Weekend/Weekday Ozone Observations in the South Coast Air Basin

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show that hydroxyl radical mixing ratios increase with decreasing NO x mixing ratios. Daytime nitric acid production has decreased between 1987 and 2000 due to the decrease in VOC mixing ratios. Now the production of HNO 3 would be expected to be lower by about 9 ppb between weekend and weekday. 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. Our simulations for EKMA are consistent with a study by Milford et al. (1994) as quoted by Finlayson-Pitts and Pitts (1999). Milford et al. found that for conditions that are similar to present-day conditions in the SoCAB on Sundays (i.e., VOC near 300 ppbC and NO x at 20 to 30 ppb) the rate of ozone production increases with decreasing NO x concentrations. The study showed that an ozone disbenefit results if present-day Sunday NO x concentrations are reduced. The Milford et al. (1994) study also shows that ozone production rates increase during the day because NO x is consumed faster than VOC, which increases the VOC to NO x ratio. Given the present-day concentrations of NO x , increases in its emissions during the day will decrease, not increase, ozone production rates. However, based on the Milford, et al. study the NO x timing hypotheses will become an important mechanism at the same point when reductions in NO x emissions lead to lower ozone production rates. Modeling studies should be made with updated emissions inventories and models to verify the our modeling results and those of Milford, et al. (1994). 1.5 Recommendations The varying emissions that occur between weekday and weekend periods provide a natural test of the ability of air quality simulation models to simulate accurately weekdayweekend differences in precursors and ozone. One of the most important applications of this test is that it helps to probe the relative sensitivity of ozone concentrations to VOC and NOx. The Coordinating Research Council has sponsored a study by ENVIRON to conduct proximate modeling of weekday/weekend ozone episodes in the SoCAB using input data collected during the SCOS97-NARSTO field study in order to determine whether proposed hypotheses explain the weekend ozone effect. Similar modeling is planned by the ARB after developing a weekend emissions inventory for SoCAB. These modeling results should be compared against the measured temporal and spatial variations of ozone and ozone precursors. These operational evaluations of modeling results should be accompanied by an evaluation of the accuracy of the temporal and spatial patterns of precursor emissions on weekdays and weekends. In addition to probing the underlying physical and chemical processes that drive the weekend effect, a series of modeling runs should be made using plausible alternative future emissions inventories for 5, 10 and 15 years into the future. These runs should be used to assess the direction and sensitivity of projected concentrations of ozone, PAN, nitric acid, particulate nitrate, and formaldehyde to future VOC and NOx controls. These assessments will be particularly important for downwind areas where emissions transported from the SoCAB could transition from VOC limited to NOxlimited and for assessing strategies that might achieve the 8-hour ozone standard in downwind areas but increase peak ozone concentrations in the central basin. Species such NOy, PAN, HCHO, and speciated hydrocarbons should be measured in these downwind area to provide corroboration of modeling results. 1-20
Table 1.4-1 Trends in Duration and Rate of Ozone Accumulation in the SoCAB on Sunday, Wednesday, and Sunday Minus Wednesday, 1981-1999 Duration of Ozone Accumulation (hours) 1 Ozone Accumulation Rate (ppb/hour) 1 Years Sun Wed Sun-Wed Sun Wed Sun-Wed 1981-84 5.5 ± 0.2 4.2 ± 0.3 1.3 ± 0.2 21.3 ± 1.0 24.3 ± 1.2 -3.1 ± 0.8 1984-89 5.3 ± 0.3 4.2 ± 0.3 1.1 ± 0.1 19.5 ± 1.2 20.6 ± 1.6 -1.1 ± 0.8 1990-94 5.2 ± 0.3 4.4 ± 0.3 0.8 ± 0.1 18.2 ± 0.9 16.3 ± 1.0 1.9 ± 0.6 1995-98 5.8 ± 0.3 4.5 ± 0.2 1.3 ± 0.2 13.8 ± 0.9 12.2 ± 0.9 1.6 ± 0.5 1 Twelve-site means and standard errors of the means. 1-21
Page 1 and 2: WEEKEND/WEEKDAY OZONE OBSERVATIONS
Page 3 and 4: obtained by DRI from the Phase II f
Page 5 and 6: TABLE OF CONTENTS Preface..........
