Atmospheric Chemistry and Physics, vol. 14, issue 5 (2014) pp. 2267-2287 Published by Copernicus GmbH
The Pico Mountain Observatory, located at 2225ma.s.l. in the Azores Islands, was established in 2001 to observe long-range transport from North America to the central North Atlantic. In previous research conducted at the observatory, ozone enhancement (>55 ppbv) in North American outflows was observed, and efficient ozone pro- duction in these outflows was postulated. This study is fo- cused on determining the causes for high d[O3] / d[CO] val- ues (∼1 ppbv ppbv−1) observed in the summers of 2009 and 2010. The folded retroplume technique, developed by Owen and Honrath (2009), was applied to combine upwind FLEX- PART transport pathways with GEOS-Chem chemical fields. The folded result provides a semi-Lagrangian view of pol- luted North American outflow in terms of physical proper- ties and chemical processes, including production/loss rate of ozone and NOx produced by lightning and thermal de- composition of peroxy acetyl nitrate (PAN). Two transport events from North America were identified for detailed anal- ysis. High d[O3] / d[CO] was observed in both events, but due to differing transport mechanisms, ozone production ten- dency differed between the two. A layer of net ozone pro- duction was found at 2kma.s.l. over the Azores in the first event plume, apparently driven by PAN decomposition dur- ing subsidence of air mass in the Azores–Bermuda High. In the second event, net ozone loss occurred during transport in the lower free troposphere, yet observed d[O3] / d[CO] was high.We estimate that in both events,CO loss through oxida- tion contributed significantly to d[O3] / d[CO] enhancement. Thus, it is not appropriate to use CO as a passive tracer of pollution in these events. In general, use of d[O3]/d[CO] as an indicator of net ozone production/loss may be invalid for any situation in which oxidants are elevated. Based on our analysis, use of d[O3] / d[CO] to diagnose ozone enhance- ment without verifying the assumption of negligible CO loss is not advisable.
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