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** | |
| (0.00744) | (0.0165) | ||
| Summer maximum ozone (ppb) | −0.0198** | ||
| (0.00887) | |||
| Fall maximum ozone (ppb) | −0.0149** | ||
| (0.00599) | |||
| Spring maximum ozone (ppb) | 0.00198 | ||
| (0.00635) | |||
| Winter maximum ozone (ppb) | 0.0194 | ||
| (0.0165) | |||
| Observations (N) | 5603 | 5603 | 5603 |
| Number of parks | 33 | 33 | 33 |
| Average maximum ozone (ppb) | 47.51 | 47.51 | 47.51 |
| Weather controls | Yes | Yes | Yes |
| Month FE | Yes | Yes | Yes |
| Park-by-year FE | Yes | Yes | Yes |
| IV | No | Yes | No |
| Kleibergen-Paap F test | — | 95.85 | — |
| R2 | 0.904 | 0.900 | 0.908 |