Page 7 and 8: Table No. Table 1.4-1 Table 1.4-2 T
Page 9 and 10: periods by day-of-week at Los Angel
Page 11 and 12: Figure 1.4-13. Hourly average nitro
Page 13 and 14: Figure 3.1-2 Mean day-of-the-week v
Page 15 and 16: Figure 4.4-1b. Mean estimated sourc
Page 17 and 18: 1. STUDY PERSPECTIVE AND SUMMARY Th
Page 19 and 20: 40 percent. Consequently, the contr
Page 21 and 22: dioxide (NO 2 ) 4 , and carbon mono
Page 23 and 24: and upwind samples before and after
Page 25 and 26: where J 1 is the photolysis frequen
Page 27 and 28: transported from an urban center to
Page 29 and 30: of the week on Friday and Saturday.
Page 31 and 32: VOC reactivity, photochemical aging
Page 33 and 34: corresponding source attributions a
Page 35: multiplicative constant. This const
Page 39 and 40: Table 1.4-3 Regression Statistics f
Page 41 and 42: O 3 Ozone Formation Chemistry NO 2
Page 43 and 44: 200 1981-84 200 1995-98 160 160 120
Page 45 and 46: 100 NO 1000 NMHC (estimated from CO
Page 47 and 48: 5 Sunday - Wednesday (1981-84) 10 T
Page 49 and 50: 10.0 8.0 NMHC/NOx 6.0 4.0 2.0 0.0 S
Page 51 and 52: Ratio of Peak Ozone to Potential Oz
Page 53 and 54: 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
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Sunday 1981-84 Time (PDT) 16 15 14
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16 Sunday 1990-94 35 Time (PDT) 15
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40 Sunday O3 Accumulation Rate (ppb
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15.0 1981-84 12.0 9.0 6.0 3.0 0.0 1
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3. CHARACTERIZATION OF THE CURRENT
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3.2 Weekday Differences in Diurnal
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timing hypothesis, which requires a
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through the conversion of NO to NO
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Weekday Differences in Diurnal Vari
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activity including [HNO 3 ]/[H 2 O
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Table 3.2-1 Day-of-the-Week Variati
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Page 113 and 114:
Page 115 and 116:
Table 3.3-1b Day-of-the-Week Differ
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120 Los Angeles - N. Main - Saturda
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160 140 Upland - Saturdays mean Ozo
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maximum 1hour ozone (ppb) 100 90 80
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NMHC/NOx ratio 14 12 10 8 6 4 2 0 S
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Los Angeles - N. Main Azusa 140 140
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Los Angeles - N. Main Azusa 140 140
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2.8 2.6 PAN (ppb) 2.8 2.6 HCHO (ppb
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NO2 Photolysis Rate Parameter 0.35
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4. WEEKDAY VARIATIONS IN THE SOURCE
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9 a.m. andnoon. Sampling was conduc
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interfaced to an Entech model 7100
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values measured at the regional sit
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into a category named "UNID." The s
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more than a few percent of the tota
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period, the amount of NOx associate
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weekend/weekday differences during
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• Day-of-the-week changes in the
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Table 4.4-1 Default Source Composit
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Table 4.4-1a Number of observations
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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
Page 161 and 162:
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
Page 177 and 178:
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
Page 193 and 194:
Page 195 and 196:
Page 197 and 198:
Pico Rivera 150 120 BC-Associated C
Page 199 and 200:
Nitrogen Oxides 800.00 700.00 600.0
Page 201 and 202:
5. REFERENCES Altshuler, S.L., T.D.
Page 203 and 204:
Fujita, E.M., J.G. Watson, J.C. Cho
Page 205 and 206:
Roberts, J.M. (1983) Measurements o
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Weekend/Weekday Ozone Observations in the South Coast Air Basin

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