Tunable diode laser absorption spectroscopy has been widely employed for gas sensing, where the gas concentration is often obtained from the absorption signal with a known or a fixed absorption path length. Nevertheless, there are also numerous applications in which the absorption path length is very challenging to retrieve, e.g., open path remote sensing and gas absorption in scattering media. In this work, a new approach, based on the wavelength modulation spectroscopy (WMS), has been developed to measure the gas absorption signal and the corresponding absorption path length simultaneously. The phase angle of the first harmonic signal (1f phase angle) in the WMS technique is utilized for retrieving the absorption path length as well as the gas absorption signal. This approach has been experimentally validated by measuring carbon dioxide (CO2) concentration in open path environment. The CO2 concentration is evaluated by measuring the reflectance signal from a distant object with hundreds of meters away from the system. The measurement accuracy of the absorption path length, evaluated from a 7-day continuous measurement, can reach up to 1%. The promising result has shown a great potential of utilizing the 1f phase angle for gas concentration measurements, e.g., open path remote sensing applications.