N-[2,5-dichloro-4-(1,1,2,3,3,3-hexafluoropropoxy)-phenyl-aminocarbonyl]-2,6-difluorobenzamide

Phototransformation in water

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

phototransformation in water

Type of information:

experimental study

Adequacy of study:

key study

Study period:

19 May 1992 to 25 Jan 1994

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Study type:

direct photolysis

Qualifier:

according to guideline

Guideline:

EPA Guideline Subdivision N 161-2 (Photodegradation Studies in Water)

Version / remarks:

October 18, 1982

Deviations:

not specified

GLP compliance:

yes (incl. QA statement)

Radiolabelling:

yes

Analytical method:

gas chromatography

high-performance liquid chromatography

mass spectrometry

other: Thin-layer chromatography (TLC)

Details on sampling:

- Sampling Times: Samples of one vessel each were taken for analysis directly after the treatment and after 3, 5.8, 7.1, 11.8, 14.8, 19.8 and 22.3 days of exposure to continuous light. The latter time interval corresponded to 44.6 calendar days or to 19.7 days Florida sunlight equivalents as calculated from the initial spectral energy distribution measurements. The trapping solutions were taken for analysis and replaced by fresh solutions at all sampling times of the vessels.

Buffers:

The photolytic behaviour of the test substance was investigated in 15 mL aqueous solution buffered at pH 7 (0.01 M phosphate buffer) where the test compound is known to be hydrolytically stable. Since the starting concentration of the test substance was less than 10-4 M, the buffers ionic strength did not exceed 0.01 M at which concentration buffer effects are negligible. The buffer was sterilized by filtration.

Light source:

Xenon lamp

Light spectrum: wavelength in nm:

>= 290

Details on light source:

- Filters used: Filters to cut-off light of less than 290 nm wavelength
- Irradiation equipment: The spectral energy distribution (300 - 700 nm) of the light source, i.e. the incident light available at the water level in all vessels, and that of natural sunlight were recorded using a portable spectroradiometer. A holographic grating monochromator and a Teflon cosine diffuser either fixed or in combination with a glass fiber optic probe were used.

Details on test conditions:

TEST SYSTEM
- Type, material and volume of test apparatus/vessels: Cylindrical vessels (sample volume = 15 mL), constructed of borosilicate glass but covered with a quartz glass lid.
- Control group: A dark control solution was stored in the same type of vessels, incorporated in an identical gas flow system and maintained in darkness at 25 °C.
- Details on temperature control: A thermocouple was fitted to one vessel for temperature control. The vessels were irradiated in sockets of a water cooled steel tank (25 °C) placed within the photolysis apparatus.
- Details of traps for volatile: All vessels were fitted with inlet and outlet ports for collection of volatiles. For the collection of volatiles all vessels were connected in line and ventilated with an air flow of about 5 mL/rninute controlled by a peristaltic pump. Incoming air was passed through a 2 N NaOH trap in order to remove carbon dioxide and a water trap for humidication. The outlet air passed three traps for absorbing volatiles in the sequence 0.1 N H2SO4, ethylene glycol and 2 N NaOH.
- Sterilisation method: Microbial infection of the solutions during application of a.i. and by ventilation of the system was avoided by adequate procedures and equipment. Subsamples of the nal test solution were tested for possible microbial contamination at the end of the experiment.

TEST MEDIUM
- Preparation of test medium: The 14C-labelled test substance was dissolved in 1.0 mL acetonitrile and thereafter its accurate amount determined by liqid scintillation counting to be 1.9392 mg. From this stock solution aliquots of 0.015 mL were added to 15 mL of buffer solution thus giving a final concentraion of the test compound of 0.0514 mg/mL. The cosolvent concentration in the aqueous system was limited to 0.1 % by volume.

REPLICATION
- No. of replicates (dark): 1
- No. of replicates (irradiated): 8 treated replicates and one contained untreated buffer only and served as temperature.

