Photochemistry of phenanthrene, pyrene, and fluoranthene in ice and snow

Atmospheric Environment 43 (2009) 2252–2259

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Photochemistry of phenanthrene, pyrene, and fluoranthene in ice and snow Keren Ram a, Cort Anastasio a, b, * a b

Graduate Group in Agricultural and Environmental Chemistry, University of California – Davis, One Shields Avenue, Davis, CA 95616, USA Atmospheric Science Program, Department of Land, Air and Water Resources, University of California – Davis, One Shields Avenue, Davis, CA 95616, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 July 2008 Received in revised form 15 December 2008 Accepted 28 January 2009

Although polycyclic aromatic hydrocarbons (PAHs) are common pollutants in snow, there is little quantitative data about their rates of photodegradation in this environment. To begin to address this gap, we have measured the degradation kinetics of phenanthrene, pyrene, and fluoranthene on ice, as these are the most abundant PAHs in arctic snow. Frozen aqueous solutions of individual PAHs, with and without added hydrogen peroxide (HOOH) as a source of hydroxyl radical (OH), were illuminated with simulated sunlight. For all three PAHs, direct photodecay is the main mechanism of degradation, while  OH-initiated indirect photodegradation is a minor sink. Rate constants (1 SE) for direct photodegradation extrapolated to midday, surface snow conditions at Summit, Greenland on the summer solstice are 3.8 (0.8)  105, 28 (3)  105, and 1.4 (0.7)  105 s1 for phenanthrene, pyrene, and fluoranthene, respectively. Apparent quantum efficiencies for photodegradation with simulated sunlight were 3.8 (0.8)  103, 4.3 (0.5)  104, and 2 (1)  105, respectively. Calculated PAH lifetimes in surface snow under Summit conditions are 1–19 h during mid-summer, but increase to >100 days in the dark winter. While the short photodegradation lifetimes in the summer suggest that there should be no appreciable PAH levels in this season, past measurements at Summit sometimes show significant levels of these PAHs in summer surface snow. This discrepancy is likely due to differences in PAH location between lab samples (where the PAHs are probably in quasi-liquid layers) and real snow (where PAHs are likely primarily associated with particulate matter). Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: PAHs Greenland Photodegradation Kinetics Persistent organic pollutants

1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are a class of toxic organic pollutants that are byproducts of incomplete fuel combustion. Due to their lipophilic nature, PAHs will often bioaccumulate, and some of the higher molecular weight PAHs are also carcinogenic (ATSDR, 1995). The sources of PAHs are mostly anthropogenic, and they are ubiquitous in all environmental media, including snow, surface waters, soil, and ambient air. Phenanthrene (PHE), pyrene (PYR), and fluoranthene (FLA) are usually the most abundant PAHs in both Arctic and lower latitude snow, and they are among the most abundant PAHs in other media (Finlayson-Pitts and Pitts, 2000; Carrera et al., 2001; Vilanova et al., 2001; Kay et al., 2008). In the atmosphere, PAHs are typically associated with fine particles, often soot, which can protect the PAHs from degradation, resulting in lifetimes on the order of months (Behymer and Hites, 1985; Bjorseth et al., 1979; Zepp and Schlotzhauer, 1979; Masclet * Corresponding author. Graduate Group in Atmospheric Science, Department of Land, Air and Water Resources, University of California – Davis, One Shields Avenue, Davis, CA 95616, USA. Tel.: þ1 (530) 754 6095; fax: þ1 (530) 752 1552. E-mail address: [email protected] (C. Anastasio). 1352-2310/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2009.01.044

et al., 2000). Typical total concentrations of PAHs in surface snow at Summit, Greenland range from 0.1 to 10 ng kg1, with ranges for PHE, PYR, and FLA of approximately 0.1–5 ng kg1 each (Jaffrezo et al., 1994; Masclet et al., 2000; Slater et al., 2002). Understanding the snowpack reactions of PAHs (and other compounds) is complicated because snow contains several reservoirs and the rates of transformation are likely different in each. These reservoirs include the interstitial firn air, the bulk ice, quasi-liquid layers (liquid-like water phases occurring around and between snow grains) and undissolved particulate matter, such as soot (Domine´ and Shepson, 2002; Dibb et al., 2007; Domine´ et al., 2008). Volatilization to the firn air (and, subsequently, the boundary layer) is likely a minor sink for most PAHs in dry snowpacks due to low temperatures and high snow area index (Domine´ et al., 2007; Meyer and Wania, 2008), although this fate may be more significant as snow undergoes metamorphosis (Herbert et al., 2006). Similarly, PAH incorporation into bulk ice is unlikely, as PAHs are large and hydrophobic (Domine´ et al., 2007). Thus most PAHs, and PAH transformations, in snow likely occur in quasi-liquid layers and in particles. While the photodegradation of PAHs on particles has been studied, the available data does not present a coherent picture, with lifetimes for the same compound ranging from several hours to

