Weekend/Weekday Ozone Observations in the South Coast Air Basin

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have shown that diesel exhaust is the largest source of black carbon (Schauer et al., 1996; Watson et al., 1998; and Fujita et al., 1998). The experimental design for apportioning ambient NOx is based upon the premise that variations in the relative contributions to ambient NOx of gasoline and diesel exhaust can be estimated from variations in concentrations of CO and black carbon. Multiple regression is used in this analysis to account for the variance in ambient NOx concentrations, based on linear combinations of CO and black carbon. The multiple regression takes the form [NOx] = b 0 + b 1 [CO] + b 2 [BC]. The b’s are the regression coefficients, representing the amount that [NOx] changes when either [CO] and [BC] changes by 1 unit with the other independent variable held constant. The b 0 is the constant, where the regression line intercepts the [NOx] axis, representing the concentration of NOx when both CO and BC are zero. The scatterplots in Figures 4.5-1 and 4.5-2 show the correlations of NO and NOx to black carbon and CO, respectively, at Azusa and Pico Rivera for different sets of weekdays and hours of the day. The correlations with CO and black carbon are higher for NOx than NO. The degree of correlation is not significantly different for a dataset consisting of only weekdays versus all days and all hours or versus 6 to 9 a.m. only. The regression coefficients were first derived using on-road measurements from the mobile van consisting of simultaneous 5-minute average concentrations of NO/NOy, CO, and black carbon in varying mix of traffic during weekdays and weekends. The dataset excludes NOy data when NO is less than 10 ppb and sampling times during the ozone accumulation period to minimize the effect of photochemical depletion of NOx on the regression. We also excluded the data from the truck stop because the ratios of NOx to black carbon for idling trucks differ considerably from trucks on the open road. The NOx to black carbon ratios that were measured on freeway loops averaged 27 ppb of NOx per µg/m 3 of black carbon compared to a ratio of 67 at the truck stop. The final dataset includes about 350 sets of observations. For the most part, the dataset consists of on-road sampling of fresh mobile source emissions so that the b 1 and b 2 in the regression equation is the average contribution to NOx per unit of CO and black carbon, respectively. Thus b 1 [CO] is the NOx associated with CO and b 2 [BC] is the NOx associated with black carbon. The dataset yielded the following regression equation: [NOx, ppb] = -1.34 + 39.42 * [CO, ppm] + 14.84 * [BC, µg/m 3 ] with standard errors for b 0 , b 1 , and b 2 of 7.32, 3.94, and 0.94, respectively, and R 2 of 0.6. These coefficients were applied to the full set of data from the mobile van and at the Azusa and Pico Rivera monitoring sites. We also examined the use of MTBE and the sum of nC 10 -nC 15 as alternative independent variables. Both of these variables are from time-integrated rather than continuous measurements, so the regression was less robust. Furthermore, there are evaporative sources of MTBE that can complicate the association with NOx. Figures 4.5-3a through 4.5-3d show the amounts of ambient NOx associated with CO and black carbon for the mobile sampling conducted on Monday, October 2, Wednesday, October 4, Saturday, October 7, and Sunday, October 8, respectively. The contributions are shown in ppb and percentage of NOx. Breaks in the data are due to advances of the tape in the aethalometer. Each advance causes a 20-minute gap in the data. The averages corresponding to each of the freeway loops and regional/background sites are in Table 4.5-1. During the overnight carryover 4-12
period, the amount of NOx associated with CO and black carbon on freeway loops is similar to that at regional sites, with CO-associated NOx ranging mostly between 50 and 60 percent on weekdays, 60 to 65 percent on Saturday, and 65 to 70 percent on Sunday. There are large spikes of BC-associated NOx for the two weekdays on freeway loops during the ozone inhibition and accumulation periods. These spikes exist to a lesser extent on Saturday and are almost entirely absent on Sunday. The percentage of CO-associated NOx for the three freeway loops during this time of day, CO2, CV2, and PO1, are 47, 35, and 41 percent, respectively, on Monday, 38, 40, and 30 percent on Wednesday, 43, 59, and 57 percent on Saturday, and 63, 68, and 59 percent on Sunday. Figure 4.5-4 shows the amounts of ambient NOx associated with CO and black carbon for six surface street loops. The loops were distributed within the Compton and Covina Loops and designed to provide comparisons of the surface street traffic with freeway traffic in the area. The percentage of CO-associated NOx ranged from 44 to 73 percent with a mean and standard error of 56 ± 4 percent. Figures 4.5-5 and 4.5-6 show the ambient NOx associated with CO and black carbon for the ambient data at the Azusa and Pico Rivera monitoring stations, respectively. The percentage of NOx associated with CO is consistently larger on weekends and the largest fractions of NOx associated with black carbon occur midday during weekdays. These patterns are consistent with the diurnal and day-of-the-week variations in relative traffic volume of gasoline and diesel powered vehicles. The scatterplots in Figure 3.5-7 show similar correlations of predicted versus