Source attribution of carbonaceous fraction of particulate matter in the urban atmosphere based on chemical and carbon isotope composition

Air quality is of large concern in the city of Krakow, southern Poland. A comprehensive study was launched by us in which two PM fractions (PM1 and PM10) were sampled during 1-year campaign, lasting from April 21, 2018 to March 19, 2019. A suite of modern analytical methods was used to characterize the chemical composition of the collected samples. The contents of 14 sugars, sugar alcohols and anhydrosugars, 16 polycyclic aromatic hydrocarbons, selected metals and non-metals and ions were analyzed, in addition to organic and elemental carbon content. The carbon isotope composition in both analysed PM fractions, combined with an isotope-mass balance method, allowed to distinguish three main components of carbonaceous emissions in the city: (1) emissions related to combustion of hard coal, (2) emissions related to road transport, and (3) biogenic emissions. The heating season emissions from coal combustion had the biggest contribution to the reservoir of carbonaceous aerosols in the PM10 fraction (44%) and, together with the biogenic emission, they were the biggest contributors to the PM1 fraction (41% and 44%, respectively). In the non-heating season, the dominant source of carbon in PM10 and PM1 fraction were the biogenic emissions (48 and 54%, respectively).

The general population is increasingly aware of the dangers associated with breathing the air in which they live.It has been proven that exposure to poor quality air can lead to numerous health problems [1][2][3][4][5][6][7] .Krakow is the 2nd largest city in Poland, with almost 1 million inhabitants.The levels of PM 2.5 and PM 10 in the urban atmosphere of Krakow, in contrast to other large European cities, are regularly exceeding the air quality standards set up by the World Health Organization 8 , mainly in the autumn and winter season.To improve the air quality in the city, the Krakow City Council introduced a total ban on solid fuels combustion from September 1st, 2019 9 .The key objective from the perspective of city residents and policymakers is to identify the sources of particulate matter emissions and their contribution to the overall pollution load of the local atmosphere.There are several approaches aimed at source allocation of suspended particulate matter based on the statistical analysis of the chemical and elemental composition of the given PM fraction.1][12][13] ).
The carbonaceous fraction of particulate matter, besides inorganic carbon (carbonate), mostly consist of organic (OC) and elemental (EC) carbon 14 .EC is considered as a primary contaminant.It is usually emitted in incomplete combustion processes of different materials (biomass, coal, oil, petrol, etc.).Organic carbon is a mixture of different organic compounds with various functional groups incl.polycyclic aromatic hydrocarbons

Carbon isotope analyses and isotope-mass balance approach
The analysed filters were aggregated into sets representing approximately monthly periods, to obtain a sufficient amount of carbon for isotope analyses (> 1 mg).The obtained results were then assigned to the heating or nonheating season.Aggregation periods and the corresponding weighted averages of the total carbon present in the aggregated samples representing both fractions are presented in Table 1S (Supplementary Information).The total carbon (TC) reservoir in the analysed aggregated samples was transformed into gaseous form (CO 2 ) which was later subjected to cryogenic purification.The method is presented in detail in 38,47 .The 13 C/ 12 C ratios of the obtained carbon dioxide were quantified using isotope ratio mass spectrometry (IRMS).The radiocarbon content was measured with accelerator mass spectrometry (AMS) on the 1.5 SDH-Pelletron Model Compact Carbon AMS 48 .The accelerator mass spectrometry analyses were done after the IRMS analysis (CO 2 was recovered after isotope ratio mass spectrometry).
The measured 13 C/ 12 C ratios are expressed as δ values defined as per mille deviations from the internationally accepted reference standard 49,50 , whereas the radiocarbon contents in the analysed samples are expressed as percent of modern carbon (pMC) defined as [51][52][53] : where 14 R SN and 14 R ON are 14 C/ 12 C isotope ratios measured in the analysed samples and the reference standard (Oxalic Acid II), respectively.The measured isotope ratios were then used to calculate the contribution of predefined emission sources of carbon using isotope-mass balance approach 38 : where: F bio ,F coal, F traff stand for mass fractions of carbon originating from biogenic emissions, coal combustion and road transport emissions, respectively.δ 13 C TC is the 13 C isotope composition of the measured PM 1 or PM 10 samples.δ 13 C bio, coal, traff stands for the assigned 13 C isotope signatures of biogenic emissions, coal combustion and road transport emissions, respectively.pMC TC signifies percent of Modern Carbon of the analysed PM 1 or PM 10 samples, whereas pMC bio, coal, traff stands for assigned percent of modern carbon values for biogenic emissions, coal combustion and road transport emissions, respectively.The mass fractions (F bio , F coal, F traff ) of the total carbon reservoir present in the monthly composite samples of PM 1 and PM 10 can be calculated from Eqs. (2-4) above, provided that all other variables are either measured or derived from other sources.
