ambient formaldehyde measurements made at a remote marine

Atmos. Chem. Phys., 6, 2711–2726, 2006www.atmos-chem-phys.net/6/2711/2006/© Author(s) 2006. This work is licensedunder a Creative Commons License.

AtmosphericChemistry

and Physics

Ambient formaldehyde measurements made at a remote marineboundary layer site during the NAMBLEX campaign – acomparison of data from chromatographic and modified Hantzschtechniques

T. J. Still1, S. Al-Haider1, P. W. Seakins1, R. Sommariva1, J. C. Stanton1, G. Mills2, and S. A. Penkett2

1School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK2School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK

Received: 9 August 2005 – Published in Atmos. Chem. Phys. Discuss.: 6 December 2005Revised: 13 June 2006 – Accepted: 13 June 2006 – Published: 6 July 2006

Abstract. Ambient formaldehyde concentrations are re-ported from the North Atlantic Marine Boundary Layer Ex-periment (NAMBLEX) campaign at Mace Head on the westcoast of Eire during August 2002. The results from twotechniques, using direct determination via gas chromatogra-phy and the Hantzsch technique, show similar trends but asignificant off set in concentrations. For westerly air flowscharacteristic of the marine boundary layer, formaldehydeconcentrations from the gas chromatographic and Hantzschtechnique ranged from 0.78–1.15 ppb and 0.13–0.43 ppb, re-spectively. Possible reasons for the discrepancy have beeninvestigated and are discussed, however, no satisfactory ex-planation has yet been found. In a subsequent laboratory in-tercomparison the two techniques were in good agreement.

The observed concentrations have been compared withprevious formaldehyde measurements in the North Atlanticmarine boundary layer and with other measurements fromthe NAMBLEX campaign. The measurements from theHantzsch technique and the GC results lie at the lower andupper ends respectively of previous measurements. In con-trast to some previous measurements, both techniques showdistinct diurnal profiles with day maxima and with an ampli-tude of approximately 0.15 ppb. Strong correlations were ob-served with ethanal concentrations measured during NAM-BLEX and the ratio of ethanal to formaldehyde determinedby the gas chromatographic technique is in good agreementwith previous measurements.

Some simple box modelling has been undertaken to inves-tigate possible sources of formaldehyde. Such models arenot able to predict absolute formaldehyde concentrations asthey do not include transport processes, but the results show

Correspondence to:P. W. Seakins([email protected])

that oxygenated VOCs such as ethanal and methanol are verysignificant sources of formaldehyde in the air masses reach-ing Mace Head.

1 Introduction

The North Atlantic Boundary Layer Experiment, NAM-BLEX, took place at Mace Head, Eire, during July and Au-gust 2002 to help quantify our understanding of photochem-ical oxidation processes in clean and moderately pollutedenvironments. Objectives included quantifying the role ofhalogen species in the marine boundary layer (MBL), study-ing the reactive nitrogen budget and formation of new par-ticles. Objectives particularly relevant for our formaldehyde(HCHO), measurements included model/measurement com-parisons for radical species and the role of reactive hydro-carbons in the MBL. Formaldehyde plays an important rolein determining radical concentrations, influencing both HOxformation and removal. Due to its importance and difficultiesin determining accurate concentrations (Gilpin et al., 1997)formaldehyde concentrations were measured using two dif-ferent techniques. The University of Leeds (UoL) used achromatographic method based on the detection of separatedHCHO with a helium ionization detector (Hunter et al., 1998,1999; Hopkins et al., 2003). The University of East Anglia,UEA, used a version of the Hantzsch reaction (Cardenas etal., 2000) where ambient HCHO was scrubbed into solutionsand derivitized by reaction with 2,4 pentadione and ammo-nia. The adduct was then detected via fluorescence at 510 nmfollowing UV excitation by a mercury lamp. The two sys-tems were located at sites approximately 200 m apart.

Published by Copernicus GmbH on behalf of the European Geosciences Union.

2712 T. J. Still et al.: Ambient HCHO at NAMBLEX

Table 1. Some previous measurements of formaldehyde in the marine environment.

Reference Campaign and Date Location Platform Technique [HCHO] in Notesclean air/pptv

Harris et al. (1992) “Polarstern” 1988 North Atlantic Ship TDLS 650 Measurement is meanvalue over 40–45◦ N.No diurnal variation

Tanner et al. (1996) NARE 1993 Nova Scotia Coastal site DNPH 200–400 Summer measure-ment

Solberg et al. (1996) EMEP 1994-5 Mace Head Coastal site DNPH 200–500 Winter measurement.Measurements atother sites showsummer maxima.

Cardenas et al. (2000) ACSOE 1996 Mace Head Coastal site Hantzsch 200–450 Summer measure-ment.

Weller et al. (2000) ALBATROSS 1996 North Atlantic Ship Hantzsch 400–500 No diurnal variationobserved.

Fried et al. (2002) NARE 1997 Newfoundland Plane TDLS 410±150Wagner et al. (2001) INDOEX 1999 Indian Ocean Ship TDLS 430±100 Diurnal variation

with amplitude of∼200 pptv observed.

In the very remote environment, methane is the majorsource of formaldehyde via the reactions:

OH + CH4O2−→ H2O + CH3O2 (R1)

CH3O2 + HO2 → CH3OOH+ O2 (R2)

CH3O2 + CH3O2 → 2CH3O + O2 (R3)

CH3O2 + CH3O2 → HCHO+ CH3OH + O2 (R4)

CH3OOH+ hν → CH3O + OH (R5)

OH + CH3OOH → HCHO+ H2O + OH (R6)

OH + CH3OOH → CH3O2 + H2O (R7)

CH3O + O2 → HO2 + HCHO (R8)

Under such low NOx conditions, the methyl peroxy radical(CH3O2) formed from the reaction of OH with CH4 pre-dominantly reacts with other peroxy radicals. Reaction withHO2 leads to CH3OOH which effectively acts as a reser-voir species on route to HCHO formation (although it canbe rained out). Self reaction leads more directly to HCHOformation. Reaction (R8) is very rapid and other reactionsof methoxy radicals do not need to be considered. Addi-tional sources of formaldehyde include higher hydrocarbonsand oxygenated VOCs such as methanol or acetaldehyde.

