A Pilot Investigation of PM Indoor/Outdoor Mass Concentration and Chemical Analysis during a Period of Extensive Fireplace Use in Athens

During the recent economic crisis in Greece, fireplaces and wood stoves have been extensively used for domestic heating even in densely populated cities like Athens. Throughout the last winter periods (especially winter 2012–2013), a persistent phenomenon of smoke haze covering many urban and suburban areas of the city was observed. In the framework of the present study, indoor and outdoor PM10 and PM2.5 measurements were conducted in an apartment in suburban Athens during December 2012–February 2013 for two periods. One period with minimal or no wood burning at fireplaces and another period with intense wood burning taking place in the area. The results highlighted the impact of biomass burning on PM mass concentration in the ambient atmosphere as well as the indoor air. OC/EC and K/EC ratios for both (indoor and outdoor) particle fractions revealed their origin from biomass burning. The most abundant ions were SO4 and NO3 followed by Ca, PO4, Na for indoor and outdoor particles with levels typical for this suburban area. Finally, Fe strongly dominated in both indoor and outdoor air while elemental enrichment factors highlighted the anthropogenic origin of trace elements. Indoor to outdoor concentration ratios, especially during the period of extensive fireplace use, showed that carbonaceous particles and some trace species (Cu, K, Na) were released in the indoor air.


INTRODUCTION
Wood burning always used to be a traditional way for domestic heating particularly in rural areas.Recently, due to the Greek economic crisis, a transition from dieselfueled domestic heating to wood burning (fireplaces or stoves) was observed, stimulated by declining income (Safari et al., 2013;Sarigiannis et al., 2014).Despite the fact that open fireplaces in apartments and houses situated in big cities are designed to be mostly decorative, they have been extensively used as a low-cost solution for domestic heating.As a result, during the last winter periods (especially winter 2012-2013), the phenomenon of smoke haze in the Athens Metropolitan area was often observed.Furthermore, non-certified firewood contaminated wood and other by-products or waste is often used in stoves and fireplaces.Consequently, the scientific interest focuses on the smoke haze chemical composition and particulate matter (PM) concentration, related to both short and long-term effects on the population's health (Sarigiannis et al., 2015).
From a literature review, a significant fraction of ambient particulate matter, ranging from 10% to over 85%, has been worldwide attributed to wood burning residential emissions, although referring to specific seasonal or local conditions (Larson and Koenig, 1994;Luhar et al., 2006;Jeong et al., 2008).Several studies have examined the impact of residential wood burning on the air quality of both rural/ forest inhabited areas (Bari et al., 2009;Caseiro et al., 2009) and large cities.Indicative of the latter are the studies of Favez et al. (2009) and Viana et al. (2013) who investigated the influence of biomass burning aerosols on PM 2.5 levels in the air of Paris and Barcelona.Concerning Greece, Saffari et al. (2013) collected urban PM 2.5 samples in Thessaloniki (northern Greece) during the winter periods of 2012 and 2013, when air pollution episodes-related to biomass burning-occurred.The results indicated a 30% increase in the PM 2.5 mass concentration as well as a 2-5-fold increase in the concentration of wood smoke tracers, while the concentrations of fuel oil tracers declined by 20-30% during 2013, reflecting the Greek economic crisis.Another recent study conducted in Thessaloniki (Sarigiannis et al., 2014) aimed at assessing the population exposure during wintertime and how this exposure is affected by the use of biomass for domestic heating.As they report, the outdoor measurements highlighted a significant increase of PM 10 (143%) and PM 2.5 (223%) during the transition from the warm to the cold period of the year 2012.
In the framework of the present study, continuous indoor and outdoor PM 10 and PM 2.5 measurements were conducted inside and outside of an Athenian apartment-house during a period of extensive fireplace use for domestic heating (December 2012-February 2013).The study aims at estimating the effect of biomass burning of PM mass concentration in the atmosphere of the area as well as the resulting influence on the indoor levels.PM origin has been examined through chemical analysis for organic (OC) and elemental carbon (EC), ionic and elemental species.To the best of our knowledge, the number of the studies investigating the consequences of extensive biomass burning for domestic heating in big Greek cities in the period of the economic crisis, is quite limited.

