RAINFOR GEM Intensive Plots Manual (pdf) - University of Oxford

CO 2 efflux calculationsWhile the EGM-calculated values are useful (and should be noted), you should always directly calculate yourfluxes from the raw data (the .dat file). To do so, take only the last 10 records of any individual measurement,verify that there is an approximately linear increase in CO 2 over time (open the .dat file in Excel and check byplotting CO 2 concentration (“CO2 Ref”) against time (“Input E”)), and use the following equation 104 105 :RucCPV10 1d= *g CO 2 m -2 h -1t10− C− t144.01* 0.36( Ta+ 273.15) A RuWhere R uc is uncorrected CO 2 efflux (g CO 2 m -2 h -1 ), t 10 and t 1 are the times (sec) since that measurement beganat records 10 and 1 chosen respectively (e.g. record 1 could be 86 s after measurement began and record 10 thefinal record of this measurement after 124 s), C 10 and C 1 are CO 2 concentrations (ppmv) at times t 10 and t 1 , P isambient pressure at time t 10 (mb, e.g. ~1013.25 mb at sea level) and T a is air temperature at time t 10 (°C). TheSRC-1 chamber volume V d is 0.0012287 m 3 from measurement of the SRC-1 and the upper part of the adapter104 The Ideal Gas Law states that n moles of any gas at ambient pressure P mb and temperature T a °C will occupy Vol m 3 whereVolnR( T + 273.15)u a= (R u =8.31432 J mol -1 K -1 is the Universal Gas Constant). Therefore, at time t 1 the CO 2 occupies a volumeCV1000000100Pn R1 1 u a+d( T 273.15)= m 3 .100PBetween times t 1 and t 10 the CO 2 concentration rises from C 1 to C 10 ppmv, so the number of moles CO 2 that enter thechamber during this time is10− nC10 − C1100P1Vd1000000 R−u( T + 273.15) ( t t )Aa1C10− C1= V1000000C10− C1100P1Vd* 44.01*36001000000 Ru( Ta+ 273.15) ( t10− t1) AC10− C1P 273Ruc* *t − t 1000 T + 273dRu( T + 273.15)75100Pn mol CO 2 . This equates to a molar flux of101amol CO 2 m -2 s -1 . The molar mass of CO 2 is 44.01 g/mol so this is equivalent tog CO 2 m -2 h -1 .44.01*3.6105 Metcalfe et al. (2009) hadd* *( ) A 22. 41101aV= g CO 2 m -2 h -1 but the 22.41 L there was forold Standard Temperature and Pressure (0°C, 1013 mb) when it should have been for new STP (0°C, 1000 mb) which would havebeen 22.7 L. These forms are essentially equivalent through the value of R u ≈100*22.41/273.