SUPPORTING INFORMATION

The photochemistry of surface freshwaters in

the framework of climate change

Davide Vione,* Andrea Scozzaro

Department of Chemistry, University of Torino, Via P. Giuria 5, 10125 Torino, Italy.

* Corresponding author. Tel. 011-6705296. Fax 011-6705242. E-mail: davide.vione@unito.it

Table of contents

Model approach and equations Page S2

Photochemical modelling parameters Page S5

Table S1 Page S5

Table S2 Page S6

Table S3 Page S7

Effects of environmental factors on aquatic photoreactions Page S8

Figure S1 Page S9

Figure S2 Page S10

Figure S3 Page S12

Figure S4 Page S13

Freshwater vs. seawater photochemistry Page S13

Figure S5 Page S14

Photochemical modelling of stratified lake water Page S15

Figure S6 Page S16

Figure S7 Page S17

Figure S8 Page S18

Figure S9 Page S19

Figure S10 Page S20

Figure S11 Page S21

References Page S22

S 2

Model approach and equations

Radiation absorption by cromophoric dissolved organic mater (CDOM), NO3− and NO2

− was

calculated by assuming competition for irradiance in a Lambert-Beer approach.S1

Moreover,

because CDOM is by far the main sunlight absorber in surface waters, at least below 500 nm,S2

the

water absorbance is mostly accounted for by CDOM itself. On this basis, the spectral photon flux

densities absorbed by CDOM, NO3−, NO2

− and a generic molecule M can be expressed as follows:

S3

]101[)()()(1 dAoCDOM

a ppλλλ −−= (S1)

]101[)()(

][)()(

)(

1

3133 dAoNONO

a pA

NOp

λλλ

λελ −

−

−=−−

(S2)

]101[)()(

][)()(

)(

1

2122 dAoNONO

a pA

NOp

λλλ

λελ −

−

−=−−

(S3)

]101[)()(

][)()(

)(

1

1 dAoMM

a pA

Mp

λλλ

λελ −−= (S4)

where p°(λ) is the mid-latitude, incident spectral photon flux density of sunlight in a given month,

)(λε i (with i = NO3−, NO2

− or M) the molar absorption coefficients of nitrate, nitrite and M, [i] the

relevant concentration values, d the water depth, and λλ 015.0

1 45.0)( −= eDOCA the specific water

absorption coefficient over an optical path length of 1 cm (DOC is the measured dissolved organic

carbon, expressed in mgC L−1

).S1

Equations (S2-S4) assume that ][)( ii λε « )(1 λA .S3

The photon

fluxes absorbed by the different species are the integrals over wavelength of the respective absorbed

spectral photon flux densities:

λλλ

dpPi

a

i

a ∫= )( (S5)

where i = CDOM, NO3−, NO2

− or M. The formation rates of hydroxyl radicals (

•OH, tot

OHR• ) by

CDOM, NO3− and NO2

−, and of CDOM triplet states (

3CDOM*, tot

CDOMR

*3 ) by CDOM were

calculated as follows:

[ ]∫−

•

−−−

••• Φ+Φ+Φ=λ

λλλ dpPPRNO

OH

NO

a

NO

a

NO

OH

CDOM

a

CDOM

OH

tot

OH)()( 2233 (S6)

CDOM

a

CDOM

CDOM

tot

CDOMPR

** 33 Φ= (S7)

The formation quantum yields of photoreactive transient species by irradiated CDOM may vary

among different aquatic environments, but such variations are more limited than the environmental

variability might suggest.S4

For our simulations we used reasonably representative values for

S 3

surface waters, namely CDOM

OH•Φ = 510)3.00.3( −×± and

CDOM

CDOM*3Φ = 0.01 (Bodrato and Vione, 2014).

The value of CDOM

OH•Φ here used takes into account all known and poorly known processes of

•OH

photoproduction by CDOM, including the photo-Fenton reactions triggered by irradiation of Fe

species.S1

For the photogeneration of •OH by nitrate we used

−

•Φ 3NO

OH =

0075.0][25.2

0075.0][10)2.03.4( 2

+

+⋅×± −

IC

IC, where [IC] (mol L

−1) is the inorganic carbon concentration

([H2CO3] + [HCO3−] + [CO3

2−]). The expression for

−

•Φ 3NO

OHtakes into account the effect of inorganic

carbon species (mostly bicarbonate) on nitrate photochemistry, including reactions with geminate

photofragments in the solvent cage.S5

The calculation of the steady-state [•OH], which takes into account both photochemical generation

