def METHOD_NAME(self, params_dict):
    """
    Invokes the SO2 Index endpoint
    :param params_dict: dict of parameters
    :returns: a string containing raw JSON data
    :raises: *ValueError*, *APIRequestError*
    """
    lat = str(params_dict['lat'])
    lon = str(params_dict['lon'])
    start = params_dict['start']
    interval = params_dict['interval']
    # build request URL
    if start is None:
        timeref = 'current'
    else:
        if interval is None:
            timeref = self._trim_to(
                formatting.to_date(start), 'year')
        else:
            timeref = self._trim_to(
                formatting.to_date(start), interval)
    uri = '%s/%s,%s/%s.json' % (SO2_INDEX_URL, lat, lon, timeref)
    _, json_data = self._client.get_json(uri)
    return json_data