WO2012069963A1

WO2012069963A1 - Sensor for assessing air pollution

Info

Publication number: WO2012069963A1
Application number: PCT/IB2011/055120
Authority: WO
Inventors: Johan Marra
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2010-11-26
Filing date: 2011-11-16
Publication date: 2012-05-31

Abstract

A sensor (1) for assessing air pollution is provided. The sensor comprises a volatile organic compound (VOC) sensing section (100) and an ultrafme particle (UFP) sensing section (200). The VOC sensing section includes an ionizing unit (120) for ionizing at least part of airborne VOCs in air passing through a first conduit (110). The VOC sensing section further includes a first electrode (130) arranged, in the first conduit, downstream of the ionizing unit. The first electrode is arranged in proximity to a second electrode (140) on which a bias voltage is applicable for enabling precipitation of at least part of the ionized airborne VOCs on the first electrode. The UFP sensing section includes a charging unit (220) for charging at least part of airborne particles in air passing through a second conduit (210). The UFP sensing section further includes a capturing element (230) arranged, in the second conduit, downstream of the charging unit. The capturing element is capable of capturing charged airborne particles. Further, the sensor comprises an evaluation unit (300) being connected to both the first electrode and the capturing element for obtaining a signal representative of the air pollution.

Description

Sensor for assessing air pollution

FIELD OF THE INVENTION

The present invention relates to a sensor for assessing air pollution. More specifically, the present invention relates to a sensor comprising a volatile organic compound (VOC) sensing section for sensing airborne VOCs and an ultrafme particle (UFP) sensing section for sensing airborne UFPs.

BACKGROUND OF THE INVENTION

Health- hazardous air pollutants comprise airborne particles, sized between 10 nm and 10 μιη, and a broad range of gases comprising mostly inorganic combustion-related gases (CO, NO_x, S0₂) and many other volatile organic compound (VOC) gases, together usually referred to in terms of the total volatile organic compound (TVOC) concentration. Depending on the individual person, the inhalation of such pollutants may trigger feelings of discomfort, induce or aggravate respiratory disorders, and can eventually severely harm human health by manifesting itself in the development of neurological, cardiovascular, and/or respiratory diseases including lung cancer. It is therefore desired to reduce human exposure to these pollutants as much as possible.

The sources of particulate and gaseous pollutants may be found outdoors and indoors. For example, outdoor sources of particulate and gaseous pollutants comprise automobile traffic, industrial emissions and heating installations for buildings. Most outdoor combustion sources are easily recognized through their emission of ultrafme particles (UFPs) with a size between about 10 nm and 300 nm. In addition, outdoor combustion sources give often rise to the emission of variable amounts of gases like CO, NO_x, and S0₂. However, outdoor VOC sources are relatively less common and are generally not associated with combustion or heating processes.

Furthermore, as most people reside at least 80 % of their time indoors, the existence of indoor air pollution is of particular importance because most exposure occurs indoors. Indoor spaces comprise homes, buildings, work places, and car cabins. Common indoor sources of air pollution also comprise processes and human activities that involve combustion or heating such as, e.g., cooking, smoking, candle burning and ironing. Again, these sources are readily recognized through their emission of UFPs. Examples of indoor sources of air pollution that do not involve combustion may be paints, building materials, decoration materials, hobby activities, deodorant spraying, cleaning, cosmetics, animals, and humans themselves, wherein few of these sources betray themselves trough the emission of UFPs but almost all of them emit VOCs, which may or may not be odorous.

Recently, UFP sensors have been introduced, as disclosed in e.g. US- 2010/0043527. However, there is still a need for more reliable sensors for the quantitative indoor measurement of volatile organic compounds (VOCs). Furthermore, there is a need for new types of detectors capable of assessing the overall severity of the existing indoor air pollution.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome, or at least alleviate, some of the above problems and to provide improved sensors or sensing devices for assessing air pollution.

This and other objects are achieved by providing a sensor having the features defined in the independent claim. Preferred embodiments are defined in the dependent claims.

Hence, according to a first aspect of the present invention, there is provided a sensor for assessing air pollution. The sensor comprises a volatile organic compound (VOC) sensing section and an ultrafine particle (UFP) sensing section. The VOC sensing section includes an ionizing unit for ionizing at least part of airborne VOCs in air passing through a first conduit. The VOC sensing section further includes a first electrode arranged, in the first conduit, downstream of the ionizing unit. The first electrode is arranged in proximity to a second electrode on which a bias voltage is applicable for enabling precipitation of at least part of the ionized airborne VOCs on the first electrode. The UFP sensing section includes a charging unit for charging at least part of airborne particles in air passing through a second conduit. The UFP sensing section further includes a capturing element arranged, in the second conduit, downstream of the charging unit. The capturing element is capable of capturing charged airborne particles. Further, the sensor comprises an evaluation unit being connected to both the first electrode and the capturing element for obtaining a signal representative of the air pollution.

