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 (O3) are
thought to have sizeable influences from distant extra-regional emission
sources or natural stratospheric O3, which complicates
design of local emission control measures. To quantify the amount of
long-range transported O3 (LRT O3), its
origin, and change over time, we conduct and analyze detailed
sensitivity calculations characterizing the response of
O3 to emissions from different source regions across the
Northern Hemisphere in conjunction with multi-decadal simulations of
tropospheric O3 distributions and changes. Model
calculations show that the amount of O3 at any location
attributable to sources outside North America varies both spatially and
seasonally. On a seasonal-mean basis, during 1990-2010, LRT
O3 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 O3 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 O3,
comparable to those from Asian emissions in recent years. Central
American emissions contribute a significant fraction of LRT
O3 in southwestern United States. In addition to the LRT
O3 attributable to emissions outside of North America,
background O3 across the continental United States is
comprised of a sizeable and spatially variable fraction that is of
stratospheric origin (29-78%).