Atmospheric chemistry of formaldehyde in the Arctic troposphere at Polar Sunrise, and the influence of the snowpack

Atmospheric Environment 36 (2002) 2553–2562

Atmospheric chemistry of formaldehyde in the Arctic troposphere at Polar Sunrise, and the influence of the snowpack A.L. Sumnera,*, P.B. Shepsona, A.M. Grannasa, J.W. Bottenheimb, K.G. Anlaufb, D. Worthyb, W.H. Schroederb, A. Steffenb, F. Domine! c, S. Perrierc, S. Houdierc a

Departments of Chemistry and Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN 47907-1393, USA b Meteorological Service of Canada, 4905 Dufferin St. Downsview, Ontario, Canada M3H 5T4 c Laboratoire de Glaciologie et Geophysique de l’Environement, B.P. 96, 38402 Saint Martin d’Heres, Cedex Grenoble, France Received 6 June 2001; received in revised form 16 September 2001; accepted 26 September 2001

Abstract The role of formaldehyde in the atmospheric chemistry of the Arctic marine boundary layer has been studied during both polar day and night at Alert, Nunavut, Canada. Formaldehyde concentrations were determined during two separate field campaigns (PSE 1998 and ALERT2000) from polar night to the light period. The large differences in the predominant chemistry and transport issues in the dark and light periods are examined here. Formaldehyde concentrations during the dark period were found to be dependent on the transport of air masses to the Alert site. Three regimes were identified during the dark period, including background (free-tropospheric) air, transported polluted air from Eurasia, and halogen-processed air transported across the dark Arctic Ocean. In the light period, background formaldehyde levels were compared to a calculation of the steady-state formaldehyde concentrations under background and low-ozone conditions. We found that, for sunlit conditions, the ambient formaldehyde concentrations cannot be reproduced by known gas-phase chemistry. We suggest that snowpack photochemistry contributes to production and emission of formaldehyde in the light period, which could account for the high concentrations observed at Alert. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Arctic chemistry; Snowpack; Snow chemistry; Formaldehyde; Troposphere

1. Introduction Over the past few years, interest in the photochemistry of the boundary layer in Polar Regions has increased substantially. This stems from continued developments in our understanding of ozone destruction at the time of polar sunrise in the marine boundary layer (Barrie et al., 1988; Impey et al., 1999; Wennberg, 1999; Foster et al., 2001). Among the important issues is the nature of the *Corresponding author. Department of Chemistry, University of California Irvine, Irvine, CA 92697-2025, USA. Tel.: +1-949-824-6347; fax: +1-949-824-3168. E-mail address: [email protected] (A.L. Sumner).

formaldehyde (HCHO) sources and sinks, since HCHO is an important source of free radicals in the relatively dry Arctic troposphere, and because it can be an important sink for free radicals such as Br atoms that play a role in Arctic boundary-layer ozone depletion (Shepson et al., 1996; Sumner and Shepson, 1999; Michalowski et al., 2000). Sumner and Shepson (1999) found that HCHO is emitted from the snowpack after polar sunrise, and that this source was likely important to the boundary layer. Although the data were somewhat ambiguous, the authors hypothesized that the HCHO source in the snowpack was photochemical in nature. Hutterli et al. (1999) found that HCHO emission from surface snow at Summit Greenland was an

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important source of HCHO for the Summit boundary layer, with estimated fluxes ranging from B1– 10
108 molecules/cm2/s. These investigators discussed evidence that HCHO emission resulted from a temperature-dependent desorption from snow that was enriched relative to equilibrium values. During the Polar Sunrise Experiment 1998 (PSE98) and ALERT2000, we conducted measurements of gas phase HCHO concentrations from the dark through the light period, in conjunction with a variety of supporting chemical and meteorological measurements, which provided an opportunity to investigate the sources and sinks of HCHO in this environment in better detail. Additional experiments were conducted during ALERT2000 in which HCHO was measured in the snowpack interstitial air and in the snowpack condensed phase to examine the role of the snowpack in generating and emitting HCHO into the boundary layer. In this paper we discuss the results of these measurements, and their impact on our understanding of formaldehyde in the Arctic marine boundary layer (MBL).

