Comparison of net CO2 fluxes measured with open- and closed-path infrared gas analyzers in an urban complex environment

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 (R2 = 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.