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. In this paper we evaluate characteristics of three optical particulate matter sensors/sizers (OPS): high-end spectrometer11-D (Grimm, Germany), low-cost sensor OPC-N2 (Alphasense, United Kingdom) and in-house developed MAQS(Mobile Air Quality System), which is based on anotherlow-cost sensor – PMS5003 (Plantower, China), under realistic conditions of strong and mild urban pollution. Results were compared against a referencegravimetric system, based on a Gemini (Dadolab, Italy), 2.3 m 3  h −1 air sampler, with two channels (simultaneously measuring PM 2.5 and PM 10 concentrations).The measurements were performed in Sarajevo, the capital of Bosnia-Herzegovina, from December 2019 until May 2020.This interval is divided into period 1 – strong pollution – and period 2 – mild pollution. The city of Sarajevo is one of the most polluted cities in Europe in terms of particulate matter: the average concentration of PM 2.5 during the period 1 was 83  µ g m −3 , withdaily average values exceeding 500  µ g m −3 . During period 2, the average concentration ofPM 2.5 was 20  µ g m −3 . These conditions represent a good opportunity to test optical devices against the reference instrument in a wide range of ambient particulate matter (PM) concentrations. The effect of an in-house developed diffusion dryer for 11-D is discussed as well. In order to analyse the mass distribution of particles, a scanning mobility particle sizer (SMPS), which together with the 11-D spectrometer gives the full spectrum from nanoparticles of diameter 10 nm to coarse particles of diameter 35  µ m, was used.All tested devices showed excellent correlation with thereference instrument in period 1, with R2 values between 0.90 and 0.99 for daily average PM concentrations. However, in period 2, where the range of concentrations was much narrower, R2 values decreased significantly, to values from 0.28 to 0.92. We have also included results of a 13.5-month long-term comparison of our MAQS sensor with a nearby beta attenuation monitor (BAM) 1020 (Met One Instruments, USA) operated by the United States Environmental Protection Agency (US EPA), which showed similar correlation and no observable change in performance over time.

Evaluation of optical particulate matter sensors under realistic conditions of strong and mild urban pollution