Dataset: GP15 Particle birefringence photon yield and particle beam attenuation coefficient from GTC CTD
Deployment: RR1814

The two particulate inorganic carbon (PIC) sensors (PIC010 and PIC011) used in this study have been extensively documented (Bishop et al. 2022). Briefly, the sensors are built on a digital WETLabs C-star 25-centimeter (cm) pathlength 6000-meter (m) rated transmissometer. A 660-nanometer (nm) laser replaced the transmissometer’s LED light source. High crossing efficiency polarizers were externally mounted to both source and receiver windows; the source polarizer is aligned with the plane of polarization of the laser and the receiver polarizer is crossed to minimize transmission of the direct beam. As light from the primary beam encounters birefringent particles, its plane of polarization is rotated and the sensor receives a signal. Voltage signals recorded by a CTD arise from four sources: (a) dark current, (b) polarizer crossing blank, (c) stray light, and (d) birefringence (beta) (Equation 1):

V_beta = V_meas - V_dark - Vc_ross - V_stray (1)

where V_meas is the raw signal from the CTD, V_dark is the reading with the beam blocked (0.007V), V_cross is the primary beam signal that is detected with no particles in the beam (~0.05V), and V_stray is light added to the beam by reflections. V_stray is assumed negligible as the primary beam is columnated and the detector receiver angle is small. In our data reduction scheme, we calculate instrument temperature, and rate of change of instrument temperature per minute. The full expression for calculation of birefringent photon yield is given by a reformulation of Equation 1:

V_{beta_corr = ((}V_meas - V_dark - V_cross•Tr)/R - V_drift - V_transient)/Tr^0.5 (2).

V_pradj = press • coefft_press (3). and V_{beta_corr_final} = V_{beta_corr} + V_pradj (4) and Beta_corr = V_{beta_corr_final} • SF ppm m^-1 (5).

Tr, is transmission measured by C-Star transmissometer (660 nm) over its 25 cm path length; As the crossing blank (V_cross) is a transmitted light signal, Tr compensates for attenuation of the crossing blank due to particles. The term, R, is the static thermal response correction calculated using instrument temperature (Bishop et al. 2022). The term V_drift is a small compensation for sensor drift during McLane pumping between the time of down and up casts (often <1 mV), V_transient is derived from thermal cycling experiment data. The term, Tr^0.5, is from Guay and Bishop (2002) and compensates for attenuation of the birefringent photon signal resulting from scattering and absorption effects of other particles in the beam. Analog voltage data (0-5 V) from these sensors is converted to physical units of ppm m^-1 (Equation 5) using scaling Factors (SF) of 448.4 ppm V^-1m^-1 and 644.7 ppm V^-1 m^-1 for PIC010 and PIC011, respectively (Bishop et al., 2022). The transmittance based corrections were no more than 20% of β_corr in surface waters and became negligible in waters below the euphotic zone.

Beta_corr is converted to PIC (nM) using by multiplying by a scale factor (SF) of 15 (Bishop et al., 2022). In this paper we describe this quantity as “birefringence PIC” or “PIC_β”. Pressure coefficients were derived using a best fit of PIC_β and McLane pump measured PIC in the mid water column (2000 m to 4000 m). These adjustments had minimal effect in the upper 500 m.

During GP15, Transmissometer CST1450 was the standard for beam attenuation coefficient (c_p) for the entire section as it was both stable and air calibrated prior to each deployment; the other transmissometers were adjusted to this standard.

Transmissometer beam attenuation coefficient calculation:

This method differs from standard procedure as it addresses temperature dependent hysteresis seen in transmissometer profiles.

cst1450_1=(cst1450-cst1450z)/cst1450r, where cst1450 = CTD measured voltage, cst1450z = the blocked beam voltage, and cst1450r is the temperature response function for the instrument

cst1450_2=cst1450_1-cum1450dr-cst1450t*0.3, where cum1450dr is voltage drift during cast, and cst1450t is a correction due to thermal hysteresis.>

tr=cst1450_2/CST1450_NetVref, Where CST1450_NetVref is Voltage the instrument reads in particle free water.

cp1450=-4*ln(tr), cp1450 is the beam attenuation coefficient calculated for this instrument.

the various quanities, CST1450_NetVref, cst1450z, cst1450r, cum1450dr, cst1450t are included in the data sets and defined in the parameter list