Weekend/Weekday Ozone Observations in the South Coast Air Basin

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• Results suggest that the weekday-weekend differences in the diurnal pattern of NO and ozone are established early in the morning and the influence of the factors related to this “constant” is maintained throughout the day. • The constant offset in the diurnal pattern of ozone and NO is likely related to the initial (i.e., 6-9 a.m.) VOC/NOx ratios. The NMHC/NOx ratio is within the range of four to eight from sunrise to the time of peak ozone so that ozone formation is VOC-limited throughout this period. Theoretical Relationship Between Precursor Emissions and Ozone • The current mixing ratios of VOC and NOx are on the VOC- limited portion of the EKMA diagram for both weekends and weekdays. The EKMA diagram shows that the decrease in NO x leads to an increase in ozone mixing ratios of about 40 ppb between weekdays and weekends, and this is consistent with the observations. • The EKMA diagram shows that that an ozone disbenefit will result if NO x emissions are decreased at current levels of VOC until the NO x mixing ratios decrease to about 10 or 12 ppb where ozone production becomes NO x - limited. • VOC control measures undertaken between 1987 and 2000 have been effective in reducing VOC mixing ratios. Ozone mixing ratios estimated from the EKMA diagram should have been reduced from 200 ppb to an average near 100 ppb in reasonable agreement with observations. The 6-9 a.m. VOC/NO x ratio decreased from near an average of 7.5 during 1987 down to between three and five during 1999-2000. • Reduced NOx mixing ratios lead to increased HO mixing ratios because there is less radical termination through the HO + NO2 reaction. The simulations show that hydroxyl radical mixing ratios increase with decreasing NO x mixing ratios. The higher HO mixing ratios lead to the more efficient formation of ozone. • The shift in VOC/NO x ratio increases the average ozone production efficiency from less than 4 to between 4 and 6. The ozone production efficiency is defined to be the ozone production divided by the sum of NO x converted to nitric acid and organic nitrates. • Daytime nitric acid production has decreased between 1987 and 2000 due to the decrease in VOC mixing ratios. At present mixing ratios of NO x and VOC, the production of HNO 3 may be reduced by lowering NO x . The difference between nitric acid production on Sunday and weekdays would be expected to be lower by about 9 ppb on weekdays. PAN, HCHO, aldehyde and organic nitrates have been reduced due to reductions in VOC mixing ratios but their mixing ratios are not strongly affected by the NO x mixing ratios at current concentrations so a weekend effect is not expected for these species. • The ozone ridgeline is well marked by a value of 10 for the indicator [HNO 3 ]/[H 2 O 2 ] and by a value of 0.2 for the indicator [H 2 O 2 ]/[HCHO]. Comparison of the current NO x and VOC mixing ratios with the EKMA diagrams shows that the indicator ratios for photochemical 3-10
activity including [HNO 3 ]/[H 2 O 2 ] and [H 2 O 2 ]/[HCHO] that ozone production is VOC limited. Weekday Variations in Empirically-Estimated Photolysis Parameter • The long-term trend in J NO2 is consistent with the trend to lower soot emissions. However, the weekday-weekend differences in J NO2 were small, and the maximum J NO2 on weekdays was slightly higher than on Sunday for the entire period from 1980 to 2000. The weekend/weekday variation in J NO2 is not consistent with the hypothesis that increased soot concentrations reduce J NO2 on weekdays. • The ratio of ozone to potential ozone has dropped markedly on weekdays from 1980 to 2000 relative to weekends, indicating that there is a more complete conversion of NO x to ozone on weekends. The long-term trend of the ratio from 1980 to 2000 is not consistent with the soot hypothesis. If all other conditions were constant, a higher J NO2 should yield a more complete conversion of NO x to ozone. The data show that this has not happened. However, as the EKMA analysis has shown this is caused by the more efficient production of ozone due to lower NO x mixing ratios rather than an increase in J NO2 . 3-11
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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
Page 55 and 56: Ambient NOx Associated with CO and
Page 57 and 58: Source Contribution Estimate (ug/m3
Page 59 and 60: 2.8 2.6 Ozone (ppb) 2.4 log NOx (pp
Page 61 and 62: 2. EVOLUTION OF THE OZONE EFFECT IN
Page 63 and 64: the SoCAB has dropped sharply as sh
Page 65 and 66: the duration of ozone accumulation,
Page 67 and 68: • NO 2 concentrations and NO 2 /N
Page 69 and 70: Table 2-1 Air Quality Parameters fo
Page 71 and 72: 0.18 Azusa, Summer 1995 12.0 0.15 1
Page 73 and 74: 250 200 150 100 50 0 1976 1977 1978
Page 75 and 76: 80 1981-84 60 40 20 0 80 Sun Mon Tu
Page 77 and 78: 1.00 1981-84 0.80 0.60 0.40 0.20 0.
Page 79 and 80: 160 1981-84 120 80 40 0 160 Sun Mon
Page 81 and 82: 5.0 1981-84 4.0 3.0 2.0 1.0 0.0 5.0
Page 83 and 84: 12.00 1981-84 11.00 10.00 9.00 8.00
Page 85 and 86: NMHC (ppbC) 1000 800 600 400 200 4-
Page 87 and 88: 10.00 1981-84 8.00 6.00 4.00 2.00 0
Page 89 and 90: Sunday 1981-84 Time (PDT) 16 15 14
Page 91 and 92: 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: Weekday Differences in Diurnal Vari
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 and 144: more than a few percent of the tota
Page 145 and 146: period, the amount of NOx associate
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
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Table 4.4-2a Number of observations
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Table 4.4-2c Number of observations
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Table 4.4-3a CMB Performance Parame
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Table 4.4-3c Source Contribution Es
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Table 4.5-2 Weekend/Weekday Differe
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800 700 600 Wednesday NO and NOy (p
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Estimated NOx Estimated NOx (ppb) 5
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VOC/NOx Ratios 6.0 5.0 4.0 Mon, 10/
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Nitrogen Oxides at Azusa 80 60 02-0
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Nitrogen Oxides at Pico Rivera 180
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Nonmethane Organic Gas at Azusa 500
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Nonmethane Organic Gas at Pico Rive
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NMOG/NOx Ratios at Azusa 20 15 02-0
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estimated contribution by source (u
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500 Nonmethane Hydrocarbons 400 NMH
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5 Azusa, Weekdays 6-9 a.m (10/2/00-
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2.0 Azusa, Weekdays 6-9 a.m (10/2/0
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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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