Radio occultation (RO) can provide high vertical resolution thermodynamic soundings of the planetary boundary layer (PBL). However, sharp moisture gradients and strong temperature inversion lead to large refractivity () gradients, and often cause ducting. Ducting results in systematically negative RO -biases due to a non-unique Abel inversion problem. Using 8-year (2006-2013) Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) RO soundings and collocated European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERA-I) data, we confirm that the large lower tropospheric negative -biases are mainly located in the subtropical eastern oceans, and quantify the contribution of ducting for the first time. The ducting-contributed -biases in the northeast Pacific (160°W~110°W; 15°N~45°N) are isolated from other sources of -biases using a two-step geometric-optics simulation. Negative bending angle biases in this region are also observed in COSMIC RO soundings. Both the negative refractivity and bending angle biases from COSMIC soundings mainly lie below ~2-km. Such bending angle biases introduce additional -biases to those caused by ducting. Following the increasing PBL height from the southern California coast westward to Hawaii, centers of maxima bending angles and -biases tilt southwestward. In areas where ducting conditions prevail, ducting is the major cause of the RO -biases. Ducting-induced -biases with reference to ERA-I comprise over 70% of the total negative -biases near the California coast where strongest ducting conditions prevail, and decrease southwestward to less than 20% near Hawaii.