3.1. Spatial redistribution impacts
To isolate the impact of spatially redistributing fossil fuel emissions, we investigated differences in annual mean CO2 fluxes and atmospheric CO2 concentrations. Quantifying the fossil fuel emissions using the Vulcan process-based model rather than using population density redistributes the emissions in space (Fig. 1, top panel). In general, the emissions are reduced over large, spatially coherent areas in the Vulcan inventory. To compensate for this reduction, individual gridcells have much higher emissions due to ‘point sources’ such as power plants or manufacturing facilities. Regional patterns exist where the Vulcan emissions shift from city centres to suburban areas, particularly in the south and east. Differences also occur due to traffic patterns: higher emissions through the central United States correspond to major highways.
Figure 1. (Top panel) Difference between A96 annual fossil fuel CO2 emissions and the Vulcan annual mean estimates (Vulcan—A96). (Bottom panel) Annual mean 30 m CO2 concentration differences (HRFF—FF95).
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Utilization of the higher resolution, process-based emissions inventory versus the population-based approach creates spatially coherent anomalies in the CO2 concentration field, with differences up to 6 ppm (Fig. 1, bottom panel). The annual mean CO2 concentration differences reflect the different spatial emission patterns of the two inventories, but are more spatially coherent across large scales when compared to the differences in the surface emissions themselves. The most noticeable differences in the CO2 concentration field are associated with large point source emission differences. Centres of high concentrations occur in the central and southeastern United States over specific locations of power plants, whereas regions of lower concentrations occur over cities with high-population density but where the power generation does not occur within city limits. Significant changes occur over heavily populated areas in California: the northcentral region has higher concentrations in the HRFF simulation whereas the southern coastline has considerably lower concentrations. The lower concentrations over southern California are advected southward along the coast by strong wind currents, creating a large spatially coherent region over the Pacific ocean. Lower concentrations in the HRFF case also occur over the Pacific northwest, the southwest and the front range of the Rocky Mountains, whereas higher concentrations are seen over the northern-central and southern-central states. Differences due to the spatially coherent emission differences are minimal, due to the small magnitude of the widespread changes.
The alternating patterns of high and low CO2 creates significant concentration gradients. For example, a gradient of more than 6 ppm occurs in central Texas, where the emissions shift from the city to more remote areas. Several other dipoles of higher and lower concentrations exist throughout the eastern United States, primarily due to shifting of emissions away from population centres to isolated large point sources (e.g. power plants, refineries, large factories).
3.2. Temporal redistribution impacts
Fossil fuel emissions vary diurnally and seasonally, and including this temporal variability will impact the resulting atmospheric CO2 concentrations. On the diurnal time scale, densely populated areas experience maximum emissions during morning and evening rush hours due to heavy motor vehicle traffic. On the seasonal time scale, the United States average Vulcan emissions have maximum emissions in July and August and lower emissions during the spring and fall. The seasonal cycle differs from the long-term mean seasonal cycle reported in Blasing et al. (2005), which showed maximum emissions in January and a secondary maximum in the summer; however, Blasing et al. showed that the magnitude of the summer maximum substantially increased between the 1980s and the 1990s. It should be noted that seasonality in the residential and commercial sectors, which contributed 10% to the total emissions, was not included in the Vulcan release version used in this study.
The seasonal cycle in the Vulcan emissions varies regionally (Fig. 2). Consistent with the U.S. total seasonal cycle, the largest emissions occur during the summer (JJA) and the smallest emissions occur in the fall (SON). Regionally coherent increases (decreases) in fossil fuel emissions persist over the majority of the country during JJA (SON); however, the magnitude of these changes is generally less than 1 μmol m−2 s−1, except at localized cities or power plants with large seasonal cycles causing differences greater than 5 μmol m−2 s−1. In the winter (DJF) and spring (MAM), the regional seasonality is much weaker, as the differences between the annual mean emissions are generally less than 10% except at individual gridcells. During the first half of the year, the eastern and western halves of the country have opposite anomalies, with the eastern states having higher emissions and the western states generally having lower emissions in DJF (and vice versa for MAM). In general, individual anomalous gridcells (dominated by large point sources or cities) follow the large-scale seasonal cycle; however, the seasonal cycle at several of these locations does vary. For example, in California, individual grid cells with emissions lower and higher than the annual mean can be seen in the summer and fall, respectively. Both large-scale seasonality and individual gridcell seasonality will alter the atmospheric CO2 concentrations.
Figure 2. Seasonal fossil fuel emissions from Vulcan, calculated at every gridcell by subtracting annual mean emission from the 3-month average and then dividing by the annual mean to convert to percent difference for each season.
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Figure 3 shows the CO2 concentration difference at 30 m above the land surface between the HRFF and FF95 simulations. Spatially coherent differences can be seen in the eastern half of the country, where the concentrations from the HRFF case are higher than the FF95 concentrations in the summer and lower in the fall. The largest difference between the two cases occurs in August, when near-surface CO2 is more than 15 ppm higher in the HRFF case at individual gridcells. On average, differences spanning 3–6 ppm are seen over the entire eastern portion of the country. In the fall when the Vulcan emissions decrease, the CO2 concentrations in the HRFF case also decrease, and large-scale differences spanning 4–6 ppm occur in the southeast, with maximum differences in November.
In certain locations, the sign of the differences remains the same throughout the year due to the different spatial allocation of fossil fuel CO2 emissions in the Vulcan versus A96 data products, but the magnitude of the differences are affected by the Vulcan seasonality. In southern California, lower concentrations persist year-round, but the magnitude of the difference varies from ∼6 ppm in the summer to greater than 15 ppm in the fall and winter. Similar features occur across the central states, with spatially coherent differences centred over cities (i.e. Denver, CO and Dallas, TX, USA). Between lower concentrations in the fall and higher concentrations in the summer, the amplitude of the seasonal difference between the HRFF and FF95 simulations exceeds 15 ppm at some locations (Fig. 4, top panel). The magnitudes of the seasonal amplitudes are spatially coherent, highlighting the regions that are significantly affected by the Vulcan seasonality. The seasonal CO2 amplitudes are greatest over densely populated regions and less dramatic over the central and western United States, where the fossil fuel emissions are smaller.
Figure 4. (Top panel) Seasonal amplitude CO2 concentration differences at 30 m (HRFF—FF95), where the magnitudes correspond to the amplitudes of spline fits to the CO2 HRFF—FF95 differences at every grid cell. (Bottom panel) Diurnal amplitude differences at 30 m, calculated by taking the amplitude of the annual mean diurnal cycle from the HRFF—FF95 differences.
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In addition to seasonal changes, the Vulcan emissions also have diurnal variability. Diurnal variations in emissions cause spatially coherent concentration anomalies over cities and highways; however, the magnitudes of the differences between the two simulations are small relative to the seasonal impacts, causing changes less than 3 ppm (Fig. 4b). Monthly maps of diurnal amplitude concentration differences indicate the seasonal variability in the diurnal cycle only causes small changes in the CO2 concentration anomalies. Although the diurnal variability does alter the atmospheric concentrations, the impact on regional scales appears to be much smaller than the impacts from seasonally varying emissions.