How Have Divergent Global Emission Trends Influenced Long-Range Transported Ozone to North America?

Several locations across the United States in noncompliance with the national standard for ground-level ozone (O-3) are thought to have sizable influences from distant extra-regional emission sources or natural stratospheric O-3, which complicate the design of local emission control measures. To quantify the amount of long-range-transported O-3 (LRT O-3), its origin, and change over time, we conduct and analyze detailed sensitivity calculations characterizing the response of O-3 to emissions from different source regions across the Northern Hemisphere in conjunction with multidecadal simulations of tropospheric O-3 distributions and changes. Model calculations show that the amount of O-3 at any location attributable to sources outside North America varies both spatially and seasonally. On a seasonal-mean basis, during 1990-2010, LRT O-3 attributable to international sources steadily increased by 0.06-0.2 ppb yr(-1) 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-3 measurements. Contributions of emissions from Europe steadily declined through 2010, while those from Asian emissions increased and remained dominant. Steadily rising NOx emissions from international shipping resulted in increasing contributions to LRT O-3, comparable to those from Asian emissions in recent years. Central American emissions contribute a significant fraction of LRT O-3 in southwestern United States. In addition to the LRT O-3 attributable to emissions outside of North America, background O-3 across the continental United States is composed of a sizable and spatially variable fraction that is of stratospheric origin (29%-78%).