Nonmethane hydrocarbons at Pico Mountain, Azores: 1. Oxidation chemistry in the North Atlantic region

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Abstract

[1] Measurements of nonmethane hydrocarbons (NMHC) at the Pico Mountain observatory at 2225 m asl on Pico Island, Azores, Portugal, from August 2004 to August 2005 (in part overlapping with the field campaign of the International Consortium on Atmospheric Research on Transport and Transformation study) were used to investigate NMHC sources and seasonal oxidation chemistry in the central North Atlantic region. Levels of anthropogenic NMHC were characteristic of the marine free troposphere. Their concentrations were low compared to continental sites at higher northern latitudes, but higher than data reported from a similarly located Pacific mountain site at Mauna Loa Observatory, Hawaii. These higher NMHC levels are indicative of a greater influence of the adjacent continents on air composition at Pico. Substantially enhanced NMHC concentrations during the summers of 2004 and 2005 were attributed to long-range transport of biomass burning plumes originating from fires in northern Canada, Alaska, and Siberia. This finding exemplifies the continuing impact of biomass burning plumes on atmospheric composition and chemistry many days downwind of these emission sources. Seasonal cycles with lower NMHC concentrations and lower ratios of more reactive to less reactive NMHC during summer reflect the higher degree of photochemical processing occurring during transport. The NMHC concentrations indicate no significant role of chlorine atom oxidation on NMHC. Ozone above 35 ppbv was measured at Pico Mountain throughout all seasons. Enhanced ozone levels were observed in air that had relatively “fresh” photochemical signatures (e.g., ln [propane]/[ethane] > −2.5). During spring-summer air that was more processed (“older” air with ln [propane]/[ethane] < −2.5) on average had lower ozone levels (down to <20 ppbv). This relationship indicates that conditions in the lower free troposphere over the mid–North Atlantic during the spring and summer lead to net photochemical ozone destruction while air is photochemically aging during transport to Pico. This behavior contrasts to that in the mid–North Pacific where other recent studies have found that the photochemistry is more nearly ozone neutral.

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