In the presence of sufficient NO, CH3O2 radicals react di-rectly with NO to form methoxy radicals and subsequentlyformaldehyde via Reaction (R8).

CH3O2 + NO → CH3O + NO2 (R9)

In a modelling study of formaldehyde production in the re-mote Indian Ocean, Wagner et al. (2002) showed that in thepresence of only 2 pptv of NO approximately 50% of CH3O2reacts via Reaction (R9).

There are two major chemical removal processes forformaldehyde. Firstly, reaction with OH (Reaction R10)which, via the subsequent rapid reaction of HCO with oxy-gen (Reaction R11), is a major route for conversion of OH toHO2.

OH + HCHO → H2O + HCO (R10)

HCO+ O2 → HO2 + CO(τ=4× 10−8 s) (R11)

Secondly, photolysis, which can act as a significant free rad-ical source. The efficiency of HCHO as a radical source de-pends on the branching ratio between the molecular and rad-ical channels (Pope et al., 2005):

HCHO+ hν → H2 + CO (R12)

HCHO+ hν → H + HCO (R13)

Formaldehyde, and carbonyls in general, have been shownto be major sources of HOx radicals in the urban winter at-mosphere (Heard et al., 2004), but formaldehyde is also animportant HO2 source in the remote free troposphere. Frostet al. noted that the importance of formaldehyde as a radi-cal source will increase at higher, increasingly drier altitudes(Frost et al., 2002).

Table 1 lists some previous formaldehyde measurementsin remote MBL environments, focusing particularly on stud-ies in the North Atlantic environment. Measurements havebeen with a variety of techniques and from airborne, ship-borne and coastal platforms. Average values range from

Atmos. Chem. Phys., 6, 2711–2726, 2006 www.atmos-chem-phys.net/6/2711/2006/

T. J. Still et al.: Ambient HCHO at NAMBLEX 2713

∼200–1000 pptv. A seasonal dependence has been observedfor formaldehyde measurements in the EMEP programme(Solberg et al., 1996) peaking during the summer months.There is conflicting evidence on meridional variations withmeasurements in the North Atlantic showing both positiveand negative variations with increasing latitude (Harris et al.,1992; Weller et al., 2000). Fried and co-workers have carriedout several airborne campaigns, their measurements show ageneral decrease in formaldehyde concentrations with alti-tude (Fried et al., 2002, 2003).

As can be seen from Table 1, a variety of experimentaltechniques have been deployed. Partially, this reflects thedifficulties in making reliable measurements on this impor-tant atmospheric intermediate. The deployment of a vari-ety of techniques has highlighted potential systematic errorsin the measurement techniques. A good review of severaltechniques and the results of a field intercomparison can befound in the work by Gilpin et al. (1997) and very recentlythe results of an HCHO field (urban) intercomparison fromthe FORMAT programme have been published (Hak et al.,2005).

2 Experimental

2.1 Site

NAMBLEX took place at the Mace Head observatory on thewest coast of Eire (53◦19′34′′ N, 9◦54′14′′ W) during Julyand August 2002. A majority of formaldehyde measure-ments were taken between 1–21 August 2002. The locationof the site is shown in Fig. 1. Air arriving between the anglesof 180 and 300◦ is free from any local land influence.

Five day back trajectories were calculated based on thewind field analysis produced by the European Centre ofMedium Range Weather Forecasts (ECMWF) and were usedto classify the origins of the air masses arriving at MaceHead. During the campaign local meteorological measure-ments were made by a wind profiler and its observationsprovided a record of the boundary layer structure, includingmeasurements of average wind speed and direction (Nortonet al., 2006). The local measurements suggest that for west-erly winds, the ECMWF trajectories and local measurementsare in good agreement. For north-easterly wind flows, char-acteristic of the early part of the campaign, local sea breezesdominated. However, the chemical signatures of typical an-thropogenic pollutants such as CO and C2H2 suggest that thesite is still receiving air broadly characteristic of origin of theair mass, albeit potentially moderated by local meteorology.

The inset to Fig. 1 shows the location of the two formalde-hyde measurements. The UEA apparatus was co-locatedwith a majority of the other instrumentation∼100 m distantand at∼10 m elevation from the average high tide mark. TheUoL instrumentation was located a further∼200 m inshoreand elevated by a further∼20 m.

2.2 UoL apparatus

The UoL instrument used during the NAMBLEX campaignwas based on a gas chromatographic (GC) system as de-scribed by Hopkins et al. (2003). The sampling inlet wasplaced 2 m above the ground and consisted of∼12 m 1/4′′

PFA tubing, and was pumped at a speed of 1 slm. The sam-ple passed through a 6.4 ml sample loop (Silco Steel). Duringinjection, helium carrier gas (CP grade, BOC, 9 ml min−1,backing pressure 50 psi, further purified via passing throughliquid nitrogen traps) was diverted through the loop, sweep-ing the sample onto the column (50 m, 0.32 mm id, 100%dimethyl polysiloxane, WCOT column, 5µm phase thick-ness, CP-Sil 5CB Chrompack, Netherlands) and refocused atthe head of the column in a liquid nitrogen trap. Followingthe elution of the untrapped air, the analytes were releasedand were separated in the column and detected using an ar-gon doped (1% Ar in He, BOC, CP grade), pulsed dischargehelium ionisation detector (pdHID) (Model D4, VICI AG,Schenkon, Switzerland). The detector flow was 30 ml min−1

maintained with a calibrated restrictor. After the elution offormaldehyde the column flow was reversed and the col-umn was back flushed (30 ml min−1, 70 s, backing pressure60 psi) to prevent water reaching the detector or the build-upof heavier weight material on the column. The sample cycletime throughout the campaign was 5.5 min.

