T.Y. Chang

Rain and snow scavenging of HNO3 vapor in the atmosphere

Abstract The coefficients for the wet removal of HNO 3 vapor from the atmosphere by rain (in-cloud and below-cloud) and snow have been derived under a number of approximations. The wet removal coefficients are parametrized in terms of precipitation rate. These coefficients are intended to represent the average scavenging from large precipitation bands or frontal systems where there is widespread weakly ascending air motion. Consequently, the derived coefficients would be appropriate, on an interim basis, for inclusion into regional or mesoscale models which include the wet removal of HNO 3 vapor. These coefficients, when combined with altitude-dependent precipitation rates and vertical profiles of HNO 3 concentrations, are also useful to estimate the flux of HNO 3 to the earths surface. The derived snow scavenging coefficients are consistent with those of recent measurements by Huebert et al. Both rain and snow scavenging coefficients are also consistent with recent observations of ionic composition in winter precipitation samples which indicated that snow removes HNO 3 vapor more efficiently than rain.

A photochemical extent parameter to aid ozone air quality management

Abstract Interest in semiempirical approaches for relating precursor emissions (VOC and NOx to ozone air quality has been renewed recently. Of particular interest is a semi-empirical approach [called Smog Production Model (SPM) or SP Algorithm] initiated by Johnson and coworkers. SPM defines a photochemical extent parameter, which provides the directional guidance on the relative effectiveness of NOx and VOC controls in reducing ozone levels. In the present paper, a modified version of SPM is introduced, and the parameters involved in SPM are evaluated using urban airshed model (UAM) simulation results. UAM results are also used to examine relationships of extent parameters to ozone reductions resulting from VOC or NOx emission reductions. A modified version of SPM is applied to ambient air quality data. With improved quantified model parameter values and improved measurements of NOy, SPM may provide important information on the relative effectiveness of precursor controls. An attractive feature of SPM is that the required data are simultaneously measured NOx NOy and ozone.

Tunnel air quality and vehicle emissions

A general relationship between roadway tunnel air quality and vehicle emissions has been derived. The model includes the effect of pollutant deposition on the tunnel surfaces and dilution from ventilation. The model is applied to air quality measurements of SO2 and particulates obtained at the Tuscarora Mountain Tunnel. It is found that, if deposition is neglected, SO2 and sulfate emission factors for both gasoline and Diesel vehicles may be underestimated by ~ 10%. The derived deposition velocities are 0.07cms−1 for SO2, 0.03cms−1 for sulfate, and ~ 0.001 cms−1 for total suspended particulates and paniculate components (except sulfate). The last value is lower than smooth-surface values quoted for aerosol deposition, and the difference between the last two values presumably reflects the approximations in the model and/or the uncertainty in its input data.

Impact of methanol vehicles on ozone air quality

Abstract A single-cell trajectory model with an updated chemical mechanism has been used to evaluate the impact on ozone air quality of methanol fueled vehicle (MFV) substitution for conventional fueled vehicles (CFV) in 20 urban areas in the U.S. Recent measurement data for non-methane organic compound (NMOC) concentrations and NMOC NO x ratios for each of the areas was used. The sensitivity of peak 1-h O3 values to variations in many of the input parameters has been tested. The functional dependence of peak 1-h O3 on NMOC NO x , ratios shows that, for many cities, the maximum O3 levels occur near the median urban-center 6–9 a.m. NMOC NO x ratios. The results of the photochemical model computations, including several methanol-fuel substitution scenarios, have been used to derive relative reactivities of methanol and formaldehyde. Per-vehicle O3 reduction potentials for MFV have also been derived. The reduction potentials and calculated percentage O3 reductions for selected MFV market-penetrations have been used to estimate the impact of any MFV market-penetration or change in MFV emission factors. All substitution scenarios evaluated lead to projections of lower peak 1-h O3 levels. Even with significant replacement of CFV by MFV, the reduction of urban O3 levels appears to be modest. However, the reductions may be significant in comparison to other available O3-reduction options.

