def constants(input_x, rtn_dict=False):
    """
    Dictionary storing commonly used constants

    Parameters
    ----------
    input_x (str): name of contant of interest
    rtn_dict (boolean): retrun complete dictionary instead of single value

    Returns
    -------
    value of constant (float) or dictionary on constants and values (dict)

    Notes
    -----
    """
    con_dict = {
        # Relative atomic mass of air (Just considering N2+O2)
        'RMM_air': (.78*(2.*14.)+.22*(2.*16.)),
        # Avogadro constant (Mol^-1)
        'AVG': 6.0221413E23,
        # Dobson unit - (molecules per meter squared)
        'mol2DU': 2.69E20,
        # Specific gas constant for dry air (J/(kg·K))
        'Rdry': 287.058,
    }
    if rtn_dict:
        return con_dict
    else:
        return con_dict[input_x]