CHAPTER 6 

FINDINGS AND DISCUSSION 
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Chapter 6. Findings and Discussion 

The MATES III Study incorporates several updates and improved methodologies compared to 

previous air toxics studies in the Basin to measure and model ambient levels of air toxics and 

their associated risks. Key elements and findings are listed below. 

6.1. Improvements to Ambient Monitoring 

• Samples were taken at ten fixed sites, once every three days. This compares to once 

every six days in previous studies. 

• Sampling occurred for two full years at eight sites, and one full year at all ten sites. 

• An updated method was used to estimate the contribution of diesel exhaust to ambient 

particulates. 

6.2. Improvements to Air Toxics Modeling 

• An updated modeling platform, CAMx, was used to model levels of air toxics throughout 

the Basin using the 2005 emissions inventory. This is the same model as used in the 

2007 Air Quality Management Plan (AQMP). 

• An updated meteorology model for the year 2005 was used. 

• Updated emissions inventories based on the 2007 AQMP were used. 

• An improved assignment of on-road truck emissions using the Caltrans/SCAG Truck 

Model was used. 

• The CAMx platform was also applied to the MATES II time frame, using a back-cast of 

the MATES III air toxics emissions inventory to give more of an “apples to apples” 

comparison of modeled air toxics risks. 

6.3. Key Findings 

• During the study period, the overall Basin cancer risk from air toxics based on the annual 

average levels calculated from the ten monitoring sites data was approximately 1,200 per 

million. 

• Diesel exhaust was the key driver for air toxics risk, accounting for over 80% of the total 

air toxics risk. 

• Of the pollutants measured, only formaldehyde was above the current chronic exposure 

levels developed by OEHHA. However, OEHHA has proposed revised limits. If the 

proposed limits are approved, all average levels of air toxics would be below established 

Reference Exposure Levels. 

• Ambient levels of most substances measured were lower over the period of the MATES 

III Study compared to that of the MATES II Study, which was conducted in 1998-99, 

reflecting the success of various control strategies to reduce exposure to air toxics. 

• Benzene and 1,3-butadiene average levels, pollutants mainly from vehicles, were down 
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50% and 73% , respectively. 

• Stationary source-related pollutants, such as methylene chloride (a solvent used in paint 

remover) and perchloroethylene (an industrial solvent used in dry cleaning) also showed 

declines of 53% and 78%, respectively. 

• Levels of elemental carbon in the PM10 particulate fraction were about 38% lower than 

those found in MATES II. About a 10% reduction was traced to a difference in 

analytical instrumentation used in MATES III. The net 28% reduction in ambient levels 

may reflect decreased emissions combined with annual meteorological differences. 

• Estimated air toxics cancer risks for the second year of the study were somewhat higher 

than the first year. It should be noted that the first year of the study had higher rainfall 

than the second year, which is expected to affect particulate measurements. This mainly 

reflects a higher estimate for diesel particulate levels in the second year. 

• The Rubidoux station showed higher levels of hexavalent chromium in the ambient 

measurements compared to MATES II, and higher levels than found at the other 

monitoring locations. Additional follow up studies by District staff identified a nearby 

cement facility as the source. A report on the additional intensive monitoring results can 

be found on the District’s web site at 

http://www.aqmd.gov/RiversideCement/RiversideCement.html. 

• Modeling analysis shows the highest risks from air toxics surrounding the port areas, with 

the highest grid cell risk about 3,700 per million, followed by the area south of Central 

Los Angeles where there is a major transportation corridor, with grid cell modeled risks 

ranging from about 1,400 to 1,900 per million. 

• Modeling analysis also showed pronounced exposures along freeways and near 

intermodal facilities. 

• While the model estimates of air toxics risk showed an overall Basin-wide reduction, 

some areas showed an increase in air toxics risk. This was shown to be the case near the 

ports, in the eastern portions of the Basin and in northern Los Angeles County. 

• Most of the monitored pollutants at the microscale sites did not show increased levels 

compared to the nearest fixed sites. The exception was the Santa Ana microscale site, 

which showed elevations of several pollutants related to vehicle emissions. 

• Risk estimates in this study do not include mortality from particulate exposure. This was 

done in the 2007 AQMP. 

6.4. Discussion and Policy Implications 

• Although there are uncertainties in the ambient estimates, diesel particulate continues to 

be the dominant toxic air pollutant based on cancer risk. This finding holds up regardless 

of methodology used. The study findings therefore clearly call for a step-up in reducing 

diesel emissions as early as practicable and as aggressively as feasible. 

• Goods movement is a significant source of diesel emissions. With the projected future 

growth in goods movement, diesel emissions may increase. The interplay between (a) 

the increase in goods movement and (b) projected emission reduction strategies will be 
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crucial in determining whether diesel exposures are reduced in the future. 

• There are several uncertainties in estimating air toxics risks. These include uncertainties 

in the cancer potencies of the substances, in the estimates of population exposure, and 

uncertainty in estimating the level of diesel particulate. 

• There is at present no direct measurement method for ambient levels of diesel particulate. 

For this study, staff used the Chemical Mass Balance (CMB) approach to estimate diesel 

particulate levels. The CMB method is considered to be the best available method based 

on current scientific knowledge and was recommended by the Technical Advisory Group 

as well. Using the CMB model takes advantage of the specific profile of chemical 

species in different emissions sources and results in a more robust statistical correlation 

of source contributions to ambient particulate matter levels. 

• The CMB method gave higher estimates of diesel particulate than did the method used in 

MATES II. Thus, the methods used in MATES II may have underestimated risk from 

diesel particulate. 

• Since diesel emissions will likely continue to be the dominant source of cancer risk from 

air toxics for the future, improvement in the methodology to estimate ambient levels of 

diesel particulates would help reduce uncertainty in risk estimates and better characterize 

future changes in risk. 

• Since the timeframe of the MATES III study, there have been numerous regulations and 

initiatives to reduce diesel exhaust emissions by national, state, and regional authorities. 

Staff expects that any future air toxics study will be able to assess the effectiveness of 

these endeavors. 

• Although the estimated Basin-wide risks declined from the MATES II period, areas near 

the ports, and inland in the eastern and northern portions of the District, actually showed 

increases in estimated air toxics risk. These are likely due to increases in emissions 

related to the additional cargo container traffic and goods movement that occurred 

between MATES II and MATES III time periods.