Xiaochun Zhang

Key factors for assessing climate benefits of natural gas versus coal electricity generation

Assessing potential climate effects of natural gas versus coal electricity generation is complicated by the large number of factors reported in life cycle assessment studies, compounded by the large number of proposed climate metrics. Thus, there is a need to identify the key factors affecting the climate effects of natural gas versus coal electricity production, and to present these climate effects in as clear and transparent a way as possible. Here, we identify power plant efficiencies and methane leakage rates as the factors that explain most of the variance in greenhouse gas emissions by natural gas and coal power plants. Thus, we focus on the role of these factors in determining the relative merits of natural gas versus coal power plants. We develop a simple model estimating CO2 and CH4 emissions from natural gas and coal power plants, and resulting temperature change. Simple underlying physical changes can be obscured by abstract evaluation metrics, thus we focus our analysis on the time evolution of global mean temperature. We find that, during the period of plant operation, if there is substantial methane leakage, natural gas plants can produce greater near-term warming than coal plants with the same power output. However, if methane leakage rates are low and power plant efficiency is high, natural gas plants can produce some reduction in near-term warming. In the long term, natural gas power plants produce less warming than would occur with coal power plants. However, without carbon capture and storage natural gas power plants cannot achieve the deep reductions that would be required to avoid substantial contribution to additional global warming. S Online supplementary data available from stacks.iop.org/ERL/9/114022/mmedia

Modeling particulate matter emissions during mineral loading process under weak wind simulation

The quantification of particulate matter emissions from mineral handling is an important problem for the quantification of global emissions on industrial sites. Mineral particulate matter emissions could adversely impact environmental quality in mining regions, transport regions, and even on a global scale. Mineral loading is an important process contributing to mineral particulate matter emissions, especially under weak wind conditions. Mathematical models are effective ways to evaluate particulate matter emissions during the mineral loading process. The currently used empirical models based on the form of a power function do not predict particulate matter emissions accurately under weak wind conditions. At low particulate matter emissions, the models overestimated, and at high particulate matter emissions, the models underestimated emission factors. We conducted wind tunnel experiments to evaluate the particulate matter emission factors for the mineral loading process. A new approach based on the mathematical form of a logistical function was developed and tested. It provided a realistic depiction of the particulate matter emissions during the mineral loading process, accounting for fractions of fine mineral particles, dropping height, and wind velocity.

Time scales and ratios of climate forcing due to thermal versus carbon dioxide emissions from fossil fuels

The Earth warms both when fossil fuel carbon is oxidized to carbon dioxide and when greenhouse effect of carbon dioxide inhibits longwave radiation from escaping to space. Various important time scales and ratios comparing these two climate forcings have not previously been quantified. For example, the global and time-integrated radiative forcing from burning a fossil fuel exceeds the heat released upon combustion within 2 months. Over the long lifetime of CO2 in the atmosphere, the cumulative CO2-radiative forcing exceeds the amount of energy released upon combustion by a factor >100,000. For a new power plant, the radiative forcing from the accumulation of released CO2 exceeds the direct thermal emissions in less than half a year. Furthermore, we show that the energy released from the combustion of fossil fuels is now about 1.71% of the radiative forcing from CO2 that has accumulated in the atmosphere as a consequence of historical fossil fuel combustion.

Radiation partitioning and its relation to environmental factors above a meadow ecosystem on the Qinghai‐Tibetan Plateau

Understanding the energy balance on the Qinghai-Tibetan Plateau is essential for better prediction of global climate change. To characterize the energy balance on the plateau, we examined the radiation partitioning over a Kobresia meadow, the most widely distributed vegetation on the plateau, for the period from 2002 to 2005. The incident solar radiation (R(s)) and net radiation (R(n)) averaged 6298 and 2779 MJ m(-2) yr(-1), respectively. The albedo averaged 0.220 annually, with a slightly low value of 0.202 in the growing season from May to September. An increase in soil water or leaf area index was correlated with a decrease of albedo over the meadow. The annual solar radiation lost 34% as longwave radiation, which was higher than values reported for lowland grasslands. The annual radiation efficiency (R(n)/R(s)) over the meadow, at an average of 0.44, was, however, much lower than that for lowland grasslands. The net longwave radiation (L(n)) and the normalized effective radiation (L(n)/R(s)) over the meadow were much higher than that for the global surface or for lowland grasslands, indicating that the longwave exchange between alpine meadow and atmosphere is the most important component of energy losses. A path analysis suggests that the water vapor pressure, air temperature, and cloud cover are the major factors governing the variations of both the net radiation and the net longwave radiation in the alpine meadow ecosystem.