def get_unit(pollutant):
    """Obtain the unit of the pollutant for plot label.

    """

    unit_dict = {'PM2.5': r'$\mu g/m^3$',
                 'PM10': r'$\mu g/m^3$',
                 'O3': 'ppb',
                 'NO2': 'ppb',
                 'SO2': 'ppb',
                 'CO': 'ppm'}

    return unit_dict[pollutant]