K. Ram, C. Anastasio / Atmospheric Environment 43 (2009) 2252–2259

more than 1000 h depending on the particle type (Behymer and Hites, 1985). Even less is known about PAH photodegradation on ice. In water PAHs undergo both direct photodegradation and indirect photo-oxidation with species such as hydroxyl radical (OH) (Neff, 1979; Fasnacht and Blough, 2002); we expect that similar reactions occur in quasi-liquid layers in/on ice. Kahan and Donaldson have shown that PAHs (anthracene and naphthalene) undergo direct photodegradation in surface quasi-liquid layers on ice, while Kla´n and coworkers have shown that OH can also transform these PAHs on ice (Dolinova et al., 2006; Kahan and Donaldson, 2007). Although these results show that PAHs on ice are photodegraded, it is difficult to extrapolate them to obtain environmentally-relevant rate constants and lifetimes. However, these results do suggest that photodegradation is an important mechanism for transformation of PAHs in high latitude snowpacks during the long, sunlit days of summer. The major objectives of this study were to: (1) quantify the rates of direct and indirect (OH) photochemical transformations of the three most abundant PAHs in Summit, Greenland snow; (2) determine the importance of direct photodegradation versus reaction with OH; (3) calculate the quantum efficiencies for PAH photodegradation on ice; and (4) compare the seasonality in PAH levels implied by our photochemical lifetimes to previously reported PAH concentration profiles for Summit surface snow. 2. Experimental methods 2.1. Materials Sodium sulfate (ACS) and acetonitrile (HPLC grade) were from Fisher. Sodium benzoate (NaBA, 99%), phenanthrene (PHE, 98%), pyrene (PYR, 98%), and fluoranthene (FLA, 99%) were from Aldrich. Water was from a Millipore Milli-Q Plus system (18.2 MU cm). 2.2. Molar absorptivity measurements Solution absorbances were measured in a 1-cm quartz cell with a Shimadzu UV-2501PC spectrophotometer for six solutions each of PHE, PYR, and FLA in 50:50 acetonitrile:Milli-Q at room temperature. Solution concentrations ranged from 30 mM to 10 mM for PHE, and 25–1000 mM for FLA and PYR; 50:50 acetonitrile:Milli-Q solution was used as reference and as a blank for each absorbance spectrum. Regressions of absorbance versus concentration were used to calculate molar absorptivity coefficients. 2.3. Direct photodegradation of PAHs: sample preparation and illumination Illumination solutions contained 10 mM NaBA (to mimic the natural sink for OH in snow), 20 mM Na2SO4 (to mimic the total ion concentration of snow) and 50 nM of phenanthrene (PHE), pyrene (PYR), or 100 nM of fluoranthene (FLA). 450 mL of a given sample solution was pipetted into 15 quartz tubes (GE 021 quartz; 5-mm ID  7-mm OD, 3-cm length) for most experiments. The tubes were sealed using aluminum foil wrapped with ParafilmÒ and frozen vertically (so that the solution did not touch the foil) in a laboratory freezer at 20  C. After freezing, tubes were set horizontally onto an aluminum plate inside a custom-designed illumination chamber (Paige Instruments) consisting of a temperature-controlled, Peltiercooled, Teflon-coated, copper block base beneath the aluminum plate and a 150 mm  150 mm GE 021 quartz window on top to transmit light onto the samples. The aluminum plate contained 8 identical parallel ‘‘trenches,’’ designated ‘‘A’’ through ‘‘G,’’ going from left to right. Seven of the trenches each held 2 quartz tubes

2253

(oriented with the bottoms of the tubes meeting in the middle of the trench), numbered ‘‘1’’ (top tube) or ‘‘2’’ (bottom tube). In trench ‘‘F’’ we put one sample tube and one tube with a thermocouple wire in (frozen) Milli-Q to monitor and maintain temperature in the chamber (typically at 10  C) during illumination. Illumination trenches ‘‘A’’ and ‘‘G’’ were usually reserved for four dark control sample tubes, which were identical to the illumination tubes but were wrapped in foil. Samples were illuminated with simulated sunlight made by passing the output from a high pressure 1000-W Xe lamp through: (1) a dichroic cold mirror (to transmit 300–500 nm light, but remove other wavelengths to reduce sample heating), (2) a thin sheet of PTFE Teflon as a diffuser, and (3) an air mass 1.5 global filter (Sciencetech). As shown in Supplemental Fig. 1, the solar simulator output roughly approximates Summit summer sunlight, although it has relatively less of the shorter UV and more of the longer UV wavelengths. After illumination, samples were melted at room temperature in the dark and immediately injected onto a Thermo-Hypersil C-18 reverse phase HPLC column attached to a Shimadzu LC-10ATVP HPLC with SPD-10AVP UV–Vis detector in order to quantify PAH levels. We used an eluent of 80:20 acetonitrile:Milli-Q at a flow rate of 0.97 mL min1 and a detection wavelength of 250 nm (PHE), 335 nm (PYR), or 286 nm (FLA). Because there was often a small amount of PAH sorption to the quartz tubes (typically 5–10% of the total PAH), after injecting each sample we rinsed the empty sample tube with 450 mL of 80:20 acetonitrile:Milli-Q, measured the PAH concentration in the rinse, and added this to that measured initially in the sample.

