Eddy-covariance flux measurements in an Italian deciduous forest using PTR-ToF-MS, PTR-QMS and FIS

Figures & data

Table 1. The time-frame 11–26.06.2013 where all three instruments were operating simultaneously. Isoprene VMR averages and standard deviations from all three instruments PTR-ToF-MS, PTR-QMS and FIS are shown in the table.

Isoprene VMR (ppb)	PTR-ToF-MS	PTR-QMS	FIS
Average VMR	1.47	1.05	1.25
Standard deviation VMR	1.84	1.05	1.10

Table 2. VOCs observed with the PTR-ToF-MS during the measurement campaign (measured with protonated masses and first number in table refer to m/z).

31.017 CH3O^+ formaldehyde	69.069 C5H9^+ isoprene
33.034 CH5O^+ methanol	71.054 C4H7O^+ MVK/MACR/croton-aldehyde
42.033 C2H4N^+ acetonitrile	73.034 C3H5O2^+ methylglyoxal
43.017 C2H3O^+ acetic acid (fragment)	73.071 C4H9O^+ methyl-ethyl-ketone/butanal
43.051 C3H7^+ typical fragment	79.051 C6H7^+ benzene
45.034 C2H5O^+ acetaldehyde	89.055 C4H9O2^+ unknown
47.052 C2H7O^+ ethanol	93.068 C7H9^+ toluene
57.049 C3H7O^+ acrolein (tentatively)	107.078 C8H11^+ C8-alkylbenzenes
59.050 C3H7O^+ acetone/propanal	121.097 C9H13^+ C9-alkylbenzenes
61.030 C2H5O2^+ acetic-acid/(methyl-formate)	137.145 C10H17^+ monoterpenes

Table 3. Averages, standard deviations (SD) as well as 90th and 10th percentiles of the mean diurnal (24 h) averages of volume mixing ratios (VMRs) and fluxes measured by PTR-TOF-MS. Data collected during the time-frame 11–26.06.2013. The standard deviations can be found in the table. In addition to the standard deviation, the following instrumental uncertainties of the absolute VMR value are also included: 10% for isoprene, 10% for acetone, 20% for acetaldehyde, 20% for methanol, 20% for MVK/MACR and 10% for monoterpenes.

Flux (nmol m^−2 s^−1)	Isoprene	Acetone	Acetaldehyde	Methanol	MVK/MACR	Monoterpenes
Average flux	5.20	0.13	0.10	0.85	0.06	0.08
SD	2.42	0.06	0.24	0.41	0.04	0.05
90th percentile	8.68	0.18	0.41	1.25	0.11	0.13
10th percentile	2.44	0.05	−0.23	0.22	0.02	0.02
VMR (in ppb)
Average VMR	1.07	3.21	1.59	6.22	0.42	0.10
SD	0.53	1.17	0.45	2.09	0.22	0.09
90th percentile	1.64	4.80	2.15	9.20	0.69	0.15
10th percentile	0.52	1.94	1.12	3.97	0.13	0.04

Table 4. The tropospheric lifetimes have been calculated for a series of VOCs by using their atmospheric reactions with the radicals OH and NO₃, with ozone and for their photolysis rate.

		Lifetimes
Compounds	OH^a	NO3^b	O3^c	Photolysis
Isoprene^d	1.4 h	1.6 h	17 h	No significant importance.
MVK^e	6.9 h	>77 days	47 h	No significant importance at ground level^l
MACR^f	4.8 h	14 days	8 days	No significant importance at ground level^l
α-pinene^g	2.6 h	0.2 h	2.5 h	No significant importance.
β-pinene^h	1.8 h	0.4 h	15 h	No significant importance.
Methanol^i	6 days	356 days	p)	No significant importance at ground level^m
Acetone^j	32 days	>4 years	p)	93 days°
Acetaldehyde^k	9 h	17 days	>43 years	8 days^o

^a[OH] = 2 × 10⁶ molecules cm⁻³ for a 12 h day-time average.

^b[NO₃] = 2.5 × 10⁸ molecules cm⁻³ (10 ppt) for a 12 h night-time average.

