Precursor reductions and ground-level ozone in the Continental United States

Figures & data

Table 1. U.S. national emissions of NO_x and VOC (EPA, 2015a, 2015b, 2015c).

Year	Total NOx	On-road NOx	Nonroad NOx	EGU^a NOx	Other fuel NOx	Industrial processes NOx	Other NOx	Total VOC	On-road VOC	Nonroad VOC	Solvent; storage and transport VOC	Other VOC
1970	26.9	12.6	2.7	4.9	5.2	0.8	0.8	34.7	16.9	1.6	9.1	7.0
1980	27.1	11.5	3.4	7.0	4.3	0.6	0.4	31.1	13.9	2.2	8.6	6.5
1990	25.5	9.6	3.8	6.7	4.2	0.8	0.5	24.1	9.4	2.7	7.2	4.8
2000	22.6	8.4	4.2	5.3	3.5	0.8	0.4	17.5	5.3	2.6	6.0	3.5
2002	24.0	10.3	4.9	4.7	2.8	0.9	0.3	20.3	4.0	3.1	5.8	7.4
2008	16.9	6.9	3.5	3.1	2.0	1.0	0.4	17.8	3.1	2.5	4.5	7.7
2010	15.0	5.7	3.4	2.5	1.9	1.1	0.4	17.9	2.8	2.3	4.2	8.6
2013	13.1	5.0	2.7	1.8	1.9	1.2	0.5	17.7	2.2	2.0	4.0	9.6
2018	10.0							15.5
2025	8.5							15.1

Notes: The EPA trend inventory utilized two mobile-source emission models (MOVES from 2002 forward and MOBILE6 prior to 2002). Other methodological changes affect VOC emissions beginning with 2002 within various categories (“miscellaneous” as categorized by EPA; included within “other”). The projections to 2018 and 2025 are based on EPA modeling inventories that represent future-year emissions based on population growth, future emission-source activity levels, and final emission control regulations (these include vacated measures but do not include rules that are under consideration or emission reductions modeled for O₃ NAAQS attainment). Emissions from vegetation and wildfires are not included. Units are million short tons (mT).

^aElectric generating unit (EGU).

Figure 1. U.S. national NO_x emissions compared with trends in 29-site and 98-site averages of the annual 98th percentile concentration of daily 1-hr maximum NO₂ concentrations (EPA, 2015d, 2015e). Fewer trend sites operated continuously from 1980 (29) than from 1990 (98).

Figure 2. U.S. national trends in average annual 98th percentile concentration of daily 1-hr maximum NO₂ concentrations compared with trends in average annual fourth-highest daily peak 8-hr O₃ (EPA, 2015e; 2015f). Fewer sites operated from 1980 (29 for NO₂ and 222 for O₃) than from 1990 (98 for NO₂ and 466 for O₃).

Table 2. Trends in daily peak 8-hr O₃ by Core Based Statistical Area (CBSA) and metric, 1990–2014.

