Non-methane hydrocarbon variability during the FIELDVOC'94 campaign in Portugal

C₂–C₆ non-methane hydrocarbons (NMHC), including isoprene, were measured in a forested area of Portugal from June 20 to July 12, 1994 as part of the FIELDVOC'94 project. The day-to-day variability of the measured NMHC was clearly linked to the air mass origin. The longest-lived ethane did not show any significant diurnal variation. The diurnal variability of the C₃–C₅ alkanes and of acetylene presented low amplitude attributed to the changeable wind direction between northwest and southeast. The short-lived alkenes showed a well-defined diurnal variation with maximum mixing ratios during night-time and minimum during daytime resulting from both dynamical and photochemical sinks. Isoprene presented a very different trend with minima of 0.1 ppbv at night and maxima up to 12 ppbv occurring between 16:00 and 18:00 (UT). In continental air masses, the contribution of isoprene to the daytime ozone production was 3- to 6-fold greater than that of the other NMHC. In air masses of marine origin, the effect of isoprene on ozone and hydroxyl radicals was significantly reduced relative to the anthropogenic NMHC, which were dominating at night. In all cases, isoprene was a more efficient consumer of nitrate radicals than these NMHC. The observed decrease of isoprene at night was consistent with the measured nitrate radical mixing ratios of a few pptv.