Tranform it into a mol fraction. Multiply by the total pressure to get the moisture's partial pressure. Look for temperatures in tables of water/ice vapor pressures. I may be wrong, but my estimates for atmospheric conditions indicate a dew point of about -57[sup]o[/sup]C.
I get around -46[sup]0[/sup]C. First, I calculated the humidity ratio (4.17x10[sup]-5[/sup], mass of water vapor to the mass of dry air) and then read from psychrometric tables. Alternately, I checked by 25362's method and getting the same result.