(equation (S6)) and scavenging by dissolved organic matter (DOM) and inorganic carbon, can be

carried out as follows:S1,S4

][][][

2

3,3,, 233

−−

•

•−•−•

•

++=

COkHCOkDOCk

ROH

OHCOOHHCOOHDOM

tot

OH (S8)

We used a reasonable value taken from the literature for the second-order reaction rate constant

between •OH and DOM,

OHDOMk •

, = (2.0±0.4)×10

4 L mgC

−1 s

−1,S6

and accepted literature values for

the corresponding second-order reaction rate constants with bicarbonate and carbonate: OHHCO

k •− ,3

=

8.5×106 L mol

−1 s

−1;

OHCOk •− ,2

3

= 3.9×108 L mol

−1 s

−1.S7

The triplet states 3CDOM* are produced by CDOM irradiation and deactivated by a number of

processes, including internal conversion and reaction with dissolved O2. The 3CDOM* deactivation

rate constant is k' ≅ 5×105 s

−1

S8 in air-equilibrated solutions at low DOC, and it is

1

*,*

3 )'(*][ 33

−×+= DOCkkRCDOMDOMCDOM

tot

CDOM. The carbonate radical (CO3

−•) is produced by

•OH oxidation of inorganic carbon (HCO3

− and CO3

2−), and by

3CDOM* oxidation of CO3

2−.S9

The

two pathways give the following formation rates for CO3−•

:

])[][(][ 2

3,3, 2333

−−••−•−

•

•− += COkHCOkOHROHCOOHHCO

OH

CO (S9)

CDOM

a

CDOM

CO

CDOM

COPCOR ][ 2

333

−•−•− =η (S10)

where CDOM

CO•−

3

η = (6.5±0.9)×10−3

L mol−1

.S1

It is tot

COR •−

3

= CDOM

COR •−

3

+ OH

COR

•

•−3

. CO3−•

is mostly scavenged

by DOM, and its steady-state concentration (mol L−1

) can thus be expressed as follows:S1

S 4

DOCk

RCO

DOMCO

tot

CO

,

3

3

3][•−

•−

=•− (S11)

where DOMCO

k,3

•− = 102 L (mg C)

−1 s

−1.S9

The formation and transformation rate of Br2•−

was described by using the following

equations.S7,S10-S13

Br− +

•OH � HOBr

•− [k12 = 1.1⋅10

10 M

−1 s

−1; k–12 = 3.3⋅10

7 M

−1 s

−1] (S12)

HOBr•−

→ Br• + OH

− [k13 = 4.2⋅10

6 M

−1 s

−1] (S13)

Br− +

3CDOM* → Br

• + CDOM

•− [k14 = 8×10

7 M

−1 s

−1] (S14)

Br− + Br

• → Br2

•− [k15 = 9⋅10

9 M

−1 s

−1] (S15)

2 Br2•−

→ Br3− + Br

− [k16 = 2⋅10

9 M

−1 s

−1] (S16)

Br2•−

+ DOM → Products [k17 = 3⋅102 L (mgC)

−1 s

−1] (S17)

Br2•−

+ NO2− → 2 Br

− +

•NO2 [k18 = 2⋅10

7 M

−1 s

−1] (S18)

From the above reaction sequence, by applying the steady-state approximation to HOBr•−

, Br• and

Br2•−

one gets the following expression for [Br2•−

]:

16

3

14

1

1312131216

2

2181721817

24

*])[][)(]([8])[(])[(][

k

CDOMkOHkkkkBrkNOkDOCkNOkDOCkBr

++++++−=

•−−

−−−

•−

(S19)

The degradation rate of M due to photochemical reactions is the following:

∑+Φ=J

JM

M

aMM JkMPR ][][ , (S20)

where [J] is the steady-state concentration of the transient species J (•OH, CO3

•−,

3CDOM* or

1O2,

calculated as described above, with [3CDOM*] = [

1O2]), MΦ the direct photolysis quantum yield of

M, JMk , the second-order reaction rate constant between M and J, and M

aP the photon flux

absorbed by M (calculated with equations (S4,S5)). The pseudo first-order photodegradation rate

constant of M is 1][ −= MRk MM , and the half-life time is M

M kt 2ln

,2/1 = .

S 5

Photochemical modelling parameters

Table S1 reports the photochemical reactivity parameters (direct photolysis quantum yields and

second-order reaction rate constants with photogenerated transients) used for the photochemical

modelling of the phototransformation of paracetamol, carbamazepine, dimethomorph, the basic

form of glutathione (GS−), and the bacteriophage virus MS2.