FIELD DESCRIPTION AND SAMPLING
Measurements were carried out from 22 nd December 2012 to 21 st February 2013, including the Christmas holiday period, characterised by extensive use of fireplaces even in large Greek cities.An apartment of 120 m 2 situated on the 3 rd floor of a 30 year-old four-store apartment house was employed, located at a suburban area of Athens (Aghia Paraskevi), densely inhabited, with block of flats prevailing, plants and evergreen trees.The apartment faces a moderatetraffic street while in a 200 m distance there is a high-traffic Avenue (Mesogion Avenue, Fig. 1).The apartment was naturally ventilated throughout the sampling period.Smoking did not take place in the apartment throughout the campaign.Electricity was used for cooking, while building's central heating system and fireplace (occasionally) were used for heating.Each time, the fireplace was active for approximately 3 to 4 hours (except for December, 22 nd , when the fireplace was active for more than 6 hours).During the campaign, olive and beech tree wood was burned.
Sampling took place indoors (in the living room, in a 5 m distance from the fireplace) and outdoors (in the apartment's balcony), Fig. 1.The pilot study consisted of three campaigns (Table S1).Campaigns 1 and 2 included respectively PM 10 and PM 2.5 simultaneous indoor and outdoor gravimetric measurements for two periods per day, period A (00:00 h-Fig.1. Map of the area and layout of the apartment.The indoor and outdoor sampling sites have been marked with an X. 12:00 h; absent or reduced fireplace activity) and period B (12:00 h-24:00 h; intense fireplace activity).The third campaign included 24-h PM 2.5 outdoor measurements, during which extensive fireplace use was significantly eliminated.

METHODS AND MATERIALS
PM mass sampling was carried out by two identical low volume (2.3 m 3 h -1 ) samplers (Derenda LVS3.1/PMS3.1-15).Mass concentration was determined gravimetrically as described in Seleventi et al. (2012) while particulate matter was collected on quartz 47 mm-filters (although by the use of this type of filters, particle mass could be overestimated).However, quartz filters are the only type of filter that can be used for further chemical analysis (OC/EC) and are recommended by EN12341:2014.Correlation of the measurements performed by the two samplers was checked through an inter-comparison pilot study (Pearson correlation r > 0.8 with p-value < 0.001).For OC/EC analysis, a Thermal/Optical Carbon Aerosol Analyser (Sunset Laboratory) was employed, operating according to NIOSH Method 5040.The instrument's limit of detection (LOD) is 0.2 µgC cm -2 and the analytical uncertainty is equal to ± (concentration × 0.05) + instrument blank concentration.
) and cations (ΝΗ 4 + , Na + , K + , Mg 2+ , Ca 2+ ) were determined using a DIONEX ICS-1100 chromatographic system, with IonPac AS22 (4 mm) and IonPac CS12A (4mm) columns, respectively.The LOD ranged between 0.01 and 0.11 µg mL -1 for anions and between 0.01 and 0.33 µg mL -1 for cations.Uncertainty (95%, k = 2) ranged between 5.61% and 9.31% for anions and between 2.63% and 10.7% for cations.For Cu, Pb, Cr, Ni, Cd, Zn, Fe and Al analysis air filters were treated following the procedure described by Melaku et al. (2008).Determinations of Cu, Pb, Cr, Ni and Cd concentrations were carried out by graphite furnace atomic absorption spectrometry with Zeeman background correction (SpectrAA 640Z; Varian, Mulgrave, Victoria, Australia), whereas Zn and Fe determinations were performed by flame atomic absorption spectrometry (SpectrAA 200;Varian).LOD values were determined according to USEPA (1997) and the values obtained (in ng per filter) were 26 for Cu, 20 for Pb, 25 for Cr, 37 for Ni, 2.3 for Cd, 375 for Zn and 1800 for Fe.For statistical calculations, values below the LOD were assigned the limit of detection divided by 2. Quality control measures included use of blank filters and the analysis of the certified reference material (CRM) NIST 1649a (urban dust).For every batch of 10 samples a blank filter and a CRM sample were run.Relative expanded uncertainty (95%, k = 2) was calculated according to EURACHEM (2000) and the values obtained were 13.2% for Cu, 20.2% for Pb, 14.1% for Cr, 13.8% for Ni, 16.9% for Cd, 15.0% for Zn and 15.5% for Fe.
Occupants' activities were daily recorded on time activity diaries (kind, time and duration of cooking/cleaning, time and duration of fireplace use, number of occupants and visitors, time and duration of opening the windows and balcony doors, operation of electric appliances etc.) while indoor and outdoor air temperature and humidity were continuously logged with a HOBO data logger.During the 2-month campaign, outdoor air temperature and relative humidity respectively ranged between 1 and 15°C (average 9.7 ± 2.9°C) and 42 and 99% (average 74 ± 11%).Correspondingly, indoor air temperature and relative humidity respectively ranged between 14 and 22°C (average 19 ± 2.5°C) and 42 and 89% (average 69 ± 8.0%).

