def find_hpo_row(sheet, hpo):
    """
    Finds the row index of a particular HPO site within
    a larger sheet.

    Parameters
    ----------
    sheet (dataframe): dataframe with all of the data quality
        metrics for the sites.

    hpo (string): represents the HPO site whose row in
        the particular sheet needs to be determined.

    Returns
    -------
    row_num (int): row number where the HPO site of question
        lies within the sheet. returns none if the row is not
        in the sheet in question but exists in other sheets.
    """
    hpo_column_name = 'src_hpo_id'
    sheet_hpo_col = sheet[hpo_column_name]

    row_num = 99999

    for idx, site_id in enumerate(sheet_hpo_col):
        if hpo == site_id:
            row_num = idx

    if row_num == 99999:  # was never found; no data available
        return None

    return row_num