A first step in calculating the contributions of the pre-defined emission sources of carbon (biogenic emissions, coal combustion and road transport) to the overall carbon budget of the analysed PM fractions of particulate matter is the appropriate assessment of 13 C and 14 C isotope signatures of those sources.For example, the δ 13 C coal values related to coal combustion emission can differ significantly on a global scale, depending on the geographical origin and age of the burned coal (from − 30.1 to − 23.3‰, as reported in Ref. 54 ).The δ 13 C coal values of hard coal mined in Poland were thoroughly characterized.They are in a relatively narrow range, from − 24.5 to − 23.3‰ 55 .The δ 13 C coal value of − 23.3‰ was chosen for this work.The δ 13 C traff values reported for road transport emissions have the range between − 28.3 and − 24.5‰ 38,54 .For this study, the average δ 13 C traff value equal − 27.6‰, derived from the data reported in 54 was adopted.This value is comparable with the results obtained by the authors in road tunnel in Krakow, where this signature was in the range from − 27.1 to − 27.8‰, depending (2) on the PM fraction analysed 56 .The emissions of carbon associated with the road transport are mainly related to combustion of liquid fuels, but also to the wear of car tires and asphalt pavement.
If only petroleum would be considered, which is a fossil fuel devoid of radiocarbon, the pMC traff value of the particles associated with combustion of such fuel would be zero, as is the case with hard coal combustion (pMC coal ).However, considering additional factors included in road emissions, such as the use of natural rubber in the automotive industry to produce car tires (approx.15-20%), as well as the use of biofuels (the regulations in force in Poland and the European Union state that at least 10% of the petrol must be a biocomponent, for diesel fuel it is 7% 57 ), it was assumed that 14 C content in the carbon emissions from road transport is constant and equal 10 pMC 23 .
The 'biogenic emissions' category includes pollen/plant spores (emitted mainly during the growing season) and secondary organic aerosol (SOA), as well as wood and biomass combustion, which is mostly present during colder months of a year.According to previous studies 38 , the δ 13 C bio values range in Krakow from − 28 to − 24‰.Constant δ 13 C bio value equal − 25‰ was adopted in this work.Due to the fact that pollen and plant spores, as well as volatile organic compounds (VOCs) emitted by living plants should have the 14 C contents resembling those in the local atmosphere (after accounting for isotope fractionation steps involved in the photosynthesis process), we adopted in the isotope-mass balance calculations the constant pMC bio value equal 105% for nonheating season.This choice was guided by the fact that radiocarbon levels in the local atmosphere (atmospheric CO 2 ) fluctuate nowadays between 100 and 117% (unpublished data).For heating season, a slightly higher pMC value equal 115% was adopted due to the fact that burning of wood is the major source of carbonaceous aerosols in the city atmosphere during this time period of a year and the mean age of wood (time elapsed from its time of formation) is in the order of 10-15 years.At that time period the pMC levels in the city atmosphere were in the order of 115-120% 58 .
Due to the fact that carbon is present also in the mineral fraction of the analysed PM 1 and PM 10 fractions of suspended particulate matter, appropriate corrections of the measured δ 13 C TC and pMC TC values need to be applied to account for the presence of calcium carbonate.Details of the correction procedure are presented in 39.The corrections derived for 19 analysed samples of PM 1 and PM 10 fractions resulted in 0.3% and 0.2‰ for pMC TC and δ 13 C TC , respectively.As they are comparable with the measurement uncertainties of isotope analyses (ca.0.2-0.3% for AMS and 0.1-0.2‰for IRMS), no adjustment of the measured δ 13 C TC and pMC TC values was made. ) and cations (Na + , K + , Mg 2+ , Ca 2+ , NH 4 + ) concentrations were analysed with isocratic ion chromatography (IC) performed with ICS-1100 instrument (Thermo Scientific), equipped with Thermo Scientific (Dionex) AS-DV autosampler and ion-exchange columns.The details of the method are presented in the Supplementary Information.
