Atmospheric Environment (v.37, #S2)

The 1997 Southern California Ozone Study (SCOS97-NARSTO) by Bart E. Croes; Eric M. Fujita (1-2).

Keywords: Emission; Meteorology; Transport; Air quality; Aerosols; PM; SCOS-NARSTO;

To improve prediction of hourly running stabilized vehicle volumes for emissions modeling, a new method was recently developed that disaggregates the period-based volumes produced by travel demand models into hourly volumes using observed traffic count data and multivariate multiple regression. In addition to a more robust method for estimating hourly volumes, the method also provides a means for introducing spatial variability of mobile emissions into air quality modeling. In this study, the new methodology is used to disaggregate travel demand volumes both temporally and spatially. The impact of the increased spatial and temporal resolution on estimated mobile emissions was then investigated for San Diego and Los Angeles. The results indicate that actual on-road running stabilized vehicle emissions exhibit less pronounced peaking during the day and a much slower decay of emissions during the late evening hours than would be predicted by the current method for allocating period volumes to specific hours within that period. The new method also identifies corridors (or clustered locations) producing the bulk of hourly emissions. This study should prove useful for further refining of control strategies for transportation conformity purposes and for improving the performance of air quality models, which tend to underpredict ozone concentrations.
Keywords: Air quality; Vehicles; SCOS97;

The Biogenic Emission Inventory Geographic Information System (BEIGIS) is a spatially and temporally resolved biogenic hydrocarbon emissions inventory model developed by the California Air Resources Board that uses California land use/land cover, leaf mass, and emission rate databases within a GIS. BEIGIS simulates hourly emissions of isoprene, monoterpenes, and 2-methyl-3-buten-2-ol (MBO, methylbutenol) at a 1 km2 resolution. When applied to the Southern California Ozone Study (SCOS97-NARSTO) domain for the 3–7 August 1997 ozone episode, the BEIGIS model predicts total biogenic volatile organic compound (BVOC) emissions of 866 tons for the warmest day (5 August). Depending on whether wildfire emissions are included in the total volatile organic compound (VOC) emissions estimate, modeled BVOC emissions comprise between 16% and 28% of the total VOC inventory. As anthropogenic VOC emissions decline in future years due to control programs, the relative significance of BVOC emissions in the development of ozone control strategies for southern California may assume greater importance.
Keywords: Biogenic hydrocarbons; Isoprene; Monoterpenes; Methylbutenol;

Ozone transport by mesoscale and diurnal wind circulations across southern California by Jay S. Rosenthal; Roger A. Helvey; Terry E. Battalino; Charles Fisk; Paul W. Greiman (51-71).
This paper reviews sea level and elevated surface observations from Point Mugu and nearby Laguna Peak in southern Ventura County, rawin/ozonesonde data from Point Mugu, and aircraft measurements in the coastal zone and offshore obtained in support of the SCOS97-NARSTO effort. Streamline analyses of SCOS97-NARSTO wind data are also presented, which reveal synoptic and mesoscale influences on air flow over the southern California area. Complex diurnal, mesoscale and synoptic wind patterns redistribute pollutants across air basins. The data show that southeasterly flow, often present from morning into early afternoon, and coastal eddies and southerly surges convey ozone and precursors westward and northward where they may appear aloft as layers with several times the concentration of ozone at the surface. This flow can transport South Coast Air Basin (SCAB) pollutants via mountain passes to the desert where mixing with pollution from the San Joaquin Valley can occur. Climatological evidence shows this tendency for southerly flow in the earlier part of the day is a recurrent feature of southern California wind patterns, and a major mechanism for transport of ozone and precursors from the SCAB to coastal, inland and desert regions to the north.
Keywords: Aircraft measurements; Eddy circulation; Ozonesonde; Radiosonde; Streamlines;

Analysis of transport patterns during an SCOS97-NARSTO episode by Dimitra Boucouvala; Robert Bornstein (73-94).
Objectives for the current study included use of SCOS97-NARSTO data to understand transport factors in the occurrence of high ozone concentrations during 4–7 August 1997. Meteorological data for the case study included observations at 110 SCOS97-NARSTO surface sites, and upper-air measurements from 12 rawinsonde and 26 profiler sites. Analysis showed that the peak ozone resulted from an infrequent combination of large-scale upper-level synoptic forcing associated with a weak local coastal 700 mb ridge. Its movements over the California South Coast Air Basin lowered and strengthened the coastal subsidence inversion and also rotated the upper-level synoptic background flow from its normal westerly onshore direction to a less common offshore easterly flow during the nighttime period preceding the peak-ozone hours. Resulting easterly upper-level background winds produced easterly surface flow directions at inland sites, so that a surface frontal convergence zone formed where this flow met the westerly onshore combined sea breeze and upslope flows. The maximum inland penetration of the convergence zone on the peak-ozone day was to the western side of the San Gabriel Mountains, the location of maximum ozone concentrations.
Keywords: SCOS97-NARSTO; Meteorological analysis; Ozone episode;

