High-resolution, routine observations from OMI offer a unique perspective on the spatial and temporal variation of regional-scale anthropogenic NO2 emissions. By comparing trends in OMI observations with those from ground-based measurements and an emissions inventory, we have shown that satellite observations are well-suited for capturing changes in emissions over time.
Average tropospheric NO2 column concentrations (molecules cm-2) from OMI and CARB surface sites for weekdays and weekends for the South Coast region (Los Angeles basin) of California (Russell et al., 2010).
OMI observations indicate that NOx concentrations have decreased significantly in urban regions of the United States between 2005 and 2011, with an average reduction of 32 ± 7%. By examining day-of-week and interannual trends, we find that these reductions can largely be attributed to improved emission control technology in the mobile source fleet; however, we also find that the economic downturn of the late 2000's has impacted emissions.
Average summertime (April–September) OMI BEHR NO2 column densities for 2005 (left) and 2011 (right) (Russell et al., 2012).
MODIS fire pixels, OMI tropospheric NO2 columns, and wind vectors during the Zaca fire on August 19, 2007. NO2 in the fire plume dwarfs the signal from Los Angeles (white circle) (Mebust et al., 2011).
June mean anomaly in NO2 from a model of soil NOx emissions (left) and from OMI (right), calculated as difference compared to the June 2005–2008 mean (Hudman et al., 2010).
Capturing the spatial variability of NOx is necessary for understanding both ozone and nitric acid formation and the transport of reactive nitrogen due to nonlinear feedbacks of NOx on hydroxyl radical concentration and ozone production. For most of its time in orbit, the operational mode of OMI has been used to infer column NO2 with a footprint at nadir that is 13 km along-track and 24 km across-track. Higher-resolution observations can be achieved if CCD detector elements in the across-track dimension are not binned on-board the instrument, giving an optics limited footprint of approximately 7×13 km2 at nadir. We have found that at this spatial scale, slant column NO2 varies by up to 1×1016 molecules cm-2. The retrieved signal to noise ratio of the spectrum is as high as 20 and is ~5 at the plume edge, similar to the ratio for binned values in the same region. The high-resolution observations are capable of distinguishing three of four large point sources in close proximity around the Rihand Reservoir, a distinction not possible in either the single orbit or six-nadir orbit operational scale average. Taking advantage of the enhanced spatial detail, we derive a chemical lifetime of 1.9 hours for an NO2 plume advected over an uninhabited desert downwind of Dubai, UAE, where the spatial gradient in column NO2 depends more on chemical processes than on emissions.
a) MODIS RGB image, b) OMI super-zoom mode slant column NO2 (SCDNO2), c) operational-scale SCDNO2,, and d) a six-orbit operational-scale average SCDNO2 over the Rihand Resovoir in India. Power plants located around the Rihand Reservoir include Singrauli and Vindhyachal (PP1 – 4200 MW), Anpara (PP2 –1600 MW), Rihand (PP3 –1000 MW pre-2006), and Obra (PP4 – 1600 MW; Valin et al., 2011).
Day of week patterns in NO2 have been reported around the world and used to characterize daily patterns in emissions. However, changes in NO2 with day of week also reflect changes in the chemical removal rate of NO2. We used the WRF-Chem model to simulate the response of column NO2 to decreases in weekend NOx emissions and compare the simulations with spatial variations in the day of week pattern of NO2 as observed by OMI in the Los Angeles Basin. We find that in the model, the absolute reduction in NO2 and its spatial variation depend on emission reductions and changes in its chemical removal via feedback on OH and RO2. While the standard WRF-Chem model predicts weekday column NO2 that is in qualitatively good agreement with observation, it is not able to accurately capture the magnitude or spatial pattern of weekend decreases in column NO2. By increasing emission of volatile organic compounds in the 2005 EPA National Emission Inventory (NEI) by a factor of two, the simulated response of column NO2 to decreased NOx emissions is in much better agreement with observations than the standard WRF-Chem scenario. Taken together, these results indicate that the observations and model provide constraints not only on NO2 emissions, but also on VOC and HOx production rates.
OMI-observed average JJA 2005–2008 weekday column NO2 (top left) and the percent decrease of column NO2 observed by OMI in the corresponding weekend average (bottom right). WRF-Chem simulation with standard (top middle, EVOC=1x NEI2005) and 2x standard VOC emissions (top right, EVOC=2x), over Southern California for June 10–24, 2006 of “weekday” column NO2 and the simulated decrease in simulated “weekend” column NO2. Anthropogenic mobile NOx emissions are decreased by 37.5% for weekend simulations while emissions of all other species are held constant. Values where decreases are less than 37.5% are not shown.