Hundreds of millions of visitors travel to U.S. national parks every year to visit America's iconic landscapes. Concerns about air quality in these areas have led to strict, yet controversial pollution control policies. We document pollution trends in U.S. national parks and estimate the relationship between pollution and park visitation. From 1990 to 2014, average ozone concentrations in national parks were statistically indistinguishable from the 20 largest U.S. metropolitan areas. Further, relative to U.S. cities, national parks have seen only modest reductions in days with ozone concentrations exceeding levels deemed unhealthy by the U.S. Environmental Protection Agency. We find a robust, negative relationship between in-park ozone concentrations and park visitation. Still, 35% of all national park visits occur when ozone levels are elevated.

中文翻译：

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美国国家公园的空气污染和参观。

每年有数亿游客前往美国国家公园，参观美国的标志性景观。对这些地区空气质量的担忧导致了严格但有争议的污染控制政策。我们记录了美国国家公园的污染趋势，并估算了污染与参观公园之间的关系。从1990年到2014年，国家公园内的平均臭氧浓度与美国20个最大的都市区在统计学上没有区别。此外，相对于美国城市，国家公园的臭氧浓度仅超过美国环保署认为不健康的水平，而几天内的减少量却很小。我们发现公园内的臭氧浓度与公园参观之间存在强烈的负相关关系。仍然，