Mass Concentration PM 10
Particle mass concentration values for both fractions are presented in Table 1.The average PM 10 concentration was observed to be higher outdoors (6.52-171 µg m -3 , average 57.4 µg m -3 ) than indoors (9.81-125 µg m -3 , average 41.4 µg m -3 ) for both time periods (A: 00:00-12:00 and B: 12:00-24:00).The results of the present study were compared to the European Commission (EC) limit value for an indicative assessment of PM levels increase, although the relative Directives refer to ambient air.In particular, 60% and 15% of the outdoor and indoor (24-hour averaged) measurements respectively exceeded the European daily limit for outdoor PM 10 (50.0 µg m -3 , Directive 2008/50/EC).In cases of fireplace use, the average indoor PM 10 concentration (62.0 µg m -3 ) of the apartment presented a two-fold increase compared to that corresponding to days without fireplace use (35.0 µg m -3 ).A comparable increase in PM 10 levels in residences with and without fireplace use has also been reported by Sarigiannis et al. (2014).The maximum indoor and outdoor values were both observed during afternoon and early night hours (period B), when extensive fireplace use occurred in the area.It is indicative that the highest outdoor concentration (171 µg m -3 ) was recorded during the afternoon and evening hours of December 25 th (Christmas holiday celebration) while for the indoor air, the maximum concentration was recorded in the afternoon of December, 22 nd (when the apartment's fireplace was active for more than 6 hours).
PM 2.5 PM 2.5 concentration was also observed to be higher outdoors (11.4-150 µg m -3 , average 36.8µg m -3 ) than indoors (2.80 and 57.3 µg m -3 , average 27.9 µg m -3 ).Both indoor and outdoor average values are higher than the EC annual limit value for outdoor PM 2.5 (25.0 µg m -3 , Directive 2008/50/EU).The average indoor PM 2.5 concentration during days with the apartment's fireplace in use was 29.0 µg m -3 , slightly elevated compared to that during days without fireplace use.In addition, contrary to PM 10 , the highest indoor and outdoor PM 2.5 (57.3 and 150 µg m -3 respectively) values were both observed during period A. Thus, peaks in indoor PM 2.5 levels are attributed either to the apartment's fireplace burning (even for the following 12-hour period) or -in absence of indoor biomass burning-to the outdoor aggravated evening atmosphere or to contamination by adjacent to the apartment stacks and the subsequent particles accumulation indoors (Guo et al., 2010).Finally, from January 28 th to February 21 st (3 rd campaign) PM 2.5 outdoor levels did not present a noteworthy variation as domestic biomass burning in the area was significantly eliminated.The daily concentration value was 30.2 ± 7.81 µg m -3 while in 25% of the days, particle concentration exceeded the EC annual limit for outdoor PM 2.5 (25.0 µg m -3 , Directive 2008/50/EU).

