Wavelength and Temperature-Dependent Apparent Quantum Yields

Dec 29, 2017 - Photolysis of dissolved organic matter is the main source of carbonyl compounds in sunlit seawater, but rates and photoefficiences are ...
0 downloads 6 Views 1MB Size
Subscriber access provided by AUSTRALIAN NATIONAL UNIV

Article

Wavelength and Temperature-Dependent Apparent Quantum Yields for Photochemical Production of Carbonyl Compounds in the North Pacific Ocean Yuting Zhu, and David John Kieber Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05462 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 41

Environmental Science & Technology

1

Wavelength and Temperature-Dependent Apparent Quantum Yields for

2

Photochemical Production of Carbonyl Compounds in the North Pacific Ocean

3 4 5 6

Yuting Zhu and David J. Kieber*

7 8 9 10 11

Department of Chemistry, State University of New York, College of Environmental

12

Science and Forestry, 1 Forestry Drive, Syracuse, New York 13210

13 14 15 16

*

17

[email protected], Phone: 1-315-470-6951, Fax: 1-315-470-6856

Corresponding Author: 1 Forestry Drive, Syracuse, New York 13210, USA,

18 19 20

ACS Paragon Plus Environment

Environmental Science & Technology

21

ABSTRACT

22

Photolysis of dissolved organic matter is the main source of carbonyl compounds in

23

sunlit seawater, but rates and photoefficiences are poorly constrained. Wavelength- and

24

temperature-dependent apparent quantum yields (AQY) were determined for

25

photochemical production of acetaldehyde, glyoxal and methylglyoxal in North Pacific

26

Ocean seawater. Wavelength-dependent AQY at 20 ˚C decreased exponentially with

27

increasing wavelength between 290 and 380 nm, from 1.29 × 10-4 to 4.12 × 10-6, 2.52 ×

28

10-5 to 6.89 × 10-7, and 3.56 × 10-6 to 1.02 × 10-7 mol (mol quanta)-1 for acetaldehyde,

29

glyoxal, and methylglyoxal, respectively. AQY decreased after 6 h irradiation at 310 nm,

30

possibly due to depletion of photochemical precursors or carbonyl photolysis. Activation

31

energies (average±95% CI) for photochemical production at 320 nm were 9.31 (±9.3),

32

26.0 (±7.5), and 34.7 (±12.8) kJ mol-1 for acetaldehyde, glyoxal and methylglyoxal,

33

respectively. The peak response for photochemical production rates in surface seawater

34

was ~325 nm, with ~30% contribution from UV-B and ~70% from UV-A. Computed

35

wavelength-integrated photoproduction rates were 0.5–0.8, 0.04–0.2 and 0.02–0.05 nmol

36

L-1 h-1 for acetaldehyde, glyoxal, and methylglyoxal under cloudless conditions in

37

August. Results can be used to determine regional-scale photochemical production rates

38

for these compounds in the surface ocean.

1 ACS Paragon Plus Environment

Page 2 of 41

Page 3 of 41

39 40

Environmental Science & Technology

INTRODUCTION Low-molecular-weight (LMW) carbonyl compounds are important gas-phase

41

species in the atmosphere that affect the oxidizing capacity of the troposphere, serve as

42

important radical precursors,1–4 and are potentially important global sources of secondary

43

organic aerosol.5,6 It has been suggested that the oceans are an important source or sink

44

for LMW carbonyl compounds including acetaldehyde, glyoxal and methylglyoxal,

45

although large uncertainties persist.7–9

46

For acetaldehyde, several field studies indicate that the oceans are a source to the

47

lower atmosphere.10–13 The ocean’s estimated contribution11 of 17 Tg y-1 to the

48

tropospheric acetaldehyde budget is low compared to that predicted from atmospheric

49

observations or models (57–125 Tg y-1).7,8 Glyoxal is much more water soluble than

50

acetaldehyde. Its mixing ratio is high (7–80 ppt) in the marine boundary layer (MBL) in

51

the remote marine environment.9,14,15 Considering its short tropospheric lifetime (~3 h),5

52

continental sources fail to account for glyoxal’s high mixing ratio; it has been suggested

53

that there is a significant missing oceanic source of glyoxal in current atmospheric