The high-performance anion-exchange chromatography with pulsed amperometric detection (HPAE-PAD) was used to analyse 14 sugars, sugar alcohols and anhydrosugars including: inositol, erythritol, xylitol, levoglucosan, arabitol, mannosan, trehalose, mannitol, galactosan, glucose, fructose, galactose, cellobiose and sucrose.Altogether 81 samples (with a diameter of 10 mm) of each fraction (PM 1 and PM 10 ) were extracted in 3 ml of Milli-Q water, ultrasonicated for 30 min, and centrifuged at 4000 rpm for 10 min.The extracts were analyzed with a Dionex™ ICS3000 (Thermo Scientific™), equipped with a CarboPac™ MA1 column, utilizing a sodium hydroxide gradient of 480-650 mM and a flow rate of 0.4 ml•min −1 .The method is described in detail in Ref. 60 .The limit of detection (LOD) was determined as the minimum concentration that was visible on the chromatogram and produced a peak height at least three times the signal to noise ratio.The LOD values were in the range of 0.004-0.05µg•m −3 .
The elemental analysis of the filter material comprised 17 elements: Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Cu, Br, Rb, Sr, As and Pb.The concentrations of the measured elements were quantified by the energy dispersive X-ray fluorescence (EDXRF) method.Details of elemental analysis are presented in the Supplementary Information.Detection limits and detailed description of the method is presented elsewhere 12,20 .

Chemical mass closure method
The mass closure method is a semi-quantitative method based on comparison of the gravimetrically determined mass concentration of a TSP (or each fraction of PM), with the sum of the measured concentrations of individual components of TSP or chosen PM fraction, respectively 61 .The mass closure method is useful to perform preliminary qualitative analysis of possible emission sources of particulate matter based on temporal and spatial variability of chemical composition of the analysed PM fraction 10,[61][62][63][64][65][66][67][68][69] .The reconstructed chemical composition of particulate matter usually consists of about seven representative chemical components.Frequently considered components are as follows: the secondary inorganic ions, elemental carbon, organic matter, crustal matter (also

Carbonaceous fraction characteristics, concentrations of polycyclic aromatic hydrocarbons and carbohydrates
The PM samples collected in the framework of this study were characterized by a high content of organic carbon in total carbon reservoir (from 75 to 88%, depending on the considered PM fraction and the month, as determined by the thermal-optical method).PM weight average during each month of the campaign is presented in Table 1.For PM 1 it was in the range of 17-43.1 µg•m −3 and for PM 10 in the range of 38.3-107.4µg•m −3 , respectively.As expected, higher PM averages were noticed during months representing heating season in Krakow.
The analysed sugars and PAHs constituted from 0.5 to 4% of the OC concentration depending on the month.The highest share (4%) was determined in November 2018 for the PM 10 fraction, while the lowest (0.5%) in October 2018 for the PM 1 fraction.In the same month (October 2018) the ratio OC/TC was also the lowest in the entire measurement period (75%).Depending on the month, from 1 to 4 saccharides and from 1 to 14 PAHs were identified in the PM 1 fraction, to be compared with 3-8 saccharides and from 8 to 15 PAHs for PM 10 fraction (cf.Table 1).The largest number of individual types of organic compounds identified in both fractions was for the period from November 2018 to February 2019, i.e., during the heating season in Krakow.
The results of carbohydrates and polycyclic aromatic hydrocarbons analyses are summarized in Tables 5S and  6S (Supplementary Information).The PAHs results are presented in Fig. 2a-d.Levoglucosan is presented in Fig. 3 as the only carbohydrate detected in the vast majority of samples (74 out of 81 for the PM 1 fraction and 78 out of 81 for the PM 10 fraction) among the 14 analysed.As seen in Figs. 2 and 3, the concentrations of all measured PAHs and levoglucosan are higher during the heating season in Krakow (autumn and winter), which is characterized by intensified combustion of coal and wood for heating purposes.Among sixteen analysed PAHs, the highest maximum concentrations in aggregated samples were recorded for Fluoranthene, Pyrene and Benzo  4.85 ng m −3 , 4.38 ng m −3 , 5.35 ng m −3 ) in PM 1 (aggregated dates: 17.12, 21.12, 25.12 and 29.12.2018).The maximum concentration of levoglucosan was 0.9 μg m −3 (in PM 1 fraction) and 1.95 μg m −3 (in PM 10 fraction), both values measured on December 17, 2018.