2.4. Actinometry Photon fluxes in the illumination chamber were measured using quartz tubes with 450 mL of 10 mM 2-nitrobenzaldehyde (2-NB) (Anastasio et al., 1994). Because the simulated sunlight is not uniform across all 7 trenches, we first used 2-NB to map variations in photon flux in each trench. Because the rate constant for 2-NB photolysis (j2NB) is independent of temperature, and is the same in ice and solution (Galbavy et al., in preparation), tubes of 2-NB were illuminated at room temperature for simplicity. At known illumination times, aliquots were removed from a tube and injected onto the HPLC to measure the 2-NB concentration (Anastasio et al., 1994). Values of j2NB for a given tube were determined from a linear regression of

ln

½2-NBt ¼ j2NB t ½2-NB0

(1)

where [2-NB]t and [2-NB]0 are the concentrations at times t and zero, respectively, in the same tube. Listed uncertainties for j2NB, and all other quantities in this work, are 1 standard error. For each position x (i.e., ‘‘A1’’–‘‘G2’’) we calculated a correction factor, F2NB,x, which describes the photon flux at that position relative to the reference position ‘‘D1’’:

F2NB;x ¼

j2NB;x j2NB;D1

(2)

Values of F2NB,x ranged from 0.76 to 1.04 and generally decreased with distance from tube ‘‘D1’’. Average values of F2NB,x from 6 mappings over the course of 8 months were used to correct PAH decay curves (section 2.5). We also measured 2-NB actinometry in tube ‘‘D1’’ on each experimental day as a proxy for the photon flux in the illumination system so that we can normalize results from different days (as well

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as to Summit sunlight conditions; Eq. (5)). The rate constant was usually determined before and after each PAH experiment: typically these values were within 5% and the former value was used. There was no significant decay of 2-NB in dark tubes. 2.5. Direct photodegradation rate constants for PAH loss (jPAH) The first-order rate constant for the direct photodecay of PAH (jPAH) in each experiment was determined using:

ln

½PAHt =F ¼ jPAH t ½PAH0 2NB;x

(3)

where for each illumination point (i.e., trench position, x) the corresponding value of F2NB,x was used. A similar procedure (with F2NB,x set to 1) was used to a determine the first-order rate constant for loss of PAH in the dark (k0 dark), which was used to determine the dark-corrected experimental photodegradation rate constant, jPAH,exp:

jPAH;exp ¼ jPAH  k0dark

(4)

This laboratory rate constant for PAH photodecay was normalized to the value expected in surface snow under clear-sky, midday, summer solstice sunlight at Summit, Greenland (jPAH,Sum) using:

jPAH;Sum

j2NB;Sum ¼ j j2NB;exp PAH;exp

(5)

2.6. Rate constant for sunlight absorbance (jabs) and apparent quantum efficiency for PAH loss (FPAH) The rate constant for sunlight absorbance, jabs (s1 molphotons mol1-PAH) describes how much sunlight each PAH absorbs under clear-sky, midday, summer solstice sunlight in surface snow at Summit, Greenland. Values were calculated using (Anastasio and Robles, 2007):

 2303 X Il 3PAH;l Dl NA

(6)

l

where NA is Avogadro’s number, Il (photons cm2 s1 nm1) is the actinic flux from the TUV model (Madronich and Flocke, 1998), 3PAH,l (L mol1 cm1) is the base-10 molar absorptivity of the PAH, and Dl (nm) is the wavelength interval for the actinic flux values. The apparent quantum efficiency for PAH photodegradation, FPAH, is the absorption-weighted average value over the solar spectrum:

FPAH ¼

jPAH;Sum jabs

(7)

2.7. Reactions of PAHs with OH We also conducted experiments to quantify the contribution of hydroxyl radical reactions to PAH loss. Illumination solutions were the same as described in section 2.3, but also included 1.0 mM (for PHE and PYR solutions) or 2.0 mM HOOH (for FLA solutions) as a source of hydroxyl radical. The loss of PAH due to direct photodegradation and reaction with OH is:

(8)

where kPAHþOH is the second-order rate constant for reaction of PAH with OH, [OH]exp is the steady-state hydroxyl radical concentration in the experiment, k0 OH,exp (¼kPAHþOH[OH]exp) is the pseudofirst order rate constant for reaction of PAH with OH in the experiment, and k0 tot,exp is the total pseudo-first order rate constant for PAH loss. Values of k0 tot,exp were determined from plots of ln([PAH]t/[PAH]0)/F2NB,x versus t in the HOOH-containing solutions. Values of k0 OH,exp were then determined using

k0,OH;exp ¼ k0tot;exp  k0dark  jPAH;exp

(9)

where k0 dark is the pseudo-first order rate constant for PAH decay in dark control samples with HOOH, which were prepared in the same way as experimental samples, but the tubes were wrapped in foil. This experimental rate constant for PAH loss due to OH reaction was normalized to midday, clear-sky, summer solstice, surface snow at Summit, Greenland using:

k0,OH;Sum ¼ k0,OH;exp

½,OHSum ½,OHexp

(10)

where [OH]Sum was estimated as 1015 M (Anastasio et al., 2007). [OH]exp was calculated using



where j2NB,Sum is the first-order photolytic decay rate constant for 2-NB under these Summit summer conditions (2.2  102 s1, calculated using modeled TUV actinic flux; Madronich and Flocke, 1998), and j2NB,exp is the 2-NB rate constant determined on each experimental day.

