Wavelength and Temperature-Dependent Apparent Quantum Yields

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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

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Environmental Science & Technology

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Wavelength and Temperature-Dependent Apparent Quantum Yields for

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Photochemical Production of Carbonyl Compounds in the North Pacific Ocean

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Yuting Zhu and David J. Kieber*

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

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

13 14 15 16

*

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[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


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ABSTRACT

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

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

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

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photochemical production of acetaldehyde, glyoxal and methylglyoxal in North Pacific

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

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

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

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

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

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

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

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

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

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

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

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

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for these compounds in the surface ocean.

1 ACS Paragon Plus Environment

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

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

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

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

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for LMW carbonyl compounds including acetaldehyde, glyoxal and methylglyoxal,

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although large uncertainties persist.7–9

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

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

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tropospheric acetaldehyde budget is low compared to that predicted from atmospheric

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

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

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

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

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

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

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

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

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

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

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

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



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

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from the atmosphere to the sea.14

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Controversies regarding acetaldehyde, glyoxal, and methylglyoxal budgets in the

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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fundamental properties of the photochemical production of carbonyl compounds in North

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

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

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acetaldehyde, glyoxal and methylglyoxal photoproduction in seawater collected from


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

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

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

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

90

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

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

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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-

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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

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

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

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

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The DNPH reagent used to derivatize carbonyl compounds was prepared following

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

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

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

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

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

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

106

to use to separate the two phases.



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

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

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

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

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

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

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

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

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

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

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

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

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

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Q.32

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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for irradiations 330 nm. Prior to an

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

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

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

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

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

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

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response wavelength -- see Predicted Wavelength-Dependent Photoproduction Rates

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

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

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



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

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

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

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wavelength for the same reason.

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

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

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

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

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

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

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was set at 20 ˚C for all irradiations.

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Wavelength-dependent AQY were calculated from: =

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(1)

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

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

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

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(L),

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

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

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

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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

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

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

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

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=

),'(

'(

+, ./01 2 3

(2)

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

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

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

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mol quanta absorbed by nitrite),

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

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

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Carbonyl Quantification. The quantification method for carbonyl compounds was

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

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

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

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

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temperature for 24 h.

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

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

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

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

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

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

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

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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


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

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

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

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˚C. The sample analysis time was 37 min.

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Activation Energy. The activation energy for the photochemical production of

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

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

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

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

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

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wavelength in the predicted wavelength-dependent photoproduction rate spectra for

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

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calculated from linear regression analysis:

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:; = :;< −

(3)

=>

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

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

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

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

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

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

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

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0.05 was used for all tests.

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

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

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

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2010–2016 obtained from the Moderate-Resolution Imaging Spectroradiometer

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

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

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

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provided in Table S1.

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Acetaldehyde and glyoxal were the main LMW carbonyl photoproducts observed in

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

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

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

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

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

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

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

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

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

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

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

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(Figure S3).



244

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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irradiation (p0.05)

267

due to a greater analytical uncertainty.

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

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

270

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

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

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

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

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

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

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suggesting that reactions involving photochemically-generated reactive oxygen species in

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

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glyoxal or methylglyoxal photochemical stability in seawater. Since carbonyl photolysis

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

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

281

should be considered lower estimates for carbonyl photoproduction.

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

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

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

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

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

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



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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).

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

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

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

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

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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.

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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

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

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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-

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

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reaction.42


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Temperature-dependent AQY for glyoxal and methylglyoxal exhibited linear

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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

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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

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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

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

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

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photosensitized secondary photochemical processes in seawater, including

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

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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.



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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.,

Wavelength and Temperature-Dependent Apparent Quantum Yields

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