Workshop on Air Quality Data Analysis and Interpretation: Workshop on Air Quality Data Analysis and Interpretation Evaluation of Emission Inventory Emission Inventories: Emission Inventories Emission inventories are routinely used for planning purposes and as input to comprehensive photochemical air quality models.
Significant biases in either VOC or NOx emission estimates can lead to poor baseline photochemical model performance and erroneous estimates of the effects of control strategies.
Essential top-down emission inventory evaluation procedure: comparison of emission estimates with ambient air quality data.
Caution: Ambient/emission inventory comparisons are useful for examining the relative composition of emission inventories; they are not useful for verifying absolute amounts unless they are combined with bottom-up evaluations. Approach: Approach Perform the following three tasks:
Compare early morning (e.g., 0700-0900 LT) ambient- and emissions-derived NMOC/NOx and CO/NOx ratios.
Compare early morning ambient- and emissions-derived relative compositions of individual chemical species and species groups.
Compare early morning ambient- and emissions-derived relative reactivities of individual chemical species and species groups.
Early morning sampling periods are more appropriate to use in these evaluations because they have the best potential to minimize the effects of upwind transport and photochemistry. Emissions are generally high, mixing depths are low, winds are usually light, and photochemical reactions are minimized.
Conduct a second evaluation following the incorporation of the recommendations made in the first evaluation, in order to verify improvement. NMHC/NOx Emissions: NMHC/NOx Emissions PCD – 1997 (Bangkok Inventory)
Total NMHC (MW=14 g/mol) on a per C basis
(268,882 ton/yr)x(1000 kg/ton)/(0.014 kg/mol) = 19.2 x 109 mol/yr
Total NOx (MW=46 g/mol as NO2)
(329,161 ton/yr)x(1000 kg/ton)/(0.046 kg/mol) = 7.16 x 109 mol/yr
NMHC/NOx =2.7 (ppbC/ppb) NMHC/NOx Emissions - Mobile: NMHC/NOx Emissions - Mobile PCD – 1997 (Bangkok Inventory)
Mobile NMHC (MW=14 g/mol C)
(232,973 ton/yr)x(1000 kg/ton)/(0.014 kg/mol) = 16.6 x 109 mol/yr
Mobile NOx (MW=46 g/mol)
(264,648 ton/yr)x(1000 kg/ton)/(0.046 kg/mol) = 5.75 x 109 mol/yr
NMHC/NOx =2.9 (ppbC/ppb) Bangkok Emission Inventory Comparison: Bangkok Emission Inventory Comparison NOx/CO
Ambient = 30 – 70 ppb/ppm
Inventory (Total) = 430
Inventory (Mobile) = 460
Ambient (slope) = 9.3 ppbC/ppb
Ambient mean, median = 22.9, 17.2
Inventory (Total) = 2.7
Inventory (Mobile) = 2.9
Let’s look at the NMHC/CO ratio in emissions!: Let’s look at the NMHC/CO ratio in emissions! Total Emissions
NMHC/CO = (19.2 x 109 mol/yr)/ (16.5 x 109 mol/yr) = 1.2 ppbC/ppb
NMHC/CO = (16.6 x 109 mol/yr)/ (12.5 x 109 mol/yr) = 1.3 ppbC/ppb
NMHC/CO = 0.5 (slope of scatter plot)
NMHC/CO = 1.3, 0.9 (Mean, median of ratio at National Housing 10T) Bangkok Emissions Inventory Conclusions: Bangkok Emissions Inventory Conclusions NOx/CO – lower for ambient than inventory
NMHC/NOx – higher for ambient than inventory
NMHC/CO – reasonably close in ambient to inventory
These results make one question the NOx portion of the inventory specifically. It seems to be high in the inventory relative to both CO and NMHC. Differences between Emission Inventories and Ambient are Common: Differences between Emission Inventories and Ambient are Common Problems with Vehicle Emissions: Problems with Vehicle Emissions Uncertainties in Evaluation of Emission Inventories: Uncertainties in Evaluation of Emission Inventories EMISSION INVENTORY UNCERTAINTY ISSUES
Spatial and temporal allocation of activities
Adjustment of emission rates for temperature and day-specific activities
Assignment of accurate and representative source speciation profiles
AMBIENT MEASUREMENTS UNCERTAINTY ISSUES
The representativeness of the monitoring sites
The influence of lower quantifiable limits and precision
The identification, misidentification, or lack of identification of all important species
Potential sampling or handling losses of total mass or individual species
COMPARISONS-RELATED UNCERTAINTY ISSUES
The matching of emissions and ambient NMOC species
The temporal matching of the emissions and ambient data
The spatial matching of the emissions and ambient data
Meteorological factors such as wind speed and direction and mixing height
The level of ambient background NMOC and NOx concentrations
Chemical reactions VOCs as tracers: VOCs as tracers
VOCs as tracers (continued): VOCs as tracers (continued) SPECIATE 3.2: SPECIATE 3.2 http://www.epa.gov/ttn/chief/software/speciate/index.html
This is a very useful tool to provide estimates of the composition of emissions from a variety of sources.
Speciates the TOC emissions from a few hundred different sources into individual organic compounds.
Also, speciates the PM emissions from a few hundred different sources into individual “elemental” contributions.
Source profiles can be exported to the Chemical Mass Balance (CMB) model. Source Contributions: Source Contributions Species contributions to sources are generally based on emission source measurements or standard source-contributions like SPECIATE.
Source characterization can be quite expensive and representative of operations during test conditions.
We will briefly discuss an option based on ambient measurements. Comparison of Source Contributions: Comparison of Source Contributions GRACE/SAFER: GRACE/SAFER Graphical Ratio Analysis for Composition Estimates (GRACE)
Correlations between acetylene (assumed to be emitted solely from vehicle exhaust) and other VOC are used to establish the minimum and maximum exhaust-related ratios of acetylene to other species. GRACE plots of each roadway-corrected species versus all others are also examined.
Source Apportionment by Factors with Explicit Restrictions (SAFER)
SAFER is a multivariate receptor model that predicts the number of sources and their composition from the ambient data. SAFER requires that these predictions be consistent with observed intercorrelations of the concentrations and with physical constraints and explicit constraints derived from GRACE.
SAFER requires large data sets, thus, the PAMS auto-GC data are well suited for this analysis.
Environ. Sci. Technol., 28, 823-832, 1994.
Plots of VOCs vs Acetylene: Plots of VOCs vs Acetylene Edge Relationship: Edge Relationship Environ. Sci. Technol., 28, 823-832 (1994). Ratios to Acetylene: Ratios to Acetylene Ambient Data for Emissions Profiles: Ambient Data for Emissions Profiles GRACE/SAFER RESULTS 1990 ATLANTA OZONE STUDY
Using ambient data, obtained three source profiles: roadway emissions (acetylene), whole gasoline (roadway-corrected 2,3-dimethylpentane), gasoline headspace vapor (n-butane).
GRACE/SAFER-derived profiles compared well to source measurements.
Source profiles used in subsequent CMB modeling.
PAMS data well suited for these analyses. VOC Source Contributions: VOC Source Contributions Roadway Whole
headspace White – model derived
Black – source derived Chemical Mass Balance Approach: Chemical Mass Balance Approach The CMB model can be quite useful in identifying various source contributions to ambient air quality measurements.
CMB has been used extensively to understand source contributions to particulate measurement, based on the elemental composition of samples.
The same approach is quite useful for understanding various source contributions to ambient VOC measurements, based on speciated VOC composition of the samples.