Ambient temperature effect on urban CO air quality

Abstract Changes in the ambient temperature cause changes in vehicle emissions mainly during the cold-start period of operation. The influence that this effect has on the seasonal variation of urban CO concentrations is examined by means of a simple air quality model. For fixed meteorological conditions, the model predicts that the atmospheric CO concentrations at a center city measuring location will increase by 5–15% for a decrease in temperature from 75° to 25°F when none of the vehicles in that vicinity are in the cold-start mode and will increase by 55–75% when all of the nearby vehicles are in the cold-start mode. An inverse variation of CO concentration with change in temperature is derived from a statistical analysis of CO data reported both for midtown Manhattan (1975–1977) and for downtown Los Angeles (1970–1975). The magnitude of the derived variation is small for both cities and corresponds to the case where none of the vehicles in the vicinity of the measuring site are in the cold-start mode. It therefore follows that the variation of CO vehicle emissions with temperature does not have an important effect on CO air quality at these two urban centers. The highest CO concentrations are observed during the colder months in most American cities and the differences in meteorological conditions between winter and summer that might contribute to this are considered. Some significant factors are that, in the winter, multiday episodes of slowest dilution occur as well as nocturnal inversions of greater stability. From these considerations it is concluded that the meteorological factors play a significant role in the observation of high CO concentrations in urban areas during the colder months.

NO/sub 2/ air quality - precursor relationship: an ambient air quality evaluation in the Los Angeles basin

An analysis of air quality data from 1970-1975 in the Los Angeles (LA) basin has been made with emphasis on factors relevant to high hourly NO/sub 2/ concentrations, (NO/sub 2/). Detailed analysis of CO and SO/sub 2/ air quality and the (NO/sub x/)/(CO) and (CO)/(SO/sub 2/) ratios reveals that high (NO/sub 2/) result mainly from vehicular sources; contributions from stationary sources to these high (NO/sub 2/) of greater than 10: occur rarely. Meteorological conditions (very low early-morning inversion base height and low wind speed) favoring the formation of high (NO/sub 2/) restrict the impact of elevated point NO/sub x/ sources on the ground level (NO/sub 2/) during the early to mid-morning hours. The overnight leftover NO/sub x/ during high NO/sub 2/ days is shown to originate largely from local sources near the monitoring sites. A regression analysis using NO/sub 2/, NO/sub x/ and HC data from downtown LA shows that a 50: reduction in (NO/sub x/) reduces high (NO/sub 2/) by 40-45:; a 50: reduction in (HC) reduces high (NO/sub 2/) by 5-10:. The present analysis supports assumptions used in an earlier generalized rollback model that related NO/sub x/ emissions to high 1-h average (NO/sub 2/) observed at downtown LA.

URBAN OZONE AIR QUALITY IMPACT OF EMISSIONS FROM VEHICLES USING REFORMULATED GASOLINES AND M85

The urban ozone air quality impact of exhaust emissions from vehicles using reformulated gasolines and flexible/variable-fuel vehicles using M85 has been studied using emissions data from the Auto/Oil Air Quality Improvement Research Program and a single-cell trajectory air quality model with two different chemical mechanisms (the updated version of Carbon-Bond-IV (CB4) and the LCC mechanisms). Peak ozone concentrations are predicted for each fuel for all combinations of the following ambient conditions: low and high atmospheric dilution or mixing height, four NMOG/NOx ratios, two each of the initial NMOG concentration, the vehicular contribution to the ambient air, and the NMOG composition of the initial ambient mixture. The ozone impact of a fuel depends strongly on the atmospheric dilution and NMOG/NOx ratio of an area. The differences in ozone impact among fuels are limited under the condition of high atmospheric dilution and a high NMOG/NOx ratio. The ozone-forming potentials (OFPs) for the exhaust emissions based on the maximum incremental reactivities (MIRs) for various fuels are generally well correlated with model-calculated peak ozone levels at a low NMOG/NOx ratio. These OFPs can serve to separate out fuels with rather different reactivities, but not fuels with comparable reactivities. Model-calculated ozone levels for various fuels based on CB4 and LCC mechanisms are relatively well correlated at low NMOG/NOx ratios, but much less so at higher ratios. Fuels with a high aromatic content, including high-toluene fuels, tend to be ranked more favorably by CB4 than by LCC. On the other hand, M85 is ranked more favorably by LCC than by CB4. Fuels with a low 90% boiling point and a low content on aromatics and olefins are generally less reactive. M85 would be an attractive fuel if the formaldehyde emissions could be curtailed significantly. (A)