The present invention is based on an understanding that a VOC sensing section and a UFP sensing section may be integrated on a single sensing technology platform and share a common evaluation unit for obtaining a signal representative of the air pollution. Indeed, as such, the presence of gaseous VOCs in air is rather unrelated to the presence of UFPs or larger particles in air. Further, their presence is generally not related to the indoor C0₂ concentration. Thus, UFPs and VOCs correspond to different classes of air pollutants and information about the UFP concentration level is therefore complementary to information about the VOC concentration level. The present invention is therefore advantageous in that it provides an integrated sensor capable of sensing both the UFP concentration and the TVOC (the total volatile organic compound/hydrocarbon)

concentration, thereby providing a detector for assessing the overall severity of the existing air pollution in relation to both the UFP concentration and the TVOC concentration.

The signal obtained by the evaluation unit is preferably an electrical signal such as an electrical current representative of, for the UFP sensing section, the amount of particles captured per unit time at (i.e. in or on) the capturing element and, for the VOC sensing section, the amount of ionized airborne VOCs precipitated per unit time on the first electrode. The sensor of the present invention is therefore capable of providing a signal indicative of either one of the concentration level of UFPs in the air passing through the second conduit and the concentration level of VOCs in the air passing through the first conduit. In other words, by providing an evaluation unit common to both the VOC sensing section and the UFP sensing section, the sensor has the dual capability of measuring, on the one hand, the VOC concentration level and, on the other hand, the UFP concentration level. The sensor of the present invention is capable of quantifying the ambient air pollution with respect to both UFPs and VOC gases (TVOC). Such a sensor provides therefore a more detailed insight in the overall state of the ambient air pollution when compared to the situation wherein only a single UFP sensor or a single VOC sensor is present.

Advantageously, the sensor may alternatively (or sequentially) activate the VOC sensing section and the UFP sensing section in order to separately monitor the VOC concentration and the UFP concentration, respectively.

The present invention is also advantageous in that it provides for an integration of a TVOC gas sensor into an existing (electrical) UFP sensor platform with which the concentration levels of UFPs and the TVOC can be separately measured. Thus, in accordance with the present invention, existing UFP sensors may be upgraded with a VOC sensing section such that, in addition to the concentration of UFPs, the concentration level of TVOC can also be measured. The present invention is also advantageous in that the integration of a VOC sensing section and a UFP sensing section into a single sensor with a shared evaluation unit is less expensive, less voluminous and more convenient for the user than the use of a plurality of individual sensors, as done in the prior art.

Further, regarding the VOC sensing section in particular, the present invention is advantageous in that the first electrode is arranged downstream of the ionizing unit in the first conduit, thereby providing a more robust and reliable measurement of the VOC concentration via the evaluation unit since the first electrode itself can be positioned at a "safe" distance away from the ionizing unit. In other words, by arranging the first electrode downstream of the ionizing unit, the first electrode can be shielded from direct exposure to the ionizing effect of the ionizing unit, thereby remaining largely insensitive to the ionizing effect of the ionizing unit, while still having exposure to ionized VOC in the air passing through the first conduit. For example, the first electrode may be shielded from direct exposure to the radiation emitted from a source of ionization, the radiation being of sufficiently high energy to induce photo -ionization of airborne VOCs.

Further, the present invention is advantageous in that it provides a more reliable sensor for the quantitative indoor measurement of the concentration of volatile organic compounds (the TVOC concentration), such as hydrocarbon gases, which is often independent of the indoor UFP concentration.

According to an embodiment, the ionizing unit and/or the charging unit may be configured to be intermittently activated, which is advantageous in that the respective lifetimes of the ionizing unit and/or the charging unit may be further increased. With the present embodiment, by periodically activating the ionizing unit and the charging unit during predetermined time intervals, any build-up of contaminants within the ionizing unit (e.g. on an optical window of a radiation source) and/or within the charging unit is retarded.

Further, the sensor may be configured to simultaneously activate the ionizing unit and deactivate the charging unit for obtaining a (current) signal indicative of the TVOC concentration in the air passing through the first conduit, and to simultaneously deactivate the ionizing unit and activate the charging unit for obtaining a (current) signal indicative of the UFP concentration in the air passing through the second conduit. In other words, the sensor may alternatively (or sequentially) infer the TVOC concentration and the UFP concentration from the corresponding signals obtained by the evaluation unit. For example, the sensor may comprise a UFP charging unit comprising a high- voltage corona electrode, a VOC ionizing unit comprising a photo -ionization (PI) lamp, and a shared evaluation unit comprising a current meter. The shared evaluation unit may then serially measure (in time) at least part of the particle-bound charge in air passing through the second conduit of the UFP sensing section (PI lamp "off, corona electrode "on") and at least part of the charge associated with ionized gases in air passing through the first conduit of the VOC sensing section (PI lamp "on", corona electrode "off) with the current meter.