2. Methods Gas phase HCHO determinations were conducted from 15 February to 26 April, 1998, and from 17 February to 5 May, 2000, at the Special Studies Laboratory (SSL) at the Canadian Forces Station at Alert, Nunavut, Canada (82.51N, 62.31W). The SSL is located 6 km southwest of the base camp, on a plateau that is 145 m above sea level. During both PSE98 and ALERT2000, HCHO gas phase concentrations were determined using a modified version of the fluorometric method described by Fan and Dasgupta (1994). Briefly, 00 sample air is pumped at 1.4 slpm through a 60 cm 14 PFA-Teflon tube containing a concentric 0.8 mm o.d. Nafion membrane tube. High purity water, into which HCHO and other small water soluble atmospheric constituents permeate, was pumped concurrent to the air flow (at B0.1 ml/min) by a peristaltic pump. Water was pumped through the membrane by positive pressure during PSE 1998 and by aspiration during ALERT2000. The latter method removes a H2O vapor dependent sensitivity, due to evaporation of water from within the Nafion tube to maintain equilibrium pressure (which changes with the ambient absolute humidity). This is not a significant problem for PSE98, as the absolute humidity at Alert during this period is always very low. The HCHO in the water then mixes with 1,3cyclohexanedione and NH+ 4 to produce a fluorescent product. This product is detected using a ThermoSeparation Products (Fluoromomonitor III) fluorimeter. Standards for gas phase calibration were prepared using a permeation device prepared from paraformaldehyde, the output from which was deter-

mined using the DNPH/HPLC method, as described in Sirju and Shepson (1995). A low flow of air passed through the device and was diluted to a final concentration of 100–1000 ppt, using zero air. The instrument was automated, to enable injection every 20 or 15 min for PSE 1998 and ALERT2000, respectively. The data were recorded using Labview software, and HCHO was quantified by measurement of peak heights. The 20 ppt detection limit was determined from the baseline signal variability (3 s). Propagation of errors leads to an estimated uncertainty of 7 (13%+20 ppt) for a 95% confidence interval, or 723% for a typical measured concentration of 200 ppt. A field intercomparison (MacDonald et al., 1998) against a tunable diode laser absorption spectrophotometric method (Harris et al., 1989) indicated an average difference between our fluorometric determinations and those from the TDLAS of 3%, well within our estimated uncertainty. Throughout this paper, gas phase HCHO ‘‘concentrations’’ are expressed as number ratios, relative to the number density of air at the surface at 245 K (3.0
1019 cm3), in parts per trillion (ppt). Snowpack interstitial air was sampled using an all-Teflon snow probe that was designed to minimize temperature changes in the snowpack from radiative heating of the probe and was constructed at Purdue University (via the Jonathan Amy Facility for Chemical Instrumentation). It is a 2.5 cm diameter 100 cm long TFE Teflon tube, composed of two 50 cm lengths, connected together 00 using a threaded aluminum coupler. The 14 PFA-Teflon sample line (which is 33 m long, and heated to B301C starting just downstream of the probe, to prevent ice condensation in the line) runs through the center of the probe, and connects to a stainless steel NPT-Swagelok fitting just above the base of the probe. The pipe thread end connects to a TFE Teflon filter holder, which is recessed (by B0.5 cm) from the end of the probe. We used 1 mm pore-size Teflon filters, which were held in place against a perforated TFE Teflon disk by a threaded stainless steel ring, to keep snow from being drawn into the probe. A type-T thermocouple was embedded in the probe tip. To position the probe, a 2.5 cm diameter hole was first bored in the snowpack using a stainless steel pipe and the probe inserted into the resulting hole. A base for the probe was made from a plexiglass frame. Experiments were conducted in which surface snow was irradiated with the output from a Photon Technologies Instruments LPS-220 150 W Xe arc lamp, filtered through Pyrex glass to remove radiation below B300 nm. We also conducted experiments using a 250 W IR heat lamp, to test for thermal desorption of adsorbates. HCHO in melted snow samples was determined using the method of Houdier et al. (2000).