jabs ¼

  d½PAH ¼ jPAH;exp ½PAH  kPAHþOH ½,OHexp ½PAH dt   ¼  jPAH;exp þ k0,OH;exp ½PAH ¼ k0tot;exp ½PAH

½,OHexp ¼

jHOOH/,OH j2NB

 j2NB;exp ½HOOH

kBAþOH ½BA

(11)

where jHOOH/OH is the measured rate constant for formation of OH from HOOH in the system at 10  C, determined as described previously (Chu and Anastasio, 2005); j2NB is the 2-NB photolysis rate constant for the HOOH photolysis experiment; [HOOH] is the concentration in the PAH illumination solution; kBAþOH is the second-order rate constant for reaction of OH with benzoate in the quasi-liquid layer at 10  C, estimated to be 3.8 (0.2)  109 M1 s1 using an Arrhenius treatment to extrapolate from solution data (20–60  C) (Ashton et al., 1995); [BA] is the concentration of benzoic acid in the solution (10 mM); and j2NB,exp is the rate constant for 2-NB photolysis for the given PAH photodegradation experiment. For the PHE, PYR, and FLA experiments with HOOH, calculated values of jHOOH/OH were 56 (4.1), 59 (4.1), and 130 (10)  106 s1, respectively, while [OH]exp values were 1.5 (0.1), 1.6 (0.1), and 6.9 (0.6)  1014 mol L1, respectively. 2.8. Summit experiment We collected surface snow (0–3 cm) at Summit, Greenland (72.58N, 38.49W) in an undisturbed area in the clean air-sector between 135 and 100 west of magnetic north, w100 m east from our weatherport laboratory tent, which stood a few meters east of the science vault. Snow was melted, spiked with 800 nM of phenanthrene, and refrozen into w1 mL ice pellets on a PTFE Teflon sheet outside in the shade at ambient temperature. Pellets were also made using Milli-Q water spiked with 800 nM of PHE. Ice pellets were put on the snow surface (after brushing off any loose snow or surface hoar) during an illumination period centered near midday (12:30 local time). At known times, pellets were collected into amber glass jars, placed in the dark in the heated lab to melt, and injected immediately onto the HPLC (section 2.3) to determine PAH concentrations.

K. Ram, C. Anastasio / Atmospheric Environment 43 (2009) 2252–2259

3. Results and discussion 3.1. Light absorbance As shown in Fig. 1 (and Supplemental Table 1), all three PAHs have enormous molar absorptivities over the solar wavelength range (i.e., above 290 nm), as described previously (Murov et al., 1993). Peak molar absorptivities for these three PAHs are on the order of 10,000–40,000 M1 cm1 (for l > 290 nm), in stark contrast to common inorganic chromophores in snow, such as nitrate and hydrogen peroxide, which have values less than 10 M1 cm1 in this range (Chu and Anastasio, 2003, 2005). Based on past work, the absorption spectrum of a compound in/on ice appears to be (at least qualitatively) very similar to its spectrum in water (though with some band broadening in/on ice) (Matykiewiczova et al., 2007b). Thus the aqueous spectra for the PAHs in Fig. 1 suggest that not only is photodegradation a likely mechanism of decay, but that it could be quite fast. 3.2. Direct photodegradation, sunlight absorption, and apparent quantum yields As shown in Fig. 2a, all three PAHs in/on ice degraded during simulated sunlight illumination in the laboratory. From linear regressions to these data we determined jPAH,exp values (1 SE) for PHE, PYR, and FLA to be 6.4 (1.3)  106, 8.8 (0.9)  105, and 2.8 (1.3)  106 s1, respectively, although the average photon fluxes were somewhat different for each PAH, as described in the caption. Fig. 2b displays the raw data for the dark control samples in the same series of experiments. On average, the dark loss of PHE, PYR, and FLA represented 36%, 11%, and 16% of the decay in the illuminated samples, respectively. The mechanisms for PAH loss in the dark are unclear (though volatilization might be responsible for some portion), but we account for these losses in our calculation of PAH photodegradation (section 2.5). After subtracting off the dark loss and normalizing to Summit, surface snow conditions (clearsky, midday, summer solstice), rate constants for direct photodecay of PHE, PYR, and FLA are 3.8 (0.8)  105, 28 (3)  105, and 1.4 (0.7)  105 s1, respectively (Table 1). The corresponding lifetimes due to direct photodegradation are 7 (1), 1.0 (0.1), and 20 (10) h, respectively, at Summit. As shown in Supplemental Table 3, the photodecay lifetime for a given PAH in the surface snow at other polar locations is largely determined by the amount of solar radiation.