Organic and Elemental Carbon (OC and EC) PM 10
The average values for OC and EC for both particle fractions are presented in Table 2. Following the mass concentration trend, both OC and EC were on average higher during period B than period A (by 1.3 and 2 times for indoor and outdoor OC respectively; by 1.8 and 2.7 times for indoor and outdoor EC respectively), because of the increased evening-time wood and biomass burning (Saffari et al., 2013).In our study, the maximum outdoor value for OC (75.2 µg m -3 ) was recorded on December 25 th (afternoon and evening), when most people remained indoors for Christmas (public holiday).Correspondingly, the maximum indoor value (50.9 µg m -3 ) was recorded on exactly the following night and morning hours (December 26 th , period A), signifying the outdoor atmosphere's influence on indoor air levels.The maximum outdoor and indoor EC concentrations (40.4 and 33.6 µg m -3 ) were noticed respectively in the afternoon and evening of December 26 th and in the afternoon of 22 nd December (when the apartment's fireplace was active for more than 6 hours).
Correlation analysis was performed on indoor and outdoor OC and EC concentrations in order to examine the presence of indoor emission OC and EC sources.The positive correlation between indoor and outdoor was similar for the two carbon fractions: r = 0.86 for OC and r = 0.87 for EC, implying the strong outdoor atmosphere's contribution to indoor levels.When outdoor OC and EC concentrations are used as independent values for indoor OC and EC concentrations, their intercepts are equal to 5.32 and 0.72, respectively, as shown in Figs.S1(a) and S1(b).Each intercept roughly represents OC and EC concentrations that originate exclusively from indoor emission sources since intercepts represent the concentration value in case of outdoor OC and EC zero concentrations (Na et al., 2005).Consequently, the ratio of OC and EC concentration intercept to the average indoor OC and EC concentration represents the contribution of indoor sources to the measured indoor concentrations (0.31 and 0.25 respectively).
OC/EC ratio is commonly used to estimate the extent of secondary organic aerosol formation as well as to indicate the dominant fuel type for primary OC emissions.In our study, OC/EC ratio for outdoor PM 10 ranged from 3.45 to 10.6 (average = 6.70), implying the wood combustion origin of particles (Salameh et al., 2014).Our values are comparable with those (8.70-13.3)determined in a study conducted in Thessaloniki city during winters 2012 and 2013, where extensive domestic fireplace use also occurred (Saffari et al., 2013).Another common finding between the two studies is that OC/EC presented elevated levels in the evening hours compared to morning ones, signifying the more intense biomass burning in the evening period due to increased residential activities.Correspondingly, OC/EC ratio for indoor PM 10 ranged between 0.92 and 16.5 (average = 5.9), implying biomass burning contribution to indoor particles In terms of mass, the total carbon (TC) is calculated as the sum of organic matter (OM) -which is equal to OC value multiplied by a factor of 1.7 (Caseiro et al., 2009) plus EC value.OM percentage to TC for PM 10 accounted for 87% and 92% for indoor and outdoor air respectively.This percentage was noticed to be slightly elevated during period A (88% indoors; 92% outdoors) compared to period B (85% indoors; 89% outdoors).
It is known that the emissions of biomass burning contribute significantly to atmospheric concentrations of K + and carbonaceous species, such as OC and EC (Zhu et al., 2012).Similarly, fossil fuel and vehicle emissions are also the dominant sources of OC and EC, though, K + concentration is minor.Therefore, K + /OC and K + /EC ratios can be used to characterize the emission sources.In the present study, the K + /OC ratio for PM 10 ranged between 0.01-0.09for indoor air and 0.02-0.07for outdoor air.Corresponding values from previous similar studies on fireplace emissions are not available; however, the ratios found in our study are comparable to those of previous studies for wood and agricultural burning exhibiting values in the range of 0.04-0.13(Echalar et al., 1995;Andreae et al., 2001).Similarly, K + /EC ratio for PM 10 was found to range between 0.04-0.49indoors and 0.01-0.53outdoors, being close to or within the range of 0.20-0.69,reported from literature for biomass burning aerosols (Andreae et al., 1983;Ram et al., 2010).

PM 2.5
The increased evening-time wood and biomass burning during period B is depicted in OC, EC levels for fine particle fraction, too (Table 2).This was also observed by Saffari et al. (2013) who studied the increased biomass burning effect for the city of Thessaloniki.As reported, night-time chemical reactions (e.g., oxidation of organic species by the nitrate radical) might potentially contribute to the OC concentration measured in the evening period.The comparable morning and evening EC levels obtained were attributed to the fact that elemental carbon can be derived from vehicular emissions as well as biomass burning and fuel combustion.As they concluded, the dominant effect of morning-time vehicular emissions may have been counterbalanced by the increased evening-time residential heating activities.In our study, outdoor EC levels were significantly elevated during afternoon and evening possibly due to the fact that significant biomass burning emissions were added to traffic (Diapouli et al., 2013).Similarly to PM 10 , the average value as well as the range of I/O ratio for EC for PM 2.5 (1.18; 0.62-14.6)was higher compared to those for OC.
OC/EC ratio for PM 2.5 ranged between 2.11 and 10.2 outdoors and 0.43 and 13.0 indoors.No remarkable differences are noticed for OC/EC ratio between periods A and B or between indoors and outdoors.Correlation analysis showed positive correlation between indoor and outdoor OC and EC (r = 0.81 and r = 0.75 respectively) as shown in Figs.S1(c) and S1(d).The ratio of the OC concentration intercept to the average indoor OC concentration is 0.42, higher than the corresponding for PM 10 .The same ratio for EC is 0.26, quite similar to that of PM 10 .
OM percentage to TC for PM 2.5 accounted for 85% and 90% for indoor and outdoor air respectively.This percentage did not demonstrate significant differences between the two periods of the day (period B: 87% indoors; 90% outdoors.period A: 83% indoors; 89% outdoors).These values are in agreement to those found in previous studies, attributing the high contribution of OM to TC to biomass burning (Zhu et al., 2012).It is remarkable that during campaign 3, when the phenomenon of intense fireplace use had been eliminated, OM percentage in TC was significantly decreased (66%).

