Research ArticleENVIRONMENTAL STUDIES

Air pollution and visitation at U.S. national parks

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Science Advances  18 Jul 2018:
Vol. 4, no. 7, eaat1613
DOI: 10.1126/sciadv.aat1613
  • Fig. 1 Trends in maximum daily 8-hour ozone concentrations and days with maximum daily 8-hour ozone exceeding 70 ppb in large metro areas and national parks.

    (A) Average annual maximum daily 8-hour ozone concentrations. (B) Average summertime maximum daily 8-hour ozone concentrations. (C) Average days per year with maximum daily 8-hour ozone concentrations exceeding 70 ppb. (D) Exceedance days at Sequoia National Park and the Los Angeles metro area. (A to C) Control for seasonality, daily weather conditions, and park- and metro-specific fixed unobservable characteristics. Shaded areas are 95% confidence intervals.

  • Fig. 2 Seasonal effects of maximum ozone (ppb) on visitation.

    (A) Mean estimate and 95% confidence intervals for the effect of maximum ozone on visitation by season. (B) Seasonal histograms of maximum ozone. The red line indicates the 70-ppb threshold. (C) Seasonal histograms of average monthly exceedance days. ***P < 0.01, **P < 0.05, and *P < 0.1.

  • 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)560356035603
    Number of parks333333
    Average maximum ozone (ppb)47.5147.5147.51
    Weather controlsYesYesYes
    Month FEYesYesYes
    Park-by-year FEYesYesYes
    IVNoYesNo
    Kleibergen-Paap F test95.85
    R20.9040.9000.908
  • 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
  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/7/eaat1613/DC1

    Supplementary Text

    Fig. S1. Unconditional trends in maximum daily 8-hour pollution and days with maximum daily 8-hour pollution exceeding 70 ppb in large metro areas and national parks.

    Fig. S2. Nonlinear impacts of ozone and weather on visitation.

    Fig. S3. Seasonal trends in control and outcome variables.

    Fig. S4. Estimated impact of exceedance days by AQI category on log visitation from 1990 to 2014.

    Table S1. Pollution monitors, visitation, upwind ozone, and weather data by national park and year.

    Table S2. Top 20 U.S. cities.

    Table S3. Estimated impact of monthly average maximum ozone concentrations (ppb) in national parks on log visitation from 1990 to 2014 (additional specifications).

    Table S4. Estimated impact of monthly average maximum ozone concentrations (ppb) in national parks on log visitation from 1990 to 2014 (robustness checks).

    Table S5. Estimated impact of alternative monthly average pollution levels in national parks on log visitation from 1990 to 2014.

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Text
    • Fig. S1. Unconditional trends in maximum daily 8-hour pollution and days with maximum daily 8-hour pollution exceeding 70 ppb in large metro areas and national parks.
    • Fig. S2. Nonlinear impacts of ozone and weather on visitation.
    • Fig. S3. Seasonal trends in control and outcome variables.
    • Fig. S4. Estimated impact of exceedance days by AQI category on log visitation from 1990 to 2014.
    • Table S1. Pollution monitors, visitation, upwind ozone, and weather data by national park and year.
    • Table S2. Top 20 U.S. cities.
    • Table S3. Estimated impact of monthly average maximum ozone concentrations (ppb) in national parks on log visitation from 1990 to 2014 (additional specifications).
    • Table S4. Estimated impact of monthly average maximum ozone concentrations (ppb) in national parks on log visitation from 1990 to 2014 (robustness checks).
    • Table S5. Estimated impact of alternative monthly average pollution levels in national parks on log visitation from 1990 to 2014.

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