Molar Absorptivity (M-1 cm-1)

2000

40000

1500 FLA

1000

30000

PYR

PYR

500

PHE

0 290

20000

310

330

350

370

390

PHE

0 290

310

330

350

Because we froze our samples relatively slowly, the PAHs should be excluded from the bulk ice during freezing and be present in quasi-liquid layers (QLL) in/on the ice samples. This partitioning assignment is consistent with results from Kahan and Donaldson (2007), where PAHs were present in surface quasi-liquid layers regardless of whether the PAHs were delivered from the gas phase to the ice surface or initially dissolved in a solution that was then frozen. Also, as shown in Supplementary Table 2, changes in sample container (Teflon beaker versus quartz tube) had no apparent effect on the rate constants for PAH decay during our experiments. The rate constant for direct photodegradation of a compound in sunlight is a product of two factors: (1) the rate constant for sunlight absorption (jabs; Eq. (6)), and (2) the apparent quantum efficiency of destruction in sunlight (FPAH; Eq. (7)). As Fig. 3 shows, the total rate constant for sunlight absorption (i.e., the area under each curve) is comparable for FLA and PYR. In contrast, the rate of sunlight absorption by PHE is much smaller compared to the other two PAHs (Fig. 3). As tabulated in Table 1, rate constants of sunlight absorption for PYR and FLA are approximately 60 times higher than jabs for PHE. Using these jabs values in conjunction with our measured rate constants for direct photodegradation of the PAHs we can calculate the apparent quantum yields for PAH destruction, FPAH (Eq. (7)). Calculated values of FPAH for PHE, PYR, and FLA are 38 (8)  104, 4.3 (0.5)  104, and 0.2 (0.1)  104, respectively. That is, the relative efficiencies for photodecay in sunlight are 173:20:1 for PHE, PYR, and FLA, respectively. Thus the ratio of rate constants for photodegradation of PHE, PYR, and FLA under Summit conditions (2.7:20:1) reflects a combination of large rates of sunlight absorption by FLA and PYR and higher quantum efficiencies for PHE and PYR. As shown in Table 1, our quantum efficiencies for PHE, PYR, and FLA in/on ice are similar to previously reported values in water at 25  C, although our values are up to a factor of 5 lower. This suggests that the photodegradation reactions in/on ice are occurring in quasi-liquid layers and that they are only weakly dependent upon temperature. Although different experimental conditions between our work and the solution studies add some uncertainty, we can use the quantum efficiency data in Table 1 to estimate activation energies for photodecay of the PAHs. The resulting Ea values for PHE, PYR, and FLA are 18  5 or 1.3  0.3 (depending on which room temperature value of FPHE is used), 30  4, and 7  4 kJ mol1, respectively. This similarity in quantum efficiencies between water and ice is different from results reported by Kahan and Donaldson (2007), who observed an increased rate of PAH photodegradation on ice versus in solution. While the reason for this apparent enhancement on ice is unclear, the photon flux in the ice QLL might have been enhanced relative to the flux at the surface of the solution, as suggested by the authors. We have not performed experiments to elucidate the mechanisms responsible for the direct photodegradation of PAHs in/on ice. However, by analogy to results in water, the ice QLL pathways likely include reactions of the excited singlet and triplet states of PAHs with dissolved molecular oxygen – possibly via electron transfer – to form oxidized products (Fasnacht and Blough, 2003). 3.3. Indirect photo-oxidation of PAHs by OH

FLA

10000

2255

370

390

Wavelength (nm) Fig. 1. Base-10 molar absorptivities for PAHs in 50:50 acetonitrile:Milli-Q water solutions at room temperature. See Supplementary Fig. 2 and Table 1 for measurements from 200 to 400 nm.

Fig. 4 displays decay plots for PAHs in illuminated ice samples made from solutions containing HOOH as an OH source. Thus these results include both the direct photodegradation (section 3.2) as well as indirect photo-oxidation by hydroxyl radical. All three compounds decay more quickly with HOOH than without: OH accounts for over 80% of PAH decay in these experiments, but the OH contribution here is larger than in the field because of the high concentrations of HOOH that we used. Normalizing these

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K. Ram, C. Anastasio / Atmospheric Environment 43 (2009) 2252–2259

a

b

-0.4

ln ( [PAH]t / [PAH]0 )

ln ( [PAH]t / [PAH]0 )

0.0

FLA

-0.8 PHE

-1.2

0.0 FLA PHE

-0.4 PYR

-0.8 -1.2

PYR

-1.6

0

20000

40000

60000

-1.6

80000

0

Illumination Time (s)

20000

40000

60000

80000

Illumination Time (s)

Fig. 2. Loss of individual PAHs in laboratory ice samples (2.4 to 11  C) under: (a) simulated sunlight and (b) dark conditions, for phenanthrene (triangles), pyrene (squares), and fluoranthene (diamonds). For each PAH, data is shown from all of the experiments performed; see Supplemental Table 2 for details. Because photon fluxes varied between experiments, data for a given PAH were all normalized to the same j2NB value before determining the rate constant for PAH decay from a linear regression of the data in these figures. Considering all illumination experiments for each compound (panel a) gives rate constants for decay for PHE, PYR, and FLA of 1.1 (0.1)  105 s1 (R2 ¼ 0.68), 9.8 (0.6)  105 s1 (R2 ¼ 0.99), and 3 (1)  106 s1 (R2 ¼ 0.34), respectively. Average 2-nitrobenzaldehyde rate constants for these three compounds are 3.7 (0.1)  103 s1, 7.1 (0.2)  103 s1, and 4.5 (0.1)  103 s1, respectively. Experimental rate constants for degradation of PHE, PYR, and FLA in the dark (panel b) are 4.2 (0.8)  106 s1 (R2 ¼ 0.54), 1 (2)  105 s1 (R2 ¼ 0.69), and 6 (7)  107 s1 (R2 ¼ 0.84), respectively.