A.L. Sumner et al. / Atmospheric Environment 36 (2002) 2553–2562

3.1. The role of transport It has been discussed by deServes (1994) that, during the dark period, HCHO can undergo long-range transport because of its long chemical lifetime (B160 days) in the dark and slow deposition to the snowpack. The background HCHO levels can thus be attributed to transport of HCHO from polluted regions even over great distances. A back trajectory analysis of the 1998 data set for the dark period indicates that the highest HCHO concentrations (195 ppt on average) occur for

50 Ozone

500

40 400 30 300

20

[Ozone], ppb

The gas phase HCHO concentrations determined for the full experiment period during ALERT2000 are shown in Fig. 1. The gas-phase concentrations ranged from 100–370 ppt during the dark period, and from B35–360 ppt after full sunrise. This is consistent with the observations for PSE98 discussed in Sumner and Shepson (1999). The formaldehyde data are segregated according to the corresponding ozone concentrations. We have identified three conditions that are represented: background conditions in which ozone levels are relatively high, partial ozone depletion conditions in which ozone drops below 35 ppb, and full ozone depletion conditions in which ozone levels are near zero concentrations. As shown in the figure, the formaldehyde behavior varies under these three different conditions.

48 h back trajectories (925 mbar) from the direction of Siberia, while those from the south (i.e. the Baffin inlet) were considerably smaller, averaging 120 ppt, suggesting that transport is important in the dark period. As shown in Fig. 2 for 3–9 March 1998, there are periodic drops in ozone (partial depletions) from background levels during the dark period that demonstrate an intriguing inverse correlation with HCHO. It has been discussed (Worthy et al., 1994; deServes, 1994) that the dark-period ozone decreases coincide with increases in CO2, CH4, and black carbon, all indicators of anthropogenic pollution, presumably arising from

[HCHO], ppt

3. Results and discussion

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200 HCHO 10

100

0 2/17/98

0 2/21/98

2/25/98

3/1/98

3/5/98

3/9/98

Date

Fig. 2. Formaldehyde (dots) and ozone data (lines) for the dark period of the Polar sunrise experiment, 1998.

Fig. 1. Ozone (panel A) and formaldehyde (panel B) concentrations for the ALERT2000 study. Formaldehyde data are segregated according to the corresponding ozone concentration. For high O3 (>35 ppb), HCHO is shown by open triangles. The filled circles represent HCHO during ozone depletion events (O3 o10 ppb). Global radiation and ambient temperature (panel C) are also shown for reference purposes by black and grey lines, respectively.

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with relatively large concentrations of HCHO, NOx, particulate matter, etc., with depleted levels of O3, but may then become depleted in NOy during long-range transport to Alert via reactions (2)–(4) below O3 þ NO2 -NO3 þ O2 ;

ð2Þ

NO3 þ NO2 2N2 O5 ;

ð3; 3Þ

N2 O5 þ surfaces-2HNO3 ðaqÞ:

ð4Þ

We examined these occurrences using the PSE98 data set, which exhibits several striking examples of HCHO– O3 inverse correlation (Sumner and Shepson, 1999). A linear regression of formaldehyde versus ozone for the entire dark period shows that the data separate into two distinct groups, as shown in Fig. 3, which we call Case 1 (solid circles) and Case 2 (open circles) events. Examination of the supporting data revealed that the behavior of the pollution tracers described by Worthy et al. (1994) was different for each type of event, as represented by the CO2 data shown in Fig. 3. Data for Case 1 type

CO 2, ppb

371 370 369

PAN, ppt

CO2, ppb

372

Case 1 Case 2

368 300

373 372 371 370 369 368 500

200

CO

A

190 180

CO2

170

B

2.0

Hg

1.6

400 300

PAN

200

150

350

40

Ozone

400

200

35

C

30

300

25

250

20

200

100

HCHO

150

25

30

35

40

45

50

Ozone, ppb

Fig. 3. Formaldehyde concentrations are plotted versus ozone for the dark period data. The HCHO data are separated depending on the relationship observed between ozone and anthropogenic precursors into Case 1 (filled circles) and Case 2 (open circles) groupings. The CO2 data for specific events on 3 and 7 March 1998 for Case 1 and Case 2, respectively, are also shown.