Duration:

22.3 d

Temp.:

24.9 °C

Initial conc. measured:

0.051 g/L

Reference substance:

not specified

Dark controls:

yes

% Degr.:

84.75

Sampling time:

22.3 d

Test condition:

Irradiated

% Degr.:

0.95

Sampling time:

22.3 d

Test condition:

Dark control

Key result

DT50:

10.25 d

Test condition:

Irradiated

Remarks on result:

other: continuous light equivalent to 18 days of latitude 30° N summer sunlight

Transformation products:

not specified

Remarks:

M7

Details on results:

An overview of the results is provided in Table 1 - Table 4 in 'Any other information on results incl. tables'
- Recovery of Radioactivity: On the average, 99.08 % of the radioactivity applied was recovered after the various exposure periods to artificial light. For the dark control the recovery after 22 days accounted for 99.88 %. By far the majority of the recovered radioactivity remained in the aqueous solution. In addition to the radioactivity in the aqueous phase volatile radioactivity in form of carbon dioxide was observed. At the end of the exposure period 1.17 % of the applied radioactivity was found to be carbon dioxide demonstrating that a small amount of the chlorophenyl—ring moiety was photolytically mineralized.

- Rate of Degradation of the Parent Molecule: Except for day 15 which is considered to be an outlier, a continuous degradation of the parent molecule with time was observed. When fitted to the exponential function, the compound showed a half-life of 10.25 days when exposed to continuous light equivalent to 9.0 days of latitude 30° N summer sunlight. No significant degradation occurred in the control solution, i.e. the parent molecule accounted after 22 days still for 99.05%.

- Characterization and Identification of Degradates: Besides the parent molecule accounting at the end of the study for 15.25 % two major radioactive fractions (unknown metabolite 1 and M7) were found. Their corresponding amounts after 22 days of exposure were 13.14 and 62.11% of the radioactivity applied corresponding to 0.007 and 0.032 ppm only. In addition mainly one minor radioactive fraction (unknown metabolite 2) was detected representing at the end of the study 7.98 % or 0.004 ppm. When further analysed, M4B proved to consist at least two degradates, unknown metabolite 2a and unknown metabolite 2b, amounting to 5.15 and 2.83% (=0.003 and 0.001 ppm). While the parent molecule and unknown metabolite 1 completely and to a major extent M7 partitioned into the dichloromethane phase, the unknown metabolite 2 remained in the aqueous phase of the photolysis solution.

- Unknown metabolite 1: Unknown metabolite 1 was neither found to be identical with any of the reference compounds nor to be identical with isolated and identified degradate from the hydrolysis study. For further identification the amount of unknown metabolite 1 was by far too low. All attempts to set up a photolysis experiment at a higher concentration even when the concentration of the co-solvent was increased up to 10% resulted in a complete adsorption of the practically water insoluble test compound within a few hours after starting the experiment.

- M7: Degradate M7 was found to be very polar. It was eluted from reversed phase column already after 5.8 minutes.

Table 1. Balance of Radioactivity and Formation of Photolytic Degradates after Exposure of 14C-labelled test substance to artificial Light (Values given in % of the radioactivity applied). 

Day	Parent substance	Unknown metabolite 1	M7	Unknown metabolite 2	CO2	NA	Total
0	99.46	0.00	0.00	0.00	0.00	0.05	99.58
3	83.69	3.10	10.18	0.00	0.04	1.64	98.65
5.8	77.23	6.73	12.21	0.00	0.11	2.05	98.34
7.1	73.48	7.76	14.56	0.00	0.14	2.85	98.79
11.8	42.17	12.39	37.91	0.00	0.32	6.42	99.21
14.8	44.83	12.52	34.70	0.00	0.65	6.47	99.18
19.8	22.64	14.63	53.35	7.68	0.92	0.00	99.22
22.3	15.25	13.14	62.11	7.98	1.17	0.00	99.66
Dark control	99.05	0.00	0.00	0.00	0.00	0.83	99.88

* Degradate fraction unknown metabolite 2 consisted of at least 2 degradates amounting to 5.15 (unknown metabolite 2a) and 2.83 % (unknown metabolite 2b) of the radioactivity applied. These figures corresponded to 0.003 and 0.001 ppm.

* The amount of unknown metabolite 1observed at the end of the study corresponded to a level of radioactivity of 0.007 ppm only.

NA: Not Analysed

Table 2. Suntest and Sunlight Intensities at the Beginning of the Study.