Generalized Rollback Modeling for Urban Air Pollution Control

A general formula is derived that can be used to calculate the reductions in emissions of inert pollutants required to achieve National Ambient Air Quality Standards (NAAQS) and to predict future urban atmospheric concentrations. The derivation incorporates the main features of atmospheric diffusion modeling and takes account of all categories of sources and their spatial distribution. In our previous paper, carbon monoxide (CO) emissions from light duty vehicles were considered separately with the approximation that emissions from other sources of CO would grow and be controlled proportionately to that of light duty vehicles. The new general formula is applied to Phoenix-Tucson using EPA data. It Is found that Phoenix-Tucson will meet the NAAQS for CO by 1985 if a 12 g/mi light duty vehicle emission standard is adopted. The EPA, using the same data in a modified rollback analysis, had predicted that Phoenix-Tucson, as well as a number of other localities, would not achieve the NAAQS even if the 3.4 g/mi ...

Urban CO Concentrations and Vehicle Emissions

The effect of the general growth of CO vehicular emissions in urban areas on the CAMP station measurements in downtown areas, where vehicular traffic is saturated is considered. With the assumption that the street-level CO concentration is derived from the sum of an urban background term and a local street-effect term, the urban background CO concentration is computed with a diffusion model by introducing a simple area source distribution. The local street-effect term is taken to be constant at a saturation emission level corresponding to a saturation traffic density when the emission per vehicle-mile and meteorological conditions are fixed. The present analysis indicates that the local street-effect term, AC, has a major role in determining street-level concentrations for pollutants, such as CO, whose air quality standard is based on maximum concentrations with averaging times of 1 hour and 8 hours. The relevance of this analysis to the abatement requirements of the Clean Air Amendments and to the drivin...

Modeling smog chamber measurements of incremental reactivities of volatile organic compounds

Abstract A series of experiments performed at the GM chamber facility provided useful data for the evaluation of two current chemical mechanisms used in airshed models (SAPRC97 and SAPRC93 mechanisms) and a test of their predictions of maximum incremental reactivities which describe the change in ozone caused by adding a small amount of a compound to a polluted urban mixture under high-NOx conditions. In general, the SAPRC97 detailed mechanism performed well in simulating the volatile organic compound (VOC) reactivity experiments for most test species; however, it had a tendency to underpredict incremental reactivities. For base-case runs containing a nine-component urban-surrogate mixture under high-NOx conditions, where maximum concentrations of either O3 or the smog produced (SP=the initial NO oxidized plus the ozone produced) were not attained during a 12-h irradiation, the SAPRC97 performed well while the SAPRC93 underestimated SP or O3 significantly. Under low-NOx conditions where SP or O3 maximums were attained, the SAPRC97 as well as the SAPRC93 underpredicted SP or O3 for runs containing the urban-surrogate mixture. Simulations of incremental reactivity experiments and special chamber runs showed that the SAPRC97 mechanism performed poorly for n-octane and some aromatic isomers such as ethylbenzene and p-xylene, while it performed well for other aromatic isomers such as toluene, m-xylene and 1,3,5-trimethylbenzene. Although, additional chamber data for aromatic isomers is needed to further clarify the parameterized chemical mechanisms for aromatic isomers, the newer SAPRC97 mechanism appears to be much improved over the older SAPRC93 mechanism for simulating aromatic chemistry.