Past and future ozone trends in California's South Coast Air Basin: Reconciliation of ambient measurements with past and projected emission inventories

Figures & data

Figure 1. Map of the South Coast Air Basin with approximate locations of relevant air quality monitoring stations.

Figure 2. Trends in summer reactive organic gas and NO_x emissions in tons per day for the South Coast Air Basin from 1975 to 2020 with 2008 base year (CARB, 2012b). Estimates for biogenic emissions are from the modeling inventory used in the 2012 SoCAB Air Quality Management Plan (SCAQMD, 2012).

Figure 3. Trends in four-site (Los Angeles N. Main, Azusa, Pomona, and Rubidoux) average summer (May–October 1990–2010) Wednesday and Sunday 6–9 a.m. ambient CO, NO_x, and CO/NO_x.

Figure 4. Trends in four-site (Los Angeles N. Main, Azusa, Pomona, and Rubidoux) average summer Wednesday and Sunday 6–9 a.m. ambient NMOC (ppbC) and NMOC/NO_x ratios in the South Coast Air Basin.

Figure 6. Trends in ozone exceedances (primary y-axis) by Sunday, Saturday, and weekdays for Los Angeles N. Main, Pomona, and Crestline and SoCAB design values (secondary y-axis).

Table 1. Ozone, products of photooxidation, and indicator ratios ^a predicted by box model simulations for 1985, 1995, 2005, and 2020 ^b

				Weekday					Sunday
					2020	2020				2020	2020
					(−10% VOC,	(−10% VOC,				(−10% VOC,	(−10% VOC,
	Time	1985	1995	2005	−50% NOx)	−75% NOx)	1985	1995	2005	−50% NOx)	−75% NOx)
Ozone, precursors, and products
O3 formation (ppb/hr)	Daily	21.8	12.0	5.4	13.5	17.4	30.1	21.5	12.6	16.7	17.2
Max O3(ppb)	Max O3	183	101	46	120	120	209	175	115	116	95
VOC (ppbC)	6–9 a.m.	947	616	261	235	235	805	523	221	199	199
NOx (ppb)	6–9 a.m.	114	101	70	35	17.5	57	51	35	18	9
PAN (ppb)	Max O3	7.2	2.1	0.4	1.9	2.7	8.4	5.2	1.7	1.4	1.8
HNO3 (ppb)	Max O3	33	23	11	12	5	14	15	12	5	2
HCHO (ppb)	Max O3	13.3	9.4	4.3	4.3	5.9	9.7	7.1	4.2	3.5	5.4
H2O2 (ppb)	Max O3	0.1	0.0	0.0	0.8	3.8	4.9	2.4	0.5	3.4	4.2
Indicators*
VOC/NOx	6–9 a.m.	8.3	6.1	3.7	6.7	13.4	14.1	10.3	6.3	11.4	22.6
NOy	Max O3	154	126	81	18	8	80	72	49	9	4
H2O2/HNO3	Max O3	0.00	0.00	0.00	0.06	0.77	0.35	0.15	0.05	0.65	2.09
HCHO/NOy	Max O3	0.22	0.16	0.10	0.24	0.46	0.37	0.29	0.23	0.41	0.81
O3/(NOy−NOx)	Max O3	5	4	4	9	18	9	8	8	17	32
Notes: ^aShade backgrounds indicate values exceeding threshold for NOx limitation (VOC/NOx > 10; NOy < 20; H2O2/HNO3 > 0.4; HCHO/NOy > 0.28; and O3/(NOy − NOx) > 7).
^bEmission reductions in 2020 from 2010 levels.

Figure 7. Mean 6–9 a.m. NMHC and NO_x mixing ratios in the SoCAB during ozone episode days referenced to the 1987 SCAQS for 2010 and earlier based on ambient CO and NO_x trends. Extrapolations to 2020 based on projected emissions from 2005 to 2020 of −10% NMHC and −40% NO_x.

Figure 8. Predicted ozone mixing ratios (ppm) and formation rates (ppb/hr) from box model ozone simulations using the initial NMHC and NO_x mixing ratios shown in Figure 7.