1.2 0.8

450 HCHO, ppt

HCHO, ppt

250

CO, ppb

ð1Þ

3

O3 þ NO-O2 þ NO2

episodes (e.g. 3 March 1998) exhibit a negative CO2–O3 correlation, with a correlation coefficient of 0.81. However, the Case 2 episodes (e.g. 7 March 1998) exhibit a strong but positive CO2–O3 correlation with a correlation coefficient of 0.84. It is clear that the behavior of CO2 is different for the two types of partial ozone depletions. Here we will examine two specific cases, from 3 and 7 March 1998 shown in Figs. 4 and 5 (Cases 1 and 2), respectively. The data for HCHO, O3, several anthropogenic tracers, and elemental Hg, which can serve as an indicator for halogen atom processing (Boudries and Bottenheim, 2000; Lu et al., 2001), are shown. For the event shown in Fig. 4 (Case 1), O3 decreased from 40 ppb on 2 March to a minimum of B30 ppb on 3 March, with a simultaneous increase in HCHO and the anthropogenic tracers, CO, CO2, and peroxyacetyl nitrate (PAN). While the HCHO data immediately prior to the event were not available, the stable ozone concentrations suggest that HCHO was likely steady at B125 ppt until the event. During the O3 decrease, the ambient temperature decreased from 201C to 321C, as the wind direction changed from southerly to northerly in the early morning hours of 3 March. The back trajectories from 2 to 3 March indicated a change in air mass origin from southerly (the direction of the Beaufort sea) to westerly, and from altitudes that may indicate contact with the Arctic Ocean during transport.

Hg, ng/m

long-range transport, most typically from Eurasia. It can be hypothesized that the O3 decrease results from removal of O3 by NOx (reaction (1)) in a polluted region, followed by weak vertical mixing during longrange transport to Alert. Such air masses may originate

b Ozone,, ppb

2556

100

15 10

12:00 AM 12:00 AM 12:00 AM 12:00 AM 12:00 AM 3/2/98 3/3/98 3/4/98 3/5/98 3/1/98 EST

Fig. 4. Ozone data (black lines, Panel C) for a Case 1-type dark period depletion. Formaldehyde (triangles, C) is inversely correlated with ozone, as are the anthropogenic tracers PAN (grey line, B), Hg (dotted line, B), CO (circles, A), and CO2 (grey line, A) and. Gaseous mercury does not appear to change significantly in concentration during the event.

369 500 450 400 350 300

A

1.6

Hg PAN

B

0.8

450

Ozone

40

HCHO, ppt

400

35

350

30

300 250 200

1.2

CO,, ppb

CO2

3

370 PAN, ppt

CO

371

200 195 190 185 180 175 2.0

Hg,, ng/m /

373 372

25

C

HCHO

20

Ozone, ppb

CO 2, ppb

A.L. Sumner et al. / Atmospheric Environment 36 (2002) 2553–2562

15

150

10

100

12:00 AM 12:00 PM 12:00 AM 12:00 PM 12:00 AM 3/7/98 3/8/98 3/9/98 EST

Fig. 5. Ozone data (black line, Panel C) for a Case 2-type dark period depletion. Formaldehyde (triangles, C) is inversely correlated with ozone. The anthropogenic tracers PAN (grey line, B), CO2 (grey line, A) and CO (circles, A) are positively correlated with ozone during the event. Gaseous mercury (dotted line, B) is also positively correlated with ozone, which may suggest that the air mass has been impacted by halogen chemistry.

