Module 3. Air Quality

Air quality varies

hourly, daily, and over longer timescales

Air quality changes over time due to:

▪ Emission sources (individual and collective).

▪ Emission density (concentration of sources in an area).

▪ Topography (geographical features affecting dispersion).

▪ Atmospheric conditions (wind, temperature, humidity, etc.).

➢Qualitative vs. Quantitative Assessment

Poor air quality reduces visibility, damages materials, harms plants, and affects health; however, regulatory programs require precise pollutant measurements.

The Main objective of a air quality assessment

is to protect human health and the environment by monitoring ambient pollutant levels and emissions.

Key Efforts for Air Quality Management

Effective programs include air quality monitoring, emissions assessment, and pollution modeling.

Pollutant levels in North America are assessed by systematically conducting long-term ambient air quality monitoring efforts. In the U.S., monitoring provides data to

1. Determine compliance with National Ambient Air Quality Standards (NAAQSs) for seven pollutant categories (criteria pollutants) in air quality control regions (AQCRs) or portions thereof

2. Determine long-term trends

3. Determine human exposures

4. Support the Air Quality Index program

5. Support emissions reduction programs

6. Determine the effectiveness of emission control programs

7. Support environmental assessments such as visibility impairment and degradation of watersheds

8. Support research efforts designed to determine potential associations between pollutant levels and adverse health and environmental effects

Monitoring Considerations

Air quality monitoring involves numerous measurements of individual pollutants over time at a few locations in organized, systematic programs. Concentration measurements are made from samples of small volumes of ambient air.

Air Quality Monitoring & Sampling

Due to the vast and dynamic nature of the atmosphere, pollutant concentrations are estimated from limited samples using fixed-site monitors.

Manual methods:

Pollutants are collected on a medium and analyzed later, and sufficient pollutant mass is required for detection.

Automated (real-time) methods:

Sampling and analysis occur simultaneously, requiring smaller sample sizes.

Factors Affecting Sampling Efficiency:

Collection efficiency depends on sampling techniques, pollutant properties, flow rates, and instrument limitations. Lower flow rates often improve efficiency, especially for gas-phase contaminants.

Optimizing Sampling & Analysis:

For accurate results, pollutants may require extended sampling duration, refrigeration, or preservatives to prevent loss before analysis. Real-time systems use low flow rates for high sensitivity and precision.

Manual Sampling (Intermittent Methods)

▪ Pollutants are collected over a fixed period (e.g., 24 hours).

▪ Analysis occurs later, so concentrations are averaged over time.

▪ Requires extended sampling durations to collect enough pollutants for detection.

▪ Useful for tracking long-term air quality trends but may miss shortterm pollution spikes.

➢Real-Time Monitoring (Continuous Methods)

▪ Measures pollutant concentrations instantly and continuously.

▪ Provides a large amount of data, which can be overwhelming.

▪ Data is averaged over set time periods (e.g., hourly, daily) for regulatory use.

▪ More effective at detecting short-term pollution peaks.

➢Factors Affecting Pollutant Measurement

Sampling Duration

Sampling Rate

Collection Efficiency

Pollutant Type

Storage and Preservation

Sampling Duration:

Longer sampling times capture broader trends, while shorter times detect rapid fluctuations.

Sampling Rate:

Affects how much air passes through the sampling device, influencing pollutant collection.

Collection Efficiency:

Some pollutants are harder to capture, requiring optimized sampling methods.

Pollutant Type:

Gaseous and particulate pollutants require different collection techniques.

Storage and Preservation:

Some samples degrade over time and need refrigeration or preservatives.

The principal objective of sampling

is to collect a pollutant or pollutants for subsequent analysis or provide a sensing environment for real-time measurements.

Pollutant averaging times vary based on

health and environmental risks

Factors affecting pollutant measurement include

sampling duration, collection efficiency, pollutant type, and preservation needs.

Challenges in air quality monitoring include

handling large data volumes, instrument sensitivity, environmental influences, and maintenance costs.

➢How air is collected for analysis

Air pollutants (gases or particles) need to be drawn into a sensing device or collected on a surface to be measured.

The process of collecting air

This process is done using a sampling train, which includes tools like a vacuum pump, flow regulator, and collecting device to capture the pollutants.

Passive Sampling:

▪ Uses simple, low-cost methods like sticky paper, dustfall jars, and lead dioxide cylinders to capture pollutants.

▪ Common in the early years but later replaced by more advanced methods.

▪ Recently, improved passive samplers (e.g., badgetype devices) are being used for personal exposure monitoring, indoor air studies, and research.

▪ They are affordable and useful for developing countries.

Grab Sampling:

▪ Collects air samples quickly (in seconds or minutes), usually for emergency situations like chemical spills.

▪ Helps identify pollutants and their concentrations at a specific moment.