		Trend in annual O3 metric (% yr^−1)
Region	CBSA	99th pct	Max 4	98th pct	95th pct	90th pct	75th pct	50th pct	10th pct
Inland Central CA	Bakersfield	–1.01	–1.04	–0.86	–0.92	–0.93	–0.72	–0.27	1.12
Inland Central CA	Fresno	–0.70	–0.73	–0.77	–0.83	–0.82	–0.50	–0.09	0.96
Inland Central CA	Sacramento	–0.99	–0.97	–0.98	–0.75	–0.68	–0.62	–0.24	0.12
New York	Bridgeport CT	–1.23	–1.21	–1.02	–0.90	–0.65	–0.36	0.06	0.76
New York	Buffalo	–1.00	–1.00	–0.90	–0.67	–0.54	–0.18	0.17	0.44
New York	New York	–1.59	–1.71	–1.72	–1.52	–1.53	–1.12	–0.56	0.10
Ohio	Cincinnati	–1.10	–1.05	–1.07	–0.74	–0.61	–0.35	0.07	0.42
Ohio	Cleveland	–1.07	–1.09	–1.10	–0.99	–0.98	–0.57	–0.10	0.18
Ohio	Columbus	–1.07	–0.99	–1.08	–1.02	–0.82	–0.50	–0.17	0.43
S California	Los Angeles	–2.60	–2.63	–2.48	–2.29	–2.01	–1.45	–0.24	1.64
S California	Riverside	–2.39	–2.43	–2.42	–2.28	–2.12	–1.70	–0.91	–0.23
S California	Santa Barbara	–1.53	–1.56	–1.69	–1.52	–1.38	–1.23	–0.96	–0.73
S Great Lakes	Chicago	–1.55	–1.29	–1.31	–0.95	–0.89	–0.68	–0.44	–0.35
S Great Lakes	Detroit	–1.10	–1.04	–0.96	–0.65	–0.43	–0.08	0.46	0.67
S Great Lakes	Milwaukee	–1.43	–1.20	–1.21	–1.01	–0.85	–0.41	–0.19	0.23
San Francisco Bay	San Francisco	–0.69	–0.69	–0.81	–0.61	–0.40	–0.02	0.03	1.16
San Francisco Bay	San Jose	–0.72	–0.74	–0.49	–0.60	–0.42	–0.18	0.13	0.93
San Francisco Bay	Santa Rosa	–1.37	–1.37	–1.13	–1.05	–0.93	–0.75	–0.86	–1.16
Southeast	Atlanta	–1.66	–1.68	–1.52	–1.63	–1.46	–1.10	–0.68	–0.10
Southeast	Birmingham	–1.15	–1.19	–1.14	–1.09	–0.92	–0.61	–0.21	0.26
Southeast	Memphis	–1.19	–1.11	–1.13	–1.19	–1.19	–1.12	–1.11	–0.52
Texas	Dallas	–1.15	–1.03	–1.07	–0.99	–0.81	–0.52	–0.34	0.01
Texas	Houston	–2.00	–1.98	–1.87	–1.40	–0.90	–0.50	–0.20	0.65
Texas	San Antonio	–0.62	–0.49	–0.59	–0.30	–0.17	0.30	0.62	0.86
Average		–1.29	–1.26	–1.22	–1.08	–0.94	–0.62	–0.25	0.33

Notes: The data are from the site with the highest fourth-highest daily peak 8-hr O₃ (maximum site) in each CBSA each year. The annual fourth-highest daily peak 8-hr O₃ is approximately equal to the 99th percentile (pct) when measurements are made 365 days per year, and is lower than the 99th percentile when measurements are made on fewer than 365 days per year. Trends that are not statistically significant (p > 0.05) are indicated in italics.

Figure 3. (a) U.S. national 1980–2013 222-site average of annual fourth-highest daily peak 8-hr O₃ vs. 29-site average of the annual 98th percentile concentration of daily 1-hr maximum NO₂ concentrations. All regressions are statistically significant (p < 0.0001). (b) National 1990–2013 466-site average of annual fourth-highest daily peak 8-hr O₃ vs. 98-site average of the annual 98th percentile concentration of daily 1-hr maximum NO₂ concentrations (EPA, 2015e, 2015f).

Figure 4. Trends in annual fourth-highest peak 8-hr O₃ at sites having the highest (x) and the lowest (open circles) values each year in eight CBSAs.

Figure 5. Annual O₃ concentration vs NO₂ concentration. The O₃ metrics are the annual fourth-highest peak 8-hr O₃ (x) and the mean peak 8-hr O₃ (open circles) concentrations from the maximum O₃ site within each CBSA. The NO₂ metric is the mean annual daily 1-hr peak NO₂ from the site with the maximum mean NO₂ concentration within the same CBSA as each O₃ site.

Table 3. Comparison of modeling predictions with empirical projections.