laboratory rates to the sunlight and OH conditions at Summit (section 2.7), and subtracting off the contribution from direct photo-oxidation, gives pseudo-first-order rate constants for indirect photo-oxidation with OH for PHE, PYR, and FLA of 1.3 (0.9)  105, 2.2 (0.3)  105, and 0.14 (0.02)  105 s1, respectively. Comparing these values to the direct photodegradation rate constants indicates that $OH is a relatively minor sink for PAHs in surface snow, accounting for approximately 25 (6)%, 8 (1)%, and 9 (4)% of PHE, PYR, and FLA degradation,

respectively, under Summit conditions (Table 1). Of course, this conclusion depends upon the steady-state concentration of OH in/ on ice, which depends on factors such as the concentration of the OH precursor (primarily HOOH), solar zenith angle, and temperature (Anastasio et al., 2007). In cases where the OH concentration is greater than the value of 1015 M used here, OH will be a more important sink for PAHs. Taking into account both direct photodegradation and the minor contribution from photo-oxidation with OH, the lifetimes for PHE, PYR, and FLA in the snowpack

Table 1 Summit-normalized results (surface snow, midday, summer solstice, clear-sky, 263 K), determined by extrapolating from our laboratory direct photodegradation and OH results to Summit, Greenland conditions. Listed uncertainties for this study are 1 standard error. jPAH,Suma (105 s1)

k0 OH,Sumb (105 s1)

% Loss from OHc (%)

stotal,Sumd

jabse (s1)

104  FPAHf (10  C, ice)

Results from Previous Solution Studiesg

(h)

104  FPAH(25  C, water)

l (nm)

Phenanthrene (PHE)

3.8 (0.8)

1.3 (0.9)

25 (6)

6 (1)

0.010

38 (8)

100 (16) 35 (5)

313 >290h

Pyrene (PYR)

28 (3)

2.2 (0.3)

8 (1)

1.0 (0.1)

0.64

4.3 (0.5)

20 (3) 22 (3) 21 (1)

313 366 300–400i

Fluoranthene (FLA)

1.4 (0.7)

0.14 (0.02)

9 (4)

19 (9)

0.62

0.2 (0.1)

1.2 (0.1) 0.02 (0.002) 0.32 (0.06)

313 366 >290h

PAH

a

Direct photodegradation rate constant for PAH (Eq. (5)). Pseudo-first-order rate constant for indirect photo-oxidation via OH (Eq. (10)). Percent of PAH loss due to reaction with OH. d Lifetime of PAH, considering both direct photodegradation and reaction with OH. e Rate constant for sunlight absorption by the PAH under Summit summer conditions. f Apparent quantum yield for PAH loss during sunlight illumination. g Single wavelength results are from Zepp and Schlotzhauer (1979), while polychromatic results are from Fasnacht and Blough (2002) and Sigman et al. (1998). Listed uncertainties are  1s. h Light output from a xenon arc lamp with a long-pass filter. i Light output from a 350-nm bulb. b c

K. Ram, C. Anastasio / Atmospheric Environment 43 (2009) 2252–2259

0.06

8.0E-04

j abs (s-1nm-1)

0.05

6.0E-04

PYR

FLA

4.0E-04

0.04

PYR

0.03

2.0E-04

0.0E+00 290

310

330

350

370

390

FLA

0.01 0.00 290

PHE

310

330

350

to ice conditions (Eq. (11)), the fact that our kPAHþOH values at 10  C are similar to those expected at room temperature suggests that these reactions are occurring in quasi-liquid layers in/on the ice and that they have small activation energies. 3.4. Field study of phenanthrene photodegradation at Summit, Greenland

PHE

0.02

370

390

Wavelength (nm) Fig. 3. Wavelength-resolved rate constants for sunlight absorption by PAHs under midday, summer solstice sunlight in surface snow at Summit, Greenland (Eq. (6)). The dashed, solid, and dotted lines represent phenanthrene, pyrene, and fluoranthene, respectively. Note that the full units of jabs at each wavelength are s1 nm1 molphotons mol1-PAH.

under midday, summer solstice conditions at Summit are calculated to be 6 (1), 1.0 (0.1), and 19 (9) h, respectively (Table 1). These very short lifetimes imply that there should be no appreciable concentrations of these PAHs in the surface snow at Summit during summer; this idea is explored in more detail in section 3.5. Combining our experimental pseudo-first-order rate constants for OH reaction (k0 OH,exp) from Fig. 4 with estimated steady-state  OH concentrations (section 2.7), we estimated the second-order rate constants for reaction of OH with the PAHs in ice (Eq. (10)). The resulting values for kPAHþOH (in units of 1010 M1 s1) are 1.3 (0.9), 2.2 (0.3), and 0.14 (0.02) for PHE, PYR, and FLA respectively, at 10  C. While we have been unable to find equivalent solution rate constants in the literature, the values for OH with aqueous benzene and naphthalene at room temperature are both w1  1010 M1 s1 (Ross et al., 1998), suggesting that kPAHþOH values for PHE, PYR, and FLA in water at room temperature are also at the diffusion-controlled limit. While it should be kept in mind that our kPAHþOH calculation depends upon assuming that the rate constant for $OH with benzoate can be extrapolated from solution

