable limits No. 5 (solid) and No. 1 (solid) and 2 (dashed) in green, two used lamps No. 3 (solid) and 4 (dashed) in red; still Figure 5. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 49 acp-2015-789-discussions-f06.png Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , presented with 2 NO (diamonds) shown for refer- 3 radicals (green), HONO (light 3 HNO NO (circles), 2 NO 2 50 HO (squares), 5 O 2 (triangles), N 2 (lilac) which are overlapped significantly by the UG5 optics – completely in the case 2 NO NO Absorption cross section (red) and quantum yield (dashed black) of species; ClO this study output (dark blue). Shown are interfering species; (gold), which clearly is not overlapped by either UG5 or BLC light sources. Additional non-interfering the spectral output of UG5 optical filteringblue), (purple) BrO and an averageof of HONO, the whilst six much BLC less lamps overlap used is in exhibited by the UV-LEDs of the BLC. Also shown is PAN Figure 6. ence. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | with a residence time of 0.96 s at temper- 2 . The red shaded area corresponding to the C ◦ NO 51 . Inset is detail of 45 to 85 C ◦ Model of thermal decomposition of PAN to acp-2015-789-discussions-f07.png atures between 0 to 150 temperature range of the UV LEDs of a BLC. Kinetic data from IUPAC (Atkinson et al., 2006) Figure 7. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 52 acp-2015-789-discussions-f08.png Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 5 O 2 ;N • 2 NO 2 HO ; I 2 NO is not shown but has the same ; ClO . Calculated from IUPAC recom- 2 H

NO 2 2 O NO 3 ; PPN N ; BrO  ; PAN 2  53 NO ; MPAN  2 NO 2 O 2 . 2 NO 2 O C(O)CH 2 3 ; CH  2 C(O)CH 3 NO Thermal decomposition profiles of IO 2 O 5 H 2

;C Figure 8. (MCPA Software Ltd.) based on 1 s residence time. Note CH mended kinetic data (Atkinson et al, 2004, 2006, 2007) using FACSIMILE kinetic modelling software J profile as CH Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 54 acp-2015-789-discussions-f09.png Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ; Yang et al.,  by altitude in any month of (b) ; Kanaya et al., (2007) • zonally and (a) 55 as are the locations of the studies of; Bauguitte et al. (c) show the same in absolute pptv values. Surface values are ; Griffin et al., (2007) and N (d) (a) and (c) GEOS-Chem model output showing the monthly maximum percentage over-reporting ; Frey et al. (2013, 2015) . simulation. Panels determined by BLC/Chemiluminescence F  2 year NO (2012) (2011) is also shown in panels month and in any of the longitudinal grid boxes. The MD160 cruise track of Hosaynali-Beygi et al., a 1 the maximum over-reporting in any month, zonal values are the maximum over reporting(2004) in any Figure 9. of Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 56 acp-2015-789-discussions-f10.png Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | . s concentration and the blue and a residence time of 1 2 42 s NO . = 0 τ concentration. The instrument interfer- 2 NO 57 C. ◦ Leighton ratio calculated for each surface model grid-box for each daylight hour for March lamp temperatures, 20 to 95 Red shows the valuesindicated calculated the without values the calculated interference with on the the interference. The interferences are calculated for different ence is characterized by a numerical solution of Eq. (5) with Figure 10. by the GEOS-Chem model as a function of the grid box