15.2. Measurement techniques

15.2.1. Sampling methods

In order to analyze air quality, we need to take samples of the ambient air without modifying its chemical composition. In order to avoid microscale effects, free air movement around the measurement site has to be provided. On the other hand, the sample inlet must be protected against rainfall and dust that can largely influence measurement data. A sampling system thus has to fulfill the following criterias (Air Quality Sampling Manual, 1997):

  • built from materials that do not react with the pollutants to be observed,

  • located in a place where free air movement is provided,

  • protect the sample from rainwater, dust and insects,

  • reliable with low maintenance requirements,

  • resistant against elements and wildlife.

Sampling time interval depends largely on the aim of the measurement. Long-term average concentration studies require samples collected during a certain time period, mostly on a daily or longer basis. This batch sampling method provides accurate results of average concentration for a period equal or longer than the sampling interval, however, cannot estimate concentration fluctuations on a finer temporal resolution. To obtain hourly or even finer concentration data, a continuous monitoring is needed where sampling is either continuous in time or performed with a very fine (1–10 s) time interval. Continuous monitoring is usually carried out in air quality investigations for public protection purposes, especially in urban areas.

Depending on the measured quantity, either passive or active sampling can be used. Passive sampling does not involve any pumping, thus the pollutants only reach the container through their own movement and diffusion. Passive sampling is an efficient tool to estimate deposition; however, concentration measurement requires a well-defined volume flux of air pumped through the measurement device, often referred to as active sampling.

15.2.2. Measurement of gas concentrations

15.2.2.1. Spectrophotometry

Spectrophotometry is based on the absorption of radiation by gases at different wavelengths. When a light with a known intensity spectrum passes the sample, the exiting radiation’s intensity spectrum can be used to determine the composition of the gas. The absorption wavelengths where intensity significantly drops are a footprint of each material, while the exact values of wavelength-dependent extinction gives information of the concentration, following the Beer–Bougert–Lambert-law:

(15.1)

where I is intensity of the radiation, c is the concentration, ε is the extinction coefficient and L is the optical path-length. Spectrophotometry consists of two main parts: infrared spectrophotometry at wavelengths larger than 800 nm, which relies on molecular rotation and vibration excitement; and ultraviolet-visible spectrophotometry in the wavelength range of 200–800 nm that is based on electron excitement and atomic absorption.

Infrared spectrophotometry is primarily used for emission measurements of most pollutant gases, like CO, CO2, NO, SO2 and hydrocarbons. It is also widely used for CO and CO2 measurement for ambient and indoor air quality investigations (Ionel and Popescu, 2010). Ultraviolet spectrophotometry is applied for the investigation of gases that have a significant absorption in the UV range, mostly NO and O3.

In a complex system containing several components like the polluted air, the absorption lines often overlap and cannot be clearly separated. One way to deal with this issue is to extend the wavelength range of the radiation and detect other absorption lines from which the components can be separately identified. Another approach is to use a chemical separation, either with a chemical filter, or with splitting the sample and entering a reactant that neutralizes one of the colliding components.

Main parts of a single beam spectrophotometer

Figure 15.5: Main parts of a single beam spectrophotometer

15.2.2.2. UV fluorescence

UV fluorescence is similar to UV spectrophotometry in a way that ultraviolet radiation is emitted into the sample; however, instead of investigating the whole absorption spectrum, only intensity at the wavelength of the component’s fluorescence is measured. In order to avoid interference with the exciting radiation, measurement is carried out in a large angle from the radiation direction. The measured light intensity in this case is directly and only emitted by the investigated gas, thus it provides an exact estimation of the concentration. UV fluorescence is mostly used for SO2-measurements, which absorbs 215 nm UV radiation and then emits fluorescent radiation between 240–420 nm with a peak of 320 nm (Ionel and Popescu, 2010). As fluorescence emits light with small intensity, the measured signal is usually amplified using photomultiplier tubes.

15.2.2.3. Chemiluminescence

Chemiluminescence is the most common way to measure the photochemical NO, NO2 and NOx concentrations. The method is based on the fact that during the reaction of NO with ozone, the resulting NO2 molecule is excited and releases radiation at 1100 nm wavelength:

NO + O3 → NO2*+ O2

(R15.1)

NO2* → NO2*+ hν (λ = 1100 nm)

(R15.2)

The released radiation is amplified by photomultiplier tubes and gives direct information about the NO concentration. The measurement is carried out in two steps: at the beginning, the sample is split and the first part passes through a converter which reduces all NO2 to NO. Then it gets into the measurement chamber where ozone is added at a constant rate. Measuring the emitted radiation intensity, the concentration of the total NOx components can be obtained. To receive information about each component, the same measurement is carried out without the converter, thus only the original NO concentration is measured. NO2 concentration is derived as a difference of measured NOx and NO levels. A similar method is available for ozone measurement using either NO or ethane (C2H4) reactant, however, due to cost and safety reasons, UV-spectrophotometry is more widely used for ozone concentration measurement (Ionel and Popescu, 2010).