54

models,9,15,16 but measured glyoxal concentrations in the open ocean are too low (0.3–5.6

55

nM).10,14,16 A nighttime direct outgassing of glyoxal from the surface microlayer was

56

suggested, but this only explains a small percentage of the missing oceanic source.17,18 A

57

similar paradox is seen for methylglyoxal. Atmospheric models indicate that only a small

58

percentage of methylglyoxal in the MBL can be explained by in situ photochemical

59

reactions;15 it has been suggested that there is likely an oceanic source.15 However,

60

methylglyoxal concentrations in the MBL and surface seawater, 10–28 ppt14,15 and 0.1–

2 ACS Paragon Plus Environment

Environmental Science & Technology

61

3.4 nM,10,14,16 respectively, suggest the opposite, namely that there should be a net flux

62

from the atmosphere to the sea.14

63

Controversies regarding acetaldehyde, glyoxal, and methylglyoxal budgets in the

64

MBL are hindered by a paucity of data regarding carbonyl compound cycling in

65

seawater. Concentrations of LMW carbonyls vary temporally and spatially in surface

66

seawater, and it has been proposed that photochemical production from dissolved organic

67

matter (DOM) and biological removal are the two main processes affecting

68

concentrations.19,20 Nevertheless, this view is supported by very few biological12,21–23 or

69

photochemical studies.10,19,24,25 Biological sources,12,23,26 photochemical losses27 and

70

atmospheric inputs8,28 are also possible, but have not been investigated in any detail.

71

Although it is considered an important source, the photochemical production of

72

carbonyl compounds in seawater is not well studied. There are a few reports of shipboard

73

measured photoproduction rates of LMW carbonyls in Sargasso Sea seawater.10,24,29

74

However, it would be premature to extrapolate these data regionally or globally, since

75

photochemical efficiency (i.e., quantum yield) data were not provided. The

76

photochemical efficiency of LMW carbonyl production has been reported for DOM-rich

77

fresh and coastal water samples,24,25 but not in open ocean waters with low DOM

78

concentrations that comprise approximately 90% of the world’s oceans and are of greater

79

significance to large-scale modeling studies. The aim of the present study was to study

80

fundamental properties of the photochemical production of carbonyl compounds in North

81

Pacific waters that are characteristic of the open ocean. Time-, wavelength- and

82

temperature-dependent apparent quantum yields (AQY) were determined for

83

acetaldehyde, glyoxal and methylglyoxal photoproduction in seawater collected from

3 ACS Paragon Plus Environment

Page 4 of 41

Page 5 of 41

Environmental Science & Technology

84

several stations. Photochemical production rates were then calculated using the AQY

85

dataset and available light and chromophoric dissolved organic matter (CDOM)

86

absorbance data. Results are discussed with respect to carbonyl cycling in seawater.

87 88 89

MATERIAL AND METHODS Chemicals and Reagents. Acetonitrile (ACN) and methanol were high

90

performance liquid chromatography (HPLC) grade (JT. Baker, Central Valley, PA). High

91

purity water (18.2 MΩ cm) was used in this study (hereafter referred to as Milli Q). This

92

water was obtained from a Millipore Q-water system containing a 0.2 µm Organex

93

attachment (Millipore, Billerica, MA). The 2,4-dinitrophenylhydrazine, DNPH (Sigma-

94

Aldrich), was recrystallized twice from heated ACN and stored in the dark at room

95

temperature. High purity carbon tetrachloride (CCl4) (Sigma-Aldrich) was used to purify

96

DNPH prior to the derivatization reaction. Carbonyl compounds were obtained as the

97

high purity standards` available from Sigma-Aldrich. All glassware used in this study

98

was rinsed several times with methanol and Milli Q, and then baked for 8 h at 550 ˚C.

99

The DNPH reagent used to derivatize carbonyl compounds was prepared following

100

the procedure outlined in Kieber et al. (1990).30 Briefly, 27.0 ± 0.3 mg recrystallized

101

DNPH was added to a solution containing 2.5 mL HPLC grade ACN, 5 mL 12 M reagent

102

grade HCl (Baker), and 12.5 mL Milli Q water. This solution was mixed with 20 mL of

103

CCl4 in a 40 mL Qorpak vial that was tightly sealed with a Teflon-lined silicone screw

104

cap. The reagent was continually stirred to extract hydrazone contaminants into the

105

carbon tetrachloride phase. The reagent was centrifuged at 800 rpm for 5 min just prior

106

to use to separate the two phases.