Ions PM 10
Among the measured ions, the most abundant in the outdoor environment were SO 4 2-(29%) and NO 3 -(19%) followed by Ca 2+ , PO 4 3-, Na + (10%).The same order and similar percentages are obtained for indoor air: SO 4 2-(29%) were followed by NO 3 -, Ca 2+ , PO 4 3-, Na + , (12-14%), indicating sources of common origin.SO 4 2-, NO 3 and Ca 2+ dominance in outdoor PM 10 fraction, amounting for similar percentages, has also been reported by Theodosi et al. (2011) who studied the ionic particle composition at an urban and suburban area of Athens.Furthermore, Cl -, NO 3 -and Mg 2+ levels found in PM 10 fraction in the present study are in agreement with those reported in previous studies conducted in Athenian suburban areas (Theodosi et al., 2011;Eleftheriadis et al., 2014).The maximum SO 4 2-, NO 3 -and K + values (11.3, 8.45 and 1.77 µg m -3 respectively) for the outdoor air were observed in the afternoon and early night hours of December, 25 th , when extensive fireplace activity was recorded in the area.Although indoor concentration for the measured ions did not present remarkable differences between the two periods, outdoor average concentration was elevated during period B for K + and Cl -(by 47%), Ca 2+ , Na + , NO 3 -and SO 4 2-(by 20-34%).In general, NO 3 -and SO 4 2-as well as ΝΗ 4 + presented comparable morning and evening concentrations.
As mentioned in Saffari et al. (2013), this trend is most likely connected with the regional secondary origin of the aforementioned ions, since they constitute components of aged atmospheric aerosol and are mostly related to largescale regional transport processes rather than local primary emissions (Ricard et al., 2002;Eleftheriadis et al., 2014 and references within).Though, it should be noted that differences in particle chemical composition exist even between ambientgenerated particles that have infiltrated indoors and their corresponding ambient particles.As Diapouli et al. (2013b) reports, this may be attributed to the physical loss mechanisms influencing the infiltration of particles of different sizes, as well as chemical transformations affecting specific PM constituents, such as changes in gas-to-particle partitioning during the infiltration of organic compounds, NO 3 -or ΝΗ 4 + (Hering et al., 2007;Lunden et al., 2008).The ionic balance, as mole equivalence, can be a useful tool to determine possible missing ionic species (Table S2).It is expressed by the ratio of the equivalent cation sum (sum of NH 4 + , K + , Na + , Mg 2+ , Ca 2+ in neq m -3 ) to the equivalent anion sum (the sum of NO 3 -, Cl -, SO 4 2-, PO 4 3in neq m -3 ).The alkaline character of aerosols (excess of positive charge) indicate that CO 3 2-is most probably the missing anion which has not been measured using IC and is expected to associate with Ca 2+ .On the other hand, the acidic character (excess of anion charge) is attributed to the presence of H + (not measured) mainly associated with SO 4 2- (Kocak et al., 2007).In the current study, the ratio of the equivalent cation to anion sum is higher than unity (1.61 indoors, 1.15 outdoors) indicating the alkaline character of aerosols in both fractions and periods.