15.2.2.4. Gas chromatography

Gas chromatography is a measurement technique for most greenhouse gases like CO2, CH4, SF6 and N2O as well as for CO. A gas chromatograph has two inlets: an inert carrier flow (usually nitrogen) and a sample injector (Figure 15.6).

Schematic diagram of a gas chromatograph

Figure 15.6: Schematic diagram of a gas chromatograph

The sample is driven through the column by the carrier flow. The walls of the column are covered with the stationary phase, a material which interacts with the gas components to be analyzed. Interaction between the sample and the stationary phase inside the column leads to different arrival time of each component at the detector, which enables their separated measurement.

There are different detectors available for gas chromatographs. A universal detector is based on the difference of thermal conductivity of the materials (TCD, Thermal Conductivity Detector). A more specific way to measure organic and hydrocarbon compounds is the Flame Ionization Detector (FID), which is a hydrogen flame located in an electric field. When carbons enter the flame, ionization occurs and a current starts between the electrodes that provide a direct electrical signal representing the concentration. Gas chromatographs with FID are the most common tools to measure CO2 and other greenhouse gas concentration (see e.g. Haszpra, et el., 2008) at worldwide stations (Figure 15.7).

Gas chromatograph

Figure 15.7: Gas chromatograph for greenhouse gas measurement at Hegyhátsál station, Hungary

15.2.3. Measurement of aerosol concentrations

Describing the pollution caused by aerosol particles require three parameters to measure:

  • total mass concentration of aerosols,

  • size distribution,

  • chemical composition.

While the former one is measured at almost all air quality observation sites, on-site analysis of aerosol chemical composition is very rare as it requires advanced analytical chemical methods and laboratories. The exact size distribution is usually not known, but the total mass of particles smaller than a given size is measured, referred to as PM10 and PM2.5 (total mass of particulate matter smaller than 10 and 2.5 microns). Size selection is performed at the inlet using a cascade impactor (Figure 15.8). As air is accelerated through multiple narrowing nozzles, small particles remain in the flow, while large particles gain enough momentum to hit the wall where they get trapped in a collection shim.

Cascade impactor

Figure 15.8: Schematic picture of a cascade impactor

Total mass concentration of particles is measured using a Teflon-coated filter and a sensitive scale that measures the mass of the filter. For monitoring purposes, high frequency (1–10 s) mass measurement is required, while batch sampling sites use high-volume samplers (HVS) that contain and protect collected aerosols over a long period to estimate average concentrations and perform advanced chemical composition analysis.

15.2.4. Remote sensing

In the recent decades, the extremely fast development of remote sensing methods led to a wide range of both ground- and space-based tools to observe weather and air quality. In atmospheric chemistry, remote sensing measurements are used to satisfy the following needs:

  • high spatial resolution air quality data to provide initial fields for transport-exchange models,

  • high spatial and temporal resolution air quality data to estimate fluxes,

  • air quality information of hardly accessible areas,

  • vertical profile measurements,

  • measurement of airborne pollutants,

  • estimation of ozone concentration in the stratosphere.

15.2.4.1. LIDAR

Light Detection and Ranging (LIDAR) is a ground-based remote sensing tool to obtain high temporal and spatial resolution concentration fields. It is based on the same idea as spectrophotometry: if a radiation is emitted, the materials absorb this radiation at a specific wavelength and scatter it back as an isotropic radiation (Figure 15.9).

LIDAR

Figure 15.9: Schematic diagram of a LIDAR system

LIDAR emits a monochromatic laser beam at the absorption wavelength of the investigated component, and measures backscatter from the atmosphere. Its high measurement frequency enables measurements in several directions within a short period (like weather radars), which can be analyzed as a continuous concentration field. In order to avoid measurement errors due to global radiation entering the detector, a differential absorption LIDAR (DIAL) is often used that emits two laser beams, one tuned to the absorption wavelength, and the other one tuned just below or over it, being the reference beam. The beams are emitted alternately, and the difference between the two backscatter values can be regarded as the backscatter of the investigated gas.

15.2.4.2. Differential optical absorption spectroscopy (DOAS)

DOAS consists of a continuous light source with known emission spectrum, usually a Xe-arc lamp, and a detector in a certain range from the source. DOAS is also based on spectrophotometry with the difference that the sample in which the emitted light suffers extinction is the atmosphere itself. The distance between the lamp and the detector ranges from several hundred meters to many kilometers. DOAS is often used to obtain average concentrations within an urban street canyon or over an industrial area.