The detection limit during the campaign was 42 pptv; thiswas calculated taking the minimum detectable peak to havea signal to noise ratio of 3:1. The system used a formalde-hyde gas phase standards generator for calibration (KIN-TEK, LaMarque, TX). A permeation tube containing poly-meric formaldehyde with a known emission rate at a set refer-ence temperature (333 K) was held in a stabilized oven (tem-perature variation<±0.1 K) flushed with a constant flow rate(10.0 sccm) of UHP nitrogen. Delivery concentrations of 4–50 ppbv (during campaign), or subsequently 2–50 ppbv (withincreased flow rate), were obtained by diluting the oven flowwith additional known flow rates of UHP nitrogen. The de-livery tube was connected to the GC sample inlet via a “tee”junction with a positive flow out of the vent, ensuring that thedelivery pressure was at one atmosphere. The precision wasattained from the standard deviation of replicate calibrationfactors and was calculated to be 3%. The uncertainty was at-tained from the errors associated with the standards generatorand was determined to be 9% (2σ).

2.3 UEA apparatus

The fluorescence technique is based on the Hantzsch reac-tion which is a liquid phase reaction of formaldehyde fol-lowed by fluorescence detection of the resulting adduct. Thistechnique requires formaldehyde to be transferred from thegas phase into liquid phase, achieved via a stripping solutionof 0.1 N H2SO4 (made up from ACS reagent grade H2SO4,Aldrich) at room temperature, which is pumped through a

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Fig. 1. Location of the Mace Head observatory and the relative positioning of the two formaldehyde instruments.

coil (45 cm long, 10 turns of 3 mm o.d. Pyrex) and forced intocontact with gaseous formaldehyde. Ambient formaldehydewas sampled at a height of∼5 m through approximately 10 mof 3/8′′ PFA tubing. The pumping speed was a total of 15 slmgiving a very short residence time in the sampling line, andonly small decrease in pressure in the sample line. The sam-ple was introduced into the instrument at 1.5 slm through a1 m length of 1/4′′ PFA tubing.

The Hantzsch process is based on the liquid phase re-action of formaldehyde with acetylacetone (2,4 pentadione)and ammonia to produce diacetyldihydrolutidine, (DDL) thatis excited at 412 nm (Hg-Phosphor 215 lamp, Jelight, USA)and the fluorescence is detected at 510 nm with a photomul-tiplier tube (Hamamatsu).

The detection limit for this instrument was in the regionof 50 pptv (Cardenas et al., 2000). The instrument was alsocalibrated using a permeation source (KIN-TEK LaMarque,TX). The calibrant introduced into the instrument via a “Tee-piece” to vent the excess and ensure the calibration was de-livered at atmospheric pressure. The mean precision andaccuracy of the instrument (based on zero noise, calibra-

tion uncertainties and stripping efficiency uncertainties) were±6.3% and±13.2%, respectively with a minimum uncer-tainty of ±25 pptv. The detection limit (three sigma of zeronoise) of the instrument varied between 38 and 69 pptv dur-ing the campaign. Zeros were performed every 5 h by scrub-bing the sample with a charcoal filter.

2.4 Intercomparisons and artefacts

During the campaign the calibration sources for both instru-ments were exchanged and compared. When used on theUoL instrument, the UEA calibration source produced val-ues within 2% of those generated using the UoL source. Itwas therefore concluded that the calibration sources were ingood agreement for the campaign period.

The two instruments also took part in a laboratory inter-comparison at the National Physical Laboratories (Tedding-ton, UK) during June 2003. In these experiments all theinstruments sampled from a common chamber (NPL Stan-dard Atmosphere Generator). HCHO concentrations in thechamber were calculated from permeation rates of a known



Fig. 2. Ozone artefact for (N) UoL apparatus, (�) UEA apparatus as determined during experiments at NPL, 2003.

standard and measured dilution flows. All the instrumentsperformed linearity and stability checks and a blind sam-pling. Various concentrations of ozone were generated andintroduced into the chamber to investigate possible artefacts.

Both the instruments responded well to the linearity (2–8 ppbv) and stability experiments (apparatus run overnight at0.5 ppbv) and reported results within experimental errors forthe blind testing, although the concentration, at∼7 ppbv, wassignificantly higher than ambient concentrations in the MBL.Both instruments showed a positive interference for ozone asshown in Fig. 2.

The magnitude of the ozone artefact on the UoL instru-ment was significantly greater, but was reproducible, bothat NPL and during subsequent experiments at Leeds. Theresults reported below for both instruments have been cor-rected for ozone measurements as recorded at Mace Head bythe FAGE group using a UV photometric analyser.

3 Results

3.1 Comparison of time series from UoL and UEA mea-surements

Figure 3 shows the formaldehyde time series for both instru-ments. Two observations are immediately apparent. Firstly,the results from the UoL are significantly higher, and out-side the combined random errors of both techniques (9% forUoL and 13% for UEA based on errors associated with the

calibration processes). Secondly there is a reasonable cor-relation between the two measurements, and in some cases,for example that shown in the inset to Fig. 3, the pattern ofvariation is almost identical.

Figure 4 shows a correlation plot of the results. Thereis significant scatter in the data which may be associatedwith sampling slightly different air masses either due to lo-cal meteorology or to differences in the sampling techniques.The UoL apparatus is essentially a grab sample taken every5.5 min, whereas UEA data are based on averaging over aone minute period. The difference in the UoL and UEA dataappears to be in an offset rather than in the gradient of thecorrelation plot.

