Abstract
Abstract A record spanning ten years of non-methane hydrocarbon (NMHC) data from the Pico Mountain Observatory (PMO), Pico Island, Azores, Portugal, was analyzed for seasonal NMHC behavior, atmospheric processing, and trends, focusing on ethane and propane. The location of this site in the central North Atlantic, at an elevation of 2225 m asl, allows these data to be used to investigate the background conditions and pollution transport events occurring in the lower free North Atlantic troposphere. The quantity ln([propane]/[ethane]) was used as an indicator of both photochemical processing and a marker for the occurrence of pollution transport events detected at the station. The Pico data were compared with three other continuous NMHC data sets from sites bordering the North Atlantic, i.e. the Global Atmospheric Watch (GAW) stations at Summit, Greenland, Hohenpeisssenberg, Germany, and Cape Verde, using ln([propane]/[ethane]) results as an indicator for the degree of photochemical processing (‘aging’) seen in the data. Comparisons of these three data sets showed some significant differences in the seasonal background and range of observed values. The statistical distribution of binned monthly data was determined, and individual sample events were then scaled to the monthly median observed value. Back trajectories, determined by the HYSPLIT model were used to investigate the geographic origin of the observed trace gases as a function of the degree of photochemical processing. Results show that PMO samples have been subjected to a diversity of air transport and aging, from highly processed air to freshly emitted air throughout the year, and in particular during summer months. The predominant air transport is from North America, with only occasional influence from continental areas located east and southeast (Europe and Africa). The available record was found to be too variable and still too short to allow deciphering NMHC trends from the data. Ethane and propane measurements at the PMO were compared with the MOZART-4 atmospheric chemistry and transport model at the appropriate time and location. The model was found to yield good agreement in the description of the lower range of atmospheric mole fractions observed, of the seasonal cycle, and the regional oxidation chemistry. However, ethane and propane enhancements in transport events were underestimated, indicating that after the ≥ 3 days of synoptic transport to PMO the spatial extent of plumes frequently is smaller than the 2.8°x2.8° (∼300 km) model grid resolution.
Highlights
Continental outflow affects atmospheric chemistry and composition far downwind of continental coastlines.The North Atlantic region has been a particular region of interest for these studies and a series of large campaigns have addressed questions concerning the processing and fate of continental emissions during transport aloft the North Atlantic Ocean, e.g. (Parrish et al, 1993; Fehsenfeld et al, 1996; Penkett et al, 1998; Fehsenfeld et al, 2006; Palmer et al, 2013)
A particular emphasis has been the study of biomass burning emissions, with findings demonstrating a significant role of boreal fires on the oxidation and radiative properties over the North Atlantic, many days downwind of their source (Val-Martin et al, 2006; Lapina et al, 2008). Monitoring and modeling of atmospheric composition in continental outflow has proven valuable to assess emission changes from anthropogenic activities in the continents bordering the North Atlantic (Simmonds et al, 2006; Slemr et al, 2006; Derwent et al, 2007, 2013; Zhang et al, 2014), and aircraft studies have shown that most of the North Atlantic is to some degree impacted by anthropogenic emissions
This study builds on the data from one regional (PMO) and three global monitoring sites that are operated within the framework of the World Meteorological Organization (WMO) Global Atmospheric Watch (GAW ) program
Summary
Continental outflow affects atmospheric chemistry and composition far downwind of continental coastlines.The North Atlantic region has been a particular region of interest for these studies and a series of large campaigns have addressed questions concerning the processing and fate of continental emissions during transport aloft the North Atlantic Ocean, e.g. (Parrish et al, 1993; Fehsenfeld et al, 1996; Penkett et al, 1998; Fehsenfeld et al, 2006; Palmer et al, 2013). Continental outflow affects atmospheric chemistry and composition far downwind of continental coastlines.The North Atlantic region has been a particular region of interest for these studies and a series of large campaigns have addressed questions concerning the processing and fate of continental emissions during transport aloft the North Atlantic Ocean, e.g. Several recent studies have provided evidence for a reversal in Northern Hemisphere emission trends of NMHC, showing that C2-C5 NMHC increased steadily during 1950 – 1970, reached their atmospheric maxima in 1970–1980 and have since declined appreciably to 42–68% of their historic maxima in the Northern Hemisphere (NH) background atmosphere (Simpson et al, 2012; Worton et al, 2012; Helmig et al, 2014a).This decline has been primarily attributed to improved containment technologies in the hydrocarbon fuel production and more stringent emissions regulations associated with the distribution and combustion of petroleum fuels, notably catalytic converters for gasoline road vehicles
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