Figure 5. Trends in summer Sunday, weekday, and Sunday minus weekday mean maximum 8-hr ozone (ppb) during May–October 1980–2010 at Los Angeles N. Main, Azusa, Pomona, Upland, Riverside, and Crestline.

Figure 9. Ozone isopleth diagram for summer conditions in Los Angeles with initial weekday and Sunday NMHC and NO_x mixing ratios from Figure 7. The two pairs of lines represent 2010 to 2020 changes of −10% VOC and −50% NO_x (indicated by box) and −75% NO_x (indicated by end of arrow). The left pair of lines represent an underestimation of VOC emission of a factor of 2 relative to the right pair of lines.

Figure 10. Ozone isopleth diagrams of VOC/NO_x, ozone production efficiency, ozone formation rate, and nitric acid for summer conditions in Los Angeles.

Figure 11. Trends in concentrations of PM_2.5 mass (µg/m) and major components at Central Los Angeles and Rubidoux. Component data from 2003 to 2006 are from the Chemical Speciation Network, and PM_2.5 mass data are from the CARB particulate matter monitoring program.

Table 2. Comparisons of ambient NMOC/NO_x ratios measured at the SoCAB Photochemical Assessment Monitoring Stations with corresponding emissions inventory (E1) ratios for the nine 5 × 5-km grid cells surrounding the monitoring site for the month of July in 2009

	1987 (SCAQS) ^a			1997 (SCOS97-NARSTO) ^b			2009 Weekdays (This Study)
Location	Ambient	EI	Ambient/EI	Ambient	Modeled^c	Ambient/Mod	Ambient	EI	Ambient/EI
Anaheim	9.3	3.7	2.5
Azusa	8.1	4.0	2.0	4.6	4.0	1.2	6.4	3.2	2.0
Burbank	9.2	3.5	2.6
Los Angeles	8.6	4.2	2.0	4.3	3.7	1.2	5.1	2.8	1.8
Claremont	8.7	4.3	2.0
Hawthorne	9.5	5.2	1.8
Long Beach	8.7	3.8	2.3
Rubidoux	8.6	2.9	3.0				5.6	2.6	2.2
Pico Rivera				2.9	4.1	0.7	4.5	2.5	1.8
Upland				3.9	3.0	1.3
Mean	8.8	4.0	2.3	3.9	3.7	1.1	5.4	2.8	2.0
Notes: Similar comparisons from the 1987 SCAQS and SCOS97-NARSTO are also shown. ^a1987 SCAQS ambient data are for 10 weekdays and one Saturday in summer 1987. ^bSCOS97-NARSTO ambient data are for the August 4–7, 1997, ozone episode (Monday–Thursday). ^cCAMx photochemical model predictions for 9-cell average surrounding the monitoring site locations.

Table 3. Trends in NMHC/NO_x ratios of the average emissions in the month of July for the nine 5 × 5-km grid cells surrounding each of the five PAMS sites, five-site mean, and for basin-wide total and on-road motor vehicle (ORMV) emissions

	2005		2009		2014		2023
	Wed	Sun	Wed	Sun	Wed	Sun	Wed	Sun
Azusa	3.3	4.1	3.2	4.1	3.7	4.7	5.4	6.3
Central LA	2.9	3.7	2.8	3.7	3.3	4.5	4.9	6.1
Pico Rivera	2.6	3.2	2.5	3.1	3.0	3.6	4.3	4.8
Upland	2.4	2.9	2.4	2.9	2.8	3.3	3.6	3.8
Rubidoux	2.5	3.0	2.6	3.1	3.2	3.8	4.7	5.1
5-Site mean	2.7	3.4	2.7	3.4	3.2	4.0	4.6	5.2
Basinwide	2.8	3.7	2.6	3.6	3.0	3.9	4.0	4.5
Basin − ORMV	2.0	2.8	1.7	2.5	1.8	2.6	2.2	3.1

Figure 12. The combined mean diurnal variations in NMOC/NO_x and isopentane/acetylene at Los Angeles N. Main and Azusa during summer 2009 for days with daily maximum temperature at downtown Los Angles of less than versus greater than or equal to 85 ˚F.