Given the good agreement on the trends in formaldehydeconcentrations, possible reasons the systematic error include;sample losses in the lines or scrubbing processes, calibrationerrors, interferences from other gases and significant blanksin the UoL apparatus.

The University of Leeds measurements showed no varia-tion with the length of sampling tube in either field or labo-ratory tests. Given that the same material was used for sam-pling in the UEA apparatus, we would not expect any signif-icant losses in that system either. Significant loss processesin the UEA instrument would be very surprising since thewhole wetted path of the sample is PFA and no loss processeson PFA have been seen in numerous laboratory tests. Thedifference in pressure between the sample drawn down the3/8′′ PFA and the calibrant delivered directly into the instru-ment is at most 10%. Both losses and differences in pressure



Fig. 3. Time series of formaldehyde concentrations from GC and fluorescence techniques. All data corrected for ozone interferences.

Fig. 4. A graph showing the weighted bivariant regression between the GC and the fluorescence technique for all data (solid line), and the1:1 line (dotted line).

would lead to an underestimation of HCHO, but would beexpected to show a difference in slope rather than an offsetin the scatter plot.

In general, calibration errors do not appear to be a problemas the calibration systems were compared during the cam-paign. The UEA system requires transfer from the gas to



Table 2. Summary of UoL and UEA formaldehyde concentrations for the various air masses encountered during NAMBLEX.

Air Mass OriginUoL UEA24 h average[HCHO]/ppbv

Range/ppbv 24 h average[HCHO]/ppbv

Range/ppbv

Continental 1.20±0.13 0.98–1.58 0.37±0.11 0.13–0.73Westerly 0.96±0.09 0.78–1.15 0.27±0.07 0.13–0.43Anticyclonic 0.92±0.15 0.72–1.31

liquid phase. This process is always highly efficient, but isslightly temperature dependent (92–98% as temperature de-creases from 298–278 K). The temperature of the laboratoryfluctuated by±5 K and variations in sampling versus calibra-tion temperature are an additional small source of error.

As described in the previous section ozone artefacts weredetected in both systems and the reported data have been suit-ably corrected. Corrections were typically of the order of300–400 pptv formaldehyde. One other possible artefact par-ticularly relevant to the MBL is the presence of water vapour.Tests performed at NPL were inconclusive and further workis planned. It should be noted that there was no systematicvariation in the offset with relative humidity. Blanks wererecorded on a daily basis for the UoL apparatus. These wereobtained by attaching a cylinder of zero nitrogen (PremierGrade, Air Products) via 1/4′′ Teflon tubing to the inlet portof the GC. A “tee” junction was placed upstream of the sam-ple inlet to restrict the upstream pressure to 1 atmosphere.The zero air was drawn into the GC via the pump in thenormal way. An ozone artefact for the UoL apparatus hasalready been described. It should be noted that blank mea-surements were made with dry gas, our planned work on hu-midity effects may determine whether this is an importantissue. We cannot rule out the possibility of other undetectedartefacts.

Obviously one, or both, instruments were subject to sys-tematic errors during NAMBLEX, but either these had beenrectified by the following summer, or are only apparent dur-ing real air, rather than laboratory sampling. We are thereforeat a loss to explain the differences in measured concentra-tions at Mace Head during NAMBLEX and in the followingsection the results of each apparatus are presented separately.The measurements are compared with other measurementsand various models in the discussion section.

3.2 Average values and diurnal profiles for UoL apparatus

The campaign can broadly be divided into three differentair masses. From 1–5 August the site experience continen-tal air masses which had originated in Scandinavia and hadpassed over northern England. This was followed by a longperiod (6–17 August) of relatively long trajectories (highwind speeds) with a significant westerly component (rang-

ing from south to north westerly). It is possible to furthersubdivide this period into air masses with NW, W, and SWorigins, however, analysis showed no significant variationin formaldehyde concentrations with these finer componentsand hence all westerlies are considered together. Over thelast few days of the formaldehyde campaign the wind wasstill westerly but from an anticyclonic weather system withlow wind speeds. Summaries of the formaldehyde concen-trations for each period can be found in Table 2.

Not surprisingly, formaldehyde concentrations were high-est during the initial continental air flow. Concentrationsof hydrocarbons and CO are also elevated during this pe-riod. Thereafter, concentrations decreased and a diurnalpattern became more apparent, Fig. 5a, with concentrationsduring the westerly air flows varying from an early morn-ing (03:00–05:30 GMT) minimum of∼850 pptv to a peakvalue of∼1050 pptv between 14:00–16:00 GMT (local timeis GMT+1). During the latter part of the anticyclonic west-erly airflows, the amplitude of the diurnal profile appeared toincrease. There were also concurrent increases in acetyleneconcentrations suggesting some local contamination duringthese periods.

3.3 Average values and diurnal profiles from UEA

Results from the fluorescence apparatus show generally sim-ilar behaviour. Concentrations are elevated at the start ofthe campaign, but appear to remain constant between 200–500 pptv from the 3–14 August. As with the UoL data for thewesterly winds, a pronounced diurnal variation is observed,as shown in Fig. 5b, with an amplitude similar to that ob-served by the UoL.

The concentrations measured on 15 August are signifi-cantly lower than other periods during the westerly airflowwith values between 50–100 pptv. After the 16 August datafrom the UEA apparatus became patchy due to intermittentlamp failure.



Fig. 5a. Diurnal variation of formaldehyde concentrations as measured by UoL during westerly airflows (6–17 August 2002). Error barsrepresent one standard deviation of concentrations at that particular timestamp.

