Dr. Xue Xiao

Civil & Environmental Engineering

1. Optimal Estimation of the Surface Fluxes of Chloromethanes

Motivation: The four chloromethanes - methyl chloride (CH₃Cl), dichloromethane (CH₂Cl₂), chloroform (CHCl₃), and carbon tetrachloride (CCl₄), are chlorine-containing gases contributing significantly to stratospheric ozone depletion and having adverse health effects. Large uncertainties in estimates of their source and sink magnitudes and temporal and spatial variations currently exist.

Technique: We implemented the Kalman filter (with the unit pulse response method) to estimate the surface fluxes of their sources and sinks on regional/global scales. The high and low frequency observations from AGAGE, SOGE, NIES and NOAA/ESRL were used to constrain the source and sink magnitudes. GEIA inventories and other bottom-up emission estimates were used to construct a priori maps of surface fluxes of these species. The Model of Atmospheric Transport and Chemistry (MATCH), driven by NCEP interannually varying meteorological fields, was used to simulate the trace gas mole fractions using the a priori emissions and to quantify the time series of sensitivities of tracer concentrations to different aseasonal, seasonal, and regional sources and sinks.

Results: The CH₃Cl inversion results indicate large emissions of 2240 ± 370 Gg yr^-1 from tropical plants. The inversion implies greater seasonal oscillations of the natural sources and sink of CH₃Cl compared to the a priori. Seasonal cycles have been derived for both the oceanic (for CHCl₃ and CH₂Cl₂) and terrestrial (for CHCl₃) sources, with summer maxima and winter minima emissions. Our inversion results show significant industrial sources of CH₂Cl₂and CCl₄ from the Southeast Asian region. Our inversions also reflect the strong effects of the 2002/2003 globally wide-spread heat and drought conditions on the emissions of CH₃Cl from tropical plants and global salt marshes, on the soil fluxes of CH₃Cl and CHCl₃, on the biomass burning sources of CH₃Cl and CH₂Cl₂, and on the derived oceanic flux of CHCl₃.

Citation: Xiao, X. (2008), Optimal estimation of the surface fluxes of chloromethanes using a 3-D global atmospheric chemical transport model, PhD thesis, MIT, Cambridge.

2. Investigation of Soil Uptake Rate of Molecular Hydrogen

Motivation: Hydrogen (H₂), a proposed clean energy alternative, warrants detailed investigation of its global budget and future environmental impacts.

Technique: The magnitudes and seasonal cycles of the major (presumably microbial) soil sink of hydrogen have been estimated from high frequency in situ AGAGE H₂ observations and also from more geographically extensive but low frequency flask measurements from CSIRO and NOAA-GMD using the Kalman filter in a two-dimensional (2-D) global transport model.

Results: Strong seasonal cycles have been deduced for the soil uptake of H₂. The soil sink is a maximum over the northern extratropics in summer and peaks only two to three months earlier in the Northern Hemisphere than in the Southern Hemisphere. The global production rate of H₂ is estimated to be 103 ± 10 Tg yr^-1 with only a small estimated interannual variation. Soil uptake represents the major loss process for H₂ and accounts for 81% of the total destruction.

Citation: Xiao, X., et al. (2007), Optimal estimation of the soil uptake rate of molecular hydrogen from the Advanced Global Atmospheric Gases Experiment and other measurements, J. Geophys. Res., 112, D07303, doi:10.1029/2006JD007241.

Research Highlights at MIT