Table S2 reports the spectral parameters used in photochemical modelling: mid-latitude sunlight

irradiance (15 July, 9 am, clear sky), molar absorption coefficients of nitrate and nitrite, wavelength

trend of the quantum yield of •OH photogeneration by nitrite, molar absorption coefficients of

paracetamol, carbamazepine and dimethomorph.

The detailed calculation procedures used for photochemical modelling are reported in the paper that

explains the functioning of the APEX software (Aqueous Photochemistry of Environmentally-

Occurring Xenobiotics).S1

Table S1. Photochemical reactivity parameters (direct photolysis quantum yields Φ and second-

order reaction rate constants k with the photogenerated transient species) of the modelled

xenobiotics: paracetamol (APAP),S14

carbamazepine (CBZ),S15

dimethomorph (DMM),S16

the basic

form of gluthathione (GS−)S17

and the bacteriophage virus MS2.S18

n/a = not applicable (GS− does

not absorb sunlight).

x

APAP CBZ DMM GS−−−− MS2

Φx, unitless 4.6×10−2

7.8×10−4

2.6×10−5

n/a 2.9×10−3

OHxk •

,, L mol

−−−−1 s

−−−−1 1.9×10

9 1.8×10

10 2.5×10

10 9.0×10

8 7.0×10

9

−•3,COx

k , L mol−−−−1

s−−−−1

3.8×108 Low Low 7.1×10

8 1.3×10

8

21, Ox

k , L mol−−−−1

s−−−−1

3.7×107 1.9×10

5 8.5×10

5 2.1×10

8 3.5×10

8

*,3CDOMx

k , L mol−−−−1

s−−−−1

1.1×1010

7.5×108 1.6×10

9 8×10

7 6.5×10

8

S 6

Table S2. Spectral parameters concerning the incident photon flux density of sunlight (p°(λ)), the

molar absorption coefficients (ε) of nitrate and nitrite, the quantum yield Φ of •OH generation by

nitrite, as well as the molar absorption coefficients of paracetamol (APAP), carbamazepine (CBZ)

and dimethomorph (DMM).S1,S14-S16

λ,

nm

p°(λ), Einstein

cm−−−−2

s−−−−1

nm−−−−1

−3NO

ε , L

mol−−−−1

cm−−−−1

−2NO

ε , L

mol−−−−1

cm−−−−1

−

•Φ 2NO

OH,

unitless

APAPε , L

mol−−−−1

cm−−−−1

CBZε , L

mol−−−−1

cm−−−−

DMMε , L

mol−−−−1

cm−−−−

292.5 8.2×10−17

6.9 8.8 0.068 1050 10700 12500

295 1.6×10−16

7.5 8.9 0.068 840 9840 12300

297.5 3.2×10−14

7.9 9.0 0.068 590 8960 12000

300 6.5×10−14

7.9 9.1 0.068 440 7980 11700

302.5 8.4×10−13

8.0 9.1 0.067 260 6970 11300

305 1.6×10−12

7.7 9.3 0.067 190 5920 10900

307.5 4.0×10−12

7.2 9.4 0.067 130 4900 10400

310 6.4×10−12

6.7 9.7 0.065 92 3790 9750

312.5 1.2×10−11

6.0 9.9 0.064 91 2870 8990

315 1.8×10−11

5.2 10.3 0.061 84 2070 8280

317.5 2.3×10−11

4.3 10.7 0.058 76 1430 7450

320 2.7×10−11

3.5 11.3 0.054 60 980 6650

322.5 3.3×10−11

2.8 12.0 0.051 52 730 5800

325 3.9×10−11

2.1 12.8 0.047 36 460 4930

327.5 4.9×10−11

1.5 13.7 0.043 18 300 4200

330 5.9×10−11

1.0 14.6 0.038 9 200 3600

333.3 5.9×10−11

0.6 16.0 0.031 5 120 2800

340 6.6×10−11

0 19.1 0.026 1 94 1630

350 7.3×10−11

0 22.6 0.025 0 0 550

360 7.8×10−11

0 22.0 0.025 0 0 210

370 1.0×10−10

0 16.6 0.025 0 0 90

380 1.1×10−10

0 9.1 0.025 0 0 36

390 1.2×10−10

0 3.4 0.025 0 0 26

400 1.8×10−10

0 0.82 0.025 0 0 26

410 1.9×10−10

0 0.17 0.025 0 0 20

420 2.1×10−10

0 0.07 0.025 0 0 19

430 1.8×10−10

0 0.06 0.025 0 0 13

440 2.3×10−10

0 0.05 0.025 0 0 10

450 2.8×10−10

0 0.05 0.025 0 0 9

460 2.8×10−10

0 0 0 0 0 4

470 2.9×10−10

0 0 0 0 0 0

S 7

Table S3. Harmful intermediates that are known to be preferentially formed (high-yield pathways)

or poorly formed (low-yield pathways) from the parent pollutants via peculiar photoreactions (d.p.