Fig. 5b. Diurnal variation of formaldehyde concentrations as measured by UEA during westerly airflows (6–16 August 2002). Error barsrepresent one standard deviation of concentrations at that particular timestamp.

4 Discussion

4.1 Comparison with previous marine boundary layer mea-surements

Some results from previous campaigns are presented in Ta-ble 1. We have focused predominantly on measurementsfrom the North Atlantic although some other campaigns areincluded.

Formaldehyde measurements at Mace Head are relativelylimited. A measurement campaign was conducted at MaceHead under the EMEP programme, with formaldehyde be-ing monitored from November to April using 2,4 DNPH car-tridges with subsequent off-line analysis by HPLC (Solberget al., 1996). Samples were collected twice weekly, eachmeasurement consisting of 8 h sampling centred around mid-day, with subsequent off line analysis at NILU. Solberg etal. reported monthly averages between 200–500 pptv over



this winter period. Direct comparison with our summerdata cannot be made, however, the winter concentrationsappear to correlate well with those made at a Norwegiansite (Birkenes), where summer concentrations reach almost1 ppbv. Solberg et al. report annual variations in formalde-hyde levels from a number of rural sites around Europe andall show summer maxima.

Formaldehyde was measured at Mace Head during theACSOE 96 and 97 campaigns. A majority of the measure-ments were made using an earlier version of the fluoromet-ric technique, and a more limited data set was also recordedusing tuneable diode laser spectroscopy (TDLS) (Cardenaset al., 2000). Typical values for westerly air masses were200–400 pptv from the fluorometric technique, whereas theTDLS produced higher, but scattered values with a range of0–1000 pptv.

A number of formaldehyde measurements were made dur-ing the North Atlantic Regional Experiment (NARE) cam-paigns. During NARE 1993, Tanner et al. (1996) mea-sured a range of carbonyl compounds using derivatization via2,4 DNPH/HPLC at Cherbourg Point, Nova Scotia. Typicalvalues for clean air at 200–400 pptv are in good agreementwith the UEA data. Measurements were only made every sixhours, so no diurnal trends are reported. Tanner et al. report arelationship between formaldehyde and ethanal with the lat-ter being approximately 50% of the formaldehyde concentra-tions. This observation will be discussed further below.

During NARE 1997 a number of airborne measurementswere made from St John’s, Newfoundland, using both TDLSand the coil/DNPH technique (Fried et al., 2002). Gener-ally the agreement between the two techniques was goodwith a gradient on the scatter plot of 0.98±0.07, however,approximately 30% of the data points lay outside the com-bined 2σ uncertainties. Median values in clean backgroundair from 0–2 km, between 35–55◦ N are 400 pptv for TDLS(range 300–900 pptv for lowest altitude measurements) and410 pptv for the CDNPH (range 200–800 pptv for lowest al-titude measurements). Average concentrations decrease withboth altitude and latitude.

Two shipborne campaigns have monitored formaldehydein the North Atlantic. The ALBATROSS campaign tookplace during October and November 1996 with the cruisebeing a meridional track from 60◦ N to 45◦ S. Formalde-hyde was monitored via a commercial apparatus based onthe Hantzsch reaction (Weller et al., 2000). A variety of dif-ferent air masses were encountered. Typical concentrationswhen the ship was between 20–40◦ N and intercepting trajec-tories similar to the westerlies and south- westerlies reach-ing Mace Head were in the region of 300–800 pptv. Harriset al. (1992) report data from the 1988 “Polar Stern” cruisewhich took place during September and October from 45◦ Ninto the southern hemisphere. Formaldehyde was predomi-nantly measured using TDLS with a small number of com-parative experiments obtained with DNPH sampling and sub-sequent analysis via HPLC. The two methods were in good

agreement. Harris et al. report a mean formaldehyde concen-tration between 40–45◦ N of 650 pptv, with concentrationsdecreasing slightly during the cruise south.

When used to compare with formaldehyde concentrationsat Mace Head, the results of other campaigns need to be in-terpreted carefully. For air and shipborne measurements vari-ation with latitude will be strongly dependent on the originsof the air mass reaching the receptor at that point and maynot be typical of longer term measurements at that latitude.Both Fried et al. (2002) and Weller et al. (2000) report a de-crease in average formaldehyde concentrations with latitude,however, because the air masses trajectories and ocean cur-rents vary across the region, this does not mean that the lat-itudinal variation will be the same over the eastern Atlantic.Finally, the EMEP data showed significant annual variationin formaldehyde concentrations (as would be expected froma photochemical intermediate) and therefore results from dif-ferent seasons should be interpreted with care.

The final campaign discussed in this section took place inthe Indian Ocean during spring 1999 (Wagner et al., 2001).Given the location, comparison with Mace Head concen-trations are difficult, however, results from the INDOEXcampaign, (where formaldehyde was measured by TDLS)are interesting for several reasons. Firstly, even for trajec-tories that had been over the open ocean for seven days,and hence lost any continental formaldehyde, concentrationswere still significant (∼500 pptv). A direct oceanic source isnot expected and therefore this is good evidence of signifi-cant HCHO production from long-lived precursors, includ-ing oxygenated VOC. Secondly, a strong positive correlationwas noted with CO for a variety of air masses including veryaged air (HCHO/CO≈3×10−3). Finally, during periods ofconsistent air flow, diurnal variations in [HCHO] were ob-served with an amplitude of approximately 200 pptv in thenorthern hemisphere, consistent with the observed ampli-tudes of both UEA and UoL instruments during clean west-erly airflows.

4.2 Comparison with other NAMBLEX measurements

The NAMBLEX campaign was characterised by relativelypolluted air during the first part of the campaign, with a pe-riod of consistent, strong, westerly, clean air and finally an-ticyclonic conditions, originating in the North Atlantic. An-thropogenic pollutants are therefore expected to be high dur-ing the initial part of the campaign, and thereafter relativelylow, although the low wind speeds under anticyclonic condi-tions may allow for influence by local sources. The acetyleneand CO concentrations shown in Fig. 6 broadly match the ex-pected behaviour.