Intermittent Sampling (Manual Sampling)

• Used widely in the U.S. from the 1950s-70s.Less expensive but requires longer collection times (e.g., 24 hours).

• Still commonly used for pollutants like PM10, PM2.5, lead (Pb), and atmospheric deposition.

• Useful in areas with limited resources and often supplements continuous monitoring.

➢Continuous Monitoring:

▪ Provides instant or near-instant results and is the best method for air quality programs.

▪ Most accurate but also very expensive to buy and maintain.

▪ Produces a huge amount of data that needs to be processed into a manageable form.

➢Importance of VOC Sampling

▪ Volatile Organic Compounds (VOCs) are a key pollutant monitored in Photochemical Assessment Monitoring Stations (PAMS), which help track air pollution trends.

1. How PM (Particulate matter) is Collected:

➢Particles in the air can be collected using different techniques, including:

Gravitational settling

▪ Filtration

▪ Inertial impaction

▪ Electrostatic precipitation

▪ Thermostatic deposition

▪ Gravitational settling –

letting particles fall naturally due to gravity.

▪ Filtration

using filters to trap particles.

Inertial impaction

forcing air to change direction so heavy particles crash into a surface.

▪ Electrostatic precipitation

using electrical charges to attract and collect particles.

Thermostatic deposition

using temperature differences to capture particles.

➢Filtration as a Common Method

▪ Filtration is the most widely used method to collect PM for air quality monitoring.

▪ A large volume of air is passed through a filter for 24 hours, and the weight of the particles is measured to determine PM concentration.

➢Hi-Vol Samplers (Older Method)

▪ In the U.S., the High-Volume Sampler (Hi-Vol) was used until 1988. It sucked in large amounts of air using a vacuum motor, trapping particles on a glass fiber filter.

▪ It collected particles of various sizes (0.3 to 100 µm) but wasn’t ideal for health-based standards.

▪ Now, Hi-Vol samplers are mainly used for lead (Pb) monitoring.

Newer Size-Selective Devices:

▪ Newer samplers focus on collecting only the particles that can enter the human lungs.PM10 samplers collect particles smaller than 10 µm, using a greased plate to remove larger particles.

Cascade Impactors:

These separate particles into different size categories using orifice plates.

Dichotomous Impactors split air into two groups:

PM10 (2.5–10 µm) and PM2.5 (less than 2.5 µm)

Automated Continuous Monitoring:

▪ Some modern devices collect particles on filter tape and measure concentrations automatically.

Examples of Automated Continuous Monitoring

▪ TEOMs (Tapered Element Oscillating Monitors)

▪ Measures PM by tracking frequency changes in a vibrating element as particles accumulate.

▪ Very accurate and precise for real-time air quality monitoring.

In the U.S., gas concentrations in air are usually measured as

mixing ratios, such as parts per million (ppm), parts per billion (ppb), or parts per trillion (ppt).

➢Mixing ratios show

how much gas is in a million (or more) parts of air, while other measurements, like milligrams per liter, are used for water and solids.

Many countries use

mass per unit volume (like µg/m³ or mg/m³) for air pollution, but mixing ratios are becoming more popular for easier comparison.

Temperature and pressure affect

gas volume, so air samples are adjusted to standard conditions (25°C and 760 mmHg) for accuracy.

➢In air quality monitoring, particle pollution is always measured

as weight per cubic meter (µg/m³).

Accuracy

is how close a measured value is to the actual value, but 100% accuracy is hard to achieve due to errors in instruments, methods, or human mistakes.

Precision

is how consistent repeated measurements are; high precision means slight variation between measurements.

Absolute error

is the difference between the measured and actual value, while relative error is the percentage difference.

Standard deviation helps measure

precision

most measurements should be within ±10%

for good precision.

Bias

happens when measurements are consistently off due to calibration or instrument errors, even if they are precise.

In air quality monitoring, accuracy includes

random errors (precision) and systematic errors (bias), and values within ±10% are usually acceptable.

Calibration is needed to

reduce errors (bias) and ensure reliable data by comparing measurements to standard reference values.

Primary standards

are directly traceable to NIST and used for high-accuracy calibration.

Secondary standards

are commonly used for field calibration due to portability and ease of use.

Airflow rate measurements must

use instruments traceable to authoritative standards (e.g., NIST) as per USEPA regulations.

Gas-phase pollutant monitors

must be calibrated using gas standards from NIST-certified sources.

Ozone (O₃) calibration requires generating

test atmospheres with an O₃ generator since O₃ gas standards are unavailable.

USEPA regulations require states to develop air quality monitoring programs to assess compliance with

NAAQSs (National Ambient Air Quality Standards).

NAAQSs

(National Ambient Air Quality Standards).

