Exceedance day fraction is the fraction of days in a month where maximum daily 8-hour ozone concentrations exceed 70 ppb. Observations decrease when we include visibility readings due to a more limited visibility monitoring by NPS. All specifications include weather controls, park-by-year FEs, and month-of-year FEs. Instrumental variation specifications instrument for each variable using ozone concentrations in upwind counties. Values in parentheses are robust SEs 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.
Exceedance day fraction | −0.845** | −2.693** | ||||
(0.335) | (1.276) | |||||
Maximum visibility (km) | −0.000236 | 0.0191 | ||||
(0.000493) | (0.0935) | |||||
Mean visibility (km) | −0.000754 | 0.0228 | ||||
(0.00123) | (0.238) | |||||
Observations (N) | 5603 | 5603 | 4761 | 4761 | 4761 | 4761 |
Number of parks | 33 | 33 | 29 | 29 | 29 | 29 |
Mean days above 70 ppb | 1.89 | 1.89 | — | — | — | — |
Maximum visibility (km) | — | — | 214.50 | 214.50 | — | — |
Mean visibility (km) | — | — | — | — | 147.20 | 147.20 |
Weather controls | Yes | Yes | Yes | Yes | Yes | Yes |
Month FE | Yes | Yes | Yes | Yes | Yes | Yes |
Park-by-Year FE | Yes | Yes | Yes | Yes | Yes | Yes |
IV | No | Yes | No | Yes | No | Yes |
Kleibergen- Paap F test | — | 33.33 | — | 5.39 | — | 4.08 |
R2 | 0.904 | 0.891 | 0.910 | 0.744 | 0.911 | 0.809 |