According to an embodiment, the capturing element may be a particle filter arranged inside a Faraday cage, the first electrode being electrically connected to the Faraday cage, the (current meter in the) evaluation unit being connected to the Faraday cage for producing a signal representative of the air pollution.

According to an embodiment, the capturing element may comprise a set of

(substantially parallel) electrodes comprising a precipitation electrode and a counter electrode. The counter electrode is supplied with a voltage for creating an electric field between the counter electrode and the precipitation electrode which enables the precipitation of at least part of the charged particles from the air passing through the second conduit onto the precipitation electrode. The precipitation electrode is electrically connected to the first electrode, and the evaluation unit is connected to the precipitation electrode for obtaining a signal representative of the air pollution.

According to an embodiment, at least part of an inner wall of the first conduit may form the second electrode, which is a convenient way of implementing the present invention. The first conduit may then preferably be made of an electrically-conductive material, such as metal, so that a bias voltage can be applied on it.

According to an embodiment, the ionizing unit may comprise a radiation source arranged to irradiate the air passing through the first conduit for inducing photo- ionization of airborne VOCs. The present embodiment is advantageous in that the degree of photo -ionization may be controlled by regulating the intensity level of the radiation provided by the radiation source. Further, as the first electrode is arranged downstream of the radiation source and substantially shielded from direct exposure to the emitted radiation, photo- emission of electrons from the first electrode is avoided, or at least reduced, thereby nullifying or at least reducing the contribution of electron emission to the measured current signal, which provides a more reliable sensor.

According to an embodiment, the sensor may further comprise a photo- detector facing the radiation source for measuring the intensity of emitted radiation from the radiation source, thereby enabling feedback-control of the radiation source with regard to its emitted radiation intensity. In particular, the photo-detector may be a ultra-violet (UV) sensor for measuring the UV intensity emitted from the radiation source (even if the radiation source itself is not dedicated to UV emission only). The present embodiment is advantageous in that, via a feedback loop, the driving voltage of the radiation source may be adjusted such that a constant radiation emission (e.g. UV emission) is obtained in the course of time, thereby enabling compensation for possible aging effects affecting the emission from the radiation source.

According to an embodiment, the ionizing unit may be arranged in a recess of the first conduit, which is advantageous in that the ionizing unit is then protected from direct exposure to the air passing through the conduit (also referred to as the "airflow" in the following). For example, if the ionizing unit comprises a radiation source equipped with an optical window through which radiation irradiates the air passing through the first conduit, the optical window inside the recess (at some distance from the airflow) is largely protected from the deposition of polymeric organic substances, gases, ions and particles from the airflow. Because the irradiation of air with ionizing radiation creates ozone gas, the arrangement of the ionizing unit in the recess also allows for a build-up of an elevated ozone gas concentration directly adjacent to the optical window which helps to continuously clean the optical window through oxidation. The ionizing unit is then less subject to contamination and maintenance of the sensor is less often required. Further, the lifetime of the ionizing unit, and thereby the sensor, is increased.

According to an embodiment, the sensor may further comprise an air filter arranged upstream of the ionizing unit for removing particulate material from the air passing through the first conduit, which is advantageous in that all air passing the ionizing unit is ensured to first pass a particle filter which removes at least a substantial part of the particulate material from the airflow. As a result, any potential contamination of the ionizing unit through the deposition of particulate material on e.g. an optical window of a radiation source or on the inner wall of the first conduit becomes reduced. Further, the air filter may be arranged to filter VOCs of a certain molecular weight, for instance of lower molecular weight (i.e. below a predetermined molecular weight threshold), thereby providing a sensor sensitive to only VOCs of a somewhat higher molecular weight (i.e. above the predetermined molecular weight threshold) which are the most health-hazardous species.

According to an embodiment, the sensor may further comprise at least one of a temperature sensor, a relative humidity sensor and a C0₂ sensor, which is for instance advantageous in that it provides additional air quality parameters that may be of interest for feedback control in, e.g., air handling units. Further, information about temperature and relative humidity may be used for the correct interpretation of the electrical current signal obtained from the current meter in the evaluation unit in terms of the concentration levels of UFPs and/or TVOC in air. In particular, the output of the temperature and relative humidity sensors may be used to account and compensate for any loss of radiation intensity from the radiation source comprised in the ionizing unit due to radiation absorption by moisture in the air. Such a loss is generally related to the absolute humidity (AH) and could be accounted for through an arithmetic exponential factor (via the Lambert-Beer law). The absolute humidity can be derived when the relative humidity and the temperature are known. Correcting for such a loss enables for a correction of the proportionality factor between the measured current signal from the current meter and the inferred TVOC concentration in the air so that the inferred TVOC concentration level becomes independent of the humidity concentration in the air.