^c[O₃] = 1.23 × 10¹² molecules cm⁻³ (50 ppb, in June 2013 we measured about 50 ppb as 24 h daily-average just above the canopy) [11,43].

^dRate constants used k_OH+isoprene = 1.0 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, k_NO3+isoprene = 7.0 × 10⁻¹³ cm³ molecule⁻¹ s⁻¹, k_O3+isoprene = 1.3 × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹ [44].

^eRate constants used k_OH+MVK = 2.0 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, k_NO3+MVK <6 × 10⁻¹⁶ cm³ molecule⁻¹ s⁻¹, k_O3+MVK = 5.2 × 10⁻¹⁸ cm³ molecule⁻¹ s⁻¹ [44].

^fRate constants used k_OH+MACR = 2.9 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, k_NO3+MACR = 3.4 × 10⁻¹⁵ cm³ molecule⁻¹ s⁻¹, k_O3+MACR = 1.2 × 10⁻¹⁸ cm³ molecule⁻¹ s⁻¹ [44].

^gRate constants used k_OH+α-pinene = 5.3 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, k_{NO3+α-pinene} = 6.2 × 10⁻¹² cm³ molecule⁻¹ s⁻¹, k_O3+α-pinene = 9.0 × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹ [44].

^hRate constants used k_OH+β-pinene = 7.9 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, k_{NO3+β-pinene} = 2.5 × 10⁻¹² cm³ molecule⁻¹ s⁻¹, k_O3+β-pinene = 1.5 × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹ [45].

ⁱRate constants used k_OH+methanol = 9.0 × 10⁻¹³ cm³ molecule⁻¹ s⁻¹, k_NO3+methanol = 1.3 × 10⁻¹⁶ cm³ molecule⁻¹ s⁻¹ [44].

^jRate constants used k_OH+acetone = 1.8 × 10⁻¹³ cm³ molecule⁻¹ s⁻¹, k_NO3+acetone <3 × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹, k_O3+acetone = 9.0 × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹ [44].

^kRate constants used k_{OH+acetaldehyde} = 1.5 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ (298 K), k_{NO3+acetaldehyde} = 2.7 × 10⁻¹⁵ cm³ molecule⁻¹ s⁻¹, k_{O3+acetaldehyde} <6 × 10⁻²¹ cm³ molecule⁻¹ s⁻¹ [44,46].

^lNo significant importance at ground level compared to reaction with OH radicals; see also discussion in text [59].

^mNo significant importance at ground level, because methanol does not absorb ground level solar light.

ⁿ[47].

^o[64].

^pThe lifetime is very long, because these compounds are known not to react with ozone with a significant rate coefficient at ambient temperature.

Figure 1. Plots of the correlation between the three instruments for the measurement of volume mixing ratios (VMRs) of isoprene: PTR-QMS vs. PTR-ToF-MS (green, y = 1.13×); FIS vs. PTR-ToF-MS (blue, y = 1.22×); FIS vs. PTR-QMS (red, y = 0.92×). The regressions lines were calculated by orthogonal regression using the procedure described in Ref. [41]. The overall uncertainties for isoprene VMR that were measured with the two PTR-MS techniques are taken to be 10%, and the overall uncertainties for isoprene VMR measured with FIS are taken to be 15%.

Figure 2. Plots of the correlation between the three instruments for the measurements of fluxes of isoprene: PTR-QMS vs. PTR-ToF-MS (green, y = 1.14x); FIS vs. PTR-ToF-MS (blue, y = 1.22×); FIS vs. PTR-QMS (red, y = 1.03×). The regressions lines were calculated as for Figure 1. See Figure 5 for information about error estimate on the flux data from the three different instruments.

Figure 5. Isoprene fluxes measured with all three instruments during summer 2013. Random error estimates for FIS, PTR-ToF-MS and PTR-QMS flux data are shown as error bars and calculated for all three instruments as described in Section 2.4.5. The authors acknowledge the use of a similar figure in a conference paper/proceeding (see the ‘Acknowledgements’ section for more details).

