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.

Exceedance
day fraction
−0.845**−2.693**
(0.335)(1.276)
Maximum
visibility (km)
−0.0002360.0191
(0.000493)(0.0935)
Mean visibility
(km)
−0.0007540.0228
(0.00123)(0.238)
Observations
(N)
560356034761476147614761
Number of
parks
333329292929
Mean days
above
70 ppb
1.891.89
Maximum
visibility (km)
214.50214.50
Mean visibility
(km)
147.20147.20
Weather
controls
YesYesYesYesYesYes
Month FEYesYesYesYesYesYes
Park-by-Year FEYesYesYesYesYesYes
IVNoYesNoYesNoYes
Kleibergen-
Paap F test
33.335.394.08
R20.9040.8910.9100.7440.9110.809