The observed average concentrations of PAHs in the PM 10 fraction are approximately 4 times lower than those observed in Krakow in winter 2014 70 .This may indicate an improvement in air quality over a period of ca. 5 years in Krakow.PAHs average concentrations determined in this study are usually lower than those recorded in other European cities in previous years (the exception is the result from Athens) [71][72][73] .The results from this study for PM 1 fraction are similar to those obtained in Brazil 74 .This study was similar in terms of the duration.Table 6S (Supplementary Information) shows the comparison of average concentrations of PAHs obtained in this study and those reported by other research teams for different areas around the world.

Source attribution of carbonaceous aerosols based on isotope-mass balance approach
The numerical results of δ 13 C TC and pMC TC analyses of the aggregated monthly samples and the data used in isotope-mass balance equations are shown in Fig. 1S and presented in Table 7S, respectively (Supplementary Information).
The measured δ 13 C TC values ranged from − 26.1‰ (April 2018) to − 24.2‰ (December 2018) for the PM 1 fraction and from − 25.5‰ (October 2018) to − 24.2‰ (February 2019) for the PM 10 fraction.Higher δ 13 C TC values were recorded in both fractions in the period from December 2018 to March 2019.Lower values of δ 13 C TC indicate the dominance of biogenic sources while higher values indicate enhanced contribution of hard coal combustion.The measured radiocarbon content in the aggregated samples, expressed in pMC, ranged from 37.8% (December 2018) to 57.9% (July 2018) for the PM 10 fraction, and for PM 1 from 46.5% (December 2018) to 62.4% (April 2018).As expected, lower levels of radiocarbon were recorded in cooler months, reflecting enhanced emissions of carbonaceous aerosols devoid of radiocarbon, associated with combustion of coal in the city for heating purposes.Figure 4 shows scatter plots illustrating mutual relations between the isotope parameters (δ 13 C TC and pMC TC ) measured in both analysed PM fractions, for the 8 months for which both isotope parameters were measured (see list in Table 1S in Supplementary Information).
As seen in Fig. 4a, the data points cluster below the 1:1 line, indicating generally higher level of radiocarbon content in PM 1 fraction when compared to the PM 10 fraction which can be linked to higher contribution of biogenic carbonaceous aerosols characterized by high pMC values in the fine fraction (PM 1 ), or higher contribution of carbonaceous aerosols associated with burning of fossil fuels devoid of radiocarbon, accumulating in the PM 10 fraction.On the other hand, in Fig. 4b δ 13 C TC values do not provide clear seasonal information, compared to the pMC results.
The set of isotope-mass balance equations (Eqs.2-4) was solved for F bio , F coal, F traff mass fractions, separately for heating and non-heating season and for both PM fractions.Heating season contained 6 aggregated monthly samples (for both fractions of PM), while non-heating season 3-in case of PM 10 and 4-in case of PM 1 (see list in Table 1S in Supplementary Information).The results are presented in Fig. 5a,b.During the non-heating season, the biogenic emissions were a dominant carbon source in the PM 1 fraction with a share of 54%.These emissions are related to the combustion of biomass, as well as to emissions from the biosphere (emissions of pollen, spores and VOCs and subsequent oxidation during the vegetation period).The results of the analyses of the sources of organic aerosols contained in the PM 1 fraction, carried out in Krakow with the use of the Q-ACSM (quadrupole aerosol chemical speciation monitor), showed that the presence of sources related to biomass combustion in the Vol:.(1234567890 13 .This would point to a dominant role of living biosphere in the biogenic emissions of carbon during spring and summer season.Emissions of carbon associated with road transport were second in the importance (approx.32%).The lowest share was attributed to emissions related to coal combustion (ca.14%).Partitioning of the emission sources of carbon present in the PM 1 fraction varies substantially during the heating season (autumn and winter).The biogenic emissions, which are mostly associated during this period  with biomass combustion, decrease to about 44%.The share of emissions related to coal combustion increases to approximately 41% during the heating season, while the share of emissions associated with the road transportation drops to ca. 15%.However, the lower emission shares obtained for road transportation in the colder seasons do not indicate a reduction car traffic in Krakow in autumn and winter.Rather, they result from an increase in the carbon reservoir in the PM 1 fraction