4 ACS Paragon Plus Environment

Environmental Science & Technology

107

Sample Collection. Seawater samples were collected at a depth of 5 m at six

108

stations in the North Pacific Ocean in August 2013 during the Deep Ocean Refractory

109

Carbon expedition aboard the R/V Melville.31 Chemical properties of the 5 m seawater at

110

the six stations are presented in Table 1. Samples were collected in Niskin bottles

111

attached to a CTD rosette equipped with Sea-Bird Electronic SBE conductivity,

112

temperature, oxygen, and pressure sensors. The URL for the cruise CTD dataset is

113

http://www.bco-dmo.org/dataset/527102/data.

114

Seawater was gravity filtered directly from the Niskin bottle through a 0.2-µm

115

POLYCAP 75 AS Nylon filter (Whatman) into two 2 L Qorpak glass bottles previously

116

rinsed by Milli Q water and muffled at 550 ˚C for 8 h. Each bottle was filled leaving 3–5

117

mL headspace, sealed with a Teflon-lined silicone screw cap, and stored at 4 °C in the

118

dark until use analysis in Syracuse, NY. POLYCAP filters were cleaned prior to use by

119

alternating rinses of ACN and Milli Q water followed by extensive flushing with Milli

120

Q.32

121

Seawater Absorption Spectra. Seawater absorbance (A) spectra were determined

122

from 240–800 nm using a SD 2000 fiber optic spectrometer (Ocean Optics, Inc.)

123

containing a 1 m flow cell pre-cleaned with Milli Q and methanol. The cell was filled

124

with seawater using a Rainin Rabbit-Plus peristaltic pump to gently pull the sample

125

through the cell. An aqueous solution of 0.7 M NaCl was used as the reference; prior to

126

its use, the NaCl solution in a quartz flask was irradiated for 8 h using a 300 W xenon

127

lamp to remove UV-absorbing impurities. Wavelength-dependent absorption coefficients,

128

aλ, were calculated from the absorbance, where aλ = 2.303A/l for a 1 m pathlength (l).

129

Absorption spectra were determined for all samples before and after each irradiation.

5 ACS Paragon Plus Environment

Page 6 of 41

Page 7 of 41

Environmental Science & Technology

130

Apparent Quantum Yield Determination. A Milli Q sample or 0.2 µm-filtered

131

seawater sample was pneumatically pushed through 1/8” O.D. Teflon tubing with ultra-

132

high purity helium (99.999%) into a rectangular quartz cell (4 mL capacity, 1 cm

133

pathlength, Spectrocell, Inc.) for at least 10 min at a flow rate of 2 mL min-1. The quartz

134

cell was periodically inverted to remove residual air bubbles. The quartz cell was sealed

135

with a screw cap containing a Teflon-lined silicone septum insert.

136

Once the quartz cell was filled with a sample, it was placed into an enclosed

137

temperature-controlled cell holder equipped with a stirrer. All irradiations were

138

performed using a model LTIX-1002W-HS 1000 W xenon lamp (Royal Philips) along

139

with a GM 252 monochromator (Spectral Energy, Corp.). A 10 nm bandwidth was used

140

for irradiations 330 nm. Prior to an

143

irradiation, samples were temperature equilibrated in the cell holder for 5 min. The

144

incoming light was blocked for dark controls incubated in the cell holder for up to 36 h.

