A22A-01 INVITED 10:20h
Evaluating NOx emissions, and emissions changes, using instrumented aircraft
The location, strength, and timing of anthropogenic NOx emissions strongly influences particulate matter and ozone formation in the continental boundary layer. Ambient measurements using instrumented aircraft will be analyzed to better quantify NOx source strength and variability on short and medium time scales. Daytime emissions from urban areas and power plants are routinely variable on time scales of hours to days and show occasional dramatic changes of 50% or more, with corresponding effects on secondary photochemistry as a result. Observations of the influence of nocturnal emissions of NOx from anthropogenic sources on secondary product formation will also be discussed.
A22A-02 INVITED 10:35h
Mobile Laboratory Characterization of Nitrogen Oxide Emissions from Motor Vehicles and Aircraft
Nitrogen oxide emissions from mobile sources can strongly influence photochemical oxidant and secondary particulate nitrate formation in the troposphere. Quantification of the impact of mobile NOx sources requires accurate characterization of mobile source emissions under real world operating conditions. We have developed a mobile laboratory equipped with fast response trace gas sensors that has been deployed to quantifyon-road NOx fleet average emissions, NOx emissions from individual on-road vehicles, and aircraft NOx emissions at airports during taxi, take-off, and tarmac operations. Measurement strategies and instrumentation will be described and results of NOx emissions from a range of on-road vehicles, urban traffic mixes, and commercial aircraft will be presented.
A22A-03 INVITED 10:50h
Weekly and Decadal Changes in NO$_{x}$ Emissions and Tropospheric Ozone
Variations in air pollutant emissions and ambient concentrations on both weekly and decadal time scales can be used to test our understanding of atmospheric responses to changes in anthropogenic forcing. Combining records of fuel sales and on-road measurements of vehicle activity and emissions, we have estimated motor vehicle emissions by hour and day of week, separately for gasoline and diesel-powered vehicles. Between 1990 and 2000, emissions of nitrogen oxides (NO$_{x}$) from motor vehicles in California decreased by more than 30%. However, NO$_{x}$ emissions from heavy-duty diesel trucks actually increased by over 40%, offsetting some of the reductions in light-duty vehicle emissions. During the past two decades, the occurrence of higher ozone levels on weekends, a phenomenon known as the weekend effect, has become more widespread in California. The effect impacted 11% of surface observation sites in 1980-84 and 38% in 1995-1999. Results of chemical transport modeling show that the primary cause of the weekend ozone effect is the large decrease in NO$_{x}$ emissions due to a ~75% reduction in diesel truck traffic on weekends. Areas where ozone formation is VOC-sensitive therefore can experience higher ozone concentrations on weekends. Long-term (decadal) changes in anthropogenic emissions have produced a shift towards greater VOC-sensitivity, and the weekend ozone effect has grown more prevalent because diesel trucks now account for over 50% of total motor vehicle NO$_{x}$ emissions.
A22A-04 INVITED 11:05h
NOx Emissions from Large Point Sources: Variability in Ozone Production, Resulting Health Damages and Economic Costs
We present a proof-of-concept analysis of the measurement of the health damage of ozone (O3) produced from nitrogen oxides (NOx = NO + NO2) emitted by individual large point sources in the eastern United States. We use a regional atmospheric model of the eastern United States, the Comprehensive Air Quality Model with Extensions (CAMx), to quantify the variable impact that a fixed quantity of NOx emitted from individual sources can have on the downwind concentration of surface O3, depending on temperature and local biogenic hydrocarbon emissions. We also examine the dependence of resulting ozone-related health damages on the size of the exposed population. The investigation is relevant to the increasingly widely used "cap and trade" approach to NOx regulation, which presumes that shifts of emissions over time and space, holding the total fixed over the course of the summer O3 season, will have minimal effect on the environmental outcome. By contrast, we show that a shift of a unit of NOx emissions from one place or time to another could result in large changes in the health effects due to ozone formation and exposure. We indicate how the type of modeling carried out here might be used to attach externality-correcting prices to emissions. Charging emitters fees that are commensurate with the damage caused by their NOx emissions would create an incentive for emitters to reduce emissions at times and in locations where they cause the largest damage.
http://www.wws.princeton.edu/mauzerall/dlm_publications.htm
A22A-05 11:20h
Aircraft Observations of NO3 and N2O5 During NEAQS/ICARTT 2004
We report the first observation of NO3 and N2O5 from an aircraft platform. The measurements were taken on the NOAA WP-3 during the ICARTT campaign in the summer of 2004. Preliminary scientific findings include the following. Concentrations of NO3 and N2O5 were often enhanced aloft relative to those made at the surface. In recently emitted NOx plumes of urban origin, for example, NO3 and N2O5 occurred at concentrations as large at 400 pptv and 3 ppbv, respectively, and in some cases constituted a significant fraction of nocturnal NOy. In other cases, rapid degradation of both compounds was observed, particularly in the presence of sulfate aerosol. Vertical profiles showed clear differences in the nocturnal nitrogen oxide chemistry between the surface layer and the free troposphere. The presentation will discuss the implications of these observations for nocturnal budgets of NOx and O3, as well as nocturnal VOC oxidation and aerosol processing.
A22A-06 11:32h
Emission inventories for nitrogen oxides from U.S. road transport: What can ambient measurements tell us?
