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.

Nursery-grown, native plant species have potential application for revegetating disturbed arid and semiarid lands.We evaluated nurserygrown fourwing saltbush [Atriplex canescens (Pursh) Nutt.], allscale saltbush [A. polycarpa (Torrey) S. Watson], bladderpod (Isomeris arborea Nutt.), honey mesquite [Prosopis glandulosa Torrey var. torreyana (L. Benson) M. Johnston], and rubber rabbitbrush [Chrysothamnus nauseosus (Pallas) Britton] transplanted to abandoned agricultural land throughout the western Mojave Desert. Two types of temporary plant enclosure for herbivory and environmental protection (plastic cones and wire cages) and three mulch treatments (straw, bark, and none) were tested at all six sites. Rubber rabbitbrush was difficult to propagate in the nursery and is not recommended for transplanting. Significant differences in plant performance occurred between sites with similar aerial environments but contrasting degrees of edaphic disturbance. Plastic cones were significantly superior to wire cages for plant vigor and survival but no differences were detected between mulch treatments. Fourwing saltbush was generally successful over all treatments and sites and is recommended for transplanting in this area. In a larger plot study, narrow augered holes led to superior survival of honey mesquite relative to wide, hand-dug holes, and plastic cones were superior to wire cages. Mortality of all species was high due to dry, but not atypical, weather during the 2 yr of the study. We conclude that transplanting without intensive irrigation does not guarantee survival of even the most successful species. Its greater cost relative to direct seeding may not be warranted for large-scale restoration of arid and semiarid environments.

Windblown fugitive dust contributes to violations of air quality standards for particulate matter <10 pm aerodynamic diameter (PM10). In the western Mojave Desert of California, approximately 1070 ha of previously filled or over‐grazed land impacted downwind metropolitan areas by wind‐driven emissions of dust. A protocol of furrowing across the wind and direct seeding of three native perennial shrubs and a bunch grass helped reduce fugitive dust emissions in this area by more than 95%. Seeded species varied from 35 to 97% of living plant cover in individual years, reflecting rainfall patterns. In areas of deep sand, Indian ricegrass (Achnatherum hymenoides Roemer & Shultes) outperformed the shrubs, while fourwing saltbush [Atriplex canescens (Pursh) Nutt.] exhibited the most widespread establishment. This revegetation was achieved in an anomalous year with above average and late rainfall that eliminated early competition from annual species and later fostered abundant shrub growth. This success was not reproducible in more normal years, when minimal disturbance protocols such as broadcasting of seed on the untilled soil surface were as effective and less costly. We conclude: (i) direct seeding can lead to plant establishment in favorable years, but is likely to fail in any given year, (ii) direct seeding should be implemented with little soil disturbance, (iii) the native fourwing saltbush is the most likely species to become established in this environment, and (iv) unpredictable rainfall and temperature require that direct seeding be backed up with alternative strategies to achieve reliable dust and PM10 mitigation in arid environments.

Dust storms, like this one pictured along a county road in the Antelope Valley, have led to serious traffic accidents and violations of air quality standards.

Aerosol samplers collect material that is locally generated as well as that transported from upwind; knowing the extent of the area from which the sample is drawn is necessary for proper interpretation of sampler data. The U.S. Environmental Protection Agency (EPA) PM 2.5 monitoring guidelines recognize a conceptual hierarchy of sampler spatial representation, but provide no objective measures of a site's spatial representativeness. A case study of a sampler tributary area in central California provides insights into the factors that determine a sampler's spatial representation. Winter diurnal cycles of fine particle concentrations at places of habitation ranging from urban cores to small farm towns show a marked cycle that can be linked to local human activity. Assessment of the possible causes of the observed cycles leads to the hypothesis that local sources dominate primary particle mass in winter samples. The hypothesis was tested using a simple model to relate routine 24-hr PM 10 and PM 2.5 samples to a sampler's surroundings. Model results indicate that even minor sources very close to a sampler will overwhelm any regional component in a sample. The results for the cases studied also demonstrate that, in winter, most coarse (PM 10-2.5 ) particles collected are less than 2 hr old, and most primary fine (PM 2.5 ) particles are less than 4 hr old. Even on days that are not truly IMPLICATIONS Aerosol modeling and source apportionment have taken on heightened importance with the promulgation of the new National Ambient Air Quality Standards for PM 2.5 . Achieving closure between aerosol model results and field observations has proven difficult in many settings. This paper explores the spatial aspects of the problem. The findings contradict conventional wisdom about multiday, high PM episodes and suggest that detailed temporal and spatial resolution of both transport fields and source distributions will be needed to reduce uncertainties in PM 2.5 and PM 10 assessments."stagnant," samplers are very strongly influenced by their immediate surroundings (distances less than 10 km), and only weakly influenced by regional emissions.The implications for interpretation of sample analyses are as follows:(1) Typical PM sampling networks are unlikely to represent regional conditions; (2) Similarity of samples in time and space between widely separated samplers probably arises from sampling analogous local environments rather than a uniformly mixed regional air mass; (3) Even weak sources near a sampler will prevent regionally representative samples, so that "background" specification in models can be strongly skewed by misapplication of sampler data; (4) Source-receptor relationships within a single modeling grid cell can cause measured and modeled source impacts at a sampler to diverge by orders of magnitude, even for grid cells as small as 1 km; and (5) Differential deposition of coarse and fine particles will skew source apportionment by chemical tracers unless the tracers and the source emissions have the same s...