= direct photolysis).

Xenobiotic Harmful intermediate(s) High-yield

pathway(s)

Low-yield pathway(s)

(if known)

Ibuprofen 4-IsobutylacetophenoneS19

d.p., 3CDOM* •

OH

Carbamazepine AcridineS20

d.p., •OH

3CDOM*

Cefazolin 5-Methyl-1,3,4-thiadiazole-2-thiolS21 d.p.

Clofibric acid HydroquinoneS22 3

CDOM*

Triclosan DichlorodibenzodioxinsS23,S24

d.p., 3CDOM* •

OH

Gemfibrozil Chain-shortening/detachment

derivativesS25

d.p.

Phenylurea

herbicides

Aldehyde derivatives

(-CH3 → -CHO)S26-S32

d.p., •OH

3CDOM*

S 8

Effects of environmental factors on aquatic photoreactions

Photochemical processes are understandably favored when the irradiance of sunlight is higher.

Therefore, when neglecting weather-related issues, photoreactions are faster in spring-summer

compared to autumn-winter.S22,S33

Long-term changes in irradiance are less straightforward to

predict. Climate change might possibly affect UVB irradiance by reducing the stratospheric ozone

levels, through higher formation of icy surfaces that favor ozone destruction and via an increase of

the atmospheric water content. However, increasing cloud cover at high latitudes could on the

contrary screen UVB radiation.S34

Still, the UVB radiation intensity at the ground is usually very

low and this radiation poorly penetrates the water columns,S2

thus it is unlikely to impact much the

photoreaction kinetics of most compounds. In contrast, it may have a role in processes that are only

triggered by UVB and take place at or near the water surface: for instance, DNA-damaging UVB

radiation affects the depth distribution of algae in Alpine lakes, which results from a compromise

between the quest for abundant photosynthetically-active radiation and the need to avoid UVB.S35

Sunlight irradiance is closely connected with the depth of the water column. Photoreactions are

faster in shallow water, because the surface water layer is strongly illuminated by sunlight

differently from the darker lower depths.S36

The attenuation of radiation in the water column

depends on depth and on the water absorbance λA . λA is largely accounted for by CDOM

absorption, and it typically shows a featureless exponential decay above 290-300 nm that can be

approximated as λλ

S

oeAA−= . The term oA is a constant that often increases with the DOC

(dissolved organic carbon). S is the spectral slope, which usually varies between 0.013-0.017

nm−1

.S37,S38

Because λA decays exponentially with increasing wavelength, the attenuation of

irradiance with depth follows the order UVB (most attenuated) > UVA > visible. An example of

how depth and DOC may affect the underwater solar radiation is provided in Figure S1.

The DOC is a key parameter for surface waters, including the photochemical processes, and it is the

most straightforward way to measure DOM. Interestingly, DOM-rich waters are usually also

CDOM-rich (such as many lakes located in the Scandinavian peninsula and in other lake-rich, high-

latitude regions of the boreal hemisphere),S39

while many DOM-poor waters are also very

transparent (such as many Alpine lakes located above the vegetation line).S40

DOM is a major •OH

and CO3•−

scavenger, while CDOM is the source of 3CDOM* and

1O2. Because of the correlation

between DOM amount and DOC, the reactions triggered by •OH and CO3

•− are favored at low

DOC, while the 3CDOM* and

1O2 processes are enhanced at high DOC.

S41

An example of the trend of the transients with varying DOC is reported in Figure S2a. Both [•OH]

and [CO3•−

] decrease with increasing DOC, because of the scavenging by DOM. The relevant DOC

trends can be approximated by functions of the form [Conc.] ∝ DOC−n

, where n ∼ 1 for [•OH] and n

∼ 2 for [CO3•−

]. The reason for the difference is that CO3•−

is inhibited by DOM twice: (i) the

scavenging of •OH by DOM inhibits CO3

•− generation, and (ii) DOM directly scavenges CO3

•−.S42

In contrast, [3CDOM*] and [

1O2] increase with DOC, up to a plateau above 20 mgC L

−1 DOC. There

is a plateau because elevated CDOM tends to absorb almost all of the incident radiation (absorption

S 9

saturation), and further CDOM increases only produce a small absorption enhancement. Moreover,

at very high DOC there is also some role of 3CDOM* scavenging by DOM.