Further, the sensor may further comprise at least one air displacement unit (such as a ventilator, a ventilation arrangement, a fan, or a pump), configured to provide (draw) a first airflow through the first conduit (of the VOC sensing section) and a second airflow through the second conduit (of the UFP sensing section).

According to an embodiment, the magnitude and sign of the bias voltage (imposed on the second electrode in the first conduit) may be selected such that the value of the obtained (current) signal is substantially nullified or minimized when air that is substantially free of VOCs is passed through the first conduit. A zero-VOC concentration can be obtained by passing the air through an activated carbon filter located upstream of the first conduit. The selected bias voltage is chosen such as to compensate for the influence on the measurement signal (normally a measured electrical current) of any other charged species (other than photo-ionized VOCs) in the airflow such as photo-emitted electrons or ions emitted from surfaces directly or indirectly irradiated by the radiation source. The bias voltage furthermore serves to create the electric field between the first electrode and the second electrode that enables the precipitation of at least part of the ionized VOCs from air onto the first electrode. The present embodiment is advantageous in that the chosen bias voltage provides for a more reliable zero signal value for the VOC sensing section of the sensor. The bias voltage obtained according to this procedure only needs to be established during the first sensor calibration immediately after sensor manufacture and requires re- calibration only during infrequently applied periodic sensor servicing. The present embodiment is particularly advantageous over prior art sensors in which frequent separate calibrations with substantially VOC-free air and with air having a specific known VOC concentration are normally required due to the presence of a non-optimized bias voltage, aging of the lamp, internal contamination, and spurious signal drifts. In addition, the present embodiment can easily account and correct for a possible electronic offset current through an additional internal calibration procedure that involves the zeroing of the airflow through the first conduit (i.e. blocking the airflow), e.g. by switching-off the air displacement unit (such as a ventilator). A zero airflow through the first conduit effectively stops the transport of charged airborne species to the sensing electrode, also when VOCs are present in the air, hence any remaining measured current signal will then be an electronic offset current which can subsequently be compensated for internally by the evaluation unit. The existence of an electronic offset current is independent of the chosen value for the bias voltage and the VOC concentration in the air. This internal compensation calibration for an electronic offset current is therefore independent of the first calibration for establishing an optimized value for the bias voltage on the second electrode. Internal electronic offset currents become quite important when very low current signals are to be measured (down to femto-amperes). In prior art devices, proper correction for electronic offset currents can frequently not be accomplished because of remaining convective airflows which effectively prevent zero airflow conditions to be attained (e.g. in devices relying on the diffusive transport of gases).

It will be appreciated that the correction for an internal electronic offset current at any arbitrary VOC concentration may be carried out automatically at any frequency and may involve the de-activation of the air displacement unit (such as a ventilator) and the simultaneous measurement of the remaining current signal, which then corresponds to the internal offset current signal for which a correction can be made. Free air convection through the first conduit during the de-activation of the air displacement unit (such as a ventilator) may be avoided (or at least very significantly reduced) due to the presence of a fibrous particle filter at the entrance of the first conduit and inside the Faraday cage, thereby blocking free airflow. In the absence of airflow, no supply of any ionized VOCs to the first electrode is possible and, thus, the measured net current signal that is corrected for the internal electronic offset current corresponds to the zero signal level. In the presence of airflow through the first conduit, the same zero signal level is obtained in substantially VOC-free air when the second electrode is supplied with the aforementioned improved (and preferably optimized) bias voltage.

Accordingly, the sensor may be configured to periodically deactivate the air displacement unit (such as a ventilator) for resetting the obtained signal to zero under conditions of zero airflows through the first conduit and the second conduit. According to an embodiment, there is provided an air handling unit comprising a sensor as defined in any one of the preceding embodiments and a control unit configured to control the air handling unit as a function of a VOC concentration and/or a UFP concentration obtainable by the sensor. In general, minimization or reduction of indoor exposure to air pollutants occurs mostly through ventilation with outdoor air. Dependent on the outdoor air pollution level, it may be beneficial to pass the ventilation air first through an air handling unit (or cleaning filter) before allowing it to enter the building. Air ventilation in buildings is usually carried out by means of mechanical air handling units which take care of air transport, heating, cooling, and air cleaning requirements. Higher ventilation levels normally incur higher expenditures of energy. To reduce (and preferably minimize) the energy demand while maintaining the indoor air quality at an optimum level (thereby safeguarding human health, well-being, and productivity), active feedback to air handling units is desirable. Feedback information advantageously comprises information about air quality parameters such as the temperature, the relative humidity, the C0₂ concentration, and the local indoor air pollution level by UFPs and VOCs. In the present embodiment, such feedback may be provided by the sensor of the present invention. The feedback may be useful for enabling demand-controlled ventilation, demand-controlled air cleaning, and demand-controlled air heating/cooling. The air handling unit may for example be an air ventilation unit, an air cleaning unit, a heating or cooling unit or even an automatically- controlled air ventilation unit.

