Bubble fluxes of methane from small ponds. ZHAO Jiayu and WANG Jiao

Transcription

1 Bubble fluxes of methane from small ponds ZHAO Jiayu and WANG Jiao /26/2018 1

2 Outline 1 Background Information 2 Experimental Design 3 Results and Discussion 4 Conclusions 2 7/26/2018

3 (Bastviken et al., 2004) Small ponds (< km 2 ) only comprise 8.6% of total area of global water distribution, but account for 15.1% for CO 2 emission and 40.6% of diffusive CH 4 emissions. (Holgerson et al., 2016) Uncertainty in estimated values: Ebullition Analysis Our goal: Quantifying CH 4 ebullition fluxes contribution from small ponds. 7/26/2018 3

4 Outline 1 Background Information 2 Experimental Design 3 Results and Discussion 4 Conclusions 4 7/26/2018

5 2.1 Study Site Latitude, Longitude Aquaculture ponds Species cultured Culture mode o N, o E 1311 km 2 (1982, second,14%) 60% (the four major chinese carps, 2218 km 2 (2017, fifth, 8%) clam, cray) Similar to Yangtze River delta Observation pond 1 Observation pond 2 5

6 2.2 Observation Method---- Floating chamber method (FC) 2017 Summer experiment 2017 Autumn experiment 75 L, 0.25 m 2 50 L, 0.13 m Winter experiment 2018 Spring experiment 50 L, 0.13 m 2 50 L, 0.13 m /26/2018 6

7 Half- hour total fluxes dc h F = p = p K dt V h m V m F : fluxes of CH 4 (μg m -2 s -1 ) p : unit conversion constant, 16/0.06 for CH 4 c : gas concentration (CH 4 : ppm) t : time (min) K : CH 4 concentration varies with time, the slope Half- hour ebullition ratio/flux Manually find all ebullition signal (1 Hz) ebullition flux Ebullition ratio= Total flux ( D - E) difference h Ebullition flux = Δt V The diffusion slope use weighted average D m h : the height of chamber ( summer: 0.25 m ; autumn: 0.33m ) V m : the molar volume for ideal gases (22.4 L/mol) A B C E

8 2.2 Observation Method---- Inverted Funnel and water equilibrium Method Ebullition flux (IF): F CCH4 V M A t V m (Wik et al., 2013) Diffusive flux (WE): F k C w C eq 7/26/2018 8

9 2.2 Observation Method---- Inverted Funnel and water equilibrium Method Observation period: 2016 summer: spring: summer: autumn: winter: spring: Water depth of different points: A pond: A 5 > A 1 > A 2 > A 3 A 4 B pond: B 5 > B 4 > B 2 > B 3 > B 1 Red five-pointed star: FC observation sites 7/26/2018 9

10 Outline 1 Background Information 2 Experimental Design 3 Results and Discussion 3.1 CH 4 ebullition concentrations 3.2 CH 4 ebullition rate 3.3 Ebullition volume 3.4 CH 4 ebullition flux 4 Conclusions 10 7/26/2018

11 3.1 Distribution of CH4 Ebullition concentration difference Summer: Autumn: Winter: Spring: /26/2018 Minimum value ppm Maximum value ppm %5 limit value 0.07 ppm (0.001 mg) Precision of UGGA ppm 11

12 3.2 Time series of CH 4 Ebullition rate Summer experiment-2017 Autumn experiment-2017 Winter experiment-2018 Spring experiment-2018 Season Summer Autumn Winter Spring Ebullition rate / bubbles m -2 hr This study (small ponds) \ (Goodrich et al., 2011) (peatland, 43 o 12.5 N, 71 o 03.5 W) 7/26/

13 3.3 Ebullition volume Ebullition volume = Total Ebullition volume ( IF) Ebullition rate Time Funnel area (FC) Mean value: 1.3 ml (Ostrovsky et al., 2008) 7/26/

14 ratio F m / mg m -2 s -1 F m / mg m -2 s -1 F m / mg m -2 s -1 Ratio Ratio Ratio 3.4 Ebullition flux Time series of Ebullition flux and ratio Summer:3.19 (±7.51) 2017 Autumn:0.19 (±0.29) Ebullition flux Ratio % 80% 60% 40% 20% 0 F m / mg m -2 s % 80% 60% 40% 20% % 80% 60% 40% 20% Winter:0.44 (±2.26) 2018 Spring: 0.27 (±0.33) Total events:670 half-hourly Ebullition events: 289 (43%) % 80% 60% 40% 20% 7/26/

15 F m / mg m -2 s Ebullition flux Time series of Ebullition flux and ratio % Summer 76% Autumn Total flux Ebullition flux Ebullition ratio:75% Season 79% 71% Winter Spring Red dot: our experimental results (Bastviken et al., 2004) 7/26/

16 3.4.2 Floating chamber vs Inverted funnel Season Method Ebullition flux ratio 2017 summer 2017 autumn 2018 winter 2018 spring IF 5.94 (±4.27) 99% FC 5.38 (±6.59) 73% IF (±0.009) 91% FC 0.24 (±0.23) 76% IF 0.29 (±0.64) 47% FC 0.76 (±1.64) 79% IF (±0.005) 58% FC 0.20 (±0.20) 71% y = 0.84 x (r = 0.73, p <0.001, n = 33) (T-test, α = 0.05) Where x represents ebullition fluxes ---- FC FC and IF methods agreed reasonably well, given 97 98% of the CH 4 measured flux resulted from bubble emissions. (Keller and Stallard, 1994) 7/26/

17 3.4.3 Compared with other literature results FC IF Data Source: (6 ponds, 38 lakes) Red dot: this study 7/26/

18 3.4.4 Environmental factors vs CH 4 ebullition flux Water temperature (daily) E = (T-20) (r = 0.56, p <0.001, n = 75) E T = T -20 E20 θs 7/26/

19 E 20 =1.63 θ s =1.19 E 20 =3.09 θ s =1.15 E 20 = 0.68 θ s =1.20 E 20 = 0.65 θ s =1.11 E 20 = 4.31 θ s =1.12 E 20 = 0.97 θ s =1.15 E 20 = θ s =1.09 E 20 = θ s =1.08 (Aben et al., 2017) 7/26/

20 3.4.4 Environmental factors vs CH 4 ebullition Rate Water depth y = x (r = -0.53, p <0.001, n = 31) 7/26/

21 According to our experimental results, ebullition was the main pathway of CH 4 emission from small ponds, the average value of ebullition flux and ebullition ratio measured by floating chamber method was 1.02 μg m -2 s -1 and 75%, respectively. Water temperature have significant correlation with the half-hourly CH 4 ebullition fluxes. But the relationship between CH 4 ebullition flux and water depth is not yet clear. 7/26/

22 7/26/