Abstract
Ten-year observations of trace gases at Pico Mountain Observatory (PMO), a free troposphere site in the central North Atlantic, were classified by transport patterns using the Lagrangian particle dispersion model, FLEXPART. The classification enabled identifying trace gas mixing ratios associated with background air and long- range transport of continental emissions, which were defined as chemical signatures. Comparison between the chemical signatures revealed the impacts of natural and anthropogenic sources, as well as chemical and physical processes during long transport,on air composition in the remote North Atlantic. Transport of North American anthropogenic emissions(NA-Anthro) and summertime wildfire plumes (Fire) significantly enhanced CO and O3 at PMO. Summertime CO enhancements caused by NA-Anthro were found to have been decreasing by a rate of 0.67 ± 0.60 ppbv/year in the ten-year period, due possibly to reduction of emissions in North America. Downward mixing from the upper troposphere and stratosphere due to the persistent Azores-Bermuda anticyclone causes enhanced O3 and nitrogen oxides. The d[O3]/d [CO] value was used to investigate O3 sources and chemistry in different transport patterns. The transport pattern affected by Fire had the lowest d [O3]/d [CO], which was likely due to intense CO production and depressed O3 production in wildfire plumes. Slightly enhanced O3 and d [O3]/d [CO] were found in the background air, suggesting that weak downward mixing from the upper troposphere is common at PMO. Enhancements of both butane isomers were found during upslope flow periods, indicating contributions from local sources. The consistent ratio of butane isomers associated with the background air and NA-anthro implies no clear difference in the oxidation rates of the butane isomers during long transport. Based on observed relationships between non-methane hydrocarbons, the averaged photochemical age of the air masses at PMO was estimated to be 11 ± 4 days.
Highlights
The central North Atlantic Ocean is a remote region where transport of pollutants from North America, Europe, and Africa has been observed
During the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT, Fehsenfeld et al, 2006) experiment, observations including ground and aircraft measurements were collected over North America, the Azores and Europe to study the impacts of long-range transport crossing the North Atlantic
We considered transports originating from North America, Europe and Africa, downward m ixing from the upper troposphere and lower stratosphere (UTLS), and North Atlantic background air
Summary
The central North Atlantic Ocean is a remote region where transport of pollutants from North America, Europe, and Africa has been observed. The meteorology in this region is usually controlled by a persistent mid-latitude anticyclone, known as the Azores-Bermuda High. There has been much effort to study the tropospheric chemistry and long-range transport over the North Atlantic. During the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT, Fehsenfeld et al, 2006) experiment, observations including ground and aircraft measurements were collected over North America, the Azores and Europe to study the impacts of long-range transport crossing the North Atlantic.
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