How have Divergent Global Emission Trends Influenced Long-range Transported Ozone to North America?

Several locations across the United States in non-compliance with the national standard for ground-level ozone (O) are thought to have sizeable influences from distant extra-regional emission sources or natural stratospheric O, which complicates design of local emission control measures. To quantify the amount of long-range transported O (LRT O), its origin, and change over time, we conduct and analyze detailed sensitivity calculations characterizing the response of O to emissions from different source regions across the Northern Hemisphere in conjunction with multi-decadal simulations of tropospheric O distributions and changes. Model calculations show that the amount of O at any location attributable to sources outside North America varies both spatially and seasonally. On a seasonal-mean basis, during 1990-2010, LRT O attributable to international sources steadily increased by 0.06-0.2 ppb yr at locations across the United States and arose from superposition of unequal and contrasting trends in individual source-region contributions, which help inform attribution of the trend evident in O measurements. Contributions of emissions from Europe steadily declined through 2010, while those from Asian emissions increased and remained dominant. Steadily rising NO emissions from international shipping resulted in increasing contributions to LRT O, comparable to those from Asian emissions in recent years. Central American emissions contribute a significant fraction of LRT O in southwestern United States. In addition to the LRT O attributable to emissions outside of North America, background O across the continental United States is comprised of a sizeable and spatially variable fraction that is of stratospheric origin (29-78%).