	NOx Emission Reduction from 2011^a					Ambient NO2 Reduction from 2011-13^b
Modeling Area	For 75 ppbv (%)	For 70 ppbv (%)	For 65 ppbv (%)	For 60 ppbv (%)	CBSA	For 75 ppbv (%)	For 70 ppbv (%)	For 65 ppbv (%)	For 60 ppbv (%)
National	44.0%	48.9%	58.0%	69.3%	National - 34 year trends^c	35.5%	52.7%	69.9%	87.2%
					National - 24 year trends^c	33.8%	48.6%	63.4%	78.2%
East	43.2%	48.4%	58.2%	68.3%
Northeast	43.2%	48.8%	58.6%	68.7%	Bridgeport-Stamford-Norw	32.7%	50.4%	68.1%	85.7%
					New York-Northern NJ - LI	18.1%	28.2%	38.3%	48.4%
					Buffalo-Niagara Falls	0%	7.2%	26.2%	45.1%
					Pittsburgh PA	17.9%	37.2%	56.5%	75.7%
Midwest	44.8%	44.8%	59.2%	73.9%	Chicago-Naperville-Joliet	NSS	NSS	NSS	NSS
					Detroit-Warren-Livonia	22.3%	36.5%	50.6%	64.8%
					Milwaukee-Waukesha-W Al	10.3%	27.3%	44.2%	61.2%
					Cincinnati-Middletown OH^d	35.8%	63.9%	92.0%	>100%
					Evansville IN-KY^4	2.7%	29.6%	56.6%	83.5%
Central	36.9%	48.8%	61.2%	73.8%	Houston-Sugar Land-Baytwn	43.2%	53.5%	63.8%	74.0%
					Dallas-Fort Worth-Arlington	44.3%	60.7%	77.2%	93.6%
					El Paso TX	0%	26.7%	58.4%	90.2%
Southeast	50.9%	50.9%	50.9%	50.9%	Atlanta-Sandy Springs-Mar^e	14.5%	26.2%	38.0%	49.7%
					Charlotte-Gastonia-Concor^e	3.8%	19.2%	34.6%	50.0%
West	42.4%	42.4%	48.3%	69.4%
Southwest	40.4%	40.4%	49.5%	81.9%		NSS	NSS	NSS	NSS
CA	64.3%	71.5%	79.2%	83.2%	Riverside-San Bernardino–O	98.6%	>100%	>100%	>100%
					Bakersfield	99.0%	>100%	>100%	>100%
					Fresno	>100%	>100%	>100%	>100%

Notes: The listed CBSAs contain the monitoring sites having the highest projected O₃ design values within each modeling area (EPA, 2015a, Tables 3A-7 through 3A-11). Cases where the observations indicate that larger NO₂ reductions would be needed compared with modeling predictions are shown in boldface type.

^aDetermined from modeling predictions listed in Table S5.

^bAmbient data for CBSAs with statistically significant (p < 0.05) 25-year O₃–NO₂ regressions. NSS, not statistically significant (p > 0.05). Zero (0%) from 2011–2013 NO₂ corresponds to peak 8-hr O₃ < target level. >100%, the target level is exceeded even when ambient NO₂ levels are zero (i.e., the regression intercept exceeds the target O₃ concentration).

^cDetermined from 90th percentile regressions shown in Figure 3.

^dThe Louisville, KY (NSS regression) CBSA tied for highest RIA projected design values for 75 and 70 ppbv O₃ targets in the Midwest modeling area.

^eRIA did not tabulate projected design values for the Southeast.

Table 4. Summary of projected ambient NO₂ reductions for meeting four alternative O₃ NAAQS levels in 89 CBSAs compared with EPA RIA modeling predictions.

