**2.1 In-situ observations**

Generally, the absorption coefficient (ag(λ), m<sup>−</sup><sup>1</sup> ) is a direct measure of CDOM absorptivity. The spectrum of ag usually spans from ultraviolet band to red band. The spectral shape is unstructured and typically decreases with increasing wavelength in an exponential fashion (Eq. (1)) [18–23].

$$\mathbf{a}\_{\mathbb{g}}(\lambda) = \mathbf{a}\_{\mathbb{g}}(\lambda\_0) \exp\left[-\mathsf{S}\_{\mathbb{g}}(\lambda - \lambda\_0)\right],\tag{1}$$

where λ0 is the reference wavelength, and Sg is the spectral slope, describing the decreasing rate of the spectrum.

The absorptivity of CDOM is first measured on a spectrophotometer as absorbance (A(λ)), a unit-less ratio of spectral radiant power transmitted through the sample across a path length (L) [24]. A(λ) is then converted to the (Napierian) absorption coefficient (ag(λ)) according to Eq. (2):

$$\mathbf{a}\_{\mathbb{B}}(\lambda) = 2.303 \mathbf{A}(\lambda) / \text{L}.\tag{2}$$

Typically, measurements are performed using a long-path length quartz cuvette (e.g., 0.1 m) or a liquid core waveguide (0.5–5 m) with submicron-filtered (0.2 or ~0.7 μm) seawater. In low-CDOM waters, measurement of the absorption spectrum of CDOM using a conventional spectrophotometer is challenging. Due to the exponential decline of absorption with increasing wavelength, absorbance values can fall below the detection threshold of the detectors used, typically 0.03–0.06 m<sup>−</sup><sup>1</sup> [25]. Devices with longer effective path lengths, such as reflective tube absorption meters (up to 25 cm [26]), liquid waveguide cells (up to 200 cm [27, 28]), and integrating cavity meters (up to 25 m [29]) attempt to resolve this problem.

A cruise was implemented in the PRE previously from 2 to 12 May 2014. A total of 148 water samples were collected (**Figure 2**) from the water surface (with sampling depth of 0 and 1 m) and the bottom (with sampling depth 1 m above the bottom). CDOM absorbance (A(λ)) was measured in a 10-cm quartz cell using a Shimadzu UV-2550 spectrophotometer. The absorption coefficient (ag(λ)) is calculated at 1 nm interval from 250 to 700 nm according to Eq. (2). Spectral slope, Sg, over wavelength range of UV (250–400 nm), noted as Sg(250–400), is used as a

**Figure 2.** *Location of the sampling stations in the May 2014 cruise in the Pearl River Estuary.*

representative of CDOM spectral slope when analyzing the spatial heterogeneity of CDOM [30].

In addition to the spectroscopic analysis of CDOM absorption, in-situ measurements of remote sensing reflectance in visible range (400–900 nm) are also conducted.
