Norbey Barahona is an electrical engineer. He has been working for five years in this centre of measures of the air quality in Bogota, doing maintenance of the measuring device, so that they are always functioning.

The network:

The network of measuring device for the air quality in Bogotá started in 1997. There are currently 14 stations in the city, although not all are fully equipped. 13 are fixed and one is mobile. At first, all the devices were chosen from the same company. But now, they are from different brands, and they are chosen through invitations to tender: the needs are announced publicly on internet, the companies present their bid to satisfy them, and the best proposition is chosen. In 2008, all the devices were updated. Now, the devices are updated little by little, when one of them breaks down, in order to use them at the maximum (according to the instructions given by the Contraloria of Bogotá, the agency in charge of the money). The problem of this is that between the failure, the publication of the invitation to tender, the selection of the device and its final installation, a lot of time passes without the measures. Once one device is changed, the old one is to be given to a university.

The devices record the PM (particles), polluting gases, and atmospheric and climatic parameters. More precisely, in the PM, they monitor the PM2.5, PM10, and TSP (Total Suspended Particles, only in one station). The polluting gases are carbon dioxide (CO2), sulphur dioxide (SO2), ozone (O3), nitrogen monoxide (NO), dioxide (NO2) and the sum of both (NOx), carbon monoxide (CO), hydrocarbons. The meteorological parameters are the global radiation, diffused radiation, UV radiation type B, temperature, relative humidity, direction and velocity of the wind, precipitations, pressure.

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The measures are sent regularly (monthly and trimestral reports) to the Secretary of Environment of the city of Bogotá, following the instructions of the Ministry of Environment of Colombia. The device itself takes the measures, makes an analysis, and sends a result (with a consistency flag). Every 10 minutes, the results are received in the centre of monitoring of the air quality of the city, where engineers analyse them and may send alerts if critical thresholds are reached.

The stations intend to respect the American EPA norms, for example in terms of altitude. The sensors must be very close to the ground. They must not be too close to the streets, because they aim at measuring the dispersion of the pollutants. Generally it is difficult to have the perfect emplacement because the sites for the stations are lent, and their location cannot be chosen. The most critical station is the one situated near the South Highway: it is the most important highway of Bogotá, used for the heavy loads transportation, and also, many industries (soap, shoes, !) are situated in this area. This survey of the air quality is a strong incentive to move the industries outside the city.

The station we visit was built in 2000-2002. It monitors all the previously mentioned parameters except the TSP and hydrocarbons. Norbey explains us in detail the functioning of all the sensors.

PM10:

The PM10 are all the particles smaller than 10 microns. The air to be monitored passed through a few rough sieves, to prevent insects and huge particles from entering the device. Then, in order to eliminate all the particles bigger than the desired size, a kind of gravity sieve is designed: the air flow is accelerated through a funnel (provoking a Venturi), till it reaches a controlled velocity. Then the air is then projected on a wall, and only the desired particles are small enough to rebound high enough and enter in the next pipe, as explained on the diagram below.

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PM10_1

At the exit of this sieve, only stay the particles smaller than 10 microns, i.e. the PM10. This air flow with PM10 is pumped onto a filter (like a blotting paper), which keeps all the particles. The filter is changed every hour (50 minutes with pumping the air flow, 5 minutes at the beginning for calibrating, 5 minutes at the end for measuring).

The dirtiness is measured through the beta attenuation: a carbon 14 source emits beta radiation, which goes through the dirty filter, where the remaining beta radiation ionises an argon gas. The dirtiness of the filter is directly proportional to the blocked beta radiation.

PM2.5:

It works exactly like the PM10. But here, a second level of gravity sieve is added. It takes the airflow of PM10, and selects only the particles smaller than 2.5 microns.

Calibration of the polluting gas monitoring devices:

The CO2 device work for a proportion of 0 to 1000 ppm (part per million), the CO device for 0 to 50 ppm, while for the other gases, it is in the range 0 to 500 ppb (part per billion). The calibration of the device is made for 80% of the maximal range, according to the EPA, i.e. for the CO, it’ll be for a proportion of 40 ppm. The calibrator produces an air with exactly the desired proportion of the polluting gas. First, the air is cleaned from all the polluting gases with various filters (molecular sieve for H2O, Purafil for H2S, SO2, NO, O3, Purakol for SO2, O3, NO2, Active charcoal for SO2, O3, H2S, heated at 517ºC for CO). This zero-air is used to dilute the polluting gas to calibrate to the desired proportion, and the final mixture is sent to the device.

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NOx:

The principle to detect it is chemiluminescence: the NO reacts with the ozone in an exothermic reaction, which produces light, proportionally to the quantity of NO.

A first measure is made to measure the proportion of NO in the air to be monitored. Then, through a chemical reaction, all the NO2 of the air is transformed into NO. This new value of NO is measured in a second time (now it is NOx, the sum of NO and NO2). And by difference, the quantity of NO2 in the air can be calculated.

CO and CO2:

It is detected through a correlation wheel:

CO_monitorThe correlation wheel rotates and it has two windows. One is of only CO, and the other one of N2. The CO absorbs the light of a specific wavelength (46-47 micrometres). In the N2, all the light of this wavelength passes. After the wheel, the light goes through the chamber full of air to be monitored. It travels around 10 metres in this air (on a folded path). And finally the beam goes to a detector. When the beam has passed through the CO window of the wheel, it is not affected by the CO of the chamber, and it is used to calibrate the detector. When the beam has passed through the N2, the absorbed light of the specific wavelength is proportional to the CO proportion in the chamber.

SO2:

The principle used here is the UV fluorescence. The SO2 molecules of the air are excited by an ultraviolet light (at 214nm). When the molecule returns to a stable state, it emits light, which is detected. The quantity of light detected is directly proportional to the proportion of SO2.