Position in suntest apparatus	Date	Light energy (watt/m2) suntest	Light energy (watt/m2) sunlight basle
1	13.5.92	6.901	17.06
2	13.5.92	6.928	16.97
3	13.5.92	7.581
4	13.5.92	7.030
5	13.5.92	7.284
6	13.5.92	6.620
7	13.5.92	6.614
8	13.5.92	7.542
9	13.5.92	7.357
10	13.5.92	6.537
Mean Stdev		7.039 0.367 (=5.21 %)	17.015

 

Table 3. Suntest and Sunlight Intensities at the End of the Study 

Position in Suntest Apparatus	Date	Light energy (watt/m2) suntest	Light energy (watt/m2) sunlight basle
1	30.06.92	7.316	14.44
2	30.06.92	7.157	14.59
3		8.351
4	30.06.92	7.815
5	30.06.92	7.440
6	30.06.92	5449
7	30.06.92	6.206
8	30.06.92	7.197
9	30.06.92	6.398
10	30.06.92	6.302
Mean StDev		6.963 0.82 (=11.77 %)	14.515

Table 4. Duration of Irradiation Period for each Test Vessel

Vessel Number	Hours continuous irrad.	Days continuous irrad.	Calendar days	Equivalent time of natural sunlight (days)
				40°N	30°N
0	0	0	0	0.00	0.00
1	72	3.0	6.0	2.61	2.71
2	139	5.8	11.6	5.05	5.24
3	171	7.1	14.3	6.21	6.45
4	282	11.8	23.5	10.24	10.63
5	354	14.8	29.5	12.86	13.34
6	474	19.8	39.5	17.21	17.87
7	536	22.3	44.7	19.46	20.20
9 (dark control)	536		44.7	19.46	20.20
10 (temp. control)	536		44.7	19.46	20.20

 

 

Validity criteria fulfilled:

yes

Conclusions:

In a photodegradation study performed in accordance with EPA 161-2, the photolytic half-life for the test substance was calculated to be 10.25 days.

Executive summary:

The direct photodegradation of the test substance was determined in a study that was performed according to EPA guideline 161-2, in compliance with GLP criteria.In this study, 14C-difluorophenyl ring-labelled test substance was dissolved in acetonitrile then diluted with sterile 0.01M phosphate buffer to give a test solution at a concentration of 0.0514 mg/mL. The co-solvent concentration in the test solution was 0.1% by volume. The test solutions were filled into 15-mL reaction vessels equipped with a cooling system to maintain a temperature of 25°C. All vessels were connected in line to a trap to collect volatiles. Irradiation was performed continuously with a xenon arc equipped with filters to cut off light with wavelengths < 290 nm. The spectral energy distribution of the light source and that of natural sunlight at 300 - 700 nm and latitude 50°N (spring, midday) were measured. Dark non-irradiated solutions were run simultaneously to check for hydrolytic degradation. At intervals up to 22.3 days, vessels were removed and analysed. Both organic and aqueous phases were quantified by LSC. The organic phase was concentrated to dryness, dissolved in acetonitrile and analysed by HPLC using a radio detector and a UV detector. To identify the degradates, the radioactivity in a number of samples was combined and the volume reduced to near dryness. The identity of the major metabolite was confirmed by GC/MS.

The material balance for the entire experiments was between 93.8% and 108.1%. The photolytic breakdown of the test substance led to formation of M7 (the major degradate) and four unknown products. Characterisation by HPLC showed that unknown metabolite 2 contained at least two products neither of which was present at > 5.2%. An attempt was made to identify the unknown metabolite 1 by repeating the experiment at higher concentrations however; even when the co-solvent was increased to 10% v/v, the low aqueous solubility of the test substance caused it to be adsorbed to the reaction vessel. Therefore, no further attempts were undertaken to isolate and identify these components. Overall, the compound showed a half-life of 10.25 days continuous light equivalent to 20.5 calendar days or to 18 days of latitude 30° N summer sunlight. No significant degradation occurred in the dark control solution, i.e. the parent molecule amounted after 44 calendar days still to 99.05%.