We note that, for this case, the Hg concentrations were stable, as is typically the case for this long-lived pollutant. The behavior of the pollution tracers, formaldehyde, and elemental mercury are consistent with the Worthy et al. (1994) hypothesis that the partial ozone depletion was caused by the transport of a polluted air mass across the dark Arctic Ocean. The data from the second case, on 7 March, is shown in Fig. 5 (Case 2). For this case, there was a similar decrease in O3 and a striking inverse correlation with HCHO. In contrast to Case 1, all other anthropogenic pollutants, including Hg, were positively correlated with O3. The behavior of the anthropogenic tracers is inconsistent with the transport of a polluted air mass. Rather, concentrations of CO, CO2, and PAN decreased, suggesting a change in the air mass origin to one that has received less impact from polluted regions. While the wind direction at the site changed from northwesterly before the event to southeasterly during the O3 depletion, the back trajectories indicate an air mass origin from the direction of the North Pole throughout the event. In the absence of sunlight, there are two known mechanisms for ozone destruction at the surface: reaction with NO and reaction with bromide on suspended particles and possibly the snowpack (Oum

2557

et al., 1998; Albert et al., 2002), as shown below O3 þ Br -BrO þ O2 :

ð5Þ

For the Case 2 event, the depletion of ozone cannot be explained by the reaction of ozone with NO, since neither CO2 nor PAN was elevated. A possibility for such cases is that O3 is destroyed by reaction with sea salt/ice surfaces (or particles), via reaction (5), as the air mass is transported within the MBL. This does not account for the behavior of HCHO or of Hg. In the dark, the most reasonable cause for increased HCHO in the boundary layer is desorption from snow grains that have HCHO concentrations in excess of thermal equilibrium values. However, the temperature decreases slightly through the event, which would only cause perhaps a small amount of HCHO uptake into the ice/ snow. It is also necessary to account for the depletion of Hg in the O3-depleted air mass. Thus, it seems most likely that these cases result from the same halogen activated O3 depletion mechanism that causes the full ozone depletion events observed after polar sunrise (Michalowski et al., 2000). In this case, the chemistry must occur at more southerly latitudes that are sunlit, followed by transport to Alert. In such an air mass, HCHO would be elevated from chlorine atom chemistry, as discussed below, while Hg would be depleted from bromine chemistry (Schroeder et al., 1998; Ariya et al., 2002). Photochemistry may have already taken place on the Siberian side of the Arctic Ocean, which is sunlit in early March, processing ozone and Hg (via Br atom chemistry) and formaldehyde precursors (via Cl atom chemistry). We note that the GOME satellite BrO data frequently show elevated BrO at latitudes significantly lower than that for Alert in early March (Richter, personal communication). The products of this photochemistry (including formaldehyde) may then be transported across the ocean in the dark just as it has been suggested that polluted air parcels are transported to Alert (Worthy et al., 1994). 3.2. Gas phase photochemistry and relationship to O3 In the full sunlit period (i.e. after 1 April), the lifetime of HCHO due to photolysis and reaction with OH (Grannas et al., 2002) is B6 h, and can be significantly shorter in the presence of Br atom chemistry. Thus, under these conditions, the HCHO detected at Alert has been produced within an B80 km radius of the measurement site (for a typical windspeed of o4 m/s), or in other words, there are no direct anthropogenic influences. We do not see any evidence for an impact of the camp generators (located 6 km to the NE) on local HCHO, and the measured HCHO does not correlate with NOx, which is a good indicator for camp air (Ridley et al., 2000).

A.L. Sumner et al. / Atmospheric Environment 36 (2002) 2553–2562

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Thus, formaldehyde concentrations observed at Alert after polar sunrise likely result from local photochemistry. As discussed in Shepson et al. (1996), HCHO can be produced from chlorine and OH radical chemistry in the gas phase, largely through reactions (6)–(11) BrCl þ hn-Br þ Cl;

ð6Þ

HONO þ hn-OH þ NO;

ð7Þ

Cl þ CH4 -CH3 OO þ HCl;

ð8Þ

OH þ CH4 -CH3 OO þ H2 O;

ð9Þ

CH3 OO þ NO-CH3 O þ NO2 ;

ð10Þ

CH3 O þ O2 -HCHO þ HO2 :

ð11Þ

However, CH3OO  can also react with HO2 under low NOx conditions to produce significant concentrations of methyl hydroperoxide (MHP), as shown below CH3 OO þ HO2 -CH3 OOH þ O2 :

