To understand how this question, you need to know how to solve this. 1 atm is equal to 760mm Hg. This means 735x 1/760=0.967 atm. It would be easy to solve this already as long as you know the right equation to use. Measuring atmospheric pressure is essential because this will have some effects on water chemistry as well as the different weather conditions that may be experienced. Various gases may dissolve more in higher atmospheric pressures as compared to low atmospheric pressures. Do remember that air temperature usually decreases in more upper atmospheres.