MM5 simulations of a SCOS97-NARSTO episode by D. Boucouvala; R. Bornstein; J. Wilkinson; D. Miller (95-117).
MM5 simulations of the 4–7 August SCOS97-NARSTO ozone episode in the SoCAB showed that a relatively rare easterly upper-level background flow influenced surface wind flow directions at in-land sites. An inland-moving surface convergence zone resulted where the offshore background flow met an inland moving westerly (sea breeze plus thermal upslope) onshore flow. Maximum inland penetration of the zone was to the San Gabriel Mountains, where it produced peak observed ozone concentrations. MM5 reproduced the main qualitative features of the evolution of the diurnal sea breeze cycle, as it reproduced the opposing easterly flow, inland movement of the sea breeze front, and retreat of the land breeze front. The accuracy of predicted surface winds and temperatures were improved by modifications of MM5 and/or its input parameters, e.g., by use of the PSU MBLI scheme, analysis nudging with NCAR/NCEP model output, accurate specification of deep-soil temperature, observational nudging with SCOS97-NARSTO surface and upper level winds and temperatures, and use of updated urban land-use patterns.
Keywords: SCOS97-NARSTO; SoCAB; MM5; Meteorological conditions;

Measurement of NO Y during the SCOS97-NARSTO by Dennis R. Fitz; Kurt Bumiller; Ash Lashgari (119-134).
Dual converter NO Y analyzers configured to also measure nitric acid were evaluated under laboratory conditions, smog chamber simulations, and during ambient monitoring. Commercial molybdenum metal converters in the NO Y configuration were found to be capable of converting all major NO Y species to NO under laboratory testing conditions. Some converters required higher-temperature set points than received from the manufacturer and several converters did not achieve 95% efficiency at the highest temperature allowed by the manufacturer (380°C). They also exhibited varying degrees of efficiency for oxidizing ammonia to NO depending on previous usage, but not operating temperature. After the field measurement program, the efficiency for converting nitric acid was found to be variable during smog chamber measurements. This was ascribed to degradation of the converter under field measurement conditions. The ambient measurement program showed a significant memory effect that contributed to a positive NO Y measurement bias. The magnitude of this memory effect appeared to be related to the degree of ambient concentrations of pollutants being sampled. Ambient nitric acid measurements determined from the dual converter approach were related to those from a tunable diode laser absorption spectrometer although the correlation was not high. The variable converter efficiency for ammonia may lead to negative biases when measuring nitric acid.
Keywords: SCOS97-NARSTO; NO Y ; Nitric acid; Chemiluminescent; Denuder; TDLAS;

Performance audits and laboratory comparisons were conducted as part of the quality assurance program for the 1997 Southern California Ozone Study (SCOS97-NARSTO) to document potential measurement biases among laboratories measuring speciated nonmethane hydrocarbons (NMHC), carbonyl compounds, halogenated compounds, and biogenic hydrocarbons. The results show that measurements of volatile organic compounds (VOC) made during SCOS97-NARSTO are generally consistent with specified data quality objectives. The hydrocarbon comparison involved nine laboratories and consisted of two sets of collocated ambient samples. The coefficients of variation among laboratories for the sum of the 55 PAM target compounds and total NMHC ranged from ±5 to 15 percent for ambient samples from Los Angeles and Azusa. Abundant hydrocarbons are consistently identified by all laboratories, but discrepancies occur for olefins greater than C4 and for hydrocarbons greater than C8. Laboratory comparisons for halogenated compounds and biogenic hydrocarbons consisted of both concurrent ambient sampling by different laboratories and round-robin analysis of ambient samples. The coefficients of variation among participating laboratories were about 10–20 percent. Performance audits were conducted for measurement of carbonyl compounds involving sampling from a standard mixture of carbonyl compounds. The values reported by most of the laboratories were within 10–20 percent of those of the reference laboratory. Results of field measurement comparisons showed larger variations among the laboratories ranging from 20 to 40 percent for C1–C3 carbonyl compounds. The greater variations observed in the field measurement comparison may reflect potential sampling artifacts, which the performance audits did not address.
Keywords: Volatile organic compounds; VOC speciation; Quality assurance; Laboratory audits; Laboratory comparisons;

