Chemical composition of airborne particles inside and outside a Swedish residence assessed by real time aerosol mass spectrometry
Summary, in English
A number of deleterious health effects have been identified from exposure to outdoor air-borne particulate matter. This issue is complicated by the fact that people are spending most of the time indoors, where particles of both indoor and outdoor origin are present. The aim of this work was to assess the differences in particle chemical composition inside and outside of the residence and gain better understanding about major contributors to the observed levels indoors. Our results showed that indoor aerosol mass concentration exceeded the outdoor values mainly due to the contribution of organic matter from indoor sources during the presence of the residents at home. The infiltration of chemical species from outdoors was not the major factor determining indoor aerosol mass concentration and chemical composition.1 INTRODUCTIONConsidering that on average in developed countries we spend about 65% (Brashe and Bis-chof) of our time in private homes, the understanding of exposure to particulate matter in homes is important, yet knowledge is sparse. Indoors, aerosol concentrations come from in-door sources, infiltrate from outdoors and can be formed through reactions of gas-phase precursors emitted both indoors and outdoors. Several characteristics and processes influence the properties of aerosols indoors, among them: active indoor sources (presence of occupants), outdoor aerosol characteristics, physicochemical particle transformations during infiltration and while indoors, ventilation type, tightness of the building envelope and particle deposition (Morawska et al., 2013). In this study, we aimed to investigate the differences in chemical composition between aerosols inside and outside of the residence and to identify the origin of major contributors to the particle levels indoors. We report preliminary results of measure-ments for a 1-month period in an occupied apartment. 2 METHODS Indoor and outdoor measurements were performed in an occupied residence in Malmö, Swe-den. It was a naturally ventilated four-room apartment (292 m3), located in a three-store con-crete building surrounded by a green zone. A Time-of-Flight Aerosol Mass Spectrometer (AMS, DeCarlo et al., 2006) was used to measure particle mass loadings and size-resolved mass distributions (size range of 50-500 nm) of indoor and outdoor organic, sulphate, nitrate, ammonium and chloride aerosols. An automatic switching valve alternated between indoor and outdoor lines with a time interval of 20 and 10 minutes in indoor and outdoor air, respectively. Both sampling lines were mounted at the ground floor level and led to the basement where the aerosol was dried and measured by AMS. Calculated residence time of the particles in line was 1.5 minutes. Indoor sampling line was heated and insulated, additional carrier flow was used to lower the resi-dence time. Indoor to outdoor (I/O) ratios were calculated and used for comparisons of differences in aerosol composition inside and outside of the residence.3 RESULTS AND DISCUSSIONThe results showed higher total average mass concentration indoors (12.9 μg/m3) compared to outdoors (5.4 μg/m3) during entire measuring period. Indoor to outdoor (I/O) ratio for or-ganics was 6.7, for nitrate 0.3, for sulphate 0.5, for ammonium 0.2 and for chloride 0.2. Or-ganic matter was the dominant species indoors, accounting for most of the total mass (92 %) due to contribution from indoor sources during the time when residents were at home, i.e. occupancy period. Figure 1A illustrates elevated particle mass concentrations when different indoor activities took place. Effects of penetration and phase change of the outdoor particle species can be observed during non-occupancy period (Figure 1B). Non-occupancy time ac-counted only to 7 % of the total monitoring period and did not have much influence on parti-cle mass concentration indoors.Ammonium nitrate (NH4NO3) and ammonium chlorine (NH4Cl) are semi-volatile aerosol species, thus, gas-to-particle partitioning depends on temperature, relative humidity, particle size and gas phase concentrations of ammonia, nitric acid and chlorine as outdoor air is transported indoors (Mozurkewich, 1993; Lunden et al., 2004). Such phase transitions are especially pronounced in the cold period of the year. The outdoor weather conditions varied during this time with Tout from -8.8 to 9.7 °C and RHout from 58 to 100 %. Indoors, Tin ranged from 20 to 26.1 °C and RHin from 27 to 50 %. Low value of I/O ratio for non-volatile sul-phate can be explained by dominating of the outdoor sources and reflects reduced infiltra-tion. The outdoor total mass concentration in urban sites measured by TOF-AMS was comparable with previous studies (Crippa et al., 2014; Jimenez et al., 2009). Some local sources, such as emissions from fireplaces by neighbours and from adjacent fast food restaurants could have contributed to the outdoor loadings. 4 CONCLUSIONSIn general, the differences in chemical composition of particles found indoors and outdoors becomes apparent from the results. Levels of organics in indoor environments were mainly influenced by indoor sources, thus, these should not be neglected when considering possible health effects. Additionally, reduced air exchange rate in the apartment in Scandinavia during wintertime enhanced aerosol accumulation and physicochemical transformation indoors.ACKNOWLEDGEMENTThis work was financed by the Swedish Research Council FORMAS (Project Dnr 942-2015-1029).