PM 2.5
Among the measured ions, the most abundant in outdoor PM 2.5 fraction were SO 4 2-(32%), Na + (17%) and NO 3 -(14%) followed by Ca 2+ (11%) and PO 4 3-(8%).In the indoor environment, Na + (24%), SO 42-(19%) and PO 4 3-(15%) present the highest contribution, followed by NO 3 -(11%), and Ca 2+ (12%).Also in this case, NO 3 -, SO 4 2-and NH 4 + presented comparable morning and evening concentrations, indicating their regional secondary origin (Saffari et al., 2013).A relatively high percentage of both indoor and outdoor SO 4 2-in fine particle fraction has been observed in similar studies conducted in Athenian apartments (Saraga et al., 2010;Seleventi et al., 2012).Literature studies refer to sulphate ion's origin from external penetration rather than indoor sources assuming that indoor SO 4 -2 originates from outdoor SO 2 emissions (vehicle exhausts, combustions, domestic central heating) converted to (NH 4 ) 2 SO 4 and (NH 4 )HSO 4 in indoor air (Chan et al., 1994;Li, 1994;Mouratidou et al., 2004).Outdoor origin is also attributed to indoor NO 3 -, although during transfer of particles indoors, removal of volatile and semi-volatile compounds (such as NH 4 NO 3 , which is known to contribute significantly to ambient PM 2.5 mass) occurs, decreasing the infiltration efficiency (Polidori et al., 2007;Diapouli et al., 2013a).Between the two periods, remarkable difference was noticed only for the outdoor K + levels which were found, on average, elevated during period B (by 32%).This is expected as K + is the major electrolyte in cell cytoplasm, which is released in large amounts of K-rich particulates in the submicron size fraction.Tsai et al. (2012) who investigated biomassburning aerosols found that potassium ions were dominant in the fine particles (0.18-1.8 µm) due to burning of vegetative material.Finally, similarly to PM 10 , PM 2.5 present an alkaline character (Cneq/Aneq = 2.23 indoors, 1.45 outdoors), for both periods of campaign 2 as well as for campaign 3 (Cneq/Aneq = 1.12).

Metals
Chemical analysis of seven heavy metals (Cu, Pb, Cr, Ni, Cd, Zn and Fe) was performed for PM 10 samples, Table S3.Among them, Fe strongly dominates in both indoor and outdoor air (81% and 82% respectively), followed by Zn (12% both indoors and outdoors) and the other metals analyzed, with contribution lower than 3%, indicating the similar elemental consistency of indoor and outdoor particles.Fe is classified among mineral components, Cu among traffic-related components (brake pad abrasion tracers), Ni among heavy fuel-oil combustion components (mostly from shipping emissions) and Cd, Zn, Cr and Pb among industrial elements (Rivas et al., 2014).Among the metals studied, Ni and Cd are regulated through WHO guidelines (WHO, 2000) with a maximum value of 1000 ng m -3 for Ni and 5.0 ng m -3 for Cd.Pb is regulated by National Ambient Air Quality Standards with a value up to 1.5 ng m -3 (US EPA, 1997) and by the 1999/30/EC European legislation directive with an annual limit value of 500 ng m -3 .Due to their negative impact on human health, the 2004/107/EC directive establishes the annual limits of 20 and 5.0 ng m -3 for Ni and Cd respectively, in the PM 10 fraction.Considering the toxic trace metals' indoor and outdoor concentrations measured in the present study, the limit values established by US-EPA, WHO and EC were not exceeded.
In order to define the most likely sources of indoor particles, enrichment factors were calculated for individual elements in terms of the average elemental composition of the upper continental crust.Despite the fact that there is no specific rule for the reference element choice, Si, Al and Fe are the most common elements used for this purpose.For each one of the examined metals, Fe was used as reference, assuming minor contributions of Fe as a potential pollutant and considering the upper continental crustal composition given by Rudnick and Gao (2003).The enrichment factor (EF) of an element E is defined according to equation 1: where E and R represent the concentrations of the examined and the reference element, respectively.If EF approaches 1, the earth's crust is the predominant source of the examined element.Operationally, given the local variation in soil composition, EF > 5 suggests that non-crustal sources contribute a significant fraction of the element (Gao et al., 2002).As the EF value increases, the contribution from noncrustal sources increases as well.Pb, Cr and Ni exhibited EFs close to 10 (respectively equal to 14.1, 12.1 and 6.2), indicating that these metals were mostly derived from resuspension of dust due to indoor human activities, as already demonstrated by the comparison of I/O ratios.For the enriched elements like Cd, Zn and Cu (EF 111.2,88.7 and 49.7,respectively) an anthropogenic origin might be suggested.As these sources occur outdoors, the high EFs calculated for the indoor environment suggest that air infiltration affects indoor air quality.

