Abstract. 

Carbonyl Sulfide (COS) is a useful tracer to estimate Gross Primary Production (GPP) because it shares part of the uptake pathway with CO₂. COS is taken up in plants through hydrolysis, catalyzed by the enzyme carbonic anhydrase (CA), but is not released. The Simple Biosphere model version 4 (SiB4) simulates COS leaf uptake using a conductance approach. SiB4 applies the temperature response of the RuBisCo enzyme (used for photosynthesis) to simulate the COS leaf uptake, but the CA enzyme might respond differently. We introduce a new temperature response function for CA in SiB4, based on enzyme kinetics with an optimum temperature. Moreover, we determine Ball-Berry model parameters for stomatal conductance (g_s) using observation-based estimates of COS flux, GPP, and g_s along with meteorological measurements in an evergreen needleleaf forest (ENF) and deciduous broadleaf forest (DBF). We find that CA has optimum temperatures of 22 °C (ENF) and 38 °C (DBF) with CA’s activation energy as 40 kJ mol^-1, which is lower than that of RuBisCo (45 °C), suggesting that air temperature changes can critically affect CA’s catalyzation activity. Optimized values for the Ball-Berry offset parameter b₀ (ENF: 0.013, DBF: 0.007 mol m^-2 s^-1) are higher (lower) than the original value (0.010 mol m^-2 s^-1) in the ENF (DBF), and optimized values for the Ball-Berry slope parameter b₁ (ENF: 16.36, DBF: 11.43) are higher than the original value (9.0) at both sites. We apply the optimized g_CA and g_s parameters in SiB4 site simulations, thereby improving the timing and peak of COS assimilation. In addition, we show that SiB4 underestimates the leaf humidity stress under conditions where high VPD should limit g_s in the afternoon, thereby overestimating g_s. Furthermore, we simulate global COS biosphere fluxes, which show smaller COS uptake in the tropics and larger COS uptake at higher latitudes, corresponding with the updates made to the CA temperature response. This SiB4 update helps resolve gaps in the COS budget identified in earlier studies. Using our optimization and additional observations of COS uptake over various climate and plant types, we expect further improvements in global COS biosphere flux estimates.