It will be appreciated that some of the features in the above described embodiments are specific to the VOC sensing section only. Thus, although these features are described in the application in combination with a sensor comprising a UFP sensing section, the present invention provides also for an advantageous VOC sensing section or VOC sensor on its own wherein at least some of the reliability, the sensitivity, the zero-value, the lifetime and the maintenance (via a reduced contamination) is improved.

Further objectives of, features of, and advantages with, the present invention will become apparent when studying the following detailed disclosure, the drawings and the appended claims. Those skilled in the art will realize that different features of the present invention can be combined to create embodiments other than those described in the following. BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing non-limiting and illustrative examples of the invention, wherein:

Fig. 1 is a schematic illustration of a sensor in accordance with a first embodiment of the present invention; and

Fig. 2 is a schematic illustration of a sensor in accordance with a second embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

With reference to Fig. 1, a sensor in accordance with an embodiment of the present invention is described.

Fig. 1 is a schematic illustration of a sensor 1 comprising a first sensing section 100 for sensing airborne VOCs and a second sensing section 200 for sensing airborne UFPs.

The first sensing section 100, hereinafter referred to as VOC sensing section 100, comprises an ionizing unit 120 for ionizing at least part of airborne VOCs in the air passing through a first conduit 110. The first conduit 110 may comprise an air inlet 112 for receiving air and an air outlet 114 for releasing air from the inside of the first conduit 110. The VOC sensing section 100 further includes a first electrode 130 arranged, in the first conduit 110, downstream of the ionizing unit 120. The first electrode 130 is also arranged in proximity to a second electrode 140 onto which a bias voltage "VpiD prec" is applicable for enabling precipitation of at least part of the ionized airborne VOCs from air on the first electrode 130.

The second sensing section 200, hereinafter referred to as UFP sensing section

200, includes a charging unit 220 for charging at least part of airborne particles in the air passing through a second conduit 210. The second conduit 210 may comprise an air inlet 212 for receiving air and an air outlet 214 for releasing air from the inside of the second conduit 210. In the example depicted in Fig. 1, the air outlet 114 of the first conduit 110 corresponds with the air outlet 214 of the second conduit 210. However, although the first conduit 110 and the second conduit 210 may share a common outlet, it will be appreciated that the present invention may be implemented with a first conduit 110 and a second conduit 210 having separate outlets. The UFP sensing section 200 further includes a capturing element 230 arranged, in the second conduit 210, downstream of the charging unit 220 and capable of capturing charged airborne particles.

The sensor 1 further comprises an evaluation unit 300 including an electrical current meter 160 being connected to both the first electrode 130 and the capturing element 230 for obtaining a signal representative of the air pollution. The evaluation unit 300 may for instance also comprise processing means (not shown) for processing the signal obtained by the current meter 160.

Referring now in particular to the VOC sensing section 100 for the detection of gaseous pollutants, the ionizing unit 120 may be a photo -ionization radiation source (or PI lamp) 122 capable of emitting radiation that is sufficiently energetic to ionize at least part of the airborne VOCs in the air passing through the first conduit 110. The radiation source 122 may be equipped with an optical window 126 arranged to enable the irradiation of the airflow passing by the optical window 126 of the radiation source 122 through the first conduit 110. The PI lamp 122 may for example be a 10.6 eV Krypton gas-filled radiation source that can be excited or powered by means of either internal or external electrodes 124 (e.g. via a potential referred to as Vpio iamp in Figure 1), thereby emitting its highly-energetic ionizing radiation through the optical window 126 into air. The optical window 126 of the PI lamp 122 may for example be a MgF₂ optical window.

For example, the PI lamp 122 may be controlled by the application of a high- frequency (100 kHz) voltage V_PID lamp between two external electrodes 124 around the PI lamp 122. In case a 10.6 eV lamp is used, at least part of those airborne VOCs that have an ionization potential less than 10.6 eV can be ionized, which normally corresponds to VOCs with a molecular mass larger than about 40 to 50 dalton.

After ionization of at least part of the airborne VOCs in the airflow, the airborne charged gaseous species can be at least partly precipitated from the airflow onto the first electrode 130 by means of an externally-applied electric field. The electric field corresponds to the field generated between the first (sensing) electrode 130 and the second electrode 140 arranged in proximity to the first electrode 130. In particular, an inner wall of the first conduit 110 may form the second electrode 140, such as illustrated in Fig. 1.

Further, in the example depicted in Fig. 1, the first (sensing) electrode 130 is in electrical contact with a Faraday cage 240 which itself is connected to the evaluation unit 300 comprising the electrical current meter 160. However, it will be appreciated that, in an alternative embodiment without any Faraday cage, the first electrode 130 may be directly connected to the (current meter of the) evaluation unit 300. Further, the sensor 1 may further comprise an air displacement unit such as a ventilator 400 or pump arranged to provide or draw a first airflow through the UFP sensing section, i.e. through the second conduit 210, and a second airflow through the TVOC sensing section, i.e. through the first conduit 110.