(SLAMS)

State and local air monitoring stations

(SLAMS) must monitor

pollutant concentrations in areas with high pollution,

dense populations,

primary emission sources, and

regional background levels.

New objectives for air quality monitoring include

assessing pollutant transport among populated areas and welfare impacts in rural/remote areas.

Spatial scales for monitoring locations include:

▪ Microscale (few meters to 100 m)

▪ Middle scale (100 m to 0.5 km)

▪ Neighborhood scale (0.5 to 4 km)

▪ Urban scale (4 to 50 km)

▪ Regional scale (tens to hundreds of kilometers).

Microscale, middle, neighborhood, and urban

are for high pollutant concentrations;

middle, neighborhood, and urban are

for high population densities and representative concentrations;

neighborhood or regional are

for background levels and pollutant transport.

Photochemical Assessment Monitoring Stations

PAMS

Photochemical Assessment Monitoring Stations (PAMS) must

be established in states with O₃ levels exceeding air quality standards.

Monitoring techniques must follow

federal reference methods (FRMs) or

approved equivalents,

meet quality assurance standards, and

report results annually to USEPA.

Federal grants support

PAMS and SLAMS and operate alongside federal monitoring activities like NAMS

NAMS

(National Air Monitoring Stations).

NAMS is an

NAMS is an

urban air quality network that provides long-term data for air quality comparisons and trends. It is often operated under USEPA contracts.

PAMS focuses on monitoring

ozone (O₃) and related pollutants in areas with high ground-level O₃ concentrations, like the East Coast, California, and several other cities.

IMPROVE network monitors

PM2.5 and visibility to assess their relationship, supported by the USEPA and other federal agencies.

CASTNet provides atmospheric data for

rural O₃ levels and dry deposition of pollutants, including sulfate, nitrate, ammonium, and O₃.

Atmospheric deposition networks like NADP/NTN and IADN focus on

monitoring pollutants in wet and dry deposition, especially for acid rain and mercury.

Air Monitoring Networks:

In the U.S., different networks track air pollution to ensure clean air. These networks are supported by federal funds and managed by the Environmental Protection Agency (EPA) and state, local, and tribal agencies.

SLAMS Network

More flexible and customized for local air monitoring needs.

NAMS Network

Focuses on long-term air quality tracking in urban areas to compare trends. Often operated under SLAMS programs.

PAMS Network

Specially designed to monitor ozone (O₃) and other air pollutants in areas with high ozone levels, mainly during summer.

Where the Data Goes from Networks

Information from these networks is reported to the EPA’s Aerometric Information Retrieval System (AIRS), which compiles and publishes air quality data.

IMPROVE Network

Tracks fine particles (PM2.5) and visibility, mainly in national parks and protected areas.

CASTNet Network

Measures ozone (O₃) and pollutants in rural areas

NADP Network

Monitors acid rain and mercury pollution.

IADN Network

A U.S.-Canada effort to track pollutants in the Great Lakes region.

Shifting Focus in Air Monitoring:

Since pollution levels of some pollutants (like lead, sulfur dioxide, and carbon monoxide) have significantly decreased, monitoring efforts are now redirected to emerging concerns like fine particles (PM2.5) and toxic air pollutants.

All air quality monitoring activities conducted at NAMS, SLAMS, and PAMS sites for criteria pollutants

must use methodologies approved by the USEPA as reference methods or their equivalent.

Primary Method (FRM) for SO₂ Monitoring:

FRM (Federal Reference Method) for sulfur dioxide (SO₂) is the spectrophotometric pararosaniline method.

➢ It involves manual collection, in which SO₂ is absorbed in a potassium tetrachloromercurate solution, forming an HgCl₂SO₃²⁻ complex.

➢It reacts with formaldehyde (HCHO) and pararosaniline hydrochloride to create a red-violet color, which is measured using a spectrophotometer. SO₂ concentration is determined using a standardized concentration curve.

FRM

Federal Reference Method

Limitations of FRM:

The manual method is cumbersome and timeconsuming. Even automated FRM monitors require significant maintenance. Many air monitoring stations use EPA-approved equivalent methods instead.

Automated & Equivalent SO₂ Monitoring Methods

Multigas Longpath Monitoring System (Approved in 2000) Uses Fourier Transform Infrared (FT-IR) Spectroscopy for remote sensing.

Measures SO₂ concentration over an open path up to 1 km. Uses an infrared (IR) beam to detect SO₂ absorption at specific wavelengths.

Other Equivalent Methods:

Conductimetry

Amperometry/Electrochemistry

UV Fluorescence Detection

Flame Photometry

Gas Chromatography

Conductimetry:

Collects SO₂ in hydrogen peroxide (H₂O₂) and measures increased conductivity due to sulfuric acid (H₂SO₄) formation.

Amperometry/Electrochemistry:

Measures electric current generated by SO₂ reaction in a collection solution.