# Chlorophyll a (chlor_a)

## Table of Contents

## 1 - Product Summary

This algorithm returns the near-surface concentration of chlorophyll-a (chlor_a) in
mg m^{-3}, calculated using an empirical relationship derived from in situ measurements of `chlor_a` and remote sensing reflectances ($R_{rs}$) in the blue-to-green region of the visible spectrum. The implementation is contingent on the availability three or more sensor bands spanning the 440 - 670 nm spectral regime. The algorithm is applicable to all current ocean color sensors. The `chlor_a` product is included as part of the standard Level-2 OC product suite and the Level-3 CHL product suite.

The current implementation for the default chlorophyll algorithm (chlor_a) employs the standard OC3/OC4 (OCx) band ratio algorithm merged with the color index (CI) of Hu et al. (2012). As described in that paper, this refinement is restricted to relatively clear water, and the general impact is to reduce artifacts and biases in clear-water chlorophyll retrievals due to residual glint, stray light, atmospheric correction errors, and white or spectrally-linear bias errors in $R_{rs}$. As implemented, the algorithm diverges slightly from what was published in Hu et al. (2012) in that the transition between CI and OCx now occurs at 0.15 < CI < 0.2 mg/m$^3$ to ensure a smooth transition.

Algorithm Point of Contact: P. Jeremy Werdell, NASA Goddard Space Flight Center

## 2 - Algorithm Description

##### Inputs:

$R_{rs}$ at 2-4 wavelengths between 440 and 670nm

##### Outputs:

`chlor_a`, concentration of chlorophyll *a* in mg/m^{-3}

##### Approach:

The `chlor_a` product combines two algorithms, the O'Reilly band ratio OCx (e.g. chl_oc4) algorithm and the Hu color index (CI) algorithm (chl_hu).

The CI algorithm is a three-band reflectance difference algorithm employing the difference between $R_{rs}$ in the green band and a reference formed linearly between $R_{rs}$ in the blue and red bands.

$CI = R_{rs}(\lambda_{green}) - [R_{rs}(\lambda_{blue}) + (\lambda_{green}-\lambda_{blue)}/(\lambda_{red}-\lambda_{blue}) * (R_{rs}(\lambda_{red})-R_{rs}(\lambda_{blue}))]$

where $\lambda_{blue}$, $\lambda_{green}$ and $\lambda_{red}$ are the instrument-specific wavelengths closest to 443, 555 and 670nm respectively.

The OCx algorithm is a fourth-order polynomial relationship between a ratio of $R_{rs}$ and `chlor_a`.

$log_{10}(chlor\_a) = a_0 + \sum\limits_{i=1}^4 a_i \left(log_{10}\left(\frac{R_{rs}(\lambda_{blue})}{R_{rs}(\lambda_{green})}\right)\right)^i$,

where the numerator, $R_{rs}(\lambda_{blue})$, is the greatest of several input $R_{rs}$ values and the coefficients,
a_{0}-a_{4}, are sensor-specific:

sensor | default * | blue | green | a0 | a1 | a2 | a3 | a4 | |
---|---|---|---|---|---|---|---|---|---|

OC4 | SeaWiFS | Y | 443>490>510 | 555 | 0.3272 | -2.9940 | 2.7218 | -1.2259 | -0.5683 |

OC4E | MERIS | Y | 443>490>510 | 560 | 0.3255 | -2.7677 | 2.4409 | -1.1288 | -0.4990 |

OC4O | OCTS | Y | 443>490>516 | 565 | 0.3325 | -2.8278 | 3.0939 | -2.0917 | -0.0257 |

OC3S | SeaWiFS | N | 443>490 | 555 | 0.2515 | -2.3798 | 1.5823 | -0.6372 | -0.5692 |

OC3M | MODIS | Y | 443>488 | 547 | 0.2424 | -2.7423 | 1.8017 | 0.0015 | -1.2280 |

OC3V | VIIRS | Y | 443>486 | 550 | 0.2228 | -2.4683 | 1.5867 | -0.4275 | -0.7768 |

OC3E | MERIS | N | 443>490 | 560 | 0.2521 | -2.2146 | 1.5193 | -0.7702 | -0.4291 |

OC3O | OCTS | N | 443>490 | 565 | 0.2399 | -2.0825 | 1.6126 | -1.0848 | -0.2083 |

OC3C | CZCS | Y | 443>520 | 550 | 0.3330 | -4.3770 | 7.6267 | -7.1457 | 1.6673 |

OC2S | SeaWiFS | N | 490 | 555 | 0.2511 | -2.0853 | 1.5035 | -3.1747 | 0.3383 |

OC2E | MERIS | N | 490 | 560 | 0.2389 | -1.9369 | 1.7627 | -3.0777 | -0.1054 |

OC2O | OCTS | N | 490 | 565 | 0.2236 | -1.8296 | 1.9094 | -2.9481 | -0.1718 |

OC2M | MODIS | N | 488 | 547 | 0.2500 | -2.4752 | 1.4061 | -2.8233 | 0.5405 |

OC2M-HI | MODIS (500-m) | Y | 469 | 555 | 0.1464 | -1.7953 | 0.9718 | -0.8319 | -0.8073 |

OC2 | OLI/Landsat 8 | 482 | 561 | 0.1977 | -1.8117 | 1.9743 | -2.5635 | -0.7218 | |