S43

The direct photolysis processes show intermediate behavior. The absorption of sunlight by a

substrate is in competition with CDOM absorption, thus elevated CDOM inhibits the direct

photolysis. However, the inhibition of the direct photolysis by CDOM is usually less important than

the inhibition of •OH and CO3

•− reactions by DOM.

S41

Figure S1. Spectral photon flux density of sunlight at different water depths, for different DOC values. The

zero-depth spectrum corresponds to a UV irradiance of 22 W m−2

, which can be observed in early April or

early September at midday (or at mid-morning or mid-afternoon in July) in mid-latitude, fair-weather

conditions. The water absorption spectrum was here approximated with the formula

λλ

015.045.0 −= edDOCA (thereby assuming dDOCAo 45.0= ),S1

where d is depth [units of cm].

Obviously, the same values of the product DOCd × give here the same attenuation. Some

representative (d, DOC) couples of values are provided near each spectrum.

S 10

Figure S2. (a) Steady-state concentrations of

•OH, CO3

•− and

3CDOM* (approximately overlapping with

1O2) upon freshwater summertime irradiation (22 W m

−2

sunlight UV irradiance), as a function of the DOC. Note the logarithmic scale on the Y-axis. First order photodegradation rate constants of (b) paracetamol, (c)

the conjugated base of glutathione (GS−), and (d) the bacteriophage MS2, as a function of the DOC (d.i. = direct inactivation). The corresponding half-life times

are reported on the right Y-axis. The contributions to photodegradation/photoinactivation by •OH, CO3

•−,

1O2,

3CDOM* and direct photolysis are highlighted in

different colors. Other water conditions in all cases: 5 m depth, 10−4

mol L−1

nitrate, 10−6

mol L−1

nitrite, 10−3

mol L−1

bicarbonate, 10−5

mol L−1

carbonate.

S 11

As far as pollutant photodegradation is concerned, Figure S2b shows as an example the behavior of

the antipyretic paracetamol, which mainly undergoes photodegradation by CO3•−

, 3CDOM* and

direct photolysis.S14

Unless otherwise specified, the "days" time units refer to summer sunny days

under mid-latitude conditions (mid-July in the northern hemisphere), in a well-mixed water column.

The reaction with CO3•−

prevails at DOC < 2-2.5 mgC L−1

, while that with 3CDOM* is most

important for higher DOC values. The direct photolysis is a secondary process, but its importance

becomes non-negligible at DOC = 2-4 mgC L−1

. The combination of the three processes produces a

minimum in the photodegradation rate constant of paracetamol at DOC = 3-4 mgC L−1

. The same is

not true for all compounds: if the 3CDOM* (and/or

1O2) reactions are less important than for

paracetamol, the kinetics can consistently slow down with increasing DOC.S44

This is for instance

what happens with the basic form of the tripeptide glutathione (GS−, Figure S2c), for which the

3CDOM* reaction is less important compared to paracetamol. GS

− does not absorb sunlight and

thus does not undergo direct photolysis, but in low-DOC environments the photoreactions triggered

by, most notably, CO3•−

are in competition with assimilation by microorganisms.S17

A similar DOC trend as for GS− is observed with the bacteriophage virus MS2 (Figure S2b).

Interestingly, while 1O2 plays a negligible role in the photodegradation of paracetamol, and similar

issues are observed with several other anthropogenic pollutants, both glutathione (as well as several

aminoacids S45

) and MS2 (as well as several other viruses S18

) are efficiently photodegraded by 1O2,

especially at high DOC.

The model results reported in Figure S2 were obtained under the assumption that (C)DOM only

acts through its total amount, without qualitative modifications. Actually, the CDOM optical

properties were only scaled for the water DOC ( λλ

015.045.0 −= edDOCA ), assuming that DOC-

normalized optical properties would not change. Similarly, the photogeneration quantum yields of •OH,

3CDOM* and

1O2 by irradiated CDOM were also kept constant with varying DOC, and the

same assumption was made for the reaction rate constants of DOM with •OH, CO3

•− and

3CDOM*.

