A Re-examination of Density Effects in Eddy Covariance Measurements of CO_2 Fluxes

Corrections of density effects resulting from air-parcel expansion/compression are important in interpreting eddy covariance fluxes of water vapor and CO2 when open-path systems are used. To account for these effects, mean vertical velocity and perturbation of the density of dry air are two critical parameters in treating those physical processes responsible for density variations. Based on various underlying assumptions, different studies have obtained different formulas for the mean vertical velocity and perturbation of the density of dry air, leading to a number of approaches to correct density effects. In this study, we re-examine physical processes related to different assumptions that are made to formulate the density effects. Specifically, we re-examine the assumptions of a zero dry air flux and a zero moist air flux in the surface layer, used for treating density variations, and their implications for correcting density effects. It is found that physical processes in relation to the assumption of a zero dry air flux account for the influence of dry air expansion/compression on density variations. Meanwhile, physical processes in relation to the assumption of a zero moist air flux account for the influence of moist air expansion/compression on density variations. In this study, we also re-examine mixing ratio issues. Our results indicate that the assumption of a zero dry air flux favors the use of the mixing ratio relative to dry air, while the assumption of a zero moist air flux favors the use of the mixing ratio relative to the total moist air. Additionally, we compare different formula for the mean vertical velocity, generated by air-parcel expansion/compression, and for density effect corrections using eddy covariance data measured over three boreal ecosystems.

A photosynthesis continuous monitoring system for CAM plants

Diurnal CO_(2) exchanges in crassulacean acid metabolism(CAM)plants are significantly different from those in C3 and C4 plants.The instantaneous short-time CO_(2) exchange of a single leaf measured by commercial portable photosynthesis measuring systems with a small leaf chamber cannot reflect the plant photosynthetic capacity for CAM plants because of the CO_(2) fixation property.Therefore,a photosynthesis continuous monitoring system with two canopy cuvettes was developed for measuring diurnal net CO_(2) exchange rates for CAM plants.To evaluate stability and applicability of the photosynthesis continuous monitoring system,continuous measurement of net CO_(2) exchange rates of plants with different photosynthetic pathways were conducted.An obligate CAM plant(Kalanchoe daigremontiana),four facultative CAM plants(Dendrobium officinale,D.chrysotoxum,D.nobile,and D.primulinum),a C3 plant(Strawberry,Fragaria ananassa),and a C4 plant(Corn,Zea mays)were selected as model plants.K.daigremontiana had a significant CO_(2) absorption during the dark period and its net CO_(2) exchange rates fluctuated around 0μmol/(m^(2)·s)during the photoperiod in a growth chamber.Net CO_(2) exchange rates of F.ananassa and Z.mays in a greenhouse gradually increased after sunrise,reaching a maximum at about 12:00,and then gradually decreased to negative values during the night time.It is interesting to observe that D.officinale in the greenhouse and growth chamber absorbed CO_(2) during both day and night times.The photosynthetic pathways of D.chrysotoxum,D.nobile,and D.primulinum were also well distinguished by this photosynthesis continuous monitoring system.The results showed that the photosynthesis continuous monitoring system is capable for quantitative evaluation of diurnal net CO_(2) exchange characteristics not only in the CAM plants but also in small size C3 and C4 plants with low net photosynthetic rates for long-time and high-accuracy measurements.