Reference 2

Endpoint:

phototransformation in water

Type of information:

experimental study

Adequacy of study:

key study

Study period:

6 Apr 1992 to Jul 26 1993

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Study type:

direct photolysis

Qualifier:

according to guideline

Guideline:

EPA Guideline Subdivision N 161-2 (Photodegradation Studies in Water)

Version / remarks:

October 18, 1982

GLP compliance:

yes (incl. QA statement)

Radiolabelling:

yes

Analytical method:

high-performance liquid chromatography

mass spectrometry

other: Thin-layer chromatography (TLC)

Details on sampling:

Samples of one vessel each were taken for analysis directly after the treatment and after 7, 10, 15 , 18, 21, 24 and 28 days of exposure to continuos light. The latter time interval corresponded to 25.9 Florida sunlight equivalents as calculated from the initial spectral energy distribution measurements. The trapping solutions were taken for analysis and replaced by fresh solutions at all sampling times of the vessels.

Buffers:

The photolytic behaviour of the test substance was investigated in 15 mL aqueous solution buffered at pH 7 (0.01 M phosphate buffer) where the test compound is known to be hydrolytically stable. Since the starting concentration of the test substance was less than 10-4 M, the buffers ionic strength did not exceed 0.01 M at which concentration buffer effects are negligible. The buffer was sterilized by filtration.

Light source:

Xenon lamp

Light spectrum: wavelength in nm:

>= 290

Details on light source:

- Filters used: Filters to cut-off light of less than 290 nm wavelength
- Irradiation equipment: The spectral energy distribution (300 - 700 nm) of the light source, i.e. the incident light available at the water level in all vessels, and that of natural sunlight were recorded using a portable spectroradiometer. A holographic grating monochromator and a Teon cosine diffuser either fixed or in combination with a glass ber optic probe were used.

Details on test conditions:

TEST SYSTEM
- Type, material and volume of test apparatus/vessels: Cylindrical vessels (sample volume = 15 mL), constructed of borosilicate glass but covered with a quartz glass lid.
- Control group: A dark control solution was stored in the same type of vessels, incorporated in an identical gas flow system and maintained in darkness at 25 °C.
- Details on temperature control: A thermocouple was fitted to one vessel for temperature control. The vessels were irradiated in sockets of a water cooled steel tank (25 °C) placed within the photolysis apparatus.
- Details of traps for volatile: All vessels were fitted with inlet and outlet ports for collection of volatiles. For the collection of volatiles all vessels were connected in line and ventilated with an air flow of about 5 mL/mninute controlled by a peristaltic pump. Incoming air was passed through a 2 N NaOH trap in order to remove carbon dioxide and a water trap for humidication. The outlet air passed three traps for absorbing volatiles in the sequence 0.1 N H2SO4, ethylene glycol and 2 N NaOH.
- Sterilisation method: Microbial infection of the solutions during application of a.i. and by ventilation of the system was avoided by adequate procedures and equipment. Subsamples of the final test solution were tested for possible microbial contamination at the end of the experiment. The test was performed by using a commercially available agar carrier for the determination of the total bacterial count.

TEST MEDIUM
- Preparation of test medium: The 14C-labelled test substance was dissolved in 1.0 mL acetonitrile and thereafter its accurate amount determined by liqid scintillation counting to be 0.5578 mg. From this stock solution aliquots of 0.015 mL were added to 15 mL of buffer solution thus giving a final concentraion of the test compound of 0.052 mg/mL. The cosolvent concentration in the aqueous system was limited to 0.1 % by volume.

REPLICATION
- No. of replicates (dark): 1
- No. of replicates (irradiated): 9 treated replicates and one contained untreated buffer only and served as temperature control.

Duration:

28 d

Temp.:

25 °C

Initial conc. measured:

0.052 g/L

Reference substance:

not specified

Dark controls:

yes

% Degr.:

78

Sampling time:

28 d

Test condition:

Irradiated

% Degr.:

1.3

Sampling time:

28 d

Test condition:

Non-irradiated

Key result

DT50:

16 d

Test condition:

irradiated

Remarks on result:

other: continuous irradiation, equivalent to 34 and 33 days during summer at latitudes 30° N and 40° N, respectively.

