# Ventilation Systems and Their Impact on Nanoparticle Concentrations in Office Buildings
## 2 Materials and Methods  
Nanoparticle concentrations were measured in twelve office buildings in the city of Umea in northern Sweden during the year 2017 and 2018. The locations of the office buildings are presented in Figure 1. The concentration and size distribution of nanoparticles were measured with 5-min intervals during two one-week periods representing both the heating and nonheating seasons. The measurements were performed in office rooms with an area of between \(\sim\)10 and \(\sim\)20 m\({}^{2}\), and no people entered these rooms during the measurements. Windows and doors were closed during the measurements, and there were no printers located in these rooms. The twelve office buildings were selected to include newer, retrofitted, and older buildings with various ventilation systems and energy efficiency. The office buildings were also chosen according to different distances from busy streets as the main sources of combustion-formed nanoparticles (CFNPs). The nanoparticles were measured using SMPS 3938 (SMPS = Scanning Mobility Particle Sizer Spectrometer, TSI Inc., Shoreview, MN, USA). This technique is described more detailed in the earlier analysis by [PERSON] et al. (2021) ([PERSON] et al., 2021) ([PERSON] et al., 2021), and it is shown in Figure 2. The nanoparticles that were measured as particle number concentration (PNC) in this study consist of fractions in different size classes in the range of 10 nm up to 461 nm. The ventilation systems in the twelve buildings were all based on mechanical ventilation with heat recovery (MVHR). It was a fan-controlled supply and exhaust air system with heat recovery. Air ducts were used to supply air and exhaust air to control the flow of air so that air pollutants were eliminated as efficiently as possible. The MVHR system was also equipped with a heat exchanger where the heat energy in the heated indoor air was used in order to heat the cold incoming air. The majority of the ventilation systems of the studied buildings were equipped with filters of class F7 (despite Building 10 with filter of class M6). For Building 1, no detailed information was available (security classified), but as it was a new building, it has filters \(\geq\)F7. More detailed information of buildings and ventilation systems were presented in the earlier analysis by [PERSON] et al. (2021) ([PERSON] et al., 2021) ([PERSON] et al., 2021). In parallel with nanoparticle measurements, the air velocity (m/s) was identified as 5-min average. The measurements were conducted with a multifunctional environmental meter AMI310 (KIMO Instruments. Sauermann Industrie SAS, Montpon-Menesterol, France) equipped with a \(\mathcal{O}\)100 mm telescopic vane probe and airflow cone (Figure 2). Ventilation flow (m\({}^{3}\)/h) was calculated by multiplying the air velocity by the cross-sectional area of the cone:  
\[\mathrm{q_{m}}=\mathrm{v_{m}}\times\left[\tau\times\left(\mathrm{d_{m}}/2 \right)^{2}\right]\times 3600 \tag{1}\]  
where the included variables are defined as follows:  
\(\mathrm{q_{m}}=\mathrm{ventilation flow}\left(\mathrm{m^{3}/h}\right)\)  
Figure 1: The study area with the location of Buildings 1–12, where indoor measurements were carried out. Basemap: © OpenStreetMap contributors. In terms of nanoparticle (hereinafter abbreviated NP) concentrations and the importance of ventilation, there are a number of factors that determine the concentrations of indoor pollutants including NPs. The main factors can be mathematically illustrated by the following differential equation ([PERSON] et al., 2009) [10]:  
\[\mathrm{V}\;\frac{dc}{dt}=\mathrm{q}\;(\mathrm{C}_{0}-\mathrm{C})+\mathrm{V}_{ \mathrm{pol}} \tag{2}\]  
where the included variables are defined as follows:  
V = volume of space  
Figure 2: Measurement of nanoparticles indoors with SMPS 3938 (**left**) and ventilation flow with AMI310 (**right**).