The nature of light and its absorption in natural waters

The most essential step in photochemistry is the absorption of light by chemical species. All photochemistry is driven by the molecular excitation that occurs from the absorption of light. Light exhibits both wave and particle properties that impart the energy available for chemical and physical reactions. Wave theory can be used to describe the propagation of light through various media, where, if optically different, the light might be refracted and/or reflected (e.g., transmission of light through the air-water interface). Diffraction of light is important for monochromatic studies of photochemistry and will be briefly discussed in a later section. Polarization of light will not be discussed.

While wave theory is useful, it is an incomplete description of light. The absorption and emission properties of light are best described in terms of light particles, or photons. Planck's research on black body radiation led to the concept of a quantum of light energy, or a photon. The energy, E, of any photon may be calculated using Planck's relationship, where the photon's energy is inversely proportional to its wavelength:

where h (6.63 x 10~24 J s) is a proportionality constant and c is the speed of light (3 x 108 m s_1). This relationship is important, because it allows us to quantify light energy reaching the Earth's surface and the surface of natural waters in terms of its wavelength. Often, the concept of the Einstein is employed in describing quanta of light, where 1 Einstein = 6.023 x 1023 photons, or a mole of photons. Many current light and UVR meters (e.g., Biospherical Instruments PUV and GUV radiometers) report radiation measurements in energy terms of watts (W) per squared area per discrete wavelength: /xW cm-2 nm-1. For our modeling example in section 7, we use units of energy in J m-2 nm-1, which are the SI units for radiant exposure.

6.2.1 The laws of photochemistry

As mentioned above, it is the absorption of light that drives photochemical reaction. The 1st law of photochemistry formulated by Grotthus and Draper, plainly states that"only the light which is absorbed by a molecule can be effective in producing photochemical change in the molecule." This is inherent in photochemistry, and perhaps taken for granted, but requires that we concern ourselves with accurate measurements of the light absorbed by the molecule(s) in natural waters. Therefore, we must not only measure the incident light at an aquatic surface, but also measure light propagation down through the water column (Figure 1). When light traveling through one transparent medium (e.g., the atmosphere) encounters a second medium in which the velocity of light is different (e.g., water) two different phenomena modify the light beam. First, a portion of the light beam is reflected at an angle (0a) equal to the angle of incidence. Second, the portion of the light beam transmitted into the new medium changes direction at the interface between the two media and is refracted. The angle of refraction (0W) is related to the angle of incidence by the different velocities of light in the two media, and the ratio of the velocity in medium 1 to the velocity in medium 2 is the refractive index. For most natural waters, this ratio is roughly 1.33, with subtle deviations due to the effects of temperature, light wavelength, and salt. Kirk [39] gives a thorough and rigorous discussion of the underwater light field, and the reader is directed to that reference, and also to Chapter 3.

The 2nd law of photochemistry, called the Stark-Einstein Law, states that "the absorption of light by a molecule is a one-quantum process, so that the sum of

Kirk [39],

the primary process quantum yields must be unity." Thus, a molecule that absorbs light and becomes electronically excited does so from only one photon, though bi-photonic absorption has been observed with laser light. The "primary" processes refer to the direct photochemical and photophysical reactions (e.g., dissociation, fluorescence, intersystem crossing) that occur due to light absorption. The quantum yield (<E>) is the amount of product (number of molecules) formed per unit time divided by the quanta of light absorbed per unit volume per unit time. In ecological photochemistry, this means that all of the photophysical and photochemical processes directly resulting from the absorption of a light photon must have individual quantum yields that add up to 1. We distinguish primary processes from "secondary" processes, which are the subsequent photochemical reactions that occur after a molecule has absorbed light energy. We describe these secondary processes as photosensitized reactions, because they occur due to the excess energy possessed by an excited chemical species. Photosensitized processes may be numerous (and actually induce further reactions), thus we typically observe quantum yields for individual processes that are quite small, and the overall quantum yield for a process such as photodegradation of CDOM is less than 1. The reason is that photodegradation of CDOM involves photosensitized processes that are not directly a result of the absorption of light energy.

We also point out the distinction between true and apparent quantum yields. An apparent quantum yield (Oa) is more appropriately used when working with CDOM, because we often do not know the true molar concentration of light-absorbing DOM molecules. That is, no molar basis exists for measuring the absorbance of CDOM, compared to measuring the absorbance for a single chemical of known molar concentration.

6.2.2 Absorbance of light

When dissolved species (denoted as S) absorb light photons, the outermost electron orbitals gain energy and electrons are elevated from their lowest energy state (the ground state, So) to a higher energy state (the excited state, denoted S*). Most ground state molecules are singlet ^Sq), meaning that they have paired electrons resulting in a total electron quantum spin of zero; thus a single spin state. The exception is molecular dioxygen (02), which is a ground state triplet molecule (T^, meaning that the molecule has unpaired electrons in its lowest energy state and may have three possible spin states (+1, 0, — 1). The excited state that is initially produced by singlet molecules is also a singlet state ^Si) where the subscript "1" refers to the relative energy level above the ground state. The higher excited energy states (%, etc.) are very transient and usually decay rapidly to the % state. Triplet states are generally longer-lived than singlet states, and we observe most photochemical reactions in CDOM from this state.