These assumptions are only valid as a first approximation, in the absence of clear indications of the

climate-change effects on these parameters. Interestingly, a recent work has shown that several

CDOM photochemical parameters are affected by the water-body trophic status. In particular, the

quantum yields of photoreactive transient photoproduction by CDOM were higher in oligotrophic

water bodies: therefore, increasing DOC might well increase the CDOM content in water, but such

an increase could be partially offset by lower CDOM photoreactivity.S46

An example of the

dependence of the steady-state [3CDOM*] on the quantum yield of

3CDOM* photogeneration by

irradiated CDOM (*3CDOM

Φ , varied in the 0.005-0.05 range) is shown in Figure S3. Unless

otherwise specified, model calculations here assumed *3CDOM

Φ = 0.01.S4

Depending on the

environment, climate change could increase the CDOM content through the browning phenomenon

or decrease it via oligotrophication that could derive from prolonged lake-water stratification. The

values of *3CDOM

Φ would probably move in the opposite direction as the CDOM amount, thereby

partially offsetting its variation.

S 12

Figure S3. Modelled steady-state 3CDOM* concentration, as a function of the water DOC, upon

freshwater summertime irradiation (22 W m−2

sunlight UV irradiance). Other water conditions:

10−4

mol L−1

nitrate, 10−6

mol L−1

nitrite, 10−3

mol L−1

inorganic carbon, 5 mgC L−1

DOC, 3 m water

depth.

The water pH may also be important for compounds that undergo acid-base equilibria. An example

is the anti-bacterial agent triclosan (pKa ∼ 8), which is used in personal care products and becomes

more photolabile as the pH increases, mostly because of an increase in the kinetics of direct

photolysis (Figure S4). The process is environmentally concerning, because the direct photolysis of

basic triclosan yields dioxins S23,S24

that are considerably more harmful than the parent compound.

Finally, increasing temperature may have a role on the kinetics of photochemical reactions. The

direct photolysis processes are unlikely to be much affected by a temperature increase, because the

energy involved in electronic transitions is much higher than the thermal energy, which is

comparable to the energy of the vibrational levels. Also the formation processes of the transient

species and the associated quantum yields are unlikely to be much affected by temperature, but the

reactions between transients and pollutants may become faster with increasing temperature if they

have an energy barrier. This is not the case for diffusion-controlled reactions, featuring rate

constants above 1010

L mol−1

s−1

, but reactions with rate constants at and below 109 L mol

−1 s

−1 can

potentially undergo a temperature-related increase.S47

S 13

Figure S4. Modeled first-order photodegradation rate constants (left Y-axis) and photochemical half-lives

(right Y-axis) for triclosan (HTric � Tric− + H

+, pKa = 8), as a function of pH. The contributions of the different

processes to photodegradation are highlighted with different colors. Other water conditions: 10−4

mol L−1

nitrate, 10−6

mol L−1

nitrite, 10−3

mol L−1

inorganic carbon, 5 mgC L−1

DOC, 3 m water depth.

Freshwater vs. seawater photochemistry

This paper focuses on freshwaters, but it is worth mentioning that in brackish waters and saltwaters

the photochemical reactions also involve halogen-containing radicals (e.g., Cl2•−

, Br2•−

and ClBr•−

)

that are produced upon oxidation of Cl− by

3CDOM*, and of Br

− by

•OH and

3CDOM*.

S48

In particular, increasing bromide concentration decreases the steady-state [•OH] and, as a

consequence, the steady-state [CO3•−

] (•OH is a major CO3

•− source via oxidation of inorganic

carbon species). The steady-state [Br2•−

] is obviously increased by increasing bromide, while

[3CDOM*] is very little modified by the Br

− trend. All these issues are shown in Figure S5.

Moreover, it has recently been shown that the Mg2+

content of seawater has the potential to strongly

affect the reactions induced by 3CDOM*, due to the formation of Mg-CDOM complexes. The

values of *3CDOM

Φ have been found to vary in either direction in the presence of Mg2+

, depending

on the CDOM type.S49

S 14

Figure S5. Modelled steady-state concentrations of •OH, CO3

•−,

3CDOM* and Br2

•−, as a function of

bromide concentration. Other water conditions: 10−4

mol L−1

nitrate, 10−6

mol L−1

nitrite, 10−3

mol

L−1

inorganic carbon, 5 mgC L−1

DOC, 3 m water depth.