Transformation products:

not specified

Remarks:

M6

Details on results:

An overview of the results is provided in Table 1 in 'Any other information on results incl. tables'

The material balance for the entire experiment was between 88.6% and 99.3%. The photolytic breakdown of the test substance led to formation of the "phenylamine" M6 and six unknown polar products. None of these products was the corresponding "phenylurea" M4, or any other known metabolite. Characterisation by TLC showed that unknow metabolite 3A contained at least five products none of which were present at> 8.2% (0.004 mg/L). An attempt was made to identify the unknown products by repeating the experiment at higher concentrations however; even when the co-solvent was increased to 10% v/v, the test substance was adsorbed to the reaction vessel. Therefore, no further attempts were undertaken to isolate and identify these components. The measured energy of sunlight was used to calibrate the Suntest burner i.e. the ratio between the measured energy of midday sunlight at 50°N (13.43 W/m2) and the energy output of the lamp (7.89 W/m2) used to convert study exposure hours to natural daylight hours.
Application of first order reaction kinetics gave a half-life of 16 days under continuous irradiation (i.e.32 calendar days), which is equivalent to 33-34 calendar days during summer at latitudes 30° N and 40° N.

Characterization and Identication of Degradates
Besides the parent molecule accounting at the end of the study for 22.02 % two major radioactive fractions (M6 and UM3A) were isolated by column chromatography. Their corresponding amounts after 28 days of exposure were 16.28 and 28.95 % of the radioactivity applied corresponding to 0.0085 and 0.0151 ppm only. In addition mainly one minor degradate (UM4A) was detected representing at the end of the study 5.51 % or 0.0029 ppm. While the parent molecule and degradate M6 partitioned into the dichloromethane phase degradate fraction UM3A and UM4A remained in the aqueous phase of the photolysis solution.
- M6: M6 is identified by co-chromatography on HPLC and TLC The identity of this degradate was proven by mass spectromety in earlier studies.
- UM3A: Unknown Metabolite fraction UM3A was found to be very polar. It was eluted from reversed phase column already after 2.5 minutes. On TLC in solvent system 2 radioactive fraction UM3A was splitted into five degradates designated as UM3A/a, UM3A/b, M3A/c, UM3A/d and UM3A/e. Their amounts were 8.29, 6.47, 5.81, 5.21 and 3.17 % of the radioactivity applied, respectively. These amounts corresponded to radioactivity levels of 0.004, 0.003, 0.003, 0.003 and 0.002 ppm. On HVE at pH 10 radioactive fraction UM3A was separated into five degradates designated as UM3A/1, UM3A/2, UM3A/3, UM3A/4 and UM3A/5 with amounts of 5.20, 5.40, 7.11, 4.98 and 6.24 %, respectively. These figures corresponded to 0.003, 0.003, 0.004, 0.003 and 0.003 ppm. Except for degradate UM3A/l all other proved to be acidic degradates i.e migrating versus the anode whereas at pH 2 no migration of the radioactivity was observed. Acylation with trichloroacetic acid resulted besides the original material (4.3%) in the formation of two major and one minor derivatives designated as UM3A-AC, UM3A-ACa and UM3A-ACb. HVE of the derivatives at pH 10 showed that none of them did migrate thus demonstrating the loss of their acidic character. In conclusion, fraction UM3A is considered not to be identical with any of the reference compounds and to consist of at least five degradates of about equal amounts none of them exceeding 0.004 ppm. However, for further identification their amounts were by far too low. All attempts to set up a photolysis experiment at a higher concentration even when the concentration of the co-solvent was increased up to 10% resulted in a complete adsorption of the practically water insoluble test compound within a few hours after starting the experiment.

Table 1. Degradation of 14C-dichlorophenyl labelled test substance in aqueous solution at pH 7

Time (days)	Percentage of Radioactivity Applied
	Parent substance	M6	UM3A*	UM4A	Unknown	CO2	Volatiles	Recovery
Irradiated Solutions
0	96.4	2.57	0	0	0.07	0	0	99.0
3	90.2	5.76	0	0	3.20	0.17	0.01	99.3
7	79.5	7.87	6.82	-	0	0.86	0.02	95.1
9.8	71.0	15.6	6.06	0	0	1.95	0.02	94.6
15	45.3	21.3	20.3	0	0	5.25	0.03	92.3
18	52.1	11.4	18.5	0	0	7.61	0.03	89.7
21	46.0	12.8	22.5	0	0	9.87	0.05	91.2
24	32.8	15.2	25.8	4.63	0	12.9	0.05	91.4
28	22.0	16.3	29.0	5.51	0	15.8	0.05	88.6
Non-Irradiated Solutions
28 days	98.7	0	0	0	0.73	0	0	99.4

*At least 5 components none> 8.3% AR

Validity criteria fulfilled:

yes

Conclusions:

In a photodegradation study performed in accordance with EPA 161-2, the photolytic half-life for the test substance was calculated to be 16 days.