ð12Þ

We calculate the fraction of time, F; that CH3OO  radicals react with NO as F¼ k10 ½NO =ðk10 ½NO þk12 ½HO2 Þ: Here we neglect the selfreaction of organic peroxy radicals as slow relative to reaction (12). For a typical daytime average [NO] in April of 7 ppt (Beine et al., 2002), and a model calculated [HO2] for those conditions of 4 ppt, we calculate F ¼ 0:61: This leads to significant concentrations of CH3OOH. From our model calculations (Grannas et al., 2002), and from the measurements of deServes (1994), we estimate a conservatively high value for [CH3OOH] at steady state of 100 ppt. Thus we also consider HCHO production from the reactions below OH þ CH3 OOH--HCHO þ H2 O þ OH;

HCHO þ OH-HCO þ H2 O;

ð17Þ

HCHO þ Br-HCO þ HBr;

ð18Þ

HCHO þ Cl-HCO þ HCl:

ð19Þ

Given HCHO’s short lifetime, we can reasonably assume that it is in steady state. From this simplified mechanism, we can derive the following expression: 8 9 F  ðk9 ½OH ½CH4 þ k8 ½Cl ½CH4

> > < = þk13b ½OH ½MHP Þ þ ðk13a ½OH ½MHP

> > : ; þk14 ½Cl ½MHP þ J15 ½MHP Þ : ½HCHO ss ¼ J16 þ k17 ½OH þ k18 ½Br þ k19 ½Cl

For the steady state calculation discussed here, we assume an [OH]=1.0
106 (Grannas et al., 2002; Zhou et al., 2002), and average light conditions on 10 April, where J16 ¼ 2:0
105 s1, and J15 ¼ 4:2
107 s1. We note that these photolysis rate coefficients are significantly larger than the values used previously (Shepson et al., 1996; Sumner and Shepson, 1999; Michalowski et al., 2000; Rudolph et al., 1999) because of the impact of the large surface albedo on photolysis rates (Simpson et al., 2002). For reaction (14), we assume that HCHO is produced with unit efficiency, and a rate constant of 5.7
1011 cm3/molecule/s (Wallington et al., 1990). In Fig. 6, we show the calculated [HCHO]ss as a function of [Cl] and [Br]. As discussed in several studies (Jobson et al., 1994; Ariya et al., 1998, 1999; Rudolph et al., 1999), gas phase chemistry under background (high O3) conditions at Alert appears to be dominated by OH radicals, but halogen atom chemistry becomes more important as O3 is depleted. These studies indicate that the Br/Cl ratio ranges from B400 to several thousand for conditions in which ozone is

ð13aÞ 8

ð13bÞ

Cl þ CH3 OOH-HCHO þ HCl þ OH;

ð14Þ

CH3 OOH þ hn--HCHO þ HO2 þ OH:

ð15Þ

Here we neglect the possible contribution from methanol oxidation. Methanol is present at Alert on the order of 200 ppt (Boudries et al., 2002); given its small OH reaction rate constant it will not contribute significantly at low [Cl], but when chlorine chemistry dominates, its oxidation could contribute an additional 10–15% to the HCHO production rate. HCHO can then be lost through photolysis, which tends to dominate under most Arctic conditions, and through reactions (17)–(19) below HCHO þ hn-H þ HCO;

ð16aÞ

HCHO þ hn-H2 þ CO;

ð16bÞ

0.9 [Br]/[Cl] = 1000

40

0.8 3

OH þ CH3 OOH-CH3 OO þ H2 O;

1.0x10

Bromine atoms/cm



0.7 0.6 60 100 ppt HCHO

0.5

140

120

0.4

160

80

0.3 [Br]/[Cl] = 100

0.2 0.1

180

200

220

0.0 0.0

0.2

0.4

0.6

0.8 1.0 1.2 1.4 3 Chlorine atoms/cm

1.6

240 5

2.0x10

Fig. 6. Isopleths of calculated steady-state formaldehyde concentrations plotted as a function of chlorine atom and bromine atom concentrations. Dotted lines show Br/Cl ratios of 1000 and 100, for reference purposes.

