  With more people living in cities, we are witnessing a decline in exposure to
nature. A growing body of research has demonstrated an association between
nature contact and improved mood. Here, we used Twitter and the Hedonometer, a
world analysis tool, to investigate how sentiment, or the estimated happiness
of the words people write, varied before, during, and after visits to San
Francisco's urban park system. We found that sentiment was substantially higher
during park visits and remained elevated for several hours following the visit.
Leveraging differences in vegetative cover across park types, we explored how
different types of outdoor public spaces may contribute to subjective
well-being. Tweets during visits to Regional Parks, which are greener and have
greater vegetative cover, exhibited larger increases in sentiment than tweets
during visits to Civic Plazas and Squares. Finally, we analyzed word
frequencies to explore several mechanisms theorized to link nature exposure
with mental and cognitive benefits. Negation words such as 'no', 'not', and
'don't' decreased in frequency during visits to urban parks. These results can
be used by urban planners and public health officials to better target nature
contact recommendations for growing urban populations.