145

Before irradiations were conducted to determine wavelength-dependent AQY, the

146

time dependence for carbonyl production was determined in 0.2-µm filtered seawater

147

from station (stn.) 5 and 27 at 310 ± 5 nm. Samples were irradiated for 3, 6, 9, 12 and 15

148

h. We determined time-dependent AQYs at 310 nm and not at 320 nm (the ~peak-

149

response wavelength -- see Predicted Wavelength-Dependent Photoproduction Rates

150

section) because monochromatic irradiations are time consuming, especially for low-

151

absorbing open-ocean seawater samples, and irradiating samples at 310 instead of 320

152

nm reduced irradiation times in half. A time-series study was also not conducted for

6 ACS Paragon Plus Environment

Environmental Science & Technology

Page 8 of 41

153

acetaldehyde, glyoxal or methylglyoxal at longer irradiation wavelengths (e.g., 380 nm)

154

because carbonyl production rates were too low to observe significant differences at short

155

irradiation times; a time-series study was not conducted for methylgyoxal at stn. 5 at any

156

wavelength for the same reason.

157

To quantify wavelength-dependent AQY, 0.2-µm filtered seawater collected from

158

stn. 5 and 27 was irradiated for 4, 5, 6, 10, 14, 18, 24, and 36 h at 290, 300, 310, 320,

159

330, 340, 360, and 380 nm, respectively. For stn. 3, 10, 15 and 18, irradiations were

160

conducted at 300, 310, 320, 330 nm. Methylglyoxal photoproduction AQY results are

161

only presented for stn. 15, 18, and 27; production rates and therefore AQY were below

162

the detection limit for stn. 3, 5, and 10. Except when noted, the cell holder temperature

163

was set at 20 ˚C for all irradiations.

164

Wavelength-dependent AQY were calculated from:  =

165



      

(1)

166

where  is the AQY for the photochemical formation of acetaldehyde, glyoxal or

167

methylglyoxal (mol (mol quanta)-1) at wavelength λ,

168

photoproduction (mol L-1 min-1), V is the volume of irradiated 0.2-µm filtered seawater

169

(L),

170

the fraction of radiation absorbed by the 0.2-µm filtered seawater, & is the average

171

absorption coefficient of CDOM (m-1) during the irradiation, and l is the pathlength of the

172

quartz cell (m). All seawater samples were optically thin in the 1 cm quartz cell at all

173

irradiated wavelengths.

174 175



 

is the rate of carbonyl

is the light flux determined by nitrite actinometry (mol quanta min-1), 1 − # $% is

Chemical Actinometry. The light flux was determined in a 1 cm quartz cell using an optically-thin chemical actinometer based on nitrite photolysis and the reaction of 7 ACS Paragon Plus Environment

Page 9 of 41

Environmental Science & Technology

176

photoproduced OH radical with benzoic acid to form salicylic acid (SA).33–35 The SA was

177

quantified by flow injection analysis using fluorescence detection with excitation at 305 ±

178

7.5 nm and emission at 410 ± 7.5 nm. The light flux was quantified from equation 2:

179



=

),'(

'(

  +,  ./01 2 3

(2)

180

where [SA] is the concentration of SA produced (mol L-1), 4 is the wavelength-

181

dependent molar absorption coefficient of nitrite (cm2 mol-1), ,56 is the wavelength-

182

dependent quantum yield for SA formation from nitrite photolysis (mol SA produced per

183

mol quanta absorbed by nitrite),

184

789 is the nitrite concentration in the actinometer solution (mol L-1).



is the light flux at wavelength λ (mol quanta s-1), and

185

Carbonyl Quantification. The quantification method for carbonyl compounds was

186

adapted from Kieber et al.30 Briefly, a 2.2 mL aliquot of the irradiated seawater sample or

187

dark control was added to a 20 µL aliquot of the 2,4-dinitrophenylhydrazine (DNPH)

188

reagent in a ~2.2 mL Qorpak vial that was capped tightly with no headspace. The lid of

189

the cap contained a Teflon-lined silicone septum. All samples were reacted at room

190

temperature for 24 h.

191

Derivatized standards (Sigma-Aldrich), dark controls, and samples were analyzed

192

using a Shimadzu Prominence high performance liquid chromatography (HPLC) system

193

with a model SPD-20A/V UV-Vis absorbance detector set in dual wavelength mode at

194

371 and 435 nm. The HPLC column consisted of a Waters 8×100 mm Nova-Pak

195

cartridge with 4 µm C18 packing placed in a Waters RCM radial compression cartridge

196

holder (Waters Associates, Milford, MA). The mobile phase consisted of solvent A (Milli

197

Q) and solvent B (ACN). The elution program was isocratic at 30% B for 3 min, 30 to

198

55% B in 5 min, isocratic at 55% B for 2 min, 55 to 90% B in 6 min, isocratic at 90% B 8 ACS Paragon Plus Environment

Environmental Science & Technology

Page 10 of 41

199

for 5 min, 90% B to 30% B in 1 min, followed by column equilibration to the initial

200

mobile phase composition for 15 min. All samples were injected using a 1.25 mL

201

injection loop. The flow rate was 1.5 mL min-1 and the column oven temperature was 40

202

˚C. The sample analysis time was 37 min.