A.L. Sumner et al. / Atmospheric Environment 36 (2002) 2553–2562

completely depleted (Jobson et al., 1994; Ariya et al., 1998, 1999; Rudolph et al. 1999; Ramacher et al., 1999). Rudolph et al. (1999) discuss that Cl atom concentrations increase roughly linearly as ozone decreases from background values, while at very-low ozone (i.e. o5 ppb), Br atom concentrations increase exponentially, since the Br atom sink via reaction with O3 becomes unimportant. Thus, at very low ozone, the Br/ Cl ratio becomes very large, resulting in net aldehyde destruction. Fig. 6 includes a line for Br/Cl ratio equal to 1000 that represents total ozone depletion conditions. An additional line for a Br/Cl ratio equal to 100 was added to represent an approximate minimum value for non-depletion conditions. We note, however, that Ramacher et al. (1999) indicate that Cl and Br concentrations approach zero as ozone levels reach background levels and so the very-low Br/Cl ratios apply to concentrations near the origin of Fig. 6. The [HCHO]ss predictions from Eq. (1) were then compared to the observed formaldehyde concentrations as a function of ozone concentration. Fig. 7 shows the ambient HCHO data for sunlit conditions in 1998 and 2000. The solid line shows a smoothed fit line to the data for reference purposes. As shown in the figure, HCHO is positively correlated with ozone for O3 o15 ppb. For ozone levels >15 ppb, HCHO is inversely correlated with ozone and then levels off at B200 ppt for background ozone concentrations. We plotted the calculated [HCHO]ss as a function of ozone using calculated Br/Cl ratios as observed (as a function of O3) by Ramacher et al. (1999), and using an absolute Cl concentration that decreases linearly from 9.2
104 cm3 to zero over the ozone range of 0– 40 ppb, estimated from Boudries and Bottenheim (2000)

and Rudolph et al. (1999). The result of this calculation, which represents our best estimates of realistic chlorine and bromine concentrations and gas-phase HCHO chemistry at Alert, is shown by the grey line in Fig. 7. The agreement between the ambient observations and the calculated formaldehyde steady-state concentration for completely ozone-depleted conditions is good. However, for ozone concentrations >2 ppb, the discrepancy between the observations and the calculation is significant, with the calculation underpredicting HCHO by B200 ppt for partial ozone depletion conditions and B100 ppt under background conditions. This discrepancy is similar to that estimated previously (Shepson et al., 1996; Sumner and Shepson, 1999). A similar analysis was presented by Rudolph et al. (1999) for our 1998 data set, but cannot be compared to this calculation as the parameters and assumptions for that model were not reported. It is now clear that either we are missing gas phase precursors to HCHO production, or there is a surface source of HCHO. 3.3. The role of the snowpack As discussed above, known gas phase chemistry cannot fully account for the observed gas-phase HCHO concentrations. At high-ozone concentrations, i.e. in the absence of halogen atom chemistry, the [HCHO] is often 200 ppt or higher, which is more than twice what we expect from OH–CH4 chemistry under low NOx conditions (represented by the isopleths near the origin in Fig. 6), i.e. where reaction (12) is important. Since HCHO must be completely locally produced under full sunlight conditions, we must look for additional sources of HCHO. Sumner and Shepson (1999) hypothesized

500

1998 Data 2000 Data

400

1400

1000

300 800 200

[Br] / [Cl]

1200 Smoothed Fit to Data

HCHO, ppt

2559

600 400

100 [Br] / [Cl] Calculated Steady State HCHO

0 0

10

20

200 0

30

40

50

Ozone, ppb Fig. 7. Formaldehyde concentrations during the light period are plotted as a function of ozone. The solid black line shows a smoothed fit to the data. The estimated Br/Cl ratio (dotted line) and literature data for [Cl] are used to calculate [HCHO]ss versus O3 (grey line).

