Introduction to Air Quality Monitoring Systems Met 163 7 May 2009 • Topics to be covered: • Basic urban air quality • Measurements of ozone • Measuring NOx • Measuring Black Carbon • Issues with monitoring systems
What is in Urban Air? O3, NO, NO2, CO, CO2, SO2, HCHO And other things like particulates and Black Carbon!
Primary Pollutants Carbon Monoxide (CO): is a tastless, colorless and odorless gas. Sources of CO include: fossil-fuel combustion (Incomplete combustion in autos, trucks and airplanes, some industrial process), biomass burning, and photolysis reactions. Concentrations of CO in urban air are typically 2-10 ppmv. On freeways and in traffic tunnels values rise to more than 100 ppmv.
Primary Pollutants Sulfur Dioxide (SO2): is a colorless gas that exhibits a taste at levels of 0.3 ppmv and a strong odor at levels greater than 0.5 ppmv. Sources of SO2 include: coal-fired power plants, automobile tailpipes, and volcanoes. Background concentrations in the troposphere range from 10 pptv to 1 ppbv. In polluted air, 1-30 ppbv.
Primary Pollutants Nitric Oxide (NO): is a colorless gas and a free radical. Precursor to tropospheric ozone. Sources include: fossil-fuel combustion/autos and planes, and biomass burning. Nitrogen Dioxide (NO2): is a brown gas with a strong odor. Absorbs short wavelengths. Intermediary between NO and O3 production.
Primary Pollutants Background concentrations range from 20 to 50 pptv. In urban areas, concentrations are 0.1-0.25 ppmv. In midmorning NO2 is more prevalent than during midday because sunlight breaks down most NO2 past midmorning. NOx = NO + NO2
Secondary Pollutants Secondary pollutants form chemically in the air. Ozone is a secondary pollutant in the troposphere. Ozone (O3): is a relatively colorless gas. Ozone exhibits an odor at concentrations of 0.02 ppmv or greater. Ozone has a sweet smell (Copy machine). Ozone is not emitted. Background concentrations in troposphere are 20 to 40 ppbv. In urban air, concentrations range from 0.01 ppmv at night to 0.5 ppm during afternoons in the most polluted cities. Typical mixing ratios are 80-100 ppb in urban air. In 2008, EPA lowered the 8 hr primary standard from 84 ppbv to 75 ppbv.
NO + RO2 NO2 + RO NO + O3 NO2 + O2 NO2 + hν NO + O O + O2 O3 Chemistry of Photochemical Smog Photochemical smog involves reactions among NOx and reactive organic gases (ROGs) in the presence of sunlight. We call ROGs volatile organic compounds (VOCs) which are emitted from cars and processing industry. On a typical day, ozone forms following emission of NO and ROGs. ROGs are broken down chemically into peroxy radicals, RO2.
EPA Standards National Ambient Air Quality Standards (NAAQS) • 75 ppb O3 for an 8-hour averaging period • 120 ppb O3 for a 1-hour averaging period
Gas Absorption: absorption occurs when radiative energy enters a substance and is converted to internal energy, increasing the temperature of the substance. Absorption removes energy from an incident beam, reducing the amount of radiation received past the point of absorption. Gases selectively absorb radiation in different portions of the electromagnetic spectrum. Ozone absorbs UV radiation at wavelengths < 0.35 μm and between 0.45-0.75 μm.
Description • UltraViolet Photometer • Model: 9810b • Released in 1996 by Monitor Labs, Inc. • Now owned by Teledyne Monitor Labs http://www.teledyne-ml.com/
Technology • Nondispersive UV photometer alternately switches a ozone scrubber in and out of measuring stream. • Scrubber uses manganese dioxide (MnO2)to destroy only the ozone and pass other common absorbers (SO2, etc). • Mercury vapor lamp as the light source with 254 nm at the center of absorption band. • Transmitted light density computes ratio of ozone scrubbed air (ozone free air) to non-scrubbed air (ozone air). • Has a microprocessor to monitor and adjust temperature and pressure.
Outlet Internal zero span module Span Port Ozone Cell with UV lamp Zero air scrubber Zero Port DFU Sample PneumaticsFlow Rate = 0.5 slpm Flow control External Pump Measurement cell absorption UV lamp source Measurement Valve Ozone Scrubber Inlet
Technology • Measurement Cell: • Mercury vapor lamp as source • vacuum photodiode as detector • A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation. • Glass tube used as absorption cell • Reference cycle: ozone is scrubbed out of sample and light intensity is determined (Io) • Sample cycle: light intensity is determined for ambient air with ozone (I).