S 15

Photochemical modelling of stratified lake water

It is interesting to assess which is the expected impact of stratification (i.e., different evolution of

epilimnion and hypolimnion) on compound photodegradation, as a function of the duration of the

summer stratification phase that is expected to become longer as a consequence of climate change.

For simplicity, it is assumed here that a dissolved compound at time zero is evenly distributed in the

whole water column, and that no emission occurs afterwards. For a given DOC value and with

reasonable hypotheses on CDOM absorption, it is possible to reproduce the underwater solar

spectrum and irradiance as a function of depth. For modelling sake, one can consider that the

epilimnion receives the equivalent of the sunlight irradiance at the ground (p°(λ), where λ is the

wavelength), which is then absorbed along the water column. In the case of the hypolimnion, the

corresponding irradiance (pth

(λ)) is that observed at the depth of the thermocline, dth [cm]:S1

thdAth pp)(110)()(

λλλ −°= (S21)

In equation (S21), A1(λ) is the water absorbance over a 1-cm path length (here it is assumed that λλ 015.0

1 45.0)( −= eDOCA ). See Figure S1 for an example of how the sunlight spectrum (more

precisely, its spectral photon flux density) varies with depth. As a simplification, it is assumed here

that sunlight travels vertically in the lake water. This is in principle not correct, but refraction at the

air-water interface shifts the light trajectories towards the vertical and the overall error entailed by

this approximation is quite small.S50

By treating epilimnion and hypolimnion as separate entities,

one can model the photochemical evolution of dissolved compounds in both compartments during

the stratification phase. If photodegradation follows a first-order kinetics, the compounds time

trends are defined by the modelled photodegradation rate constants. Just before the end of

stratification, the compound concentration in the photoreactive epilimnion (Cepi) will be lower than

that in the darker hypolimnion (Chypo > Cepi). Overturn will then restore uniform chemical

composition, and the resulting average concentration Cave will depend on Cepi, Chypo and on the

volumes of epilimnion and hypolimnion (respectively, Vepi and Vhypo):

hypoepi

hypohypoepiepi

aveVV

VCVCC

+

+= (S22)

The values of Vepi and Vhypo depend on the lake geometry. Here, two simplified cases will be

considered. The first is a rectangular, swimming-pool-like geometry (Figure S6a) where dtot = 50 m

is the whole lake depth, and dth = 15 m is the depth of the thermocline. The parallelepiped shape

ensures that minimum, maximum, and average depths coincide. During stratification, the

epilimnion has depth dth while the hypolimnion has depth dtot - dth. As a consequence of lake

geometry, the epilimnion and hypolimnion volumes are proportional to the respective depths, thus 11 )( −− −= thtotthhypoepi dddVV . Some compounds and one virus are here considered to assess

photodegradation under reasonable hypotheses for water chemistry: they are recalcitrant pollutants

carbamazepine and dimethomorph, photolabile paracetamol, the peptidic thiol glutathione, and the

S 16

MS2 virus. Two scenarios are compared, namely consistent mixing without stratification ("Mix" in

the relevant labels), and stratification followed by mixing. In the latter scenario

("Stratification+mix"), modelling considers the separate time trends in the epilimnion and

hypolimnion, as well as overturn at the end of stratification. Overturn is assumed to occur at the

given time, reported in the X-axis of each relevant plot (instantaneous mixing is considered here for

simplicity). In this case, concentration values in epilimnion and hypolimnion are averaged as per

equation (S22).

Lakes are generally not swimming-pool-like systems, however, and the hypolimnion usually

accounts for a smaller fraction of the total volume, at equal dth, compared to the case of

parallelepiped geometry. A simplified scenario could be that of a conical geometry, which can be

further simplified by assuming a 90-degree aperture angle (see Figure S6b). The overall lake

volume is here calculated as Vtot = ⅓ π (dtot)3, the hypolimnion volume as Vhypo = ⅓ π (dtot-dth)

3, and

the volume of the epilimnion as Vepi = ⅓ π [(dtot)3 - (dtot-dth)

3]. Therefore, it is =−1

hypoepi VV [(dtot)3 -

(dtot-dth)3] (dtot-dth)

−3. With dtot = 50 m and dth = 15 m, the hypolimnion makes up ∼34% of the

whole conical lake volume, compared to 70% in the case of swimming-pool geometry. The average

depths used for photochemical modelling can be obtained easily by comparing the volume formulas

of cone, truncated cone and cylinder: the average depth is in fact the height of the cylinder having

the same volume as the corresponding cone (for the whole lake and the hypolimnion) or truncated

cone (for the epilimnion). Therefore, the average depth of a lake with conical geometry is ⅓ dtot

(i.e., 16.67 m in this case), that of the hypolimnion ⅓ (dtot-dth) (i.e., 11.67 m), and that of the

epilimnion {dth (dtot)−2

[(dtot)2 - dtot dth + ⅓ (dth)

2]} (i.e., 10.95 m). From Figure S6b it is also

possible to note that sunlight passes through a water layer of depth dth before reaching the

hypolimnion, while the inclined slopes in the epilimnion play no role as there is no hypolimnion

below them. Therefore, equation (S21) applies in the case of conical geometry as well.