Development of instrumentation for simultaneous analysis of total non-methane organic carbon and volatile organic compounds in ambient air by Christophe Maris; Myeong Y. Chung; Richard Lueb; Udo Krischke; Richard Meller; Matthew J. Fox; Suzanne E. Paulson (149-158).
Here we describe the development of a new instrument to measure the total airborne non-methane organic carbon concentration (TNMOC), and the ratio of this value to the sum of speciated volatile organic compounds (VOCs) measured by standard gas chromatography. The TNMOC and sum of speciated VOC analyses are made simultaneously. Samples are collected in situ, with an inlet designed to minimize contact of samples with surfaces. Whole air samples are cryo-trapped with minimal collection of CO2, CO and CH4. Organics are desorbed and converted to CO2 using an oxidation catalyst. The resulting CO2 is analyzed with a flame ionization detector after reduction to methane. The instrument is tested and found to perform well on gas mixtures, ambient air and on smog chamber samples. The detection limit for the instrument is 35 ppbC, and the accuracy of the ratio of TNMOC to the sum of speciated VOCs is ±0.05 for most samples.
Keywords: VOCs; Hydrocarbons; Los Angeles air; Ambient air; Total organic carbon; PAMs Data; Air monitoring;

Simultaneous measurements of total non-methane organic carbon (TNMOC) and speciated volatile organic compounds (VOCs) were made with an instrument described in a companion paper. The ratio of the TNMOC to the speciated VOCs measurement indicates the “excess” VOCs that not detected by routine monitoring techniques. Measurements of ambient air were made in West Los Angeles, Burbank and Azusa, CA. Primary pollution sources are gasoline vapor and diesel exhaust were tested as well. These primary sources are, not surprisingly, measured well by the standard speciated analysis, with averages of 5–13% “excess” VOCs. The Burbank site, which is surrounded by freeways and light industry, had only 10–15% excess VOCs, as did UCLA during winter when there is minimal photochemistry. In contrast, averages of between 20% and 45% of the TNMOC was unmeasured by the standard chromatography at the UCLA and Azusa sites during summer. The dominant source of the unmeasured VOCs may be air that has been aged for more than 12 h, possibly for a day or more.
Keywords: VOCs; Hydrocarbons; Los Angeles air; Total organic carbon; PAMS data;

Speciated volatile organic compounds (VOCs) measurements were made at remote upwind locations in order to establish the boundary conditions for input into air quality simulation modeling for the 1997 Southern California Ozone Study (SCOS97-NARSTO). These measurements were made on two islands off the coast of Southern California and two coastal sites on the northern and southern end of the modeling domain. Results show that the mixing ratios and composition of VOC are impacted at these sites by local emissions as well as transported aged emissions. Of the four sites, San Nicolas Island is most representative of the boundary conditions at the western edge of the modeling domain with total identified non-methane hydrocarbons, identified oxygenated compounds (excluding formaldehyde), and unidentified hydrocarbons at about 12, 10, and 6 ppbC, respectively. Formaldehyde and acetaldehyde are on the order of 0.5 ppbv each, and C6–C9 aldehydes account for a majority of the oxygenated compounds. Comparable measurements at remote downwind locations also show that oxygenated compounds, especially C6–C9 aldehydes account for a significant fraction of total oxygenated VOC in downwind locations.
Keywords: Volatile organic compounds; Boundary conditions; Photochemical modeling;

Characterization of biogenic volatile organic compounds and meteorology at Azusa during the SCOS97-NARSTO by Anni Reissell; Clinton MacDonald; Paul Roberts; Janet Arey (181-196).
Meteorological modeling and analyses were performed for the 4–6 August and 4–6 September 1997 intensive sampling episodes of the 1997 Southern California Ozone Study (SCOS97-NARSTO). This investigation was conducted to study the connection of measured and modeled meteorological phenomenon with measured biogenic volatile organic compound (BVOC) concentrations. Trajectory and surface wind analysis for five out of the six intensive sampling days indicated that drainage flow from the mountains at nighttime and early morning affected the BVOCs measured at the Azusa, CA monitoring site. Nitric oxide emissions from vehicles precluded nighttime chemical reactions of the BVOCs at Azusa, allowing overnight accumulation of local monoterpene emissions in addition to monoterpenes transported by drainage flows. The monoterpene mixing ratios maximized in the early morning hours and the monoterpene/CO ratios showed sharp maxima at 0300–0600 h PDT on all days. In contrast, normalization of benzene to CO diminished its diurnal variation, consistent with vehicle traffic as the source for these two long-lived anthropogenic compounds. Substantial early morning methacrolein (MACR) and methyl vinyl ketone (MVK) mixing ratios (and high (MACR + MVK)/isoprene ratios) measured at Azusa on all episode days were consistent with drainage flow from elevated sites which had received NO x emissions and ozone transported from the urban basin by daytime upslope flows. Isolation of these elevated sites from the basin NO emissions as the nocturnal boundary layer was established would allow rapid nighttime nitrate radical reaction with isoprene, or slower isoprene reaction with ozone, to produce high (MACR + MVK)/isoprene ratios in drainage flows into Azusa.
Keywords: Biogenic volatile organic compounds; Meteorological modeling; Monoterpenes; Isoprene;