Technology • Beer/Lambert Law gives ozone concentration: O3= O3 concentration, ppm a = absorption coefficient of O3 at 245 nm=308 atm-1 cm-1 at 0˚C and 760 torr (760 torr=101kPa) l = optical path length, cm T = sample temperature, ˚K P = sample pressure, torr L = correction factor for O3 losses
Theory of Operation • Gas-phase Chemiluminescence detection to analyze nitric oxide (NO), total oxides of nitrogen (NOX), and nitrogen dioxide (NO2) • Instrument has a pneumatic system, an NO2-to-NO converter, a reaction cell, detector (PMT), and processing electronics
Theory of Operation • The luminescence from an activated molecular NO2 species produced by the reaction between NO and O3 : NO + O3 → NO2 + O2 • As the activated species NO2 reverts to a lower energy state, it emits broad-band radiation from 500-3000 nm. • NO concentration is proportional to the intensity of the chemiluminescent →proportional to the current output.
Parts diagram FRONT MOLYCON Catalytic converter DELAY COIL PERMAPURE DRYER OZONE GENERATOR OPTICAL BENCH REACTION CELL FAN FAN VALVE MANIFOLD NOx PRE-PROCESSOR BOARD POWER SUPPLY VOLTAGEREGULATOR BOARD MICROPROCESSOR BOARD FRONT REAR
Air flow diagram AIR INTAKE OZONE GENERATOR DRYER ASSY VALVE MANIFOLD REACTION CELL ASSY NO2 NO2 hn MOLYCON ASSY OPTICAL BENCH ASSY NO2 → NO CONVERTER SAMPLE PORT PARTICULATE FILTER DELAY COIL EXTERNAL PUMP EXHAUST PORT EXHAUST
Calibration process The analyzer must be calibrated initially and periodically to determine the reliability and accuracy of all air quality data. Single/multi point calibration: calibrated against know sources of NO and NO2 (NIST standards etc.) Establish a reliable and stable calibrating source. Calibrate the analyzer against the calibrating source. Zero/span calibration
Black Carbon Black Carbon is a primary aerosol component of Diesel Particulate Matter, a known toxin and regulated pollutant by several regulatory agencies, including the California Air Resources Board (CARB). Diesel Particulate Matter is known to cause adverse health effects in people who are exposed, including premature hospitalization, asthma attacks, bronchitis, other respiratory and cardiovascular symptoms, and premature death. Black Carbon is the second leading cause of Global Warming. Black Carbon is emitted as a primary pollutant to the atmosphere through a variety of incomplete combustion of sources and fuels; BC concentration cannot be modeled or predicted, it must be measured. Black Carbon is NOT adequately characterized through PM-2.5 mass only measurements, chemical speciation is necessary. (Magee Scientific)
Black Carbon Aerosol black carbon (“BC”) is a particulate pollutant species emitted from the combustion of any carbonaceous fuel. The name “Aethalometer” is derived from the classical Greek verb ‘aethaloun’, (αεφαλουν) meaning ‘to blacken with soot’. It is this optically-absorbing material that the ‘standard’ Aethalometer measures. The only known source of aerosol black carbon in the atmosphere is the combustion of carbonaceous fuels. There are no known significant biological, geological or meteorological sources.
The Aethalometer: The Optical Attenuation Method The optical method that we use is a measurement of the attenuation of a beam of light transmitted through the sample when collected on a fibrous filter. When calculated as shown, this quantity is linearly proportional to the amount of BC in the filter deposit. Define I0 as the intensity of light transmitted through the original filter, or through a blank portion of the filter. Define I as the intensity of light transmitted through the portion of the filter on which the aerosol deposit is collected. The ‘Optical Attenuation’ ATN is defined as ATN = 100 * ln (I0 / I )
The Aethalometer: The Optical Attenuation Method This measurement is affected by the wavelength of the light with which it is made, provided that the particle size is somewhat smaller than the wavelength. The absorption of light by a broad band absorber such as graphitic carbon is inversely proportional to the wavelength of the light used. Thus, for a given mass of black carbon [BC], the optical attenuation at a fixed wavelength λ may be written as ATN(λ) = σ (1/λ) * [BC] where [BC] is the mass of black carbon, and σ (1/λ) is the optical absorption cross-section (‘sigma’) that is wavelength dependent, and which is referred to as the ‘Specific Attenuation’. The Aethalometer model AE-16 uses a solid-state source operating in the near-infrared at a wavelength of 880 nm.
Definitions Range- the measurand interval over which a sensor is designed to respond. Span- the algebraic difference between the upper and lower range values. Resolution- the smallest change in the primary input that produces a detectable change in the output. Hysteresis- is present when the sensor output for a given input depends upon whether the input was increasing or decreasing. Imprecision or uncertainty- is a measure of the noise or scatter in the measurement. Bias- is the systematic error. The purpose of static calibration is to remove the bias and to numerically define the imprecision.