In this research, three most common hydrogen supply pathways will be compared in detail in terms of lifecycle emissions and the resulting changes in ozone air quality.

The following three hypothetical hydrogen pathways will be considered: (1) central steam methane reforming (SMR) of natural gas with pipeline delivery; (2) central SMR with liquid hydrogen truck delivery; and (3) onsite small-scale SMR. With geographic considerations in Sacramento County, California, the spatial layout of each pathway step (such as the NG-to-hydrogen conversion plant, refueling stations or onsite production stations, truck delivery routes, etc.) will be determined using GIS data and related real-world engineering/economic data. Assuming that 10% and 25% of current light duty fleet in Sacramento are hydrogen fuel cell vehicles (HFCV), the regional hydrogen demand for vehicle use can be estimated.

Ozone is formed in the atmosphere by volatile organic compounds (VOC) and nitrogen oxides (NOx) in the presence of sunlight, especially when the atmosphere is very stable. It is difficult to predict ozone pollution since it is not directly emitted to the air just like those primary criteria pollutants. However, both VOC and NOx, which are the ozone precursors, could be emitted directly from hydrogen supply pathways. Therefore, the following method will be used to estimate the resulting changes in ambient ozone air quality due to hydrogen pathways from a lifecycle analysis (LCA) perspective. First of all, the emission inventories for both VOC and NOx will be established, corresponding to each of hydrogen pathway steps. Next, using emission inventories and typical meteorological year 2 (TMY2) data for Sacramento County as inputs, a Gaussian atmospheric dispersion model will be run to predict the concentration increment at each of the receptors (these pollutant receptors are assumed to be located at the officially maintained air quality monitoring stations in Sacramento). Then, a model to explore the relationship between concentrations of ozone and its precursors will be developed and run to predict the changes in ambient ozone pollution. Finally, we can compare the ozone pollution due to various hydrogen pathways and determine the least polluted pathway; additionally, we also can know the annual distribution of changes in ozone pollution associated with each of hydrogen pathways.

Based on the predicted ozone pollution (and the other criteria pollutants in other studies), demographic data in Sacramento, and does-response functions, the social cost of a hydrogen pathway could be estimated, which is useful for planners and policymakers.