Executive summary:

The direct photodegradation of the test substance was determined in a study that was performed according to EPA guideline 161-2, in compliance with GLP criteria. In this study,14C-dichlorophenyl ring-labelled test substance was dissolved in acetonitrile then diluted with sterile 0.01M phosphate buffer to give a test solution at a concentration of 0.052 mg/mL. The co-solvent concentration in the test solution was 0.1% by volume. Samples were taken after treatment and after 3, 7, 10, 15, 18, 21, 24 and 28 days of exposure to continuous light.

The test solutions were filled into 15-ml reaction vessels equipped with a cooling system to maintain a temperature of 25°C. All vessels were connected in line to a trap to collect volatiles. Irradiation was performed continuously with a xenon arc equipped with filters to cut off light with wavelengths < 290 nm. The spectral energy distribution of the light source and that of natural sunlight at 300-700 nm and latitude 50ºN (Basel, Switzerland spring, midday) were measured. Dark non-irradiated solutions were run simultaneously to check for hydrolytic degradation. At intervals up to 28 days, vessels were removed and analysed. The solution was transferred to a separatory funnel and partitioned with dichloromethane. Both organic and aqueous phases were quantified by LSC. The organic phase was concentrated to dryness, dissolved in acetonitrile and analysed by HPLC using a radio detector and a UV detector. The mean recovery for the various steps was 92 - 101%. To identify the degradates, the sample was injected on a semi-preparative HPLC and fractions collected. These fractions were analysed by TLC and the retention times of the isolates compared with authentic reference standards. Further characterisation was attempted by high voltage electrophoresis.

The material balance for the entire experiment was between 88.6% and 99.3%. The photolytic breakdown of the test substance led to formation of M6 and six unknown polar products. Characterisation by TLC showed that the unknown metabolite UM3A, a polar metabolite, contained at least five products none of which were present at > 8.2% (0.004 mg/L). An attempt was made to identify the unknown products by repeating the experiment at higher concentrations however; even when the co-solvent was increased to 10% v/v, the test substance was adsorbed to the reaction vessel. Therefore, no further attempts were undertaken to isolate and identify these components. The measured energy of sunlight was used to calibrate the Suntest burner i.e. the ratio between the measured energy of midday sunlight at 50ºN (13.43 W/m2) and the energy output of the lamp (7.89 W/m2) used to convert study exposure hours to natural daylight hours. Application of first order reaction kinetics gave a half-life of 16 days under continuous irradiation (i.e. 32 calendar days), which is equivalent to 34 and 33 days during summer at latitudes 30° N and 40° N, respectively.

Description of key information

All available data was assessed. The available two studies are both included as key study and the average of the effect values was used as key value in a weight of evidence approach.

Average DT50 = 13.1 days, irradiation with continuous light by Xenon arc lamp (wavelength >= 290 nm), 25 °C, EPA 161 -2, Ellgehausen 1994a and 1994b

Key value for chemical safety assessment

Half-life in water:

13.1 d

Additional information

Two direct photolysis studies with continuous irradiation (xenon arc; wavelengths > 290 nm) are available for this endpoint. Both of them are Reliability 1 studies, which followed EPA 161-2 guideline and met the GLP criteria. The average of the effect values (DT50 = 13.1 days) from these studies was used as key value in a weight of evidence approach.

In one of the studies (Ellgehausen 1994a), the 14C-difluorophenyl ring-labelled test substance at 0.0514 mg/L sterile 0.01M phosphate buffer was exposed to continuous light for 22.3 days at 25 ˚C. The test substance showed a half-life of 10.25 days (equivalent to 20.5 calendar days or to 18 days of latitude 30° N summer sunlight). In the second study (Ellgehausen 1994b), the 14C-dichlorophenyl ring-labelled test substance at 0.052 mg/L sterile 0.01M phosphate buffer was exposed to continuous light for 28 days. The test substance showed a half-life of 16 days (equivalent to 32 calendar days or to 34 and 33 days during summer at latitudes 30° N and 40° N, respectively).