Measurements of hydrocarbon fluxes by a gradient method above a northern boreal forest

The boreal vegetation is considered as being a major source of hydrocarbons in the atmosphere. Measurements of vertical fluxes of isoprene and monoterpenes above a northern boreal forest are reported in this paper. Measurements were conducted in northern Finland (67°58′N, 24°14′E) near the northern timber lines for conifers. The hydrocarbon fluxes were measured by the gradient technique. The total daytime fluxes of following four monoterpenes: α- and β-pinene, carene and camphene, were typically 30–60 ng m−2 s−1. These compounds represented 60–80% of the total monoterpenes in the ambient air. The estimated uncertainty of isoprene fluxes exceeded observed fluxes several times, the mean daytime flux being 4±20 ng m−2 s−1. The error sources of the measured hydrocarbon fluxes are discussed. The most important source of uncertainty was the hydrocarbon sampling and analysis due to low concentration gradients.