Excitation in natural water photochemistry typically involves the promotion of an electron from an n or n orbital (the bonding orbitals common in aromatic and carbonyl compounds) to a higher-energy anti-bonding orbital (71*), and is referred to as n-71* or n-n* transition. In this excited state, S* has an excess of energy and the electronic orbital transitions impart dissimilar chemical reactivity to the excited molecule relative to its reactivity in the ground state. Therefore, several physical and chemical reactions may occur to release this excess energy and return the species to its ground state. Photophysical pathways are most common; in fact, most electron excitation results in the release of energy through various photophysical pathways that do not involve chemical reaction. For natural water photochemistry, the most common photophysical pathways are internal conversion, intersystem crossing, fluorescence, phosphorescence, and vibrational relaxation (cf. refs. [12,13,16,40]). CDOM fluorescence is a well-known phenomenon and has been studied extensively [41-46].

The presence of multiple types of chemical bonds in CDOM dictates its overall absorbance. Because CDOM is a heterogeneous mixture of perhaps hundreds or thousands of different compounds it is impossible to identify which of them is most responsible for the CDOM absorbance. However, several investigators have begun to use spectrophotometry and mass spectroscopy to identify individual chromophores [47-49]. Table 1 describes the maximum absorbance of certain molecular bonds and phenolic compounds that are likely to be present in CDOM derived from terrestrial sources (e.g., lignin) [12,50,51].

When chemical change does arise from molecular excitation by light absorption, it is usually due to the excitation to a triplet state rather than a singlet state. This may result from the longer lifetime of excited triplets (average lifetime of 10~3 s) versus excited singlets (average lifetime <10-10 s). Also, a molecule in either the excited singlet or triplet state may transfer its energy to a receptor molecule (R) which becomes excited (R*) and may then undergo chemical reaction or return to its ground state through one of the previously mentioned photophysical pathways. An important example is the transfer of energy from an excited species to molecular oxygen (02) that is itself very reactive due to its two

Table 1 Maximum UV absorbance of and maximum wavelength to break bonds common to CDOM (superscripts indicate reference cited; n.r., not reported)

Chromophore

Maximum UVR absorbance (nm)

Maximum wavelength (nm) to break bond

CO

<180

346

C=Ca

180

196

C-Ha

<180

290

Phenolsb

350

n.r

Aldehydesb

325

n.r.

Ketonesb

350

n.r

Sinapic acidc

302, 327

n.r.

Protocatechuic acidd

258,292

n.r

cPrecursor of lignin, ref. [50]. dLignin derivative, ref. [50], aRef. [51]. bRef. [12].

cPrecursor of lignin, ref. [50]. dLignin derivative, ref. [50], unpaired electrons in the ground state (recall that the ground state is a triplet). The transfer of energy creates singlet oxygen OO2), which is an effective quencher of triplet excited states and also very reactive (see Chapter 8).

6.2.3 Description of CDOM absorbance

CDOM is usually described in terms of its absorbance over the environmentally relevant wavelength range of 280 to 700 nm, encompassing the UV and the visible portions of the solar spectrum. Absorbance, measured by a spectrophotometer, is the log base 10 ratio of the light intensity, J0, incident on the sample to the light intensity, I, transmitted by the system:

Units of absorbance are reported as absorption coefficients (a, in m~J), reflecting the conversion of the raw absorbance ^IcdomM °f a water sample measured by a spectrophotometer into its optical density (OD, or a[/i]). The absorption coefficient of CDOM is thus:

where / is the pathlength of the cuvette, and the 2.303 value converts the absorbance from base e into base 10 logarithms. The inclusion of the pathlength allows for the variable pathlength of cuvettes used to measure absorbance (typically 1, 5, and 10 cm). Examples of CDOM absorbance from several sources are shown in Figure 2. In most cases, absorption coefficients increase proportional to DOC concentration, though some saline lakes have very high DOC concentrations and very low absorption coefficients [Morris, unpublished data]. DOC-specific absorbance is another measurement that incorporates the DOC concentration in the optical measurement, and approximates the molar absorptivity commonly used to describe the spectroscopy of other discrete chemical

-Lake Moreno East, ARG

8 15

280 300 320 340 360 380 400 420 440 460 480 500

Wavelength (nm)

Figure 2. Absorbance spectra of multiple sources of CDOM exhibit wide variation.

-Lake Moreno East, ARG

8 15

280 300 320 340 360 380 400 420 440 460 480 500

Wavelength (nm)

Figure 2. Absorbance spectra of multiple sources of CDOM exhibit wide variation.

species. The approximation is due to the heterogeneous nature of DOC. Some moieties of DOC may not absorb light and therefore would not be CDOM (Section 6.4). If a large fraction of the DOC is not CDOM, then DOC-specific absorbance is limited in its usefulness.

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