def get_atmos_url(year:int, variable ="PM25") -> str:
    """
    Constructs URL to download data from the
    Atmospheric Composition Analysis Group
    given a year and a band

    :param year: year
    :param variable: Gridmet band (variable)
    :return: URL for download
    """

    base = "http://fizz.phys.dal.ca/~atmos/datasets/V4NA03/"
    pattern = base + "V4NA03_{}_NA_{:d}01_{:d}12-RH35.nc"
    return pattern.format(variable, year, year)