203

Activation Energy. The activation energy for the photochemical production of

204

carbonyl compounds was determined at 320 (±5 nm) for acetaldehyde and glyoxal at stn.

205

5 and 27; the activation energy for the photochemical production of methylglyoxal was

206

determined at stn. 27 but not at stn. 5 due to analytical limitations. Temperature-

207

dependent AQY were determined at 10, 15, 20, 28 and 36 ˚C. The temperature

208

dependence was determined at 320 nm, corresponding to the approximate peak response

209

wavelength in the predicted wavelength-dependent photoproduction rate spectra for

210

acetaldehyde, glyoxal, and methylglyoxal (vide infra). The activation energy was

211

calculated from linear regression analysis:

212

:; = :;< −



(3)

=>

213

where A is the pre-exponential factor, Ea is the activation energy (kJ mol-1), R is the

214

universal gas constant (8.314×10-3 kJ mol-1 K-1), and T is the temperature (K).

215

Statistical Analyses. All statistical analyses, including simple linear regressions,

216

quasi-exponential fits, t-tests, and one-way ANOVA with a Tukey post-hoc test analyses,

217

were performed using Sigmplot 11.0 with the SigmaStat software package. A one-way

218

ANOVA with Tukey's post-hoc test was used for pairwise comparisons to evaluate

219

differences in the fit parameters m1 and m2 (Eq. 4) among the six stations. An α level of

220

0.05 was used for all tests.

221

9 ACS Paragon Plus Environment

Page 11 of 41

222 223

Environmental Science & Technology

RESULTS AND DISCUSSION Hydrographic station locations are shown in Figure S1, overlain with satellite-

224

derived CDOM absorption coefficient data at 320 nm calculated from the SeaCDOM

225

algorithm36 using seven-year averaged Aqua ocean color reflectance data for August

226

2010–2016 obtained from the Moderate-Resolution Imaging Spectroradiometer

227

(MODIS); reflectance data were downloaded from the NASA ocean color website

228

(https://oceancolor.gsfc.nasa.gov/). Representative CDOM spectra for all stations are

229

shown in Figure S2, and additional information regarding CDOM spectral slopes is

230

provided in Table S1.

231

Acetaldehyde and glyoxal were the main LMW carbonyl photoproducts observed in

232

all irradiated North Pacific Ocean seawater samples. Formaldehyde was not quantified

233

due to contamination. Acetone and propanal concentrations were above detection limits,

234

but irradiated seawater samples did not show significant differences compared to dark

235

controls. Photochemical production of methylglyoxal was only observed at the stations

236

with the highest CDOM absorption coefficients (15, 18 and 27).

237

All AQY were calculated accounting for CDOM losses that may have occurred

238

during an irradiation. CDOM losses were observed during irradiations at wavelengths

239

less than 360 nm, with losses ranging from 2 to 11% depending on the irradiation

240

wavelength, sample type and photon exposure. To illustrate a typical change in

241

absorbance during an irradiation, the & in a sample from stn. 27 irradiated for 10 h at

242

320 ± 5 nm was compared to & in unirradiated stn. 27 seawater and the dark control

243

(Figure S3).

10 ACS Paragon Plus Environment

Environmental Science & Technology

244

Time-Dependent AQY. It has been shown that the photochemical production of

245

several compounds in seawater (e.g., CO, CO2) are not a linear function of the photon

246

exposure.37,38 This nonlinearity results in time-dependent changes in AQY, and therefore

247

caution should be taken when comparing results from different studies. In the present

248

study, AQY for acetaldehyde and glyoxal photoproduction in seawater from stn. 5 and 27

249

were a linear function of the integrated photon flux at 310 nm during the first 6 h.

