Simultaneous eddy
covariance (EC) measurements of CO₂ fluxes made with open-path and
closed-path analyzers were done in urban area of Helsinki, Finland, in July
2007–June 2008. Our purpose was to study the differences between the two
analyzers, the necessary correction procedures and their suitability to
accurately measure CO₂ exchange in such non-ideal landscape. In
addition, this study examined the effect of open-path sensor heating on measured
fluxes in urban terrain, and these results were compared with similar
measurements made above a temperate beech forest in Denmark. The correlation
between the two fluxes was good ( R ² = 0.93) at the urban site,
but during the measurement period the open-path net surface exchange (NSE) was
17% smaller than the closed-path NSE, indicating apparent additional uptake of
CO₂ by open-path measurements. At both sites, sensor heating
corrections evidently improved the performance of the open-path analyzer by
reducing discrepancies in NSE at the urban site to 2% and decreasing the
difference in NSE from 67% to 7% at the forest site. Overall, the site-specific
approach gave the best results at both sites and, if possible, it should be
preferred in the sensor heating correction.