def mean_free_path(T, P, lamb_0=67.3, T_0=296.15, P_0=101325, S=110.4):
    
    """ calculates mean free path of the aerosol particles
    
    Parameters
    ----------
    T : float
        measurement temperature
    P : float
        measurement pressure
    lamb_0 : float
        reference mean free path
    T_0 : float
        reference temperature
    P_0 : float
        reference pressure
    S : float
        Sutherland constant of air
    
    Returns
    -------
    mean_free_path : float
        mean free path at T and P
    
    """
    
    return (lamb_0 * ((T_0 + S) / (T + S)) * (P_0 / P) * ((T / T_0)**2))