A growing number of companies have started commercializing low-cost sensors (LCS) that are said to be able to monitor air pollution in outdoor air. The benefit of the use of LCS is the increased spatial coverage when monitoring air quality in cities and remote locations. Today, there are hundreds of LCS commercially available on the market with costs ranging from several hundred to several thousand euro. At the same time, the scientific literature currently reports independent evaluation of the performance of LCS against reference measurements for about 110 LCS. These studies report that LCS are unstable and often affected by atmospheric conditions—cross-sensitivities from interfering compounds that may change LCS performance depending on site location. In this work, quantitative data regarding the performance of LCS against reference measurement are presented. This information was gathered from published reports and relevant testing laboratories. Other information was drawn from peer-reviewed journals that tested different types of LCS in research studies. Relevant metrics about the comparison of LCS systems against reference systems highlighted the most cost-effective LCS that could be used to monitor air quality pollutants with a good level of agreement represented by a coefficient of determination R2 > 0.75 and slope close to 1.0. This review highlights the possibility to have versatile LCS able to operate with multiple pollutants and preferably with transparent LCS data treatment.

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Monitoring personal exposure to particle matter (PM(2.5)) in ambient air requires performing measurements using portable monitors. In this work, the portable nephelometer SidePak™ AM510 Personal Aerosol Monitor manufactured by TSI Inc. was evaluated against a Tapered Element Oscillating Microbalance (TEOM) equipped with a Filter Dynamics Measurements System (FDMS). Conventionally, the SidePak is calibrated with respect to the Arizona Road Test Dust and then multiplied by an environmental calibration factor to yield mass concentration. To adapt this calibration to specific field conditions, we present an implementation of this calibration by introducing a growing factor correction which takes into account relative humidity and the dry and wet portions of the refractive index estimated from TEOM-FDMS measurements. PM(2.5) sampling with several SidePaks AM510 was carried out in background and rural sites in the Po Valley (Italy). Modeled SidePak data were plotted vs. reference TEOM-FDMS data which show a good agreement.

The Air Quality Directive (2008/50/EC) asks for the organisation of quality assurance programmes for air quality assessment methods at European level. Since the early 1990s the European Reference Laboratory for Air Pollution (ERLAP) of the EC's Joint Research Centre (JRC) has carried out Intercomparison Exercises (IE) for air pollution measurements on a regular basis for Member States of the EU. All European National Reference Laboratories (NRLs), joined together in the AQUILA Network, are obliged to participate in IE. More than 45 laboratories and institutes, coming from 35 European countries, have participated in the IE during the last 15 years. The results of the most recent IE which took place from 2005 to 2010 are described. Gas mixtures with some concentrations of CO, SO 2 , NOx, and O 3 were generated and measured by the participants. With the results of the participants' z'-score, En number, repeatability and reproducibility, outlier through the test of Grubb were evaluated.

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