Referring now in particular to the UFP sensing section 200, the charging unit

220 comprises a corona electrode 222 set at a potential V_cor which is sufficiently high to locally ionize the air, thereby emitting ions. The corona electrode 222 is surrounded by a porous screen electrode 221 set at a much lower potential V_scr. Ions transmitted through the porous screen electrode 221 enable the electrical diffusion charging of particles in the air passing through the annulus between the screen electrode 221 and the inner wall of the second conduit 210. In the example depicted in Fig. 1, the charging unit 220 is located downstream of the capturing element 230, which is embodied as a Faraday cage 240 comprising a particle filter for capturing the charged airborne particles. The Faraday cage is connected to the evaluation unit 300 comprising the sensitive current meter 160 which is capable of measuring the amount of precipitated charge per unit time on the first electrode 130 and/or the amount of captured charged particles per unit time in the filter of the Faraday cage 240 as an electrical current I sensor-

In the alternative embodiment shown in Fig. 2, the particle capturing element 230 is embodied as a set of substantially parallel electrodes comprising a counter electrode 232, to which a voltage V_part prec is applied, and a precipitation electrode 234 whereupon at least part of the charged particles precipitate from the air passing through the second conduit 210 under the influence of the electric field between the counter electrode 232 and the precipitation electrode 234. Here, the precipitation electrode 234 is connected to the evaluation unit 300 comprising the sensitive current meter 160 which is capable of measuring the amount of precipitated charges per unit time on the first electrode 130 and/or the amount of precipitated charged particles per unit time on the precipitation electrode 234 as an electrical current I_sensor. It will be appreciated that the sensor 2 depicted in Figure 2 is identical to the sensor 1 described with reference to Figure 1 except for the realization of the capturing unit 230.

The electrical current I_sensor corresponds to the signal of the sensor 1. The magnitude of the signal I_sensor reflects the TVOC pollution level in the air when the ionizing unit 120 (i.e. the PI lamp 122 used to ionize the airborne VOCs) is "on" and the charging unit 220 (i.e. the corona electrode 222 used to charge airborne particles) is "off. Similarly, the magnitude of the signal reflects the UFP pollution level in the air when the PI lamp 122 is "off and the corona electrode 222 used to charge airborne particles is "on". In the latter situation, the particle filter in the Faraday cage in Fig. 1 or the precipitation electrode in Fig. 2 captures charged UFPs from air and the captured amount of particle charges per unit time is recorded as the signal I_sensor. Thus, by alternating the activation of the corona electrode 222 and the PI lamp 122 (or sequentially activating the charging unit 220 and the ionizing unit 120), a series of I_sensor recordings may be obtained in the course of time, such recordings alternatively reflecting the UFP concentration level and the TVOC concentration level, respectively. The sensors 1 and 2 are therefore capable of providing information about both the UFP concentration and the TVOC concentration, thereby assessing the overall severity of the air pollution level.

Referring again to the UFP sensing section or UFP sensor part 200 of the sensor, it will be appreciated that the UFP sensing section may be identical to the sensor described in e.g. US20100043527 by the same applicant and inventor. The details of the UFP sensor described in US20100043527 are therefore incorporated herein by reference.

In case the UFP sensor part 200 only comprises a particle diffusion charging section 220 and a particle sensing section 230 (as shown in Fig. 1), the obtained signal I_sensor is proportional to the product of N and d_p,_av wherein N is the particle number concentration and d_p,av is the average particle diameter. However, in case an additional particle precipitation section is present between the UFP charging section and the particle sensing section (as disclosed in US-2010/0043527), wherein either or not part of the charged particles is precipitated from the airflow, N and d_p,_av can be inferred separately from two serially measured current signals I_sensor = Ii and I_sensor = h- A signal I_sensor can be derived from charged UFPs when the corona electrode 222 is activated (i.e. capable of emitting ions in the airflow in the second conduit 210) and UFPs are present in the airflow while, at the same time, the PI lamp 122 is switched off to prevent the sensor signal to become affected by the presence of charged VOCs. To ensure that I_sensor is nullified or minimized when N is equal to zero (i.e. for providing the sensor with a correction for an internal electronic offset current), the air displacement unit (ventilator, fan or pump) may be periodically switched off and the subsequently measured signal I_sensor is internally set to zero. Alternatively, the air

displacement unit (ventilator, fan or pump) could still be activated and the corona electrode de-activated, since no particle charging would then occur. The measured signal then also corresponds to the zero signal with respect to UFPs. Yet another possibility would be to deactivate both the air displacement unit (ventilator, fan or pump) and the corona electrode for establishing the zero current signal. According to an embodiment, the sensors 1 and 2 may further comprise a separate temperature (T) sensing element and a relative humidity (RH) sensing element (not shown). The outputs of the T and RH sensing elements may for instance be used to deduce the absolute humidity in the air, the value of which is necessary to be able to compensate for the loss of radiation intensity from the PI lamp 122 due to radiation absorption by moisture in the air. Such a loss is proportional with the absolute humidity and can be accounted for through an arithmetic exponential factor (the Lambert-Beer law).

