4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 1 A Hydrogen Economy’s Potential Environmental Impacts Chun Zhao Evan Cobb

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 2 A Hydrogen Economy

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 3 Hydrogen characteristics in the atm. Observed global hydrogen burden: 182 Tg Global sink: 74.4 Tg/yr Lifetime : 2.5 years Rahn etc Current Mixing Ratio of H 2 : 510ppbv Tropospheric hydrogen Stratospheric hydrogen

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 4 Hydrogen in troposphere The sources and sinks of hydrogen in troposphere

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 5 Note for table of sources and sinks Main source of Hydrogen: oxidation of organic compounds Main sink of Hydrogen: soil uptake Man-made sources: fossil fuel combustion CO + H 2 O H 2 + CO 2 Main chemical sink: OH + H 2 H + H 2 O Debate: How is H 2 lost? –What portion of H 2 is consumed by soil/microbial activity?

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 6 Hydrogen production from oxidation of organic compounds

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 7 Reduction in OH H 2 behaves like CO (takes up one OH and releases one HO 2 radical + H 2 0) Michael Prather, 2003.

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 8 Reduction in OH by ↓NO x Decrease in OH is largely driven by the reduction in NO x emissions Importance of HO x /NO x coupling leads to non- linear dependence of OH on NO x levels Shultz et al. 2003

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 9 Hydrogen in the Stratosphere The main hydrogen reservoirs in Stratosphere: Molecular hydrogen (H 2 ) Water vapor (H 2 O) Methane ( CH 4 )

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 10 H 2 Sources in the Stratosphere CH 4 OH, O( 1 D), Cl CH 3 O 2 HHO,OH,HCl CH 3 O 2 NO NO 2 CH 3 O O 2 HO 2 CH 2 O hv OH CO+H 2 HCO+H HCO+H 2 O

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 11 H 2 sinks the in stratosphere Reaction with OH, O( 1 D), Cl: H 2 + OH H 2 O + H H 2 + O( 1 D) OH + H H 2 + Cl HCl + H

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 12 The fate of Hydrogen in stratosphere The H 2 mixing ratio in the lower and middle stratosphere is nearly constant, the net hydrogen cycling in the stratosphere can be regarded as a loss in methane and a production of water.

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 13 Main reactions of H 2 in the stratosphere

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 14 Potential chemical changes in the stratosphere H 2 + OH → H 2 O + H “[H 2 O]…would result in cooling of the lower stratosphere, and the disturbance of ozone chemistry, which depends on heterogeneous reactions involving hydrochloric acid and chlorine nitrate on ices of H 2 O.” Tromp et al pg. 1740

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 15 Environmental Impact Overview Tropospheric Effects Reduced oxidative capacity of atm. (OH) Reductions in NO x, soot, sulfates, CO 2, O 3 Increased surface H 2 concentrations Change in atmosphere- biosphere reactions Global warming Stratospheric Effects Increase of water vapor Cooling in lower layers Enhanced ozone destruction chemistry Increase in noctilucent clouds Global warming

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 16 Impacts ALL hinge upon… Production methods of H 2 –“Clean” or “dirty” Leak rates from system –Current networks in Germany achieve 0.1% –Natural gas pipelines: % –10-20% losses possible from uncontrolled evaporation from liquid storage tanks –Complete fossil fuel replacement and 3% leak rate would ↑ H 2 emissions times.

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 17 Increased Surface H 2 H 2 source is from system leaks H 2 burden could increase by 30%- 120% Increased partial pressures of H 2 could affect microbial colonies More pronounced changes in N.H. than S.H.

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 18 Environmental Impacts Schultz et al Increased H 2 concentrations lead to a reduction in OH and an increased lifetime of CH 4 and without reductions in NO x, increases in tropospheric O 3

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 19 Global Warming Impacts Increased lifetime of CH 4 Changes of tropospheric and stratospheric ozone levels Noctilucent cloud formation (albedo change) Dependent on generation processes Dependent on level of fuel cell replacement –Massive reductions in CO, CO 2, NO x, and other combustion emission if made cleanly

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 20 GWP Increase for CH 4 Schultz et al. In some models, Τ CH4 would increase by 26% Radiative forcing of 0.5 W*m -2 Prather Doesn’t take NO x reductions into account Increases lifetime of CH 4 –0.60 ppm H 2 increase –GWP of ~0.026 W*m -2

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 21 GWP of H 2 Production Reductions of greenhouse gas emissions –How much? Increase of greenhouse emissions –Emissions of CO 2 : ↑34%, CH 4 : ↑19% –H 2 made by: Hydrolysis after electricity from coal power Gasification of coal Natural gas reforming Shultz et al. 2003

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 22 Reduction of Tropospheric O 3 Up to 50% reductions of NO x and CO by 100% fuel cell replacement of surface fleet reduces tropospheric ozone Assumes all H 2 is produced using emission-free processes Reduction of 1-8 ppbv in surface ozone throughout N.H. Shultz et al. 2003

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 23 Reduction of Stratospheric Ozone Increases of H 2 to stratosphere result in –Increase of H 2 0 –Decrease of columnar O 3 –Tromp et al Increase of H20 Decrease of O3

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 24 Noctilucent Clouds Clouds at extremely high altitude, about 85 km, that literally shine at night. They form in the cold, summer polar mesopause and are believed to be ice crystals. ( “An increase in the mesosphere of H 2 O derived from H 2 could lead to an increase in noctilucent clouds, with potential impact on Earth’s albedo and mesopheric chemistry.” –Tromp et al. 2003

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 25 Summary An H 2 economy could provide substantial improvements in local, regional, global air quality and lower greenhouse gas emissions depending upon production processes. Large uncertainties remain –NO x, CH 4, CO 2 emission changes –H 2 lifetime

4/20/2006Ga Tech - EAS Air Chemistry Group Presentation 26 Questions?