The GEOS-CHEM model is a global three-dimensional (3-D) model of atmospheric composition [Bey et al., 2001] driven by assimilated meteorological observations from the Goddard Earth Observing System (GEOS) of the NASA Global Modeling and Assimilation Office (GMAO). It has been applied to investigate a number of tropospheric chemistry issues. The Harvard Atmospheric Chemistry Modeling Group provides central management and development support for GEOS-CHEM. Model scientist is Daniel Jacob and model programmer is Bob Yantosca.

• Radiative effects of clouds on global tropospheric chemistry

  Clouds can modify the photolysis rates (J-values) that drive atmospheric chemistry. The enhanced photochemical rates are found above and in the upper levels of clouds, while reduced rates are found below optically thick clouds and absorbing aerosols. The objective of this study is to quantify the radiative effects of clouds on global tropospheric chemistry and continental pollution outflow, using a state-of-the-art three-dimensional tropospheric chemistry and transport model (GEOS-CHEM). Collaborators: Jim Crawford (LaRC), Brad Pierce (LaRC), and Daniel Jacob's group (Harvard).

• Cloud and Trace Gases Associations

  A recent analysis of trace gas distributions during NASA's TRACE-P aircraft mission (February-April, 2001) revealed significant enhancements of CO (and many other anthropogenic tracers) in cloudy regions as compared to clear areas, while distributions for ozone did not show clear differences between the two regions [Crawford et al., 2003]. I am using the RAQMS regional model to further examine these associations between cloud and trace gases in the springtime Asian pollution outflow. The GEOS-CHEM model is also used to extend this study to the globe and all seasons. Collaborators: Brad Pierce (LaRC), Jim Crawford (LaRC), Jassim Al-Saadi (LaRC), Chieko Kittaka (SAIC/LaRC), and Daniel Jacob's group (Harvard).

• Radiogenic tracers of atmospheric transport

  Lead-210 and beryllium-7 radioisotopes provide sensitive tests of convective transport, large-scale subsidence, and precipitation scavenging in global models. We have applied the GEOS-CHEM model to simulation of the global ²¹⁰Pb and ⁷Be distributions [Liu et al., 2001]. Initial simulation of ⁷Be showed that cross-tropopause transport in the GEOS-1 meteorological fields is too fast by a factor of 3-4. We achieve several improvements over previous models; in particular, we reproduce the observed ⁷Be minimum in the tropics and show that its simulation is sensitive to rainout from convective anvils. Comparisons of model results with aircraft observations up to 12 km altitude over the Pacific suggest that cirrus precipitation is important for explaining the low concentrations in the middle and upper troposphere. (Here is a movie for upper-tropospheric ²¹⁰Pb during the PEM-Tropics A period.)

• ²¹⁰Pb-⁷Be-ozone correlations

  Aerosol samples collected during recent aircraft missions (PEM-West A and B, PEM-Tropics A, and TOPSE) have revealed perplexing relationships between ²¹⁰Pb-⁷Be-ozone. The interpretation of these correlations requires a global three-dimensional model analysis. With our simulations of ²¹⁰Pb and ⁷Be [Liu et al., 2001], we have used the GEOS-CHEM global 3-D model of tropospheric chemistry to investigate the constraints offered by ²¹⁰Pb-⁷Be-ozone correlations on the sources of tropospheric ozone [Liu et al., 2004].

• Sources of tropospheric ozone along the Asian Pacific Rim

  While there are extensive studies of tropospheric ozone in many parts of the world, observations of vertical distribution of tropospheric ozone in East Asia have been very limited. Measurements of tropospheric ozone at lower latitudes of East Asia are even more scarce. Recently established ozonesonde station at the Hong Kong Observatory yields a unique data set for the South China region with weekly or more frequent ozonesonde soundings during a period of one and a half years. Among the features seen in those tropospheric ozone profiles are a frequently observed relative minimum of ozone mixing ratio (as low as 30-40 ppbv) in the upper troposphere (from about 9 to 16 km) in late autumn and winter [Chan et al., 1998], and highly enhanced ozone layers observed in the lower troposphere in spring. The former is associated with air masses coming from the tropical region. For the first time, we linked the latter with biomass burning in continental Southeast Asia which may provide the necessary precursors for the observed ozone enhancement [Liu et al., 1999, Chan et al., 2000]. This study involved Hong Kong Polytechnic University, NOAA Climate Monitoring and Diagnostics Laboratory, and Hong Kong Observatory. More recently, we have used the GEOS-CHEM global 3-D model of tropospheric chemistry to examine sources of tropospheric ozone along the Asian Pacific Rim, with a particular focus on Hong Kong [Liu et al., 2002].

