The atmospheric delays, e.g. ionospheric delay and tropospheric delay, are the dominant error sources for GNSS, especially for the Precise Point Positioning (PPP). The common practice of eliminating the ionospheric delay is to form an ionosphere- free (IF) observable, which is a linear combination of observables on two frequencies such as GPS L1 and L2. As for the tropospheric delay, the dry component can be precisely corrected by empirical models, while the wet component is usually estimated as unknowns. However, the higher-order ionospheric (HOI) terms are not totally cancelled out in the (first-order) IF observable and as such, when not accounted for, they degrade the accuracy of other parameters. The impacts of HOI delay on the estimated station coordinates and clocks, as well as satellite orbits and clocks are well documented in literature. This paper investigates the effects of HOI terms on the estimated tropospheric parameters, i.e. zenith tropospheric wet delay (ZWD), north and east gradients. For this purpose observations from over 100 stations with good global coverage were used considering various geographic and geophysical conditions. Numerical experimental results show that, HOI effects have a significant impact on the estimated tropospheric parameters, and the influence is geographically and geophysically dependent. The maximum differences of ZWD estimates reach over 20 mm during active periods like solar storm and geomagnetic storm. In addition, the north gradients are more likely to be affected by the HOI compared with east gradients. In particular, the tropospheric gradient component is most affected for low latitude station during daytime.