Road transport constitutes a relatively small fraction (about 40 percent) of U.S. nitrogen oxide emissions, yet it is an important source since 1) it is released predominately in populated urban areas, and 2) it is released simultaneously with reactive organic compounds, which provides an effective mix of precursors for photochemical ozone production. Comparisons of road transport emission inventories with ambient measurements in U.S. urban areas provide simple tests of some aspects of the inventories of these emissions. These tests indicate that the inventory for road transport and its temporal trend are surprisingly inaccurate. Most importantly, nitrogen oxide emissions are increasing over the last decade, rather than decreasing as suggested in the most recent inventories. Further, carbon monoxide emissions are over-estimated by about a factor of two, and total reactive organic emissions, while accurately estimated, are very poorly allocated to individual hydrocarbon species.
A22A-07 11:44h
Changes in Ozone Due to Reductions in Precursor Emissions: Application of the Adjoint Sensitivity Analysis Procedure
Control of oxides of nitrogen (NO$_x$) and volatile organic compounds (VOC) can significantly shift the sensitivities of ozone production and, subsequently, alter the contribution of emission source regions. Understanding these changes is critical for designing effective ozone control measures as the magnitude and ratio of precursor emissions change over time. However, because of the complexity of transport and chemistry in an urban airshed and because of the large number of parameters that can change, such shifts have been difficult to quantify. This study developed a new modeling tool, the Adjoint Sensitivity Analysis Procedure (ASAP), to efficiently predict all input data sensitivities in a three-dimensional Chemistry-Transport Model. The ASAP was applied to Southern California during a summertime air pollution episode to quantify the effects of changes in precursor emissions on ozone sensitivities. Using ASAP, we generated ranked lists of sensitivities of ozone to all emissions and all reaction rate coefficients. The ranked lists enabled us to quantify the effect of NO$_x$ and VOC emission reductions on chemical dynamics. For example, we found that a 50% reduction in NO$_x$ emissions increased ozone, but also shifted the chemistry from a VOC sensitive to a NO$_x$ sensitive regime as evidenced by the finding that ozone sensitivity to NO$_x$ emissions shifted from strongly negative to positive. Furthermore, the relative importance of NO$_x$ emissions increased compared to VOC emissions. Radical initiation in the reduced NO$_x$ case, with its higher relative abundance of VOC species, was found to be less important relative to the base case as evidenced by the drop in ranking of the sensitivity of ozone to the rate coefficients of radical initiation reactions. Changes in the distribution and sign of the adjoint function for emitted species revealed changes in the ozone source apportionment due to changes in emissions. For example, with a 50% reduction in NO$_x$ emissions, the adjoint function for nitric oxide (NO) remained positive for sources offshore and downwind of an inland receptor location, but switched from negative to positive for onshore sources, indicating a switch to a positive contribution to ozone.
A22A-08 11:56h
Heterogeneity of ozone yield with location of nitrogen oxide emission origin
Considering a summertime air pollution episode in Georgia as a case study, we examine how ozone accumulation efficiency and distance of impact depend on the intensity of a NOx source, its elevation above ground level, and its location within the state. The High-order Decoupled Direct Method in Three Dimensions (HDDM-3D) is applied in Community Multiscale Air Quality model simulations to quantify ozone accumulation efficiency by three metrics. In addition, distance of impact for each NOx source region is quantified by computing the first moment of associated ozone accumulation; the first moments are found to be similar across source regions in Georgia (140-210 km). We find that two mid-sized metropolitan regions in Georgia yield 40-70 percent more ground-level ozone per ton of NOx than two large power plants, and 20 percent more per ton than the Atlanta region, consistent with the lower emissions intensities of the mid-sized regions. However, we find that the heterogeneity of ozone yield for various locations within the Atlanta region can be even greater than the differences among regional averages. Heterogeneity of response to Atlanta emissions becomes far more pronounced if ozone accumulation is weighted by population, indicating that the per ton impact of a NOx source on potential population exposure is highly dependent on emission location. We discuss how small-scale heterogeneity of ozone yield and potential population impact complicate the formulation of sensible abatement and trading policies.
A22A-09 12:08h
Effects of Changing Emissions on Ozone and Particulates in the Northeastern United States
Emissions of nitrogen oxides (NOx) from electric power generation have decreased in recent years due to changes in burner technology and fuels used. Mobile NOx emissions assessments are less certain, since they must account for increases in vehicle miles traveled, changes in the proportion of diesel and gasoline vehicles, and more stringent controls on engines and fuels. The impact of these complicated emission changes on a particular region's air quality must be diagnosed by a combination of observation and model simulation. The New England Air Quality Study - Intercontinental Transport and Chemical Transformation 2004 (NEAQS-ITCT 2004) program provides an opportunity to test the effects of changes in emissions of NOx and other precursors on air quality in the northeastern United States. An array of ground, marine, and airborne observation platforms deployed during the study offer checks on emission inventories and air quality model simulations, like those of the Weather Research and Forecasting model coupled with online chemistry (WRF-Chem). Retrospective WRF-Chem runs are carried out with two EPA inventories, one compiled for base year 1999 and an update for 2004 incorporating projected and known changes in emissions during the past 5 years. Differences in model predictions of ozone, particulates, and other tracers using the two inventories are investigated. The inventories themselves and the model simulations are compared with the extensive observations available during NEAQS-ITCT 2004. Preliminary insights regarding the sensitivity of the model to NOx emission changes are discussed.