OC3 | OLI/Landsat 8 | 443>482 | 561 | 0.2412 | -2.0546 | 1.1776 | -0.5538 | -0.4570 |

* Y indicates operational / default algorithm for each sensor within OBPG processing

The coefficients were derived using version 2 of the NASA bio-Optical Marine Algorithm Data set (NOMAD).

For chlorophyll retrievals below 0.15 mg m^{-3}, the CI algorithm is used.

For chlorophyll retrievals above 0.2 mg m^{-3}, the OCx algorithm is used.

In between these values, the CI and OCx algorithm are blended using a weighted approach.

## 3 - Implementation

Product Short Name | chlor_a |

Level-2 Product Suite | OC |

Level-3 Product Suite | CHL |

Level-3 Masking | ATMFAIL, LAND, HILT, HISATZEN, STRAYLIGHT, CLDICE, COCCOLITH, LOWLW, CHLWARN, CHLFAIL, NAVWARN, MAXAERITER, ATMWARN, HISOLZEN, NAVFAIL, FILTER, HIGLINT |

For further details on the implementation, go to the algorithm source code or the graphical description of the algorithm implementation in the NASA ocean color processing code (l2gen).

##### Calling in L2GEN

```
```
- l2prod = chlor_a (refers to sensor-specific default CI/OCx blended algorithm)
- l2prod = chl_oc4, chl_oc3, chl_oc2, chl_hu
- each satellite will use its sensor-specific coefficients and wavelengths (e.g., SeaWiFS
defaults to OC4, OC3S, OC2S)
- to override the coefficients:
- chloc4_coef = [a0,a1,a2,a3,a4]
- chloc3_coef = [a0,a1,a2,a3,a4]
- chloc2_coef = [a0,a1,a2,a3,a4]

- to override the wavelengths:
- chloc4_wave = [array of 4 wavelengths, last will be denominator]
- chloc3_wave = [array of 3 wavelengths, last will be denominator]
- chloc2_wave = [numerator wavelength, denominator wavelength]

## 4 - Assessment

Level-2 satellite-to-in-situ match-up validation results are available for each sensor from the validation tool of the SeaWiFS Bio-Optical Archive and Storage System (SeaBASS). Links to those match-ups are provided below.

#### Algorithm Development:

OC4 | OC3M | OC4E | OC3V |
---|---|---|---|

OC3S | OC4O | OC3E | OC3O |
---|---|---|---|

OC3C | OC2M | OC2S | OC2M-HI |
---|---|---|---|

#### Algorithm Verification: internal consistency between operational algorithms

#### Algorithm Verification: comparison with previous versions

OC4 v4 and v5 correspond with SeaWiFS Reprocessings 3 (2000) and a preliminary NOMAD version, respectively.

#### Algorithm Verification: comparison with (Morel and Maritorena 2001)

## 5 - References

Hu, C., Lee, Z., & Franz, B. (2012). Chlorophyll a algorithms for oligotrophic oceans: A novel approach based on three-band reflectance difference . Journal of Geophysical Research, 117(C1). doi: 10.1029/2011jc007395

Morel, A., & Maritorena, S. (2001). Bio-optical properties of oceanic waters: A reappraisal. Journal of Geophysical Research: Oceans, 106(C4), 7163-7180. doi: 10.1029/2000jc000319

O'Reilly, J.E., Maritorena, S.,Mitchell, B. G., Siegel, D. A., Carder, K. L., Garver, S. A., Kahru, M., & McClain, C. R. (1998). Ocean color chlorophyll algorithms for SeaWiFS, Journal of Geophysical Research 103, 24937-24953, doi: 10.1029/98JC02160.

O'Reilly, J.E., & 24 co-authors (2000). SeaWiFS Postlaunch Calibration and Validation Analyses, Part 3.
NASA Tech. Memo. 2000-206892,
Vol. 11*,* S.B. Hooker and E.R. Firestone, Eds., NASA Goddard Space Flight Center, 49 pp.

Werdell, P. J., & Bailey, S. W. (2005). An improved bio-optical data set for ocean color algorithm development and satellite data product validation. Remote Sensing of Environment 98, 122-140, doi: 10.1016/j.rse.2005.07.001.