Epilimnion

Hypolimniondtot

dth

Thermocline

(a)

Epilimnion

Hypolimniondtot

dth

Thermocline

(a)

dtot

Thermocline

dth

(b)

dtot

Thermocline

dth

(b)

Figure S6. Lake geometries considered in the photochemical modelling of both stratified and

thoroughly mixed water. (a) Parallelepiped (swimming-pool-like) shape; (b) conical

shape with 90° aperture angle.

S 17

Figure S7. Comparison of phototransformation kinetics of the basic form of glutathione (GS−) (a,c) and of photoinactivation of the virus MS2 (b,d)

in lake water, under stratification vs. mixing conditions, in the case of a lake with parallelepiped shape (a,b) and with conical shape (c,d). Water

conditions: maximum lake depth dtot = 50 m, depth of the thermocline dth = 15 m (which gives a maximum hypolimnion depth of 35 m), 2 mgC L−1

DOC, 10−4

mol L−1

nitrate, 10−6

mol L−1

nitrite, 10−3

mol L−1

bicarbonate, and 10−5

mol L−1

carbonate.

S 18

Figure S8. Modelled phototransformation kinetics of carbamazepine in lake water, under

stratification vs. mixing conditions, in the case of a lake with parallelepiped shape.

Phototransformation in a thoroughly mixed water column is represented by the "Mix" curve. In

the case of stratification conditions, the evolution of xenobiotics in the epilimnion and

hypolimnion was computed separately. The "Stratification + mix" curve represents the pollutant

concentration in the whole water column, following stratification for the given time period (e.g.,

45 or 60 days) and sudden mix thereafter. Water conditions: lake depth dtot = 50 m, depth of the

thermocline dth = 15 m (which gives a hypolimnion depth of 35 m), 2 mgC L−1

DOC, 10−4

mol L−1

nitrate, 10−6

mol L−1

nitrite, 10−3

mol L−1

bicarbonate, and 10−5

mol L−1

carbonate.

S 19

Scenario 1

Scenario 2

Scenario 3

Figure S9. Modeled global precipitations following three different scenarios of future evolution of atmospheric CO2. Scenario 1: 1% increase per

year from 2018 to 2050; Scenario 2: 0.5% increase per year; Scenario 3: 0.2% increase per year. The maps show the distributions of precipitation

differences (precipitations in 2048 minus precipitations in 2018) on a global scale. Modeling used the EdGCM 3.2 code (Columbia University).S51

S 20

Figure S10. Modelled phototransformation kinetics of GS− because of evaporation (a) and outflow (c). Modelled phototransformation kinetics of

MS2 because of evaporation (b) and outflow (d). The left Y-axes report the photodegradation rate constants; the corresponding half-life times are

shown in the right Y-axes. The contributions of the different photoinduced processes to photodegradation are highlighted with different colors.

Initial water conditions in all cases: 2 mgC L−1

DOC, 10−4

mol L−1

nitrate, 10−6

mol L−1

nitrite, 10−3

mol L−1

bicarbonate, 10−5

mol L−1

carbonate.

S 21

Figure S11. Variations in the photodegradation kinetics of carbamazepine as a function of the river

flow rate Q. The left-Y axis reports the modelled photodegradation rate constants k. The circles

show sample half-life times that correspond to given values of the photodegradation rate constant

(1

2/1 2ln −= kt ). The half-life lengths reported on the right Y-axis were calculated on the basis of

the half-life times reported as circles ( 32/12/12/1 / oo QQvttvl == ). Other water conditions: 2 mgC

L−1

DOC, 10−4

mol L−1

nitrate, 10−6

mol L−1

nitrite, 10−3

mol L−1

bicarbonate, 10−5

mol L−1

carbonate.

The water depth was do = 4 m when Qo = 100 m3 s

−1, and it varied as 3 / oo QQdd = .

S 22

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