Large quantities of non-methane organic compounds are emitted into the atmosphere from biogenic sources, mainly from vegetation. These organic compounds include isoprene, C10H16 monoterpenes, C15H24 sesquiterpenes, and a number of oxygenated compounds including methanol, hexene derivatives, 2-methyl-3-buten-2-ol, and 6-methyl-5-hepten-2-one. In the troposphere these organic compounds react with hydroxyl (OH) radicals, nitrate (NO3) radicals and ozone (O3), and play an important role in the chemistry of the lower troposphere. In this article the kinetics, products and mechanisms of the tropospheric reactions of biogenic organic compounds are presented and briefly discussed.
Keywords: Biogenic volatile organic compound; Atmospheric chemistry; Hydroxyl radical; Nitrate radical; Ozone; Reaction products;

We examine long-term trends in ambient levels of peroxyacetyl nitrate (PAN) and peroxypropionyl nitrate (PPN) in southern California from the first measurements made in 1960 to those made in 1997 as part of SCOS97-NARSTO. We analyze peak concentrations, 24-h averages and monthly averages. Although the data record is incomplete with respect to frequency and duration of measurements and is limited with respect to seasonal and spatial variations, peak concentrations have decreased from ca. 60–70 ppb to ca. 5–10 ppb for PAN and from ca. 5–6 to 1 ppb or less for PPN. Twenty-four-hour averages and monthly averages also show downward trends in ambient PAN and PPN. The PPN/PAN concentration ratios range from 0.10 to 0.28 and the limited data record give no indication of a long-term temporal trend. The downward trend in peak PAN concentrations is similar to that of peak ozone concentrations during the same 37-year period and is likely to result from increasingly stringent controls on emissions of oxides of nitrogen and of volatile organic compounds that are precursors to ozone, PAN, PPN and other photochemical oxidants.
Keywords: Peroxyacetyl nitrate; Peroxypropionyl nitrate; Long-term trends; Urban air quality; Oxygenated fuels;

Ambient single particle analysis in Riverside, California by aerosol time-of-flight mass spectrometry during the SCOS97-NARSTO by Sylvia H. Pastor; Jonathan O. Allen; Lara S. Hughes; Prakash Bhave; Glen R. Cass; Kimberly A. Prather (239-258).
Single-particle measurements were made using aerosol time-of-flight mass spectrometry (ATOFMS) instruments in conjunction with the 1997 Southern California Ozone Study-North American Research Strategy for Tropospheric Ozone (SCOS97-NARSTO). The size and chemical composition of individual ambient particles in Riverside, CA during the summer of 1997 are described. Data collected using co-located micro-orifice uniform deposit impactors (MOUDI) impactors are used to scale the ATOFMS number counts, providing a unique picture of the particle population which complements information obtained using traditional sizing and composition analysis techniques in this and previous studies. Changes in single particle composition are observed over time, and compared and contrasted with observed changes in visibility, ozone, and PM10 concentrations. Details are provided on changes in the particle size and composition observed during three morning periods with low ozone and elevated PM10 versus three afternoon periods with both elevated ozone and PM10 levels between 21–23 August 1997. The ATOFMS size profiles show afternoon periods dominated by sub-μm particles composed of organic carbon coupled with ammonium nitrate, and morning periods with relatively high levels of super-μm dust particles. The observed changes in particle size and composition are consistent with differences in air mass trajectories arriving at Riverside during the morning and afternoon time periods.Temporal variations in single particle types detected over a 40 day sampling period are presented, demonstrating the type of unique information that can be obtained regarding atmospheric particle processes through long term sampling studies. The broader availability of single particle mass spectrometers coupled with recent advances in the field are now providing unique information on the associations of multiple chemical species (i.e. mixing state) within individual particles with high size and temporal resolution.
Keywords: Aerosol; Chemistry; On-line; Particulate matter; Neural network;