Referring again to the VOC sensing section 100, the sensor 1 may further comprise an air filter 180 arranged upstream of the ionizing unit 120. The air filter is advantageous in that any particulate material is first removed from the air passing through the first conduit 110 before the air reaches the ionizing unit 120.

Further, as mentioned above, the selection of the PI lamp is related to the type of VOCs (depending on the molecular weight) that can be ionized and thus measured. The extent to which different VOCs are charged or ionized depends on the particular VOC. A widely used reference VOC gas for this purpose is isobutylene, which is considered to be "an average VOC" with regard to its sensitivity to photo-ionization. The measured current I_sensor obtained when the PI lamp is activated and the corona electrode is deactivated is normally the result of the contribution of many different kinds of VOCs. The contribution of each individual VOC to I_sensor is dependent on its concentration and on the fraction of its molecules that are ionized. This charged fraction can have quite different values for different kinds of VOCs. Even when they would all have the same concentration, the various VOCs would therefore contribute differently to the measured total current I_sensor. To enable an apparent TVOC mass concentration in mg/m³ to be inferred from the measured I_sensor signal, it is normally assumed that the (total) signal obtained from the VOC sensing part 100 of the sensor comes from isobutylene and the relationship between I_sensor and the airborne isobutylene mass concentration can be calibrated for. In this way, the TVOC concentration inferred from I_sensor denotes an isobutylene-equivalent TVOC concentration. Instead of isobutylene, other gases such as toluene may be used as the reference gas.

As mentioned above, concerning the VOC sensing section 100, the ionized charged gaseous species may be at least partly precipitated from air onto the first electrode 130 that is connected to the Faraday cage 240 by applying a voltage VpiD prec to the first conduit 110, thereby creating an electric field between the first conduit 110 and the first electrode 130. However, the electric field may also be created between the first electrode 130 and a second electrode 140 (not necessarily formed by the conduit 110) arranged in proximity to the first electrode 130.

In any case, the voltage bias V_{PID P}rec applied to the second electrode may advantageously be chosen such that, when no VOCs are present in the first conduit 110 while the PI lamp 122 and the pump 400 are activated (the corona electrode 222 being deactivated), the signal I_sensor, corrected for a possibly existing internal electronic offset current, becomes zero. For this purpose, it will be appreciated that an activated PI lamp normally generates photo-emitted electrons (and some positive ions as well) from the various internal surfaces, such as the portion of the inner wall of the first conduit 110 facing the PI lamp 122, that become irradiated. The bias voltage V_PID prec is normally a modestly high voltage and has a positive value with respect to the reference voltage on the sensing electrode. The selection of the bias voltage V_PID prec is preferably performed when the ventilator 400 is activated and when the air drawn through the first conduit 110 is free from VOCs. Elimination of VOCs from air is, for instance, possible by passing the air through an activated carbon filter. This calibration for the bias voltage V_PID prec provides a reliable zero sensor signal obtained at zero TVOC concentration, which is very stable in the course of time and needs checking and/or adjustment only during infrequently applied periodic instrument servicing.

Apart from this "zero calibration", a "span-calibration" may be performed to derive the proportionality factor between the TVOC mass concentration in air and the measured sensor signal I_sensor. This can, for instance, be done with a known concentration of the reference gas isobutylene in the air passing through the first conduit (PI lamp and air displacement unit both activated, corona electrode de-activated). Using the then obtained proportionality factor enables the deduction of an isobutylene-equivalent TVOC mass concentration from the sensor signal I_sensor.

Preferably, a "span calibration", wherein the current signal due to a known

VOC concentration in air is measured, is not performed too frequently. The need for less frequent "span calibrations" is the result of various measures that reduce internal

contamination of the ionizing unit comprising a PI source, thereby increasing its

maintenance- free lifetime. These measures include the intermittent operation of the air displacement unit (such as a ventilator ) and the PI source, the shielding of the optical window of the PI source from direct exposure to flowing air by positioning the PI source and its optical window in the recess 116, the maintenance of a low radiation emission intensity from the PI source, the use of a particle filter at the entrance of the first conduit, and the maintenance of a constant radiation emission strength from the PI source through feedback control by a UV photo-detector facing the optical window of the PI source.

