Air pollution and visitation at national parks

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 1990-2014, 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.每年有数以亿计的游客前往美国国家公园，领略美国标志性的自然景观。针对这些区域的空气质量担忧，催生了严格却颇具争议的污染管控政策。本研究梳理了美国国家公园的污染变化趋势，并估算了污染与公园游客到访量之间的关联。1990年至2014年间，美国国家公园的平均臭氧浓度在统计学意义上与美国20个最大都会区的平均臭氧浓度并无显著差异。此外，相较于美国城市，国家公园中臭氧浓度超出美国环境保护署（U.S. Environmental Protection Agency）认定不健康标准的天数仅出现了小幅减少。研究发现，公园内的臭氧浓度与游客到访量之间存在稳健的负相关关系。但仍有35%的国家公园游客到访发生在臭氧浓度偏高的时段。