In addition to our laboratory studies, we also performed one experiment to examine phenanthrene decay in ice pellets at Summit. Fig. 5 shows that there is no significant statistical difference in the photolytic degradation of PHE between ice pellets made from melted snow and pellets made from Milli-Q. Based on these rates of decay, the lifetime of PHE during this Summit illumination is 3.4 (0.1) h, which is in good agreement (within approximately a factor of two) with the lifetime calculated for Summit conditions based on our lab experiments (Table 1). In the pellets made from purified water (Milli-Q), PHE should decay only via direct photodegradation, while in the pellets made from snow there is also the possibility of indirect photo-oxidation of PHE via oxidants such as $OH and 1O2* (Domine´ and Shepson, 2002; Anastasio et al., 2007; Kahan and Donaldson, 2007). However, the fact that the rate of decay is the same in both types of pellets indicates that direct photodecay is the dominant mechanism for PHE loss and that oxidants in the snow grains play an insignificant role. This Summit result is broadly consistent with our laboratory experiments, where OH made only a small contribution to PAH decay when extrapolated to Summit conditions; however, it does suggest that the true OH contribution to PHE decay listed in Table 1 lies at the lower end of the uncertainty range (i.e., <10%). In contrast,  OH is likely an important sink for many organic compounds that only weakly absorb actinic radiation (Anastasio et al., 2007), such as polychlorinated biphenyls (PCBs) (Matykiewiczova et al., 2007a). 3.5. Expected and observed PAH seasonality in Greenland snow While the PAH lifetimes at Summit described in sections 3.2 and 3.4 are for midday on the summer solstice, we have also extrapolated our results to estimate PAH direct photodegradation lifetimes throughout the course of a year. As Fig. 6 shows, 24-h average lifetimes for the three PAHs are very short during the summer (<2 d) but increase to very long times in winter (>100 d). Given

0.0

ln ( [PHE]t / [PHE]0 )

ln ( [PAH]t / [PAH]0 )

-0.1

-0.6

-1.1

-1.6

FLA

PHE

PYR -2.1

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Dark Snow

-0.5

-1.0 Snow

-1.5 0

2000

Milli-Q

4000

6000

8000

10000

12000

0

2000

4000

6000

8000

10000 12000 14000 16000

Exposure Time (s)

Illumination Time (s) Fig. 4. Loss of PAHs in illuminated laboratory ice samples containing 1.0 or 2.0 mM HOOH as an OH source. Rate constants for loss of PAH for phenanthrene (triangles), pyrene (squares), and fluoranthene (diamonds) were 2.1 (0.3)  104 s1 (R2 ¼ 0.94), 4.5 (0.4)  104 s1 (R2 ¼ 0.95), and 1.1 (0.09)  104 s1 (R2 ¼ 0.95), respectively. Experimental rate constants for 2-nitrobenzaldehyde loss for PHE, PYR, and FLA were 2.0 (0.07)  103 s1, 2.1 (0.05)  103 s1, and 4.6 (0.2)  103 s1, respectively.

Fig. 5. Photodegradation of phenanthrene in ice pellets at Summit, Greenland on August 2, 2005. Samples were placed on the surface snow and exposed to sunlight starting at 10:00 a.m. local time. Rate constants (1 SE) for PHE decay in ice pellets made from Milli-Q (open circles) and surface snow (open diamonds) are 7.8 (1.0)  105 s1 (R2 ¼ 0.97) and 8.3 (0.2)  105 s1 (R2 ¼ 1.00), respectively. Ice pellets made from snow and kept in the dark (Dark Snow) showed no significant loss of phenanthrene (2 (4)  106 s1, R2 ¼ 0.73).

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24-hr average lifetime (d)

100

4. Conclusions

10

1

0.1

0.01 Jan

Mar

May

Jul

Sep

Nov

Jan

Month Fig. 6. Calculated 24-h-average direct photodegradation lifetimes of PAHs in surface snow at Summit, Greenland for phenanthrene (triangles), pyrene (squares), and fluoranthene (diamonds). Values were determined by scaling the summer solstice lifetimes to other times of year using TUV actinic fluxes. Error bars are smaller than the symbols. Values are shown on the 20th day of each month, except for January, February, November, and December, where values on the 5th and 20th day are shown.