If the correlation between formaldehyde and CO, observedduring the INDOEX campaign by Wagner et al. (2001) ap-plied under NAMBLEX conditions, then formaldehyde con-centrations during westerly airflows would be predicted tobe in the region of 200–300 pptv, in good agreement with the



Fig. 6. Signatures of anthropogenic tracers. Acetylene and CO data were provided by the University of York and Mace Head research station,respectively.

UEA data. However, Fig. 6 shows a strong correlation be-tween CO and acetylene, a primary pollutant. Therefore COconcentrations at Mace Head may correlate more stronglywith primary sources rather than photochemical productionand the relationship between formaldehyde and CO observedduring highly aged air in the Indian Ocean may not apply.

Methane levels are likely to be slightly elevated in pol-luted air masses, but formaldehyde is also formed during theatmospheric oxidation of virtually all higher hydrocarbonsand oxygenated VOCs (OVOC), and due to their higher pho-tochemical loss rates, these latter species are likely to be thedominant influence on formaldehyde concentrations.

A good example of a formaldehyde precursor is acetalde-hyde (ethanal). Both photolysis and reaction with OH canlead to formaldehyde formation:

CH3CHO+ hνO2−→ CH3O2 + CO+ HO2 (R14)

OH + CH3CHOO2−→ CH3C(O)O2 + H2O (R15)

CH3C(O)O2 + RO2 → CH3 + CO2 + products (R16)

Figure 7a shows a comparison of the time series for ac-etaldehyde and formaldehyde, with Fig. 7b showing the cor-responding correlation plots with the UoL data. Acetalde-hyde was measured by the University of York (Hopkins etal., 2003; Lewis et al., 2005) concentrating VOC and OVOCfrom ∼1 l of air onto a solid trap followed by rapid thermaldesorption, separation via GC with detection by FID. Thegeneral correlation between the CH3CHO and H2CO data

from the UoL is good with both showing the same gradualdecline in concentrations at the beginning of the campaign,followed by consistent and relatively low levels during theperiods of westerly air flow.

The correlation plot of acetaldehyde con-centrations with the UoL formaldehyde data([HCHO]=(1.25±0.29)[CH3CHO]+0.46±0.19) showsan intercept of 0.46 ppbv on the HCHO axis. An interceptwould be expected given that a significant fraction offormaldehyde will be formed from methane or from frag-mentation processes that by-pass acetaldehyde production.

A significant positive correlation between formalde-hyde and acetaldehyde was observed by Tanner etal. (1996) during measurements at a coastal site inNova Scotia during NARE 1993. They reported[HCHO]ppbv=1.9[CH3CHO]ppbv+0.22 with anr coefficientof 0.62. Errors on the gradient and intercept are not reported,but from the scatter of the plot, are likely to be at least±33%and therefore comparable with our parameters derived ob-servations between acetaldehyde and the UoL formaldehydedata. Monthly averaged formaldehyde and acetaldehyde con-centrations were reported by Solberg et al. (1996) from anumber of remote European sites including Mace Head. Inall cases formaldehyde concentrations were greater than ac-etaldehyde, typically about a factor of two during the summermonths.

The UEA formaldehyde concentrations are comparable invalue with the measured acetaldehyde concentrations. Given



Fig. 7a. Time matched series of formaldehyde (UoL) and acetaldehyde (University of York).

Fig. 7b. Correlation plot of UoL formaldehyde vs. acetaldehyde. The regression equation is [HCHO]=1.25(±0.07)×[CH3CHO]+0.46(±0.03), where the errors are 1σ .

our current understanding of formaldehyde and acetaldehydephoto oxidation these observations appear to be incompati-ble.

Methanol is another significant OVOC precursor forformaldehyde. Reaction with OH leads to HCHO via re-action of either CH3O isomer formed by the initial abstrac-tion reaction. In the MBL reaction of Cl atoms with hy-drocarbons and OVOCs may also need to be considered(Ramacher et al., 1999). Cl initiated reactions tend to beless selective in the position of abstraction, but in gen-

eral form the same type of radicals as OH initiation. Al-though concentrations of Cl atoms are much lower thanOH in the MBL (typically 5×103 cm−3 vs. 1×106 cm−3),the magnitude of the abstraction rate coefficients (typically>5×10−11 cm3 molecule−1 s−1 (Qian et al., 2002; Seakinset al., 2004)) is such that Cl initiated chemistry can be signif-icant.



Fig. 8a. Comparison of measured and modelled formaldehyde for 9–10 August 2002 (JD221–222).

Fig. 8b. Comparison of measured and modelled formaldehyde for 15–21 August 2002 (JD227–233).

4.3 Modelling

The simplicity of the chemistry in the MBL allows for theconstruction of relatively simple models to test our under-standing of photochemistry in this environment. The idealmolecules for comparison are those with very short lifetimes,so that transport can be ignored and realistic comparisons canbe made with box models. Hydroxyl and hydroperoxy radi-cals are ideal examples and comparisons between model pre-dictions and measurements have been made in a number ofenvironments, including the marine boundary layer (Carslawet al., 2002; Heard et al., 2004; Sommariva et al., 2004). Thelifetime of formaldehyde (typically∼4 h under midday con-ditions) is such that transport should really be considered,

however, zero dimensional box models give an indication ofthe expected concentrations from various chemical schemesand highlight the relative importance of various formationand removal channels. A good example of a box model isdescribed by Wagner et al. (2001, 2002) and used to interpretshipborne measurements in the remote Indian Ocean fromthe INDOEX campaign. In this case methane was the domi-nant source of formaldehyde and good agreement (typicallywithin 20%) was found between measurements and modelledformaldehyde concentrations. However, as shown in Table 3,in other campaigns the models can either under or over pre-dict formaldehyde concentrations.

