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Home
Contact Us
Ag Engines
Air Quality
Air Toxics
AQ Plans
Area Designations
Application Forms
Ask Eric Wality
Board
Burn Info
Calendar
CEQA Planning
Employment
Grant Programs
Hearing Board Kid's Zone
Particle Pollution
Permit
Rules 'n Regs
SB 700
Yuba-Sutter Transit

 

2003 AIR QUALITY ATTAINMENT PLAN
CHAPTER II - AIR MONITORING

II.1      INTRODUCTION

This chapter of the California Clean Air Act Attainment Plan Update will look at NSVAB air quality monitoring data and results from the past three years (2000-2002).

This 2003 Plan Update is primarily concerned with the pollutant ozone for which the NSVAB has been designated nonattainment. This plan update will also examine the pollutant PM10.

The air quality data contained in Appendix B was provided by the CARB website at www.arb.ca.gov/adam/welcome.html. This section contains air quality statistics for the NSVAB.  Statistics are reported in units of concentration.

The Ambient Air Quality Standards establish the concentration at which the pollutant is known to cause adverse health effects to sensitive groups within the population, such as children and the elderly. Both the California and federal governments have adopted health-based standards for the criteria pollutants, which include ozone, particulate matter (PM10 and PM2.5) and carbon monoxide. In general, the air quality standards are expressed as a measure of the amount of pollutant per unit volume of air.  For example, the particulate matter standard is expressed as micrograms of particulate matter per cubic meter of air (ug/m3) and the ozone standard is expressed as parts per million (ppm).

II.2      OZONE MONITORING

Ozone is a colorless gas with a pungent odor. It is the chief component of urban smog. Ozone is not directly emitted as a pollutant, but is formed in the atmosphere when precursor emissions, hydrocarbons and nitrogen oxides, react in the presence of sunlight. Generally, low wind speeds or stagnant air coupled with warm temperatures and cloudless skies provide for the optimum conditions. As a result, summer is generally the peak ozone season. Because of the reaction time involved, peak ozone concentrations often occur far downwind of the precursor emissions. Therefore, ozone is a regional pollutant that often impacts a widespread area. In addition to adverse health effects, ozone causes damage to open vegetation, building surfaces, exposed rubber surfaces, and certain exposed plastics.

Meteorology (weather) and topography (the lay of the land) play major roles in ozone formation. When the weather is warm and the winds are light, a vertical downward motion of air and a natural cooling of the earth’s surface act together to form an inversion that traps pollutants. Sunlight then causes a chemical reaction between the hydrocarbons and nitrogen oxides to form ozone.

The Sacramento Valley is shaped like an elongated bowl. Temperature inversion layers can clamp a lid on the bowl, allowing air pollution to rise to unhealthy levels. Weather conditions cause air pollution concentrations to fluctuate widely from day to day and season to season.

Topography alone gives the NSVAB great potential for trapping and accumulating air pollutants. The strong inversions typical of NSVAB summers are caused by subsidence, the slow sinking of air causing compressional warming. The surface inversions typical of winter are formed primarily at night as air is cooled when it comes in contact with the earth’s cold surface. These are called radiation inversions.

Temperature inversions prevent pollutants from rising and being diluted vertically. Thus, pollutants remain trapped in the layer of air where people breathe. Summer subsidence inversions occur on over 90% of summer days; they persist throughout the day and tend to intensify during the afternoon. Winter radiation inversions occur on over 70% of winter nights, but are usually destroyed by daytime heating, bringing a rapid improvement in air quality by afternoon. Both types of inversion mechanisms may operate at any time of the year, and in the fall both may occur together to produce the heaviest pollution potential.

Recognizing the adverse health impacts of daylong exposure, the United States Environmental Protection Agency promulgated an 8-hour ozone standard in 1997 as a successor to the 1-hour standard, which was established in 1979.

Litigation delayed the implementation of the national 8-hour ozone standard proposed in 1997. EPA issued a proposed rule in May 2003. The proposed rule does not identify, or designate, areas that do not meet the new standard. Designations for attainment and nonattainment areas will occur by April 15, 2004, under a separate process. 

Ozone Summary

Figure 2 shows the placement of the air monitoring stations operating from 2000 through 2002 in the NSVAB.  The placement of the ozone monitors appears evenly distributed throughout the NSVAB. Currently there are eleven ozone monitors operating in the NSVAB.  Shasta County has three monitors, one located in Redding, one in Anderson and one in Lassen Volcanic Park; Butte County has two monitors, one located in Chico and one located in Paradise; Sutter County has three monitors, located in Yuba City, Pleasant Grove (removed prior to January 2003) and one on the Sutter Buttes; Tehama County has two monitors, one in downtown Red Bluff and one on the Tuscan Buttes; Glenn County has one monitor in Willows; and Colusa County has one monitor in the town of Colusa. 

Appendix A, Ozone Tables and Graphs depict, by county, 3-year ozone air quality data including: maximum 1-hour and 8-hour concentrations; days above State standard; days above national 1-hour and 8-hour standard.