Figure 3. Average diurnal variations of VOCs (PTR-ToF-MS measurements), temperature and short wavelength radiation during the period of the PTR-ToF-MS measurements (11–26.06.2013). Error bars show the standard deviation on the average of the hourly means of the fluxes. ‘Hour of day’ is in UTC.

Figure 4. Isoprene volume mixing ratios (VMRs) measured with all three instruments in June 2013. Overall uncertainties for isoprene VMR measured with the two PTR-MS techniques are taken to be 10%. Overall uncertainties for isoprene VMR measured with FIS are taken to be 15%.

Table 5. Summary of the results obtained in this investigation and the comparison with other studies. Fluxes and VMR values for isoprene, acetone, acetaldehyde, methanol, MVK/MACR and monoterpenes.

	Isoprene	Acetone	Acet-ald.	Methanol	MVK/MACR	Monoterp.	Ref.
Flux (nmol m^−2 s^−1)
Average flux	5.20	0.13	0.10	0.85	0.06	0.08	This work
	6.466	0.335	0.228	1.168	0.314	0.209	[24]
	11.294*				0.119*		[23]
	0.3–1.2	0.9–2.2	1.1–2.3	3.2–3.8		0.5–1.8	[65]
	0.025	−0.013	0.133	1.655	0.041	1.235	[22]
	30.950**						[66]
	*Only day-time.
	**Only emission potential (basal emission rate).
VMR (in ppbV)
Average VMR	1.07	3.21	1.59	6.22	0.42	0.10	This work
	1.19				0.21		[23]
	0.10–0.14	1.0–2.1	0.4–1.3	1.6–3.5		0.1–0.3	[65]

Figure A1a. Temperature at the tower top in June 2013 during the measurement campaign.

Figure A1b. Relative humidity at the tower top in June 2013 during the measurement campaign.

Figure A1c. Wind speed at the tower top in June 2013 during the measurement campaign.

Figure A1d. Solar radiation at the tower top in June 2013 during the measurement campaign.

Figure A2. Solar global irradiation, precipitation amount, and temperature monthly means observed at the EMEP/GAW measuring station in the JRC-Ispra in 2013, compared to the 1990–1999 period ± standard deviations (the Figure is from the ref. EUR report [15]).

Table A3. Multi-component calibration mix. (an 11-component VOC standard mixture from Apel-Riemer Environmental Inc., Denver, USA). Uncertainty is a conservative estimate of the combination of the uncertainties of the gravimetric preparation and analysis.

Compounds	Conc./VMR (ppb)	Uncertainties
Acetaldehyde	1084	±5%
Methanol	1015	±5%
Isoprene	1040	±5%
Acetone	1003	±5%
Acetonitrile	994	±5%
Methyl Ethyl Ketone	1001	±5%
Benzene	1045	±5%
Toluene	1023	±5%
m-Xylene	980	±5%
alfa-Pinene	1008	±5%
1,3,5-Trimethylbenzene	961	±5%

Table A4. Detection limits for the PTR-ToF-MS: Exemplary results from a PTR-ToF-MS calibration performed on 06.06.2013 for all of the VOCs measured in this investigation is shown here. ‘ncps’ is an abbreviation for “normalised counts per second (normalised to 10⁶ cps of (H₂O)H⁺ and (H₂O)₂H⁺ ions). Limits of detection (LOD, 2 s) and precision (1 s) values were calculated for a 0.1 s signal integration time.

		Sensitivity		Precision at
Compounds	m/z	(ncps ppbV^−1)	LOD (ppbV)	About 20 ppbV
Methanol	33.034	11.1	1.36	2.86
Acetonitrile	42.0344	20.1	0.33	1.27
Acetaldehyde	45.034	13.9	1.04	2.44
Acetone	59.0497	18.1	0.34	1.26
Isoprene	69.0704	8.4	0.43	1.98
2-Butanone	73.0653	15.2	0.32	1.99
Benzene	79.0548	10.0	0.29	1.70
Toluene	93.0704	11.2	0.26	1.58
m-Xylene	107.0861	12.2	0.20	1.47
1,3,5-Trimethylbenzene	121.1017	12.5	0.22	1.84
Alfa-pinene	137.133	5.2	0.61	2.92

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