Indoor to Outdoor Relationship
The indoor to outdoor concentration ratio (I/O) indicates the relative strength of influence between indoor to outdoor sources on indoor air.Indoor PM levels can be affected either by particles penetration from outdoors into the building through ventilation (Halios et al., 2013) or by indoor emissions.In terms of mass, I/O ratio is observed to be higher for PM 2.5 compared to PM 10 (in average 0.97 and 0.80 respectively), while a different behavior during the day between the two fractions is observed (Fig. 2).Particularly for PM 10 , the average I/O ratio value and the number of cases this ratio exceeding unity were higher during period B, as opposed to PM 2.5 , where higher values were observed during night and early morning hours (period A).This can be attributed to the fact that fine particles tend to accumulate indoors during night (Diapouli et al., 2011;2013a) as well as because coarse particles re-suspension due to human activities occurred mainly during afternoon and early evening.In general, indoor and outdoor PM concentrations followed a similar variation pattern excluding days when the apartment's fireplace was in use.Pearson correlation coefficient r between indoor and outdoor PM 10 and PM 2.5 was found to be 0.30 and 0.37 respectively (p < 0.05) (all days included) but when excluding days with active fireplace indoors, r increases to 0.62 and 0.51 (p < 0.05), implying outdoor sources' contribution to the indoor particle levels.
Indoor to outdoor (I/O) ratio for the carbonaceous PM components is expected to depict the biomass burning source effect on both indoor and outdoor levels.I/O ratio for OC in PM 10 (range: 0.41 to 5.82, average 1.09) and PM 2.5 (range: 0.55 to 2.80, average 1.28) had its highest values during early evening hours due to extensive fireplace use or during late night/morning periods, after days with indoor fireplace activity.The average I/O values for EC in PM 10 and PM 2.5 (2.02 ± 4.24; 1.18 ± 1.96) were found to be similar or higher compared to those for OC.Finally, by comparing the two periods of the day, I/O ratio for OC for both PM 10 and PM 2.5 fractions was noticed to be higher during period A (Table 2), possibly because of the lower temperature in the house during night and morning hours, which favors the gas to particle conversion of organic carbon (Demerjian and Mohnen, 2008).On the contrary, I/O ratio for EC in PM 10 was observed to be higher during period B of the day when fireplace was in use leading to elemental carbon production; the corresponding ratio for PM 2.5 followed similar 12h variability.The prevailing outdoor atmosphere's contribution in ionic PM composition is depicted by SO 4 2indoor to outdoor ratio value which was quite lower than unity (0.70 ± 0.21 for PM 10 and 0.76 ± 0.42 for PM 2.5 ) indicating the penetration of outdoor air inside.In the cases of Ca 2+ , Na + , Ca 2+ , K + and Mg 2+ , the I/O ratio was close or above unity (Fig. 3).The strong contribution of outdoor sources in indoor levels is also signified by the high indoor to outdoor correlation for SO 4 2-(r = 0.79 for PM 10 ; 0.87 for PM 2.5 , p < 0.05) and NO 3 -(r = 0.83 for PM 10 ; 0.82 for PM 2.5 , p < 0.05).Positive indoor to outdoor correlation was also observed for Cl -although being stronger for fine fraction (r = 0.57 for PM 10 and r = 0.86 for PM 2.5 ).
Indoor to outdoor relationship for the elemental components of PM shows both indoor and outdoor anthropogenic activities contribution.In particular, Pb, Zn and Cd show a moderate correlation between indoor and outdoor concentrations (r ranging from 0.60 to 0.66, p < 0.05), suggesting that, regardless of the strong effects of indoor sources, a fraction of their indoor concentrations may be attributed to infiltration of outdoor particles containing these metals.Similar conclusions may be drawn for Fe, for which a lower correlation (r = 0.52, p < 0.05) was calculated.The considerably higher Pb, Cr and Fe indoor-to-outdoor concentration ratios (difference in I/O ratios higher than 0.3) calculated for the sampling period A (late night and morning hours) compared to those of the sampling period   S3) are attributed to several domestic processes carried out mainly during morning hours, such as cleaning, dusting, washing and vacuuming, affecting the coarse particle concentration.Resuspension of particles during indoor activities has been found to be an important factor influencing the indoor concentrations mainly of coarse particles in tracked areas (Thatcher and Layton, 1995).Other investigators have also reported I/O ratios higher than those expected, pointing out the importance of non-apparent indoor sources (Wallace, 1996).It is stated by Geller et al. (2002) that coarse particles, which are characterized by lower infiltration rates as well as higher deposition velocities (Abt et al., 2000), settle inside and do not become re-suspended except transiently, so indoor concentrations of the associated metals are expected to be lower than those outdoors.Metals found in the coarse particles deriving from fireplaces do not seem therefore to contribute significantly to indoor metal concentrations.Comparing the I/O ratios corresponding to the afternoon and early evening hours (period B) it is pointed out that in relation to Fe (which constitutes a mineral component, I/O = 0.97) only Cu and Zn are characterized by significantly high ratios (1.54 and 1.29, respectively).Our observations point to the type of wood used or possibly the remaining ash suspended indoors is responsible for this enrichment in Cu and Zn.It has been shown that wood burning contributes to fine particles emissions with significant metal content of the fireplace smoke, since trees absorb these elements from water, soil or agricultural practices (Schmidl et al., 2008;Goncalves et al., 2010).