15.2.4.3. Satellite observations

Total mass of pollutants in a vertical column can be obtained using spectrophotometers on satellites that calculate concentrations based on wavelength-dependent radiation extinction throughout the whole atmosphere from the infrared to the ultraviolet range. As forecast-oriented meteorological satellites measure radiation intensities at a wide range of wavelengths, air quality data can be obtained using their multi-channel sensors, like the ones on the METEOSAT, GOES, MetOp or MODIS satellites. Most satellite data post-processing tools provide methods to create aerosol or carbon monoxide (CO) concentration maps using the satellite intensity data at the absorption wavelengths of the selected material. In addition, air quality oriented sensors have also been developed to perform more detailed observation, especially in the UV range, which is poorly measured for forecast purposes, but gets a high importance in atmospheric photochemistry.

A polar orbiter satellite designated primarily for atmospheric chemistry purposes was ESA’s Envisat launched in 2002. Its data was used in a wide range of projects including climate change, atmospheric dispersion, hydrology, cartography and agricultural research. Envisat finished its mission in 2012. Based on its success, the POES (Polar Operational Environmental Satellites) project was created, including both NOAA and MetOp satellites launched between 2002–2012. They host numerous sensors for weather and air quality monitoring, vegetation analysis and several other environmental research purposes.

Satellite spectrophotometers are able to measure vertical total concentration of different pollutants. Most pollutants only occur in the low levels, thus the vertical total mass is usually representative for the near-ground concentrations. However, vertical concentration profile becomes extremely important in the case of ozone measurement because of the significant difference between stratospheric and tropospheric ozone.

The total vertical ozone concentration, measured by space-based UV-spectrophotometers is described with a single value, the Dobson Unit (DU). 1 DU refers to a vertical total mass of gas that would occupy a 10 micron thick layer on the surface under standard temperature and pressure. As the stratosphere contains significantly more ozone than the troposphere, the vertical total ozone concentration expressed in DU is representative for the stratospheric ozone (Figure 15.10).

Average total vertical ozone concentration of June 2000

Figure 15.10: Average total vertical ozone concentration of June 2000, measured by space-based UV-spectrophotometer (source: NASA)

However, there is a way to obtain tropospheric ozone concentrations from satellite measurement based on the Doppler broadening effect: if the radiation is absorbed by particles that move at relatively high speed due to their thermal motion, the Doppler effect shifts the wavelength of the absorbed radiation. In the stratosphere, where density is low and thermal motion is significant, Doppler broadening results in wider absorption bands. Thus, with measuring the shape of the absorption line, the ratio of low-level and high-level ozone can be estimated.

While geostationary satellites perform continuous monitoring of the observed area’s air quality, low-level polar orbiters are able to perform much more sophisticated measurements at a designated area. Equipped with space-based LIDAR-s, they are able to measure detailed 3D concentration profiles, and their fast movement enables them to collect data from thousands of kilometers within a few minutes of time. It leads to high resolution vertical cross-sections along the satellite path that provide wealthy information about air quality patterns (Figure 15.11.). During measurement campaigns, even more detailed 3D concentration data is obtained using airplanes equipped with LIDAR-s, which is used in critical air pollution situations, or to validate satellite results.

15.2.5. Rainwater analysis

Measuring the atmospheric concentrations and surface fluxes of different materials usually gives a good estimation about the health and environmental impact of the pollution. However, wet deposition has to be also taken into account for soluble pollutants, especially for acidic components, that produce their effect mainly through wet deposition.

Rainwater analysis consists of two main parts: pH measurement to estimate acidic wet deposition often referred to as acidic rain; and composition measurement including water hardness to estimate wet deposition of soluble gases and minerals. Rainwater pH is measured using electronic pH-meters that rely on the electric conductivity of the water and provide a direct electronic signal that enables automatic monitoring.

Rainwater composition is measured using spectrophotometric and analytical chemical tools to analyze the solution. The most important ions that have a significant wet deposition are Cl-, SO42-, NO3-, NH4+, Na+, K+, Mg2+ and Ca2+. Measurement of the heavy metal, especially lead content of rainwater is also important to estimate their deposition and environmental accumulation.

Figure 15.11: Vertical cross-section of aerosol concentrations on April 17, 2010; based on LIDAR measurements performed by the CALIPSO satellite. Besides high-level clouds in northern Europe and the Mediterranean, the air pollution over France and the volcanic plume of the Eyjafjallajökull eruption is also visible. (source: NASA)

Vertical cross-section of aerosol concentrations on April 17, 2010