Optics and biogeochemistry, (the use & misuse of optical proxies)

Transcription

1 Optics and biogeochemistry, (the use & misuse of optical proxies) Emmanuel Boss, UMaine. Optical measurements: measurements of EM radiation from UV->IR. Why use optics? Provides ability to observe oceans with high spatial and temporal resolution: a. From space: global view of the ocean surface on a ~daily time scale. b. In situ: sub-meter and sub-second. Challenge: need to qualify the relationship between an optical properties and the parameter of interest.

2 Laundry list of optical proxies: Temperature NIR radiance. Nitrate, Sulphides UV absorption. DOM, Hydrocarbons fluorescence (UV-ex, VIS-em), absorption. PM, POC, C phyto attenuation, scattering, ocean color. Phytoplankton pigments fluorescence, absorption, ocean color. Particulate size tendencies or distribution spectrum of attenuation, near forward scattering, ocean color, spikes. Particulate composition (index of refraction) back-scattering to scattering ratio, degree of polarization. Particulate packing attenuation + near-forward scattering.

3 What is a proxy? What are the hallmark of a good proxy? What should YOU do before you decide to use a proxy? How do you estimate the uncertainties associated with a proxy? In this talk I will focus on some proxies for particulate concentration.

4 Direct and inverse approaches in optics: e.g. Given particles angular scattering well posed Angular scattering particles present Bohren and Huffman, 1987 ill posed What kind of approach is the use of a proxy?

5 What particles do we have in the ocean? Phytoplankton Variable in shape, size, pigment/cell and pigment composition. Variable in scattering and absorption properties

6 What particles do we have in the ocean? Non-algal particles: Organic and inorganic. Sand Silt clay Aggregates <- packaging Variable in scattering and absorption properties

7 Proxies for particulate mass Data set: Alliance of Coastal Technology (ACT) East and West coast sites, HI and Great lake. Optics is a standard method to measure turbidity, a primary determinant of water quality (e.g. ISO-7027). Model-PM /PM c(660) b b (700) b s (880) 5% 2% 1% 2% 50% 16% 9% 21% 95% 54% 36% 51% Why is there variance in the relationships? Boss et al., 2008

8 Chlorophyll fluorescence: Falkowski and Raven, 1998 F(Chl)=[chl] x a*(l ex ) x F Sackman et al., 2008, BGD

9 Chlorophyll as indicator of biomass: Phytoplankton are plastic: adjust intercellular chlorophyll content to cell requirements (e.g. Cullen 1982). Chl/C for a given light level -> specific growth-rate (Geider, 1987) Kitchen and Zaneveld (1990, JGR)

10 b bp as indicator of phytoplankton biomass: Martinez-Vicente et al., 2013

11 1 st order variability in optical properties is due to concentration. What is the range of changes in concentration? What else affects optical properties (2 nd order variability)? Composition (index of refraction) Size Shape Internal structure packaging

12 Mie theory tells us that the relationship between optical properties and mass is composition and size dependent: b bp /Volume b p /Volume D 3 1/D b sp /Mass All curves are resonant curves Highest sensitivity for micron sized particles Size of max response varies

13 c p (660) vs. mass: Theory: Observations: Stemmann & Boss, /-50% Hill et al., 2011

14 c p (660) vs. POC: +/-50% Cetinic et al., 2012

15 An aside: the b b enigma (or paradox): Based on Mie theory, backscattering should be dominated by inorganic particles and sub-micron particles (the least known of the bunch). Yet b bp correlates well with [chl] and POC (>0.7mm): Huot et al., 2008 Stramski et al., 2008

16 Possible explanation for the b b enigma: 1. Mie results are correct. However, all particles in the open ocean covary, hence the tight relationship. 2. Mie theory is not applicable. Organic particle actually backscatter more than we ascribe to them.

17 Comparing observation and modeling of phytoplankton cultures Whitmire et al., 2009

18 Composition index of refraction Zaneveld et al., 2002, OOXVI. Compiled from: Aas (1983) Carder et al. (1972) Carder et al. (1974) Babin et al., 2003

19 Mie theory: the b bp /b p is very sensitive to n and less so to the PSD: Twardowski et al., 2001

20 Observations: English Channel Jersey shore Boss et al., 2004 Loisel et al., 2007 Varies from: phytoplankton inorganic particles.

21 Internal structure: Suggest phytoplankton could contribute significantly to b bp Meyer, 1979, Dall Olmo et al., 2009 Kitchen and Zaneveld, 1992

22 Shape: Suggest phytoplankton could contribute significantly to b bp From Clavano et al.,2007

23 Packing 2 Latimer (1985) For marine aggregates size and solid fraction correlate. 4mm -points having size-f as in Maggi, 2007, or Khelifa and Hill, 2006.

24 Hypothesis: aggregation reduces dependence of mass proxies on size. Field manipulation: Experiment One LISST Pumped (red) and the other not (blue) Control both instruments deployed unpumped Slade et al.,2011

25 Effect on beam attenuation (2m depth at DMC) control On average, observed beam attenuation increases by 30% when aggregates are broken. Significantly smaller change than expected from Mie (x10 from 100->10mm).

26 Aggregate modeling : 4mm 2 Latimer (1985) For marine aggregates size and solid fraction correlate. -points having size-f as in Maggi, 2007, or Khelifa and Hill, 2006.

27 Theoretical calculations: monodispersion Aggregates Observed range Single grain Mass normalized beam attenuation for aggregates assuming a relationship between solid fraction and size as in Khelifa and Hill, 2006 (blue lines) and solid particles (red lines). Solid lines denote particles with n=1.05+i0.0001, dashed lines n= and dotted lines n= Each data point represent a population of particle all of a single size. Boss et al.,2009

28 Theoretical calculations: populations Observed range 0.2mm=<D<=200mm Mass normalized beam attenuation for populations of aggregates assuming a relationship between solid fraction and size as in Khelifa and Hill, 2006 (blue lines) and populations of solid particles (red lines) both as function of power-law exponent of the disaggregated particle populations. Solid lines denote particles with n=1.05+i0.0001, dashed lines n= and dotted lines n= Note: model is sensitive to size of primary particle, D max, F(D max ) and acceptance angle.

29 Laboratory experiment: aggregation Start with <D>~7mm clay Add salt Slade et al.,2011 As aggregate size changes from 7 70mm c p /mass and b bp /mass stay constant (<size> confirmed by microscopy).

30 Summary Optical proxies are extremely useful in oceanography as they provide relevant time and spatial scales not possible by biogeochemical measurements. One needs to make sure relationships are valid in one s environment (yes, take samples). Theory is critical to ground observations and understand observed dependencies. But, theory is an idealization

31 Questions? 愚者不問, 問者不愚 The fool does not ask, he who asks is no fool