# Presentation: 306757

## Direct measurements of ABL and free troposphere air over the 

- Bakwin, P. S., Davis, K. J., Yi, C., Wofsy, S. C., Munger, J. W., Haszpra, L. andBarcza, Z. 2002. Carbon dioxide budget in the continental atmosphere. - Tellus in review.
- Bakwin, P. S., Tans, P. P., Hurst, D. F. and Zhao, C. 1998. Measurements of carbon dioxide on very tall towers: results of the NOAA/CMDL program. - Tellus 50B: 401-415.
- Gerbig, C., Lin, J. C., Wofsy, S. C., Daube, B. C., Andrews, A. E., Stephens, B. B., Bakwin, P. S. and Grainger, A. 2002. Constraining regional to continental scale fluxesof CO2 with atmospheric observations over a continent:  A receptor-oriented analysis of COBRA data. - J. Geophys. Res. in review.
- Hurwitz, M. D., Ricciuto, D. M., Davis, K. J., Wang, W., Yi, C., Butler, M. P. and Bakwin, P. S. 2002. Advection of carbon dioxide in the presence of storm systems over a northern Wisconsin forest. - Journal of Atmospheric Science in review.

**Acknowledgements**
- We are grateful to D. Fitzjarrald, R. Strand, P. Tans, R. Teclaw, S. Wofsy and the State of Wisconsin Educational Communications Board for theoretical and technical help with this study.  This work was supported by grants from the National Oceanic and Atmospheric Administration (NAO6-GP-409), the National Science Foundation (TECO; DEB-9814194) and the Department of Energy (NIGEC; DEFC03-90ER61010 and DOE/DE-FG02-97ER62457).

- Continuous measurements of the mixing ratio of CO2 in the ABL
- have been conducted at the WLEF tower in northern Wisconsin
- since 1995 (*ref 1*).  The (monthly average) CO2 concentration at the
- WLEF tall tower shows a more pronounced seasonal cycle than
- observed in the marine boundary layer at the same latitude or at
- Niwot Ridge, in the Rocky Mountains approximately 1000 km to
- the west (Figure 1).  The pronounced seasonal cycle measured
- within the ABL is reflective of both influence from the global
- atmosphere and local surface flux (*ref 2*).  Indeed, the pattern
- is consistent with the surface being a net CO2 sink in the summer
- and source in winter.

- Direct measurements of ABL and free troposphere air over the
- WLEF tower in August 2000 show substantial mixing ratio gradients
- (Figure 2A, B; *ref 3*)  This is consistent with removal of CO2 by
- photosynthesis and addition of H2O by evapotranspiration from the
- surface.  Over several synoptic cycles, the mean gradients are
- maintained, however there is significant fluctuation from day to day
- (Figure 2C). During low pressure, with associated frontal passage
- and storms, ABL CO2 (Cm) was very similar to free troposphere CO2
- (Ct). This is expected as there should be strong vertical mixing at such
- times (*ref 4*). During intervening fair weather intervals when the area
- was under the influence of high pressure systems, Cm was drawn
- below Ct over several days.  With passage of the next frontal system,
- Cm again approached Ct. Over the entire interval, the average Cm was
- lower than Ct. This was consistent with CO2 flux measurements
- indicating that the land surrounding the tower was a net sink for CO2
- averaged over the month of August.

- Figure 3.  Monthly averages of C (Cm-Ct; open
- circles) and q (qm-qt; closedsquares) for 2000.  Cm
- and qm were obtained through continuous
- measurementsat 396 m from the tower.  Ct was
- obtained by monthly averages of CO2 collected at
- Niwot Ridge, CO.  RUC weather-forecasting data
- from the geopotentialheights of 3200 to 7600 m
- was used to obtain qt.

- Figure 1.  Monthly averages of CO2 at Niwot Ridge,
- CO, the marine boundary layer (MBL) at 44.4 °N,
- and from 396 m from the WLEF tower (Cm).

**	****	****	****    ****Final Comments**
- Method could be vastly improved with direct measurements of Ct
- and/or transport models of free troposphere CO2.
- The addition of several more tower sites could provide continental
- scale estimates of NEE.
- This method could potentially be applied to heterogenous landscapes
- such as urban and mountainous areas.

- Figure.  Monthly and annual NEE for 2000. Hatched bars
- represent tower eddy-covariance measurements from 122 m.
- FNEE was calculated using Ct from Niwot Ridge (black bars)
- and MBL (blue bars). qt was obtained from RUC data, Fq, Cm
- and qm were obtained by averaging the 24 h observations
- from the WLEF tower for a given month.

- Stable isotopes provide an independent check on whether
- gradients in CO2 reflect surface fluxes.  If the CO2 gradients
- were generated by photosynthesis and respiration, then the
- gradients of 13C between the ABL and the free troposphere
- should be consistent with surface exchange. The isotopic
- composition of the net CO2 exchanged between the free
- troposphere and the ABL () can be obtained from mass
- and isotope balance considerations:
- 			  =				(4)
- where,  is the isotope ratio of CO2 in the ABL (m) and
- the free troposphere (t), respectively. Flask measurements
- of CO2 and isotopes within the ABL and the free
- troposphere were used to calculate net for each month
- of 2000 (Table 1).  Note that this calculation depends
- only on the gradients- it does not require knowledge
- of the flux. The net flux of CO2 and net for each month
- can be used to calculate the corresponding isotopic
- signature associated with the yearly net uptake of CO2
- by this region , which was –15.0 ‰.

- The net isotope effects associated with net ecosystem exchange at the
- surface can be related to the isotope effects associated with respiration
- and photosynthesis by the following isotope mass balance approach,
-  			net =					5.
- Where  is the isotope ratio of respired CO2 and p is the isotopic
- composition of CO2 fixed in gross photosynthesis, and , is the ratio
- of gross fluxes of photosynthesis to respiration.
- We offer a sample calculation: with  = -1.04, and P = -25 ‰
-  a value of r = -25.4 ‰ would satisfy the seasonal net storage of –15.0 ‰.
- This might occur, for example, if the gross CO2 released was 91 % from
- biological decomposition with r =-25.0 ‰ and 9 % from fossil fuel
- combustion with ff =-29.6 ‰. The input parameters, especially , need
- to be better constrained by measurements before the results of such a
- calculation can be taken seriously. However, this illustrates the
- additional power that an isotope budget can provide in studies of
- regional net carbon exchange.

- mCm - tCt
-      Cm - Ct

- Results of these calculations are presented in Figure 4.