A.L. Sumner et al. / Atmospheric Environment 36 (2002) 2553–2562

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that HCHO may be produced from condensed phase photochemistry in the snowpack, involving OH radical oxidation of some unknown organic substrate. During the course of ALERT2000, experiments were conducted involving snowpack interstitial air vertical profiles, in both the dark and light periods, and controlled irradiation of the surface snow during the dark period, using a Xe-arc lamp. As discussed in Boudries et al. (2002), Grannas et al. (2002), and Guimbaud et al. (2002), there is solid evidence for a light-dependent flux of HCHO, CH3CHO, and CH3C(O)CH3 from the snowpack, as determined from the vertical profiles, and a strong diel cycle of these species in the snowpack air. However, Hutterli et al. (1999) concluded that HCHO can be emitted from the snowpack as a result of temperature-dependent adsorption/desorption. Couch et al. (2000) also found that this can be important for midlatitude snow, but attributed the HCHO emission from the snowpack to snow crystal metamorphosis driven by temperature gradients. To test the possibility for photochemical production of HCHO, we irradiated a B7 kg pile of snow collected after a fresh snowfall using the Xe-arc lamp. HCHO was measured in the condensed phase in the pile as well as in the snow interstitial air. The results are shown in Fig. 8,

500

where we plot the difference in gas phase concentration between the snow pile and the ambient air. As shown in Fig. 8a, the snow pile HCHO concentration in the gas phase increased by B277 ppt upon irradiation. Condensed-phase HCHO in the snow pile was also found to increase slightly (B15%) during a duplicate experiment. However, as shown from the snowpack temperature data, there was also an increase of 5.31C in the snow temperature near the sample inlet. To test for temperature-dependent desorption, we also heated a duplicate snow pile using an IR lamp; the results are shown in Fig. 8b. As shown in the figure, with a slightly larger increase in snow temperature, there was a factor of B7 smaller HCHO increase in the snow interstitial air. We do not intend for the quantitative results to have any particular significance, as the lamp output was not quantified during the experiment. However, this result makes it clear that it is possible to photochemically produce HCHO in the snowpack.

4. Conclusions This study has shown that, in the dark period, ambient HCHO concentrations are influenced by long-

A Heat and Radiation

Average Enrichment = 2 7 7 ppt pp o delta T = 5 . 3 C

400

-28 -30 -32

200 100

-34

0

-36 -28

500

B Heat only

Average Enrichment = 40 pp t o delta T = 5 . 8 C

400 300

-30

Snow Temperature, oC

[ H C HO ]snow - [ H C HO ] ambient, ppt

300

-32

200 -34

100 0

-36 -1 0 Lamp On

0

10

20

30

40

Time from Lamp Ignition, hours

Fig. 8. Interstitial air formaldehyde enhancements (snowpack air concentrationFambient air concentration, shown by dots) for snow pile irradiation experiments plotted as a function of time from lamp ignition. The snowpack temperature is shown by black lines. The period during which the snow pile is exposed to the lamp output is shaded in grey. Panel A shows data for the Xe-lamp experiment. Panel B shows the heating experiment using an IR heat lamp.

A.L. Sumner et al. / Atmospheric Environment 36 (2002) 2553–2562

range transport. We observed cases of the transport of polluted air to the measurement site and also air that may have been influenced by photochemistry at lower latitudes. We also find that the gas-phase HCHO concentrations observed during the sunlit period cannot be fully accounted for on the basis of known gas phase chemistry. Our measurements in the snowpack and snowpack interstitial air indicate that snowpack photochemistry likely contributes to the high HCHO concentrations in the sunlit period. We note that, as described in Guimbaud et al. (2002) and Strong et al. (2002), the boundary layer is very stable during the measurement period. We estimate that HCHO mixes in the vertical direction by only B20 m in the time frame of one HCHO photochemical lifetime. Thus, snowpack emission likely only affects the boundary layer concentrations on this approximate scale. It would be useful to determine the HCHO concentrations in the boundary layer over the lowest B100 m, to test this. It is, however, clear that measurements made in the near-surface environment are significantly impacted by air–snow exchange processes and therefore these processes need to be better characterized. If it is the case that carbonyl compounds are produced from the oxidation of dissolved organic matter in the snow, the chemical nature of this carbon source should be characterized.

Acknowledgements We thank A. Gallant, J. Deary, the Meteorological Service of Canada, and all the personnel of CFS Alert for technical and logistic support. We gratefully acknowledge the National Science Foundation Office of Polar Programs and Atmospheric Chemistry program (OPP-9818257) for financial support of this work.

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