The State standard allows only one exceedance per year on average at any site within the Air District in the preceding three-year period. This is meant to take into account year-to-year weather fluctuation and any exceptional exceedances. The California Air Resources Board has established three categories of exceptional exceedances: (1) “exceptional events” (i.e. forest fires); (2) “extreme concentration events”; and (3) “unusual concentration events”.

Ozone trends are variable and unique for each district within the NSVAB. During the past three-year period, the Butte County Paradise monitor, and the Tehama County Red Bluff monitor experienced the highest number of ozone violations in the basin. Ozone concentrations in the NSVAB have remained relatively constant over the past three years while population and vehicle miles traveled (VMT) have increased during the same period. Shasta County ozone violations significantly decreased in the past three years. The decreases in ozone concentrations are largely due to favorable meteorological conditions during this time period.

As explained in Chapter IV - Transport of Pollutants, ozone violations in the NSVAB have been classified as transport from the Broader Sacramento Area. The California Air Resources Board (ARB) has defined the impacts of transported air pollution from the Broader Sacramento Area to air districts in the Northern Sacramento Valley (also known as Upper Sacramento Valley). The ARB’s most recent assessment, published in March 2001, is discussed further in Chapter IV. 

II.3      PM10 MONITORING

Particulate Matter (PM10) refers to particles with an aerodynamic diameter of 10 microns or smaller. For comparison, the diameter of a human hair is about 50 to 100 microns. PM10 is a mixture of substances that includes: elements such as carbon, lead, and nickel; compounds such as nitrates, organic compounds, and sulfates; and complex mixtures such as soil and diesel exhaust. These substances occur in the form of solid particles or as liquid droplets. Primary particles are emitted directly into the atmosphere. Secondary particles result from gases that are transformed into particles through physical and chemical processes in the atmosphere.

PM2.5 includes a subgroup of particles that are less than 2.5 microns in aerodynamic diameter. Fine particulate matter poses an increased health risk because it can be deposited deep into the lung and may contain substances that are particularly harmful to human health. The EPA promulgated two new national PM2.5 standards in 1997. EPA plans to make final designations by December 15, 2004 based on data from 2001-2003, to reflect the most recent three years of data.

PM10 Summary

Appendix B, Particulate Matter Tables and Graphs depict, by county, three-year PM10 air quality statistics including: Maximum 24-hour Concentration; Maximum Annual Geometric and Arithmetic Mean; Estimated Days Above State 24-hour Standard; and Days Above National 24-hour Standard.

PM10 trends are also unique and variable for each district within the NSVAB. In comparison to ozone, PM10 concentrations do not relate well to growth in population or increased vehicle usage. High PM10 concentrations do not always occur in high population areas. Again, weather and topography play an important role in the fluctuation of air pollution concentrations from day to day and season to season.

In the past three years the Yuba City, Glenn and Chico monitoring stations had the highest number of estimated days above the State PM10 standard. Yuba City and Red Bluff had the highest annual averages. The NSVAB has had only one national 24-hour PM10 standard exceedance since 1987. This national exceedance occurred in Colusa County in 1999 and was significantly influenced by wildfires in the area. Because many of the sources that contribute to ozone also contribute to PM10, future ozone emission controls may improve PM10 air quality.

II.4      Emission Inventory

The California Air Pollution Control and Air Quality Management Districts and the California Air Resources Board (ARB) develop the emission inventory and associated emissions projections jointly. The California Emission Forecasting System (CEFS) is the computer tool used to develop the projections; the emission estimates are based on the most currently available growth and control data. For mobile sources, CEFS integrates the emission estimates from the EMFAC model. The emission projections are based on the 1999 inventory with updates as of November 2002.

In the following tables are forecast emissions for the Sacramento Valley Air Basin for Reactive Organic Gases (ROG) and Oxides of Nitrogen (NOx) for several source categories. The annual average emissions are reported in tons per day for the years 2010, 2015 and 2020. The projected emissions show a downtrend for both ROG and NOx, which are the precursor emissions for ozone.

REACTIVE ORGANIC GASES PROJECTED EMISSION INVENTORY
SACRAMENTO VALLEY AIR BASIN

 
All emissions are represented in Tons per Day and reflect the most current data provided to ARB.

*  Emissions from natural sources are excluded.

OXIDES OF NITROGEN PROJECTED EMISSION INVENTORY
SACRAMENTO VALLEY AIR BASIN
 

All emissions are represented in Tons per Day and reflect the most current data provided to ARB.

*  Emissions from natural sources are excluded.

II.5      BACKGROUND INFORMATION: OZONE AIR QUALITY INDICATORS

Air Quality Indicators

There are a number of ways to look at how ozone levels have changed over time, and assessing progress in attaining the State ozone standard. These are the Expected Peak Day Concentration (EPDC), and two exposure indicators (population-weighted and area-weighted). Background information is provided below, along with an overview of the calculation procedure.