measured NOx at Azusa and Pico Rivera. The predicted values are reasonably correlated with measured value with a slope of 0.70 and 0.83 at Pico Rivera and Azusa, respectively. We particularly focused on locations that are regionally representative and times that coincide with ozone accumulation. At Industry Hills, the most regionally representative of the mobile sampling sites, the percentages of CO-associated NOx in the 10:30 to 11:15 a.m. samples are 44, 48, 62, and 72 percent for Monday, Wednesday, Saturday, and Sunday, respectively. Based upon STI’s analysis of the study period meteorology, Monday, October 2 is meteorologically similar to Sunday, October 8, and Wednesday, October 4 is similar to Saturday, October 7. The Saturday/Wednesday ratios of the CO and BC associated NOx are 1.01 and 0.58, respectively. The Sunday/Monday ratios of the CO and BC associated NOx are 0.84 and 0.25. These ratios are summarized in Table 4.5-2 along with the ratios for Azusa and Pico Rivera. The three locations are very consistent with each other in the weekend/weekday differences in the relative contribution of NOx associated with CO and black carbon. These results show that the contribution of gasoline-powered vehicles to ambient NOx on a typical Saturday during the ozone accumulation period is comparable to their contribution on weekdays. This conclusion is consistent with traffic counts, which show comparable light-duty gasoline vehicle traffic volumes on weekday and Saturday during this time of day. The contribution of gasoline-powered vehicles to ambient NOx is about 25 percent lower on Sunday compared to weekdays. In contrast, the contributions of diesel vehicles during the ozone accumulation period to ambient NOx on Saturday and Sunday are about one-half and one-third of its weekday contribution, respectively. 4.5.2 Source Apportionment of NOx by Chemical Mass Balance In addition to apportionment of VOC, we also apportioned NOx to gasoline and diesel exhaust by including NOx in the source profiles. Apart from reactions of NOx that occur in the 4-13
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WEEKEND/WEEKDAY OZONE OBSERVATIONS
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obtained by DRI from the Phase II f
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TABLE OF CONTENTS Preface..........
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Table No. Table 1.4-1 Table 1.4-2 T
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periods by day-of-week at Los Angel
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Figure 1.4-13. Hourly average nitro
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Figure 3.1-2 Mean day-of-the-week v
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Figure 4.4-1b. Mean estimated sourc
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1. STUDY PERSPECTIVE AND SUMMARY Th
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40 percent. Consequently, the contr
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dioxide (NO 2 ) 4 , and carbon mono
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and upwind samples before and after
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where J 1 is the photolysis frequen
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transported from an urban center to
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of the week on Friday and Saturday.
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VOC reactivity, photochemical aging
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corresponding source attributions a
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multiplicative constant. This const
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Table 1.4-1 Trends in Duration and
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Table 1.4-3 Regression Statistics f
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O 3 Ozone Formation Chemistry NO 2
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200 1981-84 200 1995-98 160 160 120
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100 NO 1000 NMHC (estimated from CO
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5 Sunday - Wednesday (1981-84) 10 T
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10.0 8.0 NMHC/NOx 6.0 4.0 2.0 0.0 S
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Ratio of Peak Ozone to Potential Oz
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Nitrogen Oxides at Azusa, 9/30/00 t
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Ambient NOx Associated with CO and
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Source Contribution Estimate (ug/m3
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2.8 2.6 Ozone (ppb) 2.4 log NOx (pp
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2. EVOLUTION OF THE OZONE EFFECT IN
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the SoCAB has dropped sharply as sh
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the duration of ozone accumulation,
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• NO 2 concentrations and NO 2 /N
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Table 2-1 Air Quality Parameters fo
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0.18 Azusa, Summer 1995 12.0 0.15 1
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250 200 150 100 50 0 1976 1977 1978
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80 1981-84 60 40 20 0 80 Sun Mon Tu
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1.00 1981-84 0.80 0.60 0.40 0.20 0.
