Sub-grid-scale features of anthropogenic emissions of NOx and VOC in the context of regional eulerian models

Model uncertainty is a major issue concerning regional-scale air quality simulation. One major source of uncertainty in regional Eulerian models is due to sub-grid-scale (SGS) effects related to anthropogenic emissions. Regional models typically have horizontal grid resolution (Δ) of 20–80 km. Since NOx chemistry in plumes is nonlinear and often diffusion-limited, the sudden dilution of plumes over regional grid dimensions, as in current models, can lead to a fundamental distortion of their chemistry, resulting in over-production of ozone, peroxides, sulfates and nitrates in the source region, and a related over-depletion of NOx. The corresponding model uncertainty over the whole regional domain remains unquantified. In this paper, we use high-resolution information from urban and regional emission inventories and plume field studies to examine SGS features of the emissions of anthropogenic NOx and VOC (volatile organic compounds), and of their mesoscale dispersion and chemistry. Such examination provides useful insight into some of the main sources of SGS uncertainty, as well as guidance for reducing it. The mesoscale chemistry of power plant plumes is very diffusion-limited, being controlled by VOC entrainment from the background. The crosswind spread of large point-source plumes typically takes 4–6 h to reach 30 km in convective conditions, and at least a full diurnal cycle to reach 80 km. For Δ much larger than 20–30 km, regional models will not be able to capture the essence of the behavior of rural point-source plumes even with plume-in-grid treatment, or to resolve the NOx emissions from many large power plants in urban peripheries from the urban VOC emissions. Within urban areas, there is progressive improvement in the resolution of the important ratio VOC/NOx as Δ is decreased below 20 km. The nature of these emissions-related SGS features suggests that significant gain in regional model accuracy should result by limiting Δ to 20–30 km in the regional domain, by the use of finer nested gridding in metropolitan sub-domains, and by a reactive plume-in-grid treatment of major point-source emissions.