pbratios: Processing Pb isotope ratio data

#' Pb isotope ratio data in the Northen Hemisphere.
#'
#' Pb isotope ratio data measured in PM10 from different countries in the Northern Hemisphere. This
#' dataset is used internally by the function \code{isoplot.pm10}.
#'
#' @format A data.table with 169 observations and 9 columns. The columns are: \describe{
#'   \item{\code{Sample}}{ Sample name or sampling location.} \item{\code{Pb20x20y}}{ Six columns
#'   reporting Pb isotope ratio values for the ratios 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb,
#'   206Pb/207Pb, 207Pb/206Pb and 208Pb/206Pb.} \item{\code{Ref}}{ Reference number indicating the
#'   source of the data. A list of references is reported at the "Source" section of this document.}
#'   \item{\code{area}}{ A factor label for the geographical area of the sample.} }
#'
#' @source Data sources are indicated by the column \code{Ref}: \enumerate{ \item Bollhofer and
#'   Rosman, Isotopic source signatures for atmospheric lead: The Northern Hemisphere. Geochimica et
#'   Cosmochimica Acta, 2001, 65, 1727-1740. \item Carignan, Simonetti and Gariepy, Dispersal of
#'   atmospheric lead in northeastern North America as recorded by epiphytic lichens. Atmospheric
#'   Environment, 2002, 36, 3759-3766. \item Mukai, Machida, Tanaka, Vera and Uematsu, Lead isotope
#'   ratios in the urban air of eastern and central Russia. Atmospheric Environment, 2001, 35,
#'   2783-2793. \item Mukai, Tanaka and Fujii, Regional Characteristics of Sulfur and lead isotope
#'   ratios in the atmosphere at several chinese urban sites. Environmental Science and Technology,
#'   2001, 35, 1064-1071. \item Zhu, Tang, Lee, Zhang and Zhang, Lead concentrations and isotopes in
#'   aerosols from Xiamen, China. Marine Pollution Bulletin, 2010, 60, 1946-1955. }
#'
#' @examples
#' data(pm10nh)
#' summary(pm10nh)
#'
"pm10nh"