Further, for extending the maintenance- free lifetime of the air pollution sensor, both the corona electrode 222 and the PI lamp 122 (or in other words the charging unit 220 and the ionizing unit 120) may be preferably operated according to an intermittent mode of operation wherein they are only activated during, at most, a few percent of the total time of an operation cycle. Indeed, activation of the air pollution sensor during only a few percent of the total time of an operation cycle is normally sufficient for most practical applications with respect to the air handling strategy in e.g. buildings wherein the concentrations of the various air pollutants do not normally exhibit rapid fluctuations in the course of time.

It will be appreciated that the provision of fibrous air filters in combination with the presence of an axial air displacement unit (such as a ventilator ) may serve to maintain fixed airflows through the various conduits of the sensor when the air displacement unit is activated because the filters act as passive loads. Use of a ventilator instead of a pump is preferred because of its much lower noise level.

The sensor of the present invention may be part of an air handling unit used for e.g. air cleaning. The present invention is therefore applicable for controlling the air handling strategy in buildings and homes, specifically with respect to "green" buildings wherein the control of any air ventilation system is then performed in an energy- efficient way. The present invention is also applicable for the monitoring and control of air pollution in buildings, homes and cars (providing awareness about the level of air pollution and, preferably, control of an air cleaner for filtering such pollution) and industrial facilities (for air pollution assessment and control in workplaces for the protection of workers' health). Even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. The described embodiments are therefore not intended to limit the scope of the invention, as defined by the appended claims.

Claims

CLAIMS:

1. A sensor (1) for assessing air pollution, the sensor comprising:

a first sensing section (100) for sensing volatile organic compounds, including an ionizing unit (120) for ionizing at least part of airborne volatile organic compounds in air passing through a first conduit (110), and a first electrode (130) arranged, in the first conduit (HO), downstream of the ionizing unit (120) and in proximity to a second electrode (140) on which a bias voltage is applicable for enabling precipitation of at least part of the ionized airborne volatile organic compounds on the first electrode (130);

a second sensing section (200) for sensing ultrafme particles, including a charging unit (220) for charging at least part of airborne ultrafme particles in air passing through a second conduit (210), and a capturing element (230) arranged, in the second conduit (210), downstream of the charging unit (220) and capable of capturing charged airborne particles; and

an evaluation unit (300) being connected to both the first electrode (130) and the capturing element (230) for obtaining a signal representative of the air pollution.

2. The sensor of claim 1, wherein the ionizing unit (120) and the charging unit (220) are configured to be intermittently activated.

3. The sensor of any one of the preceding claims, being configured to

simultaneously activate the ionizing unit (120) and deactivate the charging unit (220) for obtaining a signal indicative of the concentration of volatile organic compounds in the air passing through the first conduit (110), and to simultaneously deactivate the ionizing unit (120) and activate the charging unit (220) for obtaining a signal indicative of the

concentration of ultrafme particles in the air passing through the second conduit (210).

4. The sensor of any one of the preceding claims, wherein the capturing element (230) is a particle filter arranged inside a Faraday cage (240), the first electrode (130) being electrically connected to the Faraday cage (240) and the evaluation unit (300) being connected to the Faraday cage (240) for producing a signal representative of the air pollution.

5. The sensor of any one of claims 1 to 3, wherein the capturing element (230) is a set of electrodes comprising a precipitation electrode and a counter electrode.

6. The sensor of any one of the preceding claims, wherein at least part of an inner wall of the first conduit (110) forms the second electrode (140).

7. The sensor of any one of the preceding claims, wherein the ionizing unit (120) comprises a radiation source (122) arranged to irradiate the air passing through the first conduit (110) for inducing photo -ionization of airborne volatile organic compounds.

8. The sensor of claim 7, further comprising a photo-detector (170) facing the radiation source (122) for measuring the intensity of emitted radiation from the radiation source (122).

9. The sensor of any one of the preceding claims, wherein the ionizing unit (120) is arranged in a recess (116) of the first conduit (110).

10. The sensor of any one of the preceding claims, further comprising an air filter (180) arranged upstream of the ionizing unit (120) for removing particulate material from the air passing into the first conduit (110).

11. The sensor of any one of the preceding claims, further comprising at least one of a temperature sensor, a relative humidity sensor and a C0₂ sensor.

12. The sensor of any one of the preceding claims, further comprising at least one air displacement unit (400) configured to provide a first airflow in the first conduit (110) and a second airflow in the second conduit (210).

13. The sensor of any one of the preceding claims, wherein the magnitude and sign of the bias voltage are selected such that the value of the obtained signal is substantially nullified or minimized when air that is substantially free of volatile organic compounds is passed through the first conduit (110).

14. The sensor of claim 13 when appended to claim 12, being configured to periodically deactivate the air displacement unit (400) for resetting the obtained signal to zero under conditions of zero airflows through the first conduit (110) and the second conduit (210).

15. An air handling unit comprising:

the sensor of any one of claims 1 to 14, and

a control unit configured to control the air handling unit as a function of a concentration of volatile organic compounds and/or a concentration of ultrafme particles obtainable by the sensor.