Region	Number of CBSAs	Number statistically significant	For 75 ppbv	For 70 ppbv	For 65 ppbv	For 60 ppbv
Total^a	89	75	14 (16%)	22 (25%)	32 (36%)	46 (52%)
Central^b	12	10	3 (25%)	5 (42%)	6 (50%)	9 (75%)
Midwest^c	14	12	3 (21%)	5 (36%)	5 (36%)	8 (57%)
Northeast^d	25	24	1 (4%)	4 (16%)	7 (28%)	10 (40%)
Northern California^e	15	11	5 (33%)	5 (33%)	5 (33%)	8 (53%)
Southeast	9	9	0 (0%)	0 (0%)	2 (22%)	4 (44%)
Southern California^f	6	4	1 (17%)	1 (17%)	2 (33%)	2 (33%)
Southwest	6	3	1 (17%)	2 (33%)	4 (67%)	4 (67%)
West^g	2	2	0 (0%)	0 (0%)	1 (50%)	1 (50%)

Notes: The table lists the number of CBSAs (% of total in region) for which the observational analyses predict that larger reductions of NO₂ will be needed compared with model predictions.

^aIf only the 75 CBSAs with statistically significant (p < 0.05) O₃–NO₂ regressions are considered, the corresponding results are 9 (12%) for 75 ppbv O₃, 17 (23%) for 70 ppbv, 24 (32%) for 65 ppbv, and 37 (49%) for 60 ppbv. If only the 38 CBSAs with the most statistically significant O₃–NO₂ regressions (r² > 0.5; p < 0.0001) are considered, the corresponding results are 3 (8%) for 75 ppbv O₃, 5 (13%) for 70 ppbv, 7 (18%) for 65 ppbv, and 13 (34%) for 60 ppbv.

^bHouston, Dallas–Fort Worth, and Austin at 75 ppbv (Austin not statistically significant at p< 0.05). In addition, Kansas City and New Orleans at 70 ppbv; Little Rock at 65 ppbv; El Paso, Oklahoma City, and Tulsa at 60 ppbv.

^cChicago, Indianapolis, and Nashville, at 75 ppbv. In addition, Cincinnati and Memphis at 70 and 65 ppbv; Huntington–Ashland WV, Cleveland, OH, and Evansville, IL, at 60 ppbv.

^d Bridgeport, CT, and Portland, ME, at 75 ppbv (Portland not statistically significant at p < 0.05). In addition, Manchester, NH, and Richmond, VA, at 70 ppbv; Springfield, MA, Reading, PA, and Harrisburg, PA, at 65 ppbv; Providence, RI, Philadelphia, PA, and Pittsburgh, PA at 60 ppbv.

^e Inland central California plus San Francisco Bay area. Bakersfield, Visalia, Fresno, Modesto (San Joaquin Valley) and Sacramento at 75, 70, and 65 ppbv. In addition, Stockton, San Jose, and San Francisco at 60 ppbv.

^f Riverside, CA, at 75 and 70 ppbv. In addition, San Diego, CA, at 65 and 60 ppbv.

^g Data available from Portland, OR, and Minneapolis–St. Paul, MN.

Environmental Protection Agency. 2015a. National Ambient Air Quality Standards (NAAQS). http://www.epa.gov/air/criteria.html (accessed January 2015).

 Google Scholar

Environmental Protection Agency. 2015b. Technical support document (TSD): Preparation of emissions inventories for the version 6.1, 2011 Emissions Modeling Platform. http://www.epa.gov/ttn/chief/emch/2011v6/2011v6.1_2018_2025_base_EmisMod_TSD_nov2014_v6.pdf (accessed January 2015).

 Google Scholar

Environmental Protection Agency. 2015c. ftp://ftp.epa.gov/EmisInventory/2011v6/ozone_naaqs/reports/ and ftp://ftp.epa.gov/EmisInventory/2011v6/v1platform/reports/2018_emissions/ ( accessed January 2015).

 Google Scholar

Environmental Protection Agency. 2015d. National Emissions Inventory (NEI) air pollutant emissions trends data. http://www.epa.gov/ttn/chief/trends/index.html (accessed January 2015).

 Google Scholar

Environmental Protection Agency. 2015e. National trends in nitrogen dioxide levels. http://www.epa.gov/airtrends/nitrogen.html (accessed January 2015).

 Google Scholar

Environmental Protection Agency. 2015f. National trends in ozone levels. http://www.epa.gov/airtrends/ozone.html (accessed January 2015).

 Google Scholar