Expected Peak Day Concentration (EPDC)

The EPDC represents the maximum ozone concentration expected to occur once per year, on average. The EPDC is based on a statistical calculation of ambient ozone data collected at each monitoring site in the district. The EPDC is useful for tracking air quality progress at individual monitoring locations. Because it is based on a robust statistical calculation, it is relatively stable, thereby providing a trend indicator that is not highly influenced by year-to-year changes in meteorology.

The EPDC is calculated using ozone data for a three-year period (the summary year and two years immediately before the summary year). The data included in the calculation are daily maximum 1-hour ozone observations. However, when three years of data are not available, an EPDC can be calculated using only one or two years of data. The EPDC is computed using a statistical procedure that fits an exponential-tail model to the upper tail of the distribution of concentrations. The fitted distribution then is used to determine analytically the concentration that is expected to recur at a one-in-one year rate.

An EPDC with a valid label (Y) indicates that the data meets the designation criterion for complete and representative data. An EPDC that is not valid (N) means that it doesn’t meet this criterion and indicates incomplete and potentially unrepresentative data. An invalid EPDC cannot be used for the purpose of determining attainment status, but can provide useful information for evaluating long-term air quality trends at individual sites.

Exposure Indicators

The exposure indicators provided are the population-weighted (PWE) and area-weighted exposure (AWE) indictors. These are intended to provide an indication of the potential for chronic adverse health impacts. Unlike the EPDC which tracks progress at individual locations, the population-weighted and area-weighted exposure indicators consolidate hourly ozone monitoring data from all sites within the district into a single exposure value. The result is a value representing the average potential exposure in an area, which in this case, is a district. The term “potential” is used, because daily activity affects an individual’s exposure. For example, being indoors during peak ozone concentrations will decrease a person’s exposure to outdoor ozone concentrations.

The purpose of the population-weighted indicator is to characterize the potential average outdoor exposure per person to concentrations above the level of the state ozone standard. The population-weighted exposure indicator represents a composite of exposures at individual locations that have been weighted to emphasize equally the potential exposure for each individual in an area. In contrast, the purpose of the area-weighted exposure indicator is to characterize the potential average annual outdoor exposure per unit area. The area-weighted exposure indicator represents a composite of exposures at individual locations that have been weighted to emphasize equally the potential exposure in all portions of the district. 

The exposure analysis is based solely on ambient (outdoor) ozone data. The calculation methodology assumes that an “exposure” occurs when a person experiences a 1-hour ozone concentration outdoors that is higher than 0.09 ppm, the level of the State standard. The PWE and AWE consider both the level and duration of ozone concentrations above the State standard. The annual exposure is the sum of all the hourly exposures during the year and presents the results as an average per exposed person or average per exposed unit land area.

II.6      OVERVIEW OF CALCULATION METHODOLOGY FOR THE POPULATION-WEIGHTED AND AREA-WEIGHTED EXPOSURE INDICATORS

The Time Period: The population-weighted and area-weighted exposure indicators are computed as an annual value for each year.

Air Quality Data:  The air quality data used for computing the exposure indicators are hourly ozone data. All available data for sites in the district are used, regardless of whether the data meet designation criterion for complete and representative data. Because the individual exposure values are interpolated from data for several monitoring sites, it is not critical that the data for all the sites be complete for all hours.

Census Data:  The exposure computations are based on census data collected by the federal government. For the years from 1985 to 1999, the population statistics are based on the 1990 census.  For the years 2000, 2001, 2002, population data from the 2000 census was used.

The federal government has divided the nation into census tracts for the purpose of counting population and obtaining demographic information. Each of these census tracts has associated with it:  a (1) centroid of the census tract, (2) the population residing within the census tract, and (3) the land area of the census tract. The population within each census tract is used in computing the annual population-weighted exposure, whereas, the land area of the census tract is used in computing the annual area-weighted exposure. The centroid of the census tract is used in computing both exposure indicators.

Calculation Procedure for Population-Weighted Exposure:  Hourly ozone concentrations are interpolated to each census tract centroid.  Hourly ozone exposures are computed for each centroid by subtracting the value of the State ozone standard (0.09 ppm) from each interpolated hourly concentration. If negative, the result is set equal to zero. The hourly exposures for each census tract are multiplied by the number of people residing in the census tract. These hourly exposures are then added together and divided by the total population of all of the census tracts for which interpolated exposure values are available.

The result represents an hourly population-weighted exposure for the district. The hourly exposures are aggregated into a daily population-weighted exposure. The daily exposures are then aggregated into an annual population-weighted exposure. This is done for each year from 1985 through 2002, for which data are available.

Calculation Procedure for Area-Weighted Exposure: The procedure for computing the area-weighted exposure is similar. In this case, the hourly exposures for each census tract are multiplied by the square kilometer land area of the census tract. These hourly exposures are then added together and divided by the total land area of all of the census tracts for which interpolated exposure values are available. The result represents an hourly area-weighted exposure for the district. The hourly exposures are aggregated into a daily area-weighted exposure. The daily exposures are then aggregated into an annual area-weighted exposure. This is done for each year from 1985 through 2002, for which data are available.