In this study we have used the Master Chemical Mech-anism (MCM) (Saunders et al., 2003) to simulate the



Table 3. Summary of the results from the measured-modelled comparisons of formaldehyde from the literature.a Daily mean.b Value readfrom a graph (Peak diurnal value).c Result from summer intensive.

Authors Environment Model constraints Typical [HCHO]/pptv Under/Over predic-tion

Weller et al. (2000) Marine, Atlantic48◦ N–35◦ S

Photochemical box model basedon CH4 + CO photo oxidation.

580±160a Under predicted by afactor of 2

Ayers et al. (1997) Coastal, Cape Grim,Australia

Photochemical box model basedon CH4 + CO photo oxidation in-cludes dimethyl sulphide mecha-nism

400±50b Under predicted by afactor of 2

Wagner et al. (2002) Marine, Indian Ocean MCM 2. Photochemical oxida-tion of CH4 and NMVOC

200±70a Over predicted by12%

Zhou et al. (1996) Free Tropospheric air Simple box model based onCH4 chemistry, constrained toCH3COOH

211± 104c Over predict

Liu et al. (1992) Free Tropospheric air Photochemical box model basedon CH4

105±42 Over predict

Jacob et al. (1996) South Atlantic(Aircraft)

Photochemical box modelincludes CH4, inorganic chem-istry and NMVOCs (exc.CH3OH+CH3CHO)

110 Over predict

Fried et al. (2003) Marine, Pacific(Aircraft)

NASA Langley box model, con-strained by observations includ-ing oVOCs

(Average 2–4 km)196±161

Equivalent

Frost et al. (2002) Marine, North At-lantic, (Aircraft)

Photochemical box model in-cludes NMHC, and constrainedto measurements. However somevalues were taken from othercampaigns.

(Average 0–2 km)410±0.11

Under predicted by afactor of 2

formaldehyde concentration based on two models. The“clean” model used only a very small subset of the MCMbased on methane and CO chemistry whilst the “full-oxy”model was based on a mechanism originating from CO,CH4, 23 hydrocarbons (including isoprene) (Lewis et al.,2005), DMS, chloroform and three oxygenated compounds(methanol, acetaldehyde and acetone) (Hopkins et al., 2003;Lewis et al., 2005). During the simulations the model wasconstrained to 15 min averages of the hydrocarbons, H2,O3, NO, NO2, H2), temperature and the measured photol-ysis rates of O3, NO2, HONO, HCHO, CH3COCH3 andCH3CHO. The model included parameters for heterogeneousuptake and dry deposition and was run for several days (typi-cally 2–3) to initialize the values of some species which werenot constrained.

Figures 8a and b show comparisons of the models with theexperimental data for two periods. Figure 8a is for JD221–222 during the westerly airflows when both instruments ob-served a pronounced diurnal variation. For these conditionsthe full-oxy model is clearly in better agreement with theUEA data, although the shape of the diurnal pattern doesnot produce the diurnal profiles observed by either technique,

particularly the decrease in concentration in the afternoonand evening of JD221.

Figure 8b shows a comparison of the clean and full-oxymodels with the experimental data for the period 15–21 Au-gust. The predictions from the full-oxy model are in reason-able agreement with the formaldehyde levels observed fromUoL apparatus. In general the model does not do a particu-larly good job in predicting the fine structure of the formalde-hyde concentrations (an exception being the sharp increase inconcentrations in the early afternoon of 16 August, observedby both experimental techniques), but this is to be expectedfrom a box model.

Figure 8b also highlights the significant contribution thatthe higher hydrocarbons and particularly the oxygenatedspecies make to the predicted formaldehyde concentration.This observation is in good agreement with the analysis byWagner et al. (2002), where even in the much less complexconditions of the remote Indian Ocean, only 77% of HCHOoriginates from methane, the other 23% coming from ethane,ethene, propene, isoprene, acetone and DMS, all at concen-trations significantly lower than those encountered at MaceHead. Our model highlights the importance of acetaldehyde



on formaldehyde generation, but unfortunately acetaldehydewas not considered by Wagner.

A rate of production analysis also emphasises the domina-tion (>70%) of the CH3O2+NO reaction as the loss mech-anism for CH3O2 for [NO] in the region of 10 pptv. Un-der these conditions formation of the CH3OOH reservoir islimited and therefore the potential for carbon removal viaCH3OOH dry deposition is relatively small.

There have been several recent formaldehyde measure-ment/model comparisons in the North Atlantic environment(Weller et al., 2000; Frost et al., 2002; Fried et al., 2003)in which all under predict measured formaldehyde concen-trations to some degree. Weller et al. compared measuredaverage formaldehyde concentrations of 580±160 pptv froma North Atlantic cruise with calculated values based on a boxmodel. On the basis of measured alkane concentrations, theyargued that methane should be the major source, but notedthat alkenes (not measured) could be a significant source.The standard methane model under predicted the measure-ments by 250 pptv. Better agreement was found by introduc-ing a formaldehyde production channel from Reaction (R2);

CH3O2 + HO2 → HCHO+ H2O + O2 (R17)

based on arguments by Ayers et al. (1997). Making channel(Reaction R17) 40% of the total reaction increased averageformaldehyde concentrations by 80 pptv. Increasing the ratecoefficient for OH abstraction and changing the branchingratios for the OH + methylhydrogenperoxide reaction withinvalues suggested in the literature increased formaldehydeby 50 pptv over the base model. Frost et al. generally un-derestimated airborne formaldehyde measurements from theNARE 97 campaign by between 130 and 180 pptv (∼ factortwo) using a methane/hydrocarbon model. They carried out asensitivity analysis and concluded that it was not possible tobridge the gap between measurement and model within thecurrently understood uncertainties in the model parametersbut noted that VOCs not measured or considered in the modelcould represent the missing source. Finally, Fried et al. reportgenerally good agreement between measurement and modelfor the TOPSE 2000 experiment, although again there is sig-nificant under prediction at high latitudes over North Atlanticregions (Fried et al., 2003).