• Transport pathways for Asian combustion outflow over the Pacific

  TRACE-P (TRAnsport and Chemical Evolution over the Pacific) is an aircraft mission over the western north Pacific conducted in March-April 2001 by the NASA Global Tropospheric Experiment (GTE) . Together with Isabelle Bey , Bob Yantosca , and my advisor Daniel Jacob , we have provided chemical tracer forecasting for TRACE-P in the field (see a picture of me in Martin Schultz's Hongkong photo gallery and also Jim Crawford's TRACE-P photo collection ) with the GEOS-CHEM global 3-D model. It has been of particular value for flight planning and will enhance the value of the TRACE-P data set for testing global 3-D model predictions [Jacob et al., 1999].

  Before the mission, we had conducted tagged CO (and O_x) tracer simulations (2^ox2.5^o resolution) for the periods of February-April 1994/2000. The results were presented in the first TRACE-P Science Team Meeting (LARC/NASA, June 26-28, 2000) and the TRACE-P simulation workshop (NCAR, Dec.4-6, 2000). Recently, we have conducted similar simulations for spring 1998 and the TRACE-P period. 1994/1998/2000/2001 simulations (including different phases of ENSO) are now being used to diagnose major transport pathways for Asian combustion outflow over the Pacific in spring and provide an interannual perspective on the TRACE-P results. In addition, we have conducted simulations for the full year of 1996 to place the TRACE-P results in a seasonal context. The model output for March 1994 are presented here as animations showing the transport and transformation of North American fossil fuel CO , European fossil fuel CO , Asian fossil fuel CO , Asian biomass burning CO , and other biomass burning CO in the troposphere (500hPa).

Harvard TRACE-P web page

• Liu, H., J.H. Crawford, et al., Sensitivity of the simulation of global tropospheric chemistry to cloud vertical distribution and optical properties, manuscript in preparation for J. Geophys. Res., 2005.

• Liu, H., J.H. Crawford, et al., Radiative effect of clouds on tropospheric chemistry in a global 3-D chemical transport model, to be submitted to J. Geophys. Res., 2005.

• Liu, H., D.J. Jacob, J.E. Dibb, A.M. Fiore, and R.M. Yantosca, Constraints on the sources of tropospheric ozone from ²¹⁰Pb-⁷Be-ozone correlations, J. Geophys. Res., 109, doi:10.1029/2003JD003988, 2004. [Abstract] [Full text (pdf)]

• Liu, H., Asian outflow of ozone and carbon-monoxide to the Pacific: Origins and pathways, thesis, Dept. of Earth and Planetary Sciences, Harvard University, Cambridage, MA, 2003.

• Liu, H., D.J. Jacob, I. Bey, R.M. Yantosca, B.N. Duncan, and G.W. Sachse, Transport pathways for Asian pollution outflow over the Pacific: Interannual and seasonal variations, J. Geophys. Res., 108(D20), 8786, doi:10.1029/2002JD003102, 2003. [Abstract (html)] [Full Text (pdf)]

• Liu, H., D.J. Jacob, L.Y. Chan, S.J. Oltmans, I. Bey, R.M. Yantosca, J.M. Harris, B.N. Duncan, and R.V. Martin, Sources of tropospheric ozone along the Asian Pacific Rim: An analysis of ozonesonde observations, J. Geophys. Res., 107(D21), 4573, doi:10.1029/2001JD002005, 2002. [Abstract] [Full Text (pdf)]

• Bey, I., D. J. Jacob, R. M. Yantosca, J. A. Logan, B. Field, A. M. Fiore, Q. Li, H. Liu, L. J. Mickley, and M. Schultz, Global modeling of tropospheric chemistry with assimilated meteorology: Model description and evaluation, J. Geophys. Res., 106, 23,073-23,096, 2001. [Abstract] [full text (pdf)]

• Liu, H., D.J. Jacob, I. Bey, and R.M. Yantosca, Constraints from ²¹⁰Pb and ⁷Be on wet deposition and transport in a global three-dimensional chemical tracer model driven by assimilated meteorological fields, J. Geophys. Res., 106, 12,109-12,128, 2001. [Abstract] [full text (pdf)]

• Chan, L.Y., C.Y. Chan, H.Y. Liu, S.A. Christopher, S.J. Oltmans, and J.M.Harris, A case study on the biomass burning in southeast Asia and enhancement of tropospheric ozone over Hong Kong, Geophys. Res. Lett., 27(10), 1479-1482, 2000 [Abstract] [full text (pdf)]

• Liu, H., W.L. Chang, S.J. Oltmans, L.Y. Chan, and J.M. Harris, On springtime high ozone events in the lower troposphere from southeast Asian biomass burning, Atmos. Environ., 33(15), 2403-2410, 1999. [Abstract] [full text (pdf)]

• Chan, L.Y., H.Y. Liu, K.S. Lam, T. Wang, S.J. Oltmans, and J.M.Harris, Analysis of the seasonal behavior of tropospheric ozone at Hong Kong, Atmos. Environ., 32(2), 159-168, 1998. [Abstract] [full text (pdf)]

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