def ppm_to_pa(ppm, pres):
    """
    Convert ppm (umol mol^-1) to a partial pressure

    Args:
        ppm (float) : ppm of gas [umol mol^-1]
        pres (float) : atmospheric pressure [Pa]

    Returns:
        pa (float) : partial pressure of gas [Pa]
    """

    pa = ppm * 1e-6 * pres

    return pa