Our modelling work emphasises the importance of oxy-genated species and particularly acetaldehyde and methanolin generating significant concentrations of formaldehyde. Wewould therefore suggest that these species, measured val-ues of which were not considered in the above modellingstudies, as a potential source of missing formaldehyde andrecommend that concurrent measurements of all oxygenatedspecies are considered for future campaigns.

During the night (from 20:00–04:00 GMT) deposition andentrainment are the only removal processes for formalde-hyde. From the averaged diurnal formaldehyde profiles for

westerly air masses (Figs. 5a, b) both experimental tech-niques show a decrease of approximately 150 pptv formalde-hyde during this period. Assuming that physical processescan be represented by a first order loss, this allows a predic-tion of the initial concentration at the start of the depositiononly period.

[HCHO]0 − 150= [HCHO]0e−kt (1)

For a sum of deposition and entrainment equal to1×10−5 s−1, based on the reported deposition and entrain-ment velocities of 0.4 cm s−1 (Wagner et al., 2002) and aboundary layer height of 800 m (Norton et al., 2006), Eq. (1)predicts [HCHO]0 of 600 pptv, in between the two obser-vations. Currently, uncertainties in boundary layer height(±200 m) and deposition velocities (up to 100%) are suchthat the calculation range can encompass both observations.However, reductions in such uncertainties could help differ-entiate between the two observations.

4.4 Implications for HOx production

Formaldehyde is an important photolytic source of radicalsin the troposphere (Reaction R13). The MCM has beenused to quantify the influence of the measured formalde-hyde levels on [HOx]. Variations in formaldehyde levelshave relatively little influence on OH radical concentrationsas while increased formaldehyde generates more HOx rad-icals, formaldehyde is also a significant HO sink via Reac-tion (R10). Using either set of formaldehyde measurementschanged the predicted OH concentration by<2% under typ-ical westerly conditions. Increased formaldehyde concentra-tions have a more pronounced positive effect on HO2 chem-istry, although the relationship is not 1:1, being bufferedby relatively complex radical interconversion. Increasingformaldehyde concentrations by 50% produces between a15–25% increase in HO2 concentrations. It should be notedthat currently (Sommariva, 2004) the MCM tends to over-predict HO2 concentrations by up to a factor of two. As notedearlier, formaldehyde may have a more pronounced influenceon HOx production in other environments e.g. where a lackof ozone or water prevents OH generation via O(1D) produc-tion.

5 Conclusions

Formaldehyde concentrations, measured by two differenttechniques during the NAMBLEX campaign appear to becorrelated, but exhibit a significant offset. Joint calibrationsand subsequent intercomparison experiments have failed toidentify the cause of the systematic errors present in one, orboth sets of apparatus. The laboratory intercomparisons atNPL in the presence of water vapour were inconclusive andfurther work in planned in this area in a new atmosphericchamber in Leeds.



Formaldehyde continues to be a difficult molecule to mea-sure with intercomparisons reporting varying levels of agree-ment. A recent comparison from the BERLIOZ campaign(Grossmann et al., 2003) reported formaldehyde measure-ments from a DOAS system to be 30% higher than thosefrom a commercial Hantzsch system. Still more recently,an intercomparison in Milan (Hak et al., 2005) gave goodagreement between FTIR, DOAS, Hantzsch and DNPH tech-niques with the slopes of the Hantzsch and DOAS regressionline not significantly differing from one. However, it shouldbe noted that formaldehyde concentrations during this inter-comparison (2–13 ppbv) were significantly higher than thoseencountered at Mace Head. More work is required to iden-tify systematic errors in formaldehyde detection, especiallyat low concentration levels.

Comparison with previous formaldehyde measurements inthe North Atlantic suggests typical concentrations between300–800 pptv under clean background conditions. The UEAdata lie at the low end of this range, the UoL data just abovethe upper limit.

Previous campaigns have shown a strong positive corre-lation between formaldehyde and acetaldehyde and with aHCHO to CH3HCO ratio of approximately 2:1. A good cor-relation has been observed between the UoL formaldehydedata and acetaldehyde measurements from the University ofYork. The ratio of concentrations is in agreement with previ-ous work.

Both instruments show a distinct diurnal profile duringperiods of consistent westerly airflow with an amplitude of150–250 pptv. This is the expected behaviour of a photo-chemical intermediate, but such behaviour has not alwaysbeen observed in previous campaigns, either due to samplingfrequency or because of fluctuations in the concentrations ofthe various air masses arriving at the receptor. This latterfactor is especially important in shipborne cruises.

Modelling studies from the latter part of the campaignpredict concentrations between the two experimental mea-surements, but more importantly, highlight the importanceof oxygenates in formaldehyde production. The involvementof oxygenated species in formaldehyde production may ex-plain some of the under predictions of simple methane onlymodels. The concentrations of formaldehyde predicted bythe MCM for an earlier part of the campaign are in betteragreement with UEA data.

Acknowledgements.The authors would like to acknowledge thehelp of P. G. Quincey, N. Martin at NPL and NERC for fundingthe NAMBLEX project. T. J. Still and S. Al-Haider thank NERCand the Kuwaiti Government respectively for the funding of PhDstudentships.

Edited by: P. Monks

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ambient formaldehyde measurements made at a remote marine

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