250

However, nonlinearity was observed at longer irradiation times (Figure S4) because

251

carbonyl photoproduction rates decreased faster than a310. This resulted in a decrease in

252

the AQY at 310 nm (Figure 1), which differed between the two stations, particularly with

253

respect to acetaldehyde. For stn. 5, AQY decreased 41 and 41% for acetaldehyde and

254

glyoxal, respectively, whereas for stn. 27 time-dependent AQY decreased less, 22 and

255

35%, respectively, after 15 h irradiation compared to the average AQY obtained after 3 h

256

of irradiation. In all cases, t-tests showed that for both acetaldehyde and methylglyoxal

257

AQY at 15 h were significantly lower than AQY determined within the first 6 h of

258

irradiation (p0.05)

267

due to a greater analytical uncertainty.

268

Time dependent decreases in AQY were due to a depletion of photochemical

269

precursors and/or possibly increased rates of carbonyl photolysis, neither of which was

270

quantified. Regarding carbonyl photolysis, it was previously shown27 that formaldehyde,

271

acetaldehyde, and propanal were photochemically stable in pure water, and therefore

272

primary photolysis is not expected to affect photoproduction AQY for these compounds

273

in seawater. Unfortunately, no data are available regarding potential secondary

274

photosensitized losses of carbonyl compounds. The estimated half-life of acetaldehyde in

275

coastal seawater is 1400 d with respect to its reaction with the hydroxyl radical,27,39

276

suggesting that reactions involving photochemically-generated reactive oxygen species in

277

seawater may not be important. However, there is no direct evidence for acetaldehyde,

278

glyoxal or methylglyoxal photochemical stability in seawater. Since carbonyl photolysis

279

rates were not determined in this study (or any other published seawater study), it is not

280

known if carbonyl photolysis affected AQY, and as such, AQY values reported here

281

should be considered lower estimates for carbonyl photoproduction.

282

To provide context for field results, the 1000 W light source used in this study is

283

~one order of magnitude more intense than solar noon sunlight in August at the surface

284

seawater in the Northwest Pacific Ocean on a cloudless day (e.g., for a 1 h irradiation at

285

305–315 nm, the photon exposure with the lamp is 0.0903 vs 0.0113 mol quanta m-2 from

286

the sun). Therefore, although not directly comparable, carbonyl photoproduction AQY

287

are expected to decrease slowly in North Pacific seawater when exposed to solar

12 ACS Paragon Plus Environment

Environmental Science & Technology

288

irradiation (e.g., no observable changes in AQY or rates are expected during one day of

289

exposure of surface seawater to solar irradiation in August).

290

Zhang et al. (2006)40 suggested that direct photodecarbonylation of simple carbonyl

291

compounds are a potential source of CO in the marine environment. However, simple,

292

LMW carbonyl compounds are photochemically stable in high purity laboratory water;27

293

and only methylglyoxal showed a significant decrease in AQY with extended photon

294

exposure, but it’s concentrations (0.1–3.4 nM) and photoproduction rates (vide infra) are

295

quite low in surface seawaters. Therefore, this pathway is likely to be at best only a minor

296

contributor to the CO photochemical production rate in the oceans.

297

Temperature-Dependent AQY. The dependence of AQY on the reaction

298

temperature for acetaldehyde and glyoxal was investigated at 320 nm with seawater from

299

stn. 5 and 27; the AQY temperature dependence for methylglyoxal photoproduction was

300

only determined at stn. 27. Photoproduction AQY for acetaldehyde at stn. 5 were

301

relatively insensitive to temperature (Figure 2), with the slope of the Arrhenius plot not

302

significantly different from zero (p>0.05, Table 2), indicating no temperature dependence

303

within experimental uncertainty. For stn. 27, AQY for acetaldehyde exhibited a very

304

slight temperature dependence (p=0.029), with an activation energy (Ea ± 95% CI, 13.2 ±

305

11.5 kJ mol-1) comparable to that for a molecular-diffusion controlled reaction in aqueous

306

solution (~10 kJ mol-1).41 Together these results suggest that the photochemical

307

production of acetaldehyde in North Pacific seawater mainly occurred through a non-

