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Modern electrochemical gas sensors hold great potential for improving practices in air quality (AQ) monitoring as their low cost, ease of operation and compact design can enable dense observational networks and mobile measurements. Despite that, however, numerous studies have shown that the performance of these sensors depends on a number of factors (e.g. environmental conditions, sensor quality, maintenance and calibration), thereby adding significant uncertainties in the reported measurements and large discrepancies from those recorded by reference-grade instruments. In this work we investigate the performance of electrochemical sensors, provided by two manufacturers (namely Alphasense and Winsen), for measuring the concentrations of CO, NO2, O3 and SO2. To achieve that we carried out collocated yearlong measurements with reference-grade instruments at a traffic AQ monitoring station in Nicosia, Cyprus, where temperatures ranged from ca. 0 ∘C in the winter to almost 45 ∘C in the summer. The CO sensors exhibit the best performance among all the ones we tested, having minimal mean relative error (MRE) compared to reference instruments (ca. −5 %), although a significant difference in their response was observed before and after the summer period. At the other end of the spectrum, the SO2 sensors reported concentration values that were at least 1 order of magnitude higher than the respective reference measurements (with MREs being more than 1000 % for Alphasense and almost 400 % for Winsen throughout the entire measurement period), which can be justified by the fact that the concentrations of SO2 at our measuring site were below their limit of detection. In general, variabilities in the environmental conditions (i.e. temperature and relative humidity) appear to significantly affect the performance of the sensors. When compared with reference instruments, the CO and NO2 electrochemical sensors provide measurements that exhibit increasing errors and decreasing correlations as temperature increases (from below 10 to above 30 ∘C) and RH decreases (from >75 % to below 30 %). Interestingly, the performance of the sensors was affected irreversibly during the hot summer period, exhibiting different responses before and after that, resulting in a signal deterioration that was more than twice that reported by the manufacturers. With the exception of the Alphasense NO2 sensor, all low-cost sensors (LCSs) exhibited measurement uncertainties that were much higher, even at the beginning of our measurement period, compared to those required for qualifying the sensors for indicative air quality measurements according to the respective European Commission (EC) Directive. Overall, our results show that the response of all LCSs is strongly affected by the environmental conditions, warranting further investigations on how they are manufactured, calibrated and employed in the field.

Size resolved measurements of aerosol particles are essential for understanding key processes in the atmosphere, and assessing their potential impacts on human health and climate. This has led to an increasing demand for compact and portable, yet cost-effective, instruments for characterizing the atmospheric aerosol. A commonly employed instrument for sizing aerosol particles in the sub-micrometer range is the Differential Mobility Analyzer (DMA). Commercial DMAs weigh several kilograms, and have a cost higher than 10 kEUR. When employed in aerosol size spectrometers, they attribute a poor time resolution to the system (in the range of several minutes), as they require scanning of their operating conditions. This limits their ability to probe rapidly-changing aerosols caused by certain processes. One way to overcome this limitation is to employ several DMAs operated at fixed conditions (instead of one in which the operating conditions are scanned), but doing so multiplies the weight, the size and the cost of the entire system. Here we report on the design and testing of a lightweight, miniaturized and cost-effective Parallel-Plate DMA. The weight of this DMA is reduced to less than 1 kg, whereas its manufacturing cost is two orders of magnitude less than that of their commercial counterparts.

Aerosol particles properties depend strongly on their particle size and they have significant effects on both, human health and environment. Nanometer sized particles possess special electrical, optical, and/or magnetic properties. This is one of the reasons which started the interest towards studying aerosol particles in the nanometer size range (Chen and Pui, 1995). The most efficient tool for determining the size of aerosol particle in the sub-micrometer and nanometer range is the differential mobility analyzer (DMA). This popular tool has two coaxial cylindrical electrodes between of them a potential difference is applied and forces the charged polydisperse aerosol to migrate from one electrode to another. Only those particles which have an electrical mobility in a narrow range, the will pass through the classifier (Stolzenburg, 1988). Classifying aerosols according to their electrical mobility dates back to the first half of the 20th century and from that time plethora different DMAs have been build and their performances have been tested according to their transfer function and size resolution. One major limitation of classical DMAs is the time it takes to scan over the entire size range to get the size distribution of the aerosol. This is especially leading to the loss of information if the aerosol is changing its size and/or concentration rapidly. This happens for instance during new particle formation events, or also when the measurement takes place on fast moving platforms, such as cars, or airplanes.The present work evaluates the performance of two different, newly developed DMA types, that aim towards overcoming this limitation. This is done by replacing the classic design of a single monodisperse outlet DMA to a multiple monodisperse outlet DMA. In our case the DMAs have three monodisperse outlets and are 3D-printed (namely, the 3MO-DMA) (Chen et al., 2007; Giamarelou et al., 2012; Barmpounis et al., 2016; Bezantakos et al., 2016). The 3MO-DMA is not only a fast response instrument able to sizing three different sizes ranges at the same time but also is a cost-effective and lightweight instrument suitable to get measurements not only ground based but also on Unmanned Aerial Vehicles or balloons.