CONCLUSIONS
The present work aims at studying the indoor and outdoor air quality of an apartment during a period of extensive fireplace use in a densely populated city, Athens.PM 10 and PM 2.5 indoor and outdoor gravimetric measurements and chemical analysis were conducted for two periods per day: a period characterized by absent or reduced fireplace activity in the area and a period with intense fireplace activity in the area.
The results highlighted the effect of biomass burning on PM mass concentration in the atmosphere of the area as well as the resulting influence on the indoor levels.On average, outdoor PM 10 and PM 2.5 concentration levels were observed to be (up to two times) higher during the hours of intense fireplace activity in the area (afternoon and early night hours).The apartment's fireplace use contributed to indoor PM levels but even while not in use, the indoor air was substantially influenced by the burden outdoor atmosphere.OC to EC ratio as well as K + to EC ratio for both (indoor and outdoor) particle fractions revealed their origin from biomass burning.I/O ratio for OC and EC presented different behavior between the two 12 h periods of the day, due to processes controling the primary and secondary aerosol production indoors.The high contribution (> 85%) of organic matter (OM) percentage to total carbon (TC) for both particle fractions highlights the effect of extensive fireplace use on outdoor and indoor aerosol composition.This conclusion is verified by the fact that during the last stage of the campaign, when the intense fireplace use had been substantially eliminated, OM percentage in TC was significantly decreased (66%).
The ionic composition of indoor and outdoor particles reveals their common origin and depicts the suburban atmospheric aerosol character of the area.The relatively high percentage of indoor SO 4 2-is attributed to external penetration (vehicle exhausts, combustions, domestic central heating) rather than indoor sources, while its relatively low I/O ratio indicates the reduced penetration of outdoor air inside.Outdoor origin is also attributed to indoor NO 3 -, although after transfer of nitrate salts indoors, decomposition of nitrate compounds to their gaseous precursors occurs, decreasing the indoor concentration.
Finally, the limit values for toxic trace metals established by US-EPA, WHO and EC were not exceeded in the present study.The high enrichment factors calculated for the indoor environment indicated dust resuspension and anthropogenic activities sources contribution, while highlighting the significant infiltration rates, especially during periods of intensive use of fireplaces.
Overall, the results of this study indicate that domestic wood burning has an effect on PM mass concentration and chemical composition in the atmosphere of an urban area, as well as on the residential air quality.Therefore, air quality control strategies should be addressed by the public authorities, especially in periods of massive fireplace use in large cities.

DISCLAIMER
Reference to any companies or specific commercial products does not constitute financial and personal conflict of interests.

Fig. 2 .
Fig. 2. Time variation of PM 10 (Campaign 1) and PM 2.5 (Campaign 2) mass concentration.Days with fireplace activity in the apartment are marked with a cycle.Missing values are due to instrumentation failure.

Fig. 3 .
Fig. 3. Average indoor to outdoor ratio values for PM mass and main components concentration.

Table 1 .
Statistics for PM 10 and PM 2.5 mass concentration for the indoor and outdoor air and indoor to outdoor ratio (I/O) values.

Table 2 .
OC and EC average concentration, I/O and OC/EC during the campaigns.85% of the samples within the range of 3-70), either by the apartment's fireplace emissions or by the aggravated atmosphere of the area.When calculated only for the days with the apartment's fireplace use, OC/EC ratio is slightly lower (4.8),possibly due to the predominance of EC production.