that the typical monthly accumulation of snow at Summit is 5– 10 cm (Dibb and Fahnestock, 2004), and that the e-folding depth for the wavelengths of sunlight that are most responsible for PAH decay (Fig. 3) is approximately 7–9 cm (Galbavy et al., 2007), we would expect: (1) virtually no photolytic decay of PAHs in winter before they are buried beneath the snow photic zone, but (2) rapid destruction of PAHs during the summer. Thus our photochemistry results suggest that PAH concentrations in surface snow would be very low in the summer and highest in the winter. This seasonality will be modified somewhat because rates of PAH deposition to the Greenland snowpack are highest in spring (due to transport of emissions from Eurasia) and summer (due to boreal forest fires) (Jaffrezo et al., 1994; Masclet et al., 2000; Slater et al., 2002), but given the rapid photodegradation, we expect negligible concentrations of PAHs in summer. PAH levels in Summit snow from previous reports generally agree with this expected seasonality, but not always (Jaffrezo et al., 1994; Masclet et al., 2000; Slater et al., 2002). These reports show that in most years there is typically a late-winter or spring peak of PAH levels in the surface snow and, often, a summertime minimum. However, the observed seasonal variations in PAH concentration profiles are typically not as strong as Fig. 6 suggests and there can be concentration peaks in any season, including summer. These field observations, especially the occasional appearance of summer peaks, suggest that PAHs in the Greenland summer snowpack do not always degrade quickly, in contrast with our expectations. Why might PAH photodegradation be slower in the field compared to in our experiments? One likely reason is that the PAHs in Summit snow are likely largely associated with fine soot particles on ice grains (Bjorseth et al., 1979; Baek et al., 1991), in contrast to our experiments, where the PAHs probably existed in quasi-liquid layers in/on the ice. Compared to quasi-liquid layers, photodegradation should be slower in the soot particles both because quantum efficiencies for PAH destruction tend to decrease with decreasing solvent polarity (Zepp and Schlotzhauer, 1979) and because there might be significant light attenuation within the soot particles. More generally, since it takes weeks to transport material from northern hemispheric emission sources to Summit, the labile pools of PAHs in these emissions will be destroyed during transit and thus the PAHs deposited to the snowpack will be the most recalcitrant.

Our laboratory results show that phenanthrene, pyrene, and fluoranthene all photodegrade relatively quickly in/on sunlit ice, with lifetimes under Summit summer conditions of 1–19 h in surface snow. Direct photodegradation is the main mechanism of decay for these PAHs in snow, while reaction with OH is a minor sink. Of the three PAHs we studied, pyrene has the shortest lifetime, due to both a high rate constant for sunlight absorption and a relatively high quantum efficiency for photodegradation. Though our results suggest rapid degradation of PAHs in snow during summer, field observations of PAH seasonality suggest that this is not always the case. In part, this difference might be because PAHs in our experiments were present in quasi-liquid layers in the ice, while in the field PAHs are more likely associated with particles in the snowpack. It is unclear how differences in location for OH and PAHs in experimental samples versus on real snow grains might affect the evaluation of the OH contribution to PAH decay in the field. Finally, it should be noted that degradation of PAHs does not necessarily decrease overall toxicity since the photochemical reaction products of organics are typically more water soluble, and can be more toxic, than those of the parent compounds (Dolinova et al., 2006). Melting of seasonal snowpacks will release PAHs and their photoproducts into surface waters, which could represent a human and ecosystem health hazard, especially in a warming world with reduced amounts of permanent snow. Acknowledgements Funding was provided by the Office of Polar Programs at the National Science Foundation (Grant #0455055), the Donald Crosby Fellowship at UC Davis, and the UC Davis and Humanities Graduate Research Award. We thank the Danish Polar Board and Greenlandic Home Rule Government for permission to work in Greenland; the 109th NY ANG for air transport to Greenland; and VECO Polar Resources for logistical support. Special thanks to Eddie Galbavy for his work at Summit, to Liang Chu for HOOH photolysis data, and to Jon Bower for the lamp output calculations. Appendix. Supplementary material Supplementary data associated with this article can be found in the online version, at doi:10.1016/j.atmosenv.2009.01.044. References Agency for Toxic Substances and Disease Registry (ATSDR), 1995. Toxicological Profile for Polycyclic Aromatic Hydrocarbons (PAHs). United States Department of Health and Human Services, Public Health Services, Atlanta, GA. Available from: http://www.atsdr.cdc.gov/toxprofiles/tp69.html. Anastasio, C., Faust, B.C., Allen, J.M., 1994. Aqueous-phase photochemical formation of hydrogen peroxide in authentic cloud waters. Journal of Geophysical Research-Atmospheres 99, 8231–8248. Anastasio, C., Galbavy, E.S., Hutterli, M.A., Burkhart, J.F., Friel, D.K., 2007. Photoformation of hydroxyl radical on snow grains at Summit, Greenland. Atmospheric Environment 41, 5110–5121. Anastasio, C., Robles, T., 2007. Light absorption by soluble chemical species in Arctic and Antarctic snow. Journal of Geophysical Research-Atmospheres 112. Ashton, L., Buxton, G.V., Stuart, C.R., 1995. Temperature dependence of the rate of reaction of OH with some aromatic compounds in aqueous solution. Evidence for the formation of a pi-complex intermediate? Journal of the Chemical Society-Faraday Transactions 91, 1631–1633. Baek, S.O., Goldstone, M.E., Kirk, P.W.W., Lester, J.N., Perry, R., 1991. Phase distribution and particle size dependency of polycyclic aromatic hydrocarbons in the urban atmosphere. Chemosphere 22, 503–520. Behymer, T.D., Hites, R.A., 1985. Photolysis of polycyclic aromatic hydrocarbons adsorbed on simulated atmospheric particulates. Environmental Science & Technology 19, 1004–1006. Bjorseth, A., Lunde, G., Lindskog, A., 1979. Long-range transport of polycyclic aromatic hydrocarbons. Atmospheric Environment 13, 45–53.

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