Hi, can this detect ppb level concentrations? looking for atmospheric composition measurements.

The working condition of the probe is -20~50℃，RH15%~90%。
And the PCB is not waterproof.
This means you could exposed it under the sum, but the whole sensor is not waterproof, so the rain may damage the sensor.

I have an H2S and SO2 probe. Is it normal to have a 0 value of concentration level(ppm) if there is no gas subjected to the probe? And if I test the H2S probe to a SO2 gas, will the concentration value change or will it also be zero?

The H2 sensor's resolution is 1PPM=0.1PPB, so it could not detect the ppb level concentrations.