Ten-year chemical signatures associated with long-range transport observed in the free troposphere over the central North Atlantic

B. Zhang, R. C. Owen, J. A. Perlinger, D. Helmig, M. Val Martín, L. Kramer, L. R. Mazzoleni, C. Mazzoleni
2017 Elementa: Science of the Anthropocene  
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
more » ... enic 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 O 3 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 O 3 and nitrogen oxides. The d [O 3 ]/d [CO] value was used to investigate O 3 sources and chemistry in different transport patterns. The transport pattern affected by Fire had the lowest d [O 3 ]/d [CO], which was likely due to intense CO production and depressed O 3 production in wildfire plumes. Slightly enhanced O 3 and d [O 3 ]/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. Zhang et al: Ten-year chemical signatures associated with long-range transport observed in the free troposphere over the central North Atlantic Art. 8, page 2 of 20 Zhang et al: Ten-year chemical signatures associated with long-range transport observed in the free troposphere over the central North Atlantic Art. 8, page 3 of 20 Zhang et al: Ten-year chemical signatures associated with long-range transport observed in the free troposphere over the central North Atlantic Art. 8,
doi:10.1525/elementa.194 fatcat:pwczljoin5albho2zdhrss4hiy