Table 1 Estimated impact of monthly average maximum ozone concentrations (parts per billion) in national parks on log visitation from 1990 to 2014.

All specifications include weather controls, park-by-year FE and month-of-year FEs. Column 2 instruments in-park monthly average maximum ozone using ozone concentrations from upwind counties. Column 3 examines the effects of ozone by season. Values in parentheses are robust SEs (standard errors) clustered at the park using a bootstrap procedure with 500 replications. ***P < 0.01, **P < 0.05, and *P < 0.1. The Stock-Yogo weak identification critical value for the Kleibergen-Paap F test is 16.38 for a 10% maximal IV bias relative to ordinary least squares.

Maximum ozone (ppb)−0.0162**−0.0394**
Summer maximum ozone (ppb)−0.0198**
Fall maximum ozone (ppb)−0.0149**
Spring maximum ozone (ppb)0.00198
Winter maximum ozone (ppb)0.0194
Observations (N)560356035603
Number of parks333333
Average maximum ozone (ppb)47.5147.5147.51
Weather controlsYesYesYes
Month FEYesYesYes
Park-by-year FEYesYesYes
Kleibergen-Paap F test95.85