No abstract
Methane (CH 4) and carbon monoxide (CO) mixing ratios were measured at an air quality monitoring station near the Mt. Wilson (MW) Observatory in southern California starting in the spring of 2007. Diurnal variation and mixing ratio correlation (R 2 ¼ 0.81) were observed. The correlation results observed agree with previous aircraft measurements collected over the greater Los Angeles (LA) metropolitan area. The consistent agreement between CH 4 and CO indicates these gases are well-mixed before reaching the sampling site and the emission source contributions of both compounds are reasonably constant. Since CH 4 and CO are considered non-reactive on the time scale of dispersion within the LA urban area and their emission sources are likely to be similarly distributed (e.g., associated with human activities) they are subject to similar scales of atmospheric transport and dilution. This behavior allows the relationship of CH 4 and CO to be applied for estimation of CH 4 emissions using well-documented CO emissions. Applying this relationship a ''top-down'' CH 4 inventory was calculated for LA County based on the measurements observed at MW and compared with the California Air Resources Board (CARB) ''bottom-up'' CH 4 emissions inventory based on the Intergovernmental Panel on Climate Change recommended methodologies. The ''top-down'' CH 4 emissions inventory is approximately onethird greater than CARB's ''bottom-up'' inventory for LA County. Considering the uncertainties in both methodologies, the different CH 4 emissions inventory approaches are in good agreement, although some under and/or uninventoried CH 4 sources may exist.
Four heavy-duty and medium-duty diesel vehicles were tested in six different aftertreament configurations using a chassis dynamometer to characterize the occurrence of nucleation (the conversion of exhaust gases to particles upon dilution). The aftertreatment included four different diesel particulate filters and two selective catalytic reduction (SCR) devices. All DPFs reduced the emissions of solid particles by several orders of magnitude, but in certain cases the occurrence of a volatile nucleation mode could increase total particle number emissions. The occurrence of a nucleation mode could be predicted based on the level of catalyst in the aftertreatment, the prevailing temperature in the aftertreatment, and the age of the aftertreatment. The particles measured during nucleation had a high fraction of sulfate, up to 62% of reconstructed mass. Additionally the catalyst reduced the toxicity measured in chemical and cellular assays suggesting a pathway for an inverse correlation between particle number and toxicity. The results have implications for exposure to and toxicity of diesel PM.
Emissions from four heavy-duty and medium-duty diesel vehicles were tested in six different aftertreatment configurations using a chassis dynamometer. The aftertreatment included four different diesel particle filters (DPF) and two prototype selective catalytic reduction (SCR) devices for NO(x) control. The goal of the project was to fully characterize emissions from various in-use vehicles meeting the 2007 particulate matter (PM) standard for the United States and California and to provide a snapshot of emissions from 2010 compliant vehicles. The aftertreatment devices all worked as designed, realizing significant reductions of PM and NO(x). The DPF realized > 95% PM reductions irrespective of cycle and the SCRs > 75% NO(x) reductions during cruise and transient modes, but no NO(x) reductions during idle. Because of the large test matrix of vehicles and aftertreatment devices, we were able to characterize effects on additional emission species (CO, organics, and nucleation mode particles) from these devices as a function of their individual characteristics. The two predicting parameters were found to be exhaust temperature and available catalytic surface in the aftertreatment, which combine to create varying degrees of oxidizing conditions. The aftertreatments were not found to incur a fuel penalty.
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