Table 2 Estimated impact of exceedance day fraction, maximum visibility, and mean visibility on log visitation from 1990 to 2014.

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.