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160 1981-84 120 80 40 0 160 Sun Mon
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5.0 1981-84 4.0 3.0 2.0 1.0 0.0 5.0
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12.00 1981-84 11.00 10.00 9.00 8.00
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NMHC (ppbC) 1000 800 600 400 200 4-
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10.00 1981-84 8.00 6.00 4.00 2.00 0
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Sunday 1981-84 Time (PDT) 16 15 14
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16 Sunday 1990-94 35 Time (PDT) 15
Page 93 and 94: 40 Sunday O3 Accumulation Rate (ppb
Page 95 and 96: 15.0 1981-84 12.0 9.0 6.0 3.0 0.0 1
Page 97 and 98: 3. CHARACTERIZATION OF THE CURRENT
Page 99 and 100: 3.2 Weekday Differences in Diurnal
Page 101 and 102: timing hypothesis, which requires a
Page 103 and 104: through the conversion of NO to NO
Page 105 and 106: Weekday Differences in Diurnal Vari
Page 107 and 108: activity including [HNO 3 ]/[H 2 O
Page 109 and 110: Table 3.2-1 Day-of-the-Week Variati
Page 115 and 116: Table 3.3-1b Day-of-the-Week Differ
Page 117 and 118: 120 Los Angeles - N. Main - Saturda
Page 119 and 120: 160 140 Upland - Saturdays mean Ozo
Page 121 and 122: maximum 1hour ozone (ppb) 100 90 80
Page 123 and 124: NMHC/NOx ratio 14 12 10 8 6 4 2 0 S
Page 125 and 126: Los Angeles - N. Main Azusa 140 140
Page 127 and 128: Los Angeles - N. Main Azusa 140 140
Page 129 and 130: 2.8 2.6 PAN (ppb) 2.8 2.6 HCHO (ppb
Page 131 and 132: NO2 Photolysis Rate Parameter 0.35
Page 133 and 134: 4. WEEKDAY VARIATIONS IN THE SOURCE
Page 135 and 136: 9 a.m. andnoon. Sampling was conduc
Page 137 and 138: interfaced to an Entech model 7100
Page 139 and 140: values measured at the regional sit
Page 141 and 142: into a category named "UNID." The s
Page 143: more than a few percent of the tota
Page 147 and 148: weekend/weekday differences during
Page 149 and 150: • Day-of-the-week changes in the
Page 151 and 152: Table 4.4-1 Default Source Composit
Page 153 and 154: Table 4.4-1a Number of observations
Page 155 and 156: Table 4.4-1c Number of observations
Page 157 and 158: Table 4.4-2a Number of observations
Page 159 and 160: Table 4.4-2c Number of observations
Page 161 and 162: Table 4.4-3a CMB Performance Parame
Page 163 and 164: Table 4.4-3c Source Contribution Es
Page 165 and 166: Table 4.5-2 Weekend/Weekday Differe
Page 167 and 168: 800 700 600 Wednesday NO and NOy (p
Page 169 and 170: Estimated NOx Estimated NOx (ppb) 5
Page 171 and 172: VOC/NOx Ratios 6.0 5.0 4.0 Mon, 10/
Page 173 and 174: Nitrogen Oxides at Azusa 80 60 02-0
Page 175 and 176: Nitrogen Oxides at Pico Rivera 180
Page 177 and 178: Nonmethane Organic Gas at Azusa 500
Page 179 and 180: Nonmethane Organic Gas at Pico Rive
Page 181 and 182: NMOG/NOx Ratios at Azusa 20 15 02-0
Page 183 and 184: estimated contribution by source (u
Page 185 and 186: 500 Nonmethane Hydrocarbons 400 NMH
Page 187 and 188: 5 Azusa, Weekdays 6-9 a.m (10/2/00-
Page 189 and 190: 2.0 Azusa, Weekdays 6-9 a.m (10/2/0
Page 191 and 192: Ambient NOx Associated with CO and
Page 193 and 194: Ambient NOx Associated with CO and
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Ambient NOx Associated with CO and
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Pico Rivera 150 120 BC-Associated C
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Nitrogen Oxides 800.00 700.00 600.0
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5. REFERENCES Altshuler, S.L., T.D.
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Fujita, E.M., J.G. Watson, J.C. Cho
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Roberts, J.M. (1983) Measurements o
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Weekend/Weekday Ozone Observations in the South Coast Air Basin

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