To cite package 'smoothie' in publications use:

Gilleland E (2013). “Two-dimensional kernel smoothing: Using the R package smoothie.” NCAR Technical Note, TN-502+STR, 17pp. doi:10.5065/D61834G2.

Corresponding BibTeX entry:

  @TechReport{,
    title = {Two-dimensional kernel smoothing: Using the R package
      smoothie},
    author = {Eric Gilleland},
    institution = {NCAR Technical Note, TN-502+STR, 17pp.},
    year = {2013},
    doi = {10.5065/D61834G2},
  }