Nonmethane hydrocarbons at Pico Mountain, Azores: 2. Event-specific analyses of the impacts of mixing and photochemistry on hydrocarbon ratios

摘要

Nonmethane hydrocarbon (NMHC) measurements at Pico Mountain observatory are analyzed in combination with FLEXPART transport simulations. The results are used to assess the utility of NMHC as indicators for the occurrence of atmospheric transport events and photochemical processing on an event-specific basis. Continuous in situ observations were made in lower free tropospheric air that had experienced a wide range of photochemical aging and traveled 3–10 days since last encountering significant emissions. They therefore provide a data set suitable for investigating the interaction of emissions, transport, and mixing in aged air parcels. Measurements of ethane, propane, and n-butane during April 2005 are analyzed in detail to assess the degree to which the magnitude and variability of hydrocarbon ratios can be quantitatively understood as resulting from the mixing of emissions that have experienced varying degrees of photochemical oxidation. The age distribution of emissions was estimated with FLEXPART; resulting hydrocarbon ratios were simulated by applying first-order decay to these emissions using the method of Parrish et al. (2007). These simulations capture the magnitude and large variations of observed hydrocarbon ratios. The relationship between the distribution of the timing of upwind emissions and measures of photochemical age calculated using these hydrocarbon ratios is discussed using the FLEXPART age distributions. The results demonstrate that continuous NMHC observations can be used to derive measures of photochemical age that can be quantitatively interpreted without the need for assumptions regarding mixing. Such analyses are valuable for studying OH levels and the distribution of upwind emission timing during pollution transport events.