308

thermal reaction involving a primary photolysis or diffusion-controlled free radical

309

reaction.42

13 ACS Paragon Plus Environment

Page 14 of 41

Page 15 of 41

Environmental Science & Technology

310

Temperature-dependent AQY for glyoxal and methylglyoxal exhibited linear

311

Arrhenius behavior (Figure 2). AQY increased significantly with temperature, by a

312

factor of 1.5 and 1.7 per 10 ˚C, with an Ea of 26 and 35 kJ mol-1 for glyoxal and

313

methylglyoxal, respectively. These activation energies are ~50 % to more than double

314

that observed for CO photoproduction in seawater (12.2 kJ mol-1)40, nitrate (~20 kJ mol-

315

1 43,44

316

hydroxyl radical with benzoic acid to form SA at circumneutral pH (~12–18 kJ mol-1),33

317

or aqueous reactions between the OH radical and chloramines (6–9 kJ mol-1)46 or natural-

318

water organic matter isolates (14–15 kJ mol-1).47

)

or nitrite (~14 kJ mol-1)43,45 photolysis in aqueous solution, the reaction of the

319

Although significantly higher than many natural water photoreactions reported in

320

the literature, the relatively high Ea for glyoxal and methylglyoxal photoproduction are

321

not unique; similar or even higher Ea values have been observed for other known

322

photosensitized secondary photochemical processes in seawater, including

323

dimethylsulfide (DMS) photolysis32 and photoproduction of hydrogen peroxide.37 Toole

324

et al. (2003)32 observed AQY for DMS photolysis in the Sargasso Sea doubled with a

325

temperature increase of 20 ˚C, with an Ea of 23–25 kJ mol-1. Kieber et al. (2014)37 also

326

found a strong temperature dependence for H2O2 photoproduction in seawater from

327

several sources (average ~22 kJ mol-1), which was suggested to result from the thermal

328

disproportion of superoxide as the rate limiting step.

329

These results suggest that glyoxal and methylglyoxal photoproduction rates are not

330

controlled by primary photolysis or radical reactions involving highly reactive species

331

such as the OH radical, but rather they are regulated by photosensitized or thermal

332

reactions with significant energy barriers.

14 ACS Paragon Plus Environment

Environmental Science & Technology

333

Page 16 of 41

Wavelength-Dependent AQY. Wavelength-dependent AQY for acetaldehyde,

334

glyoxal, and methylglyoxal photoproduction from 290–380 nm (stn. 5, 27) and from

335

300–330 nm (stn. 3, 10, 15, 18) are presented in Figure 3. Only stn. 15, 18 and 27 data

336

are presented for methylglyoxal, since they were the only stations that showed

337

measurable photochemical production of methylglyoxal under the irradiation conditions

338

used in this study.

339

Wavelength-dependent AQY for acetaldehyde, glyoxal and methylglyoxal

340

decreased exponentially with increasing wavelength. Acetaldehyde photoproduction

341

AQY ranged from 1.29 x 10-4 at 290 nm to 4.12 x 10-6 mol (mol quanta)-1 at 380 nm,

342

which were approximately one to two orders of magnitude greater than for glyoxal (2.52

343

× 10-5 to 6.89 × 10-7 mol (mol quanta)-1) and methylglyoxal (3.56 × 10-6 to 1.02 × 10-7

344

mol (mol quanta)-1), respectively.

345

Wavelength-dependent AQY data were fit to a single exponential decay function:

346

 = # (@A B@1 (9C,))

(4)

347

m1 and m2 were obtained from linear regression analysis of -lnΦλ = m1+ m2(λ-290),

348

where m1 is the y-intercept at 290 nm and m2 is the spectral slope. Fitting parameters are

349

presented in Table 3 for m1 and m2; fitted lines for wavelength-dependent AQY are shown

350

in Figure 3. All regressions were significant for all compounds and stations with p380 nm.40,49,57 When leveling off was

385

previously observed, AQY data were fit to a quasi-exponential fit40 or two exponential

386

functions.49 In the present study, these fits did not improve or only marginally improved

387

AQY spectral fits for carbonyl photoproduction (e.g.,