associated with dominant emissions from the biomass and coal combustion for heating purposes in the city during this period (average concentration of PM 1 increases from 11.9 to 19.0 μg m −3 , to be compared with an increase from 34.0 to 47.2 μg m −3 for PM 10 fraction).Similar relations between emission sources are observed for the PM 10 fraction of the suspended particulate matter.During the heating season, the mass fractions of carbon derived from the isotope-mass balance approach amounts to 44% (coal combustion), 37% (biogenic emissions) and 19% (road transport).For the non-heating season the biogenic emissions dominate, with approx.48%, while the share of carbon emissions associated with road transportation is around 24%.However, the calculated mass fractions of combustion of coal during the non-heating season appear unexpectedly high (approximately 28%), when compared to the PM 10 fraction in similar studies carried out in Krakow (cf.Table 2) (where the contribution of the coal combustion during summer ranges from 5 to 10%), and to the PM 1 fraction in this study (14%).Thus, the results from different studies are comparable to the results of PM 1   37,38   .However, the comparison between PM 10 collected during winter 2018/2019 season in urban background site and presented in 37 , as well as PM 10 from this study during heating-season 2018/2019 shows very similar values (discrepancy of up to 2%) (Table 2).
To sum up, the above-presented results show that during the heating season emissions from coal combustion had the biggest contribution to the PM 10 carbonaceous aerosol reservoir (44%), and together with the biogenic emissions they were the biggest contributors to PM 1 (41% and 44%, respectively).In the non-heating season, the dominant source of carbon were biogenic emissions (48% of PM 10 , 54% of PM 1 ).
Table 3 shows the comparison of the signatures used in the previous studies conducted in Krakow and the signatures used for this work.We can see that most of the signatures are comparable to each other (at least in terms of the assumed uncertainties), the only exception is δ 13 C traff , where the value assumed in previous studies  (− 30 ± 1‰) differs from that adopted for this publication (− 27.6‰), which might indicate one of the reasons for the differences between the results in previous researches conducted in Krakow and the results obtained in this work.
To assess the uncertainties associated with the contribution of three emission sources discussed above, the sensitivity analysis was used (see Fig. 2S, Supplementary Information).The maximum influence parameter was defined which is the ratio of the calculated change of the given component of the emissions derived from the isotope mass balance to the assumed range of the given parameters (pMC and 13 C value).The calculated biogenic component of the emissions was the most sensitive function of the assumed radiocarbon signature of this component.The maximum influence parameter was equal 0.4% per 1 pMC.The road transport component of the emissions appeared to be less sensitive to the assumed pMC signature of this component (Fig. 2S Supplementary Information).
For the 13 C isotope composition, the assumed isotopic signature of the coal combustion source in heating season has the highest influence on the calculated coal combustion contribution reaching up to 14.9% per 1‰, this translates to ca. 3% uncertainty of the derived component of the balance (assuming the measurement uncertainty of δ 13 C values 0.2‰).In conclusion, the sensitivity analysis showed that the uncertainty of the estimated carbon balance components for the analysed period is in the order of a few percent.Details of the method are presented in 38 .

Attribution of PM 1 and PM 10 sources based on mass closure method
The seasonal attribution of sources of the analysed PM 1 and PM 10 fractions, based on the chemical mass closure approach, is presented in Fig. 6a,b.
Organic matter constituted the largest share of all the components of the analysed samples of particulate matter.In PM 10 it was in the range from 27% (spring) to 57% (autumn), while in PM 1 from 44% (summer) to 64% (winter).Furthermore, the method showed seasonal variability of elemental carbon concentration: an increase from 3.1% in spring to 5.1% in winter for PM 10 and from 6.1% in spring to 6.8% in winter for PM 1 , with the highest percentage of EC for this fraction recorded in autumn (7.3%).For SIA, an increase was observed from 11% in spring to 19% in winter for PM 10 , with the highest share of SIA (22%) occurring in autumn.On the other hand, SIA within the PM 1 fraction increased from 15% in spring to 22% in winter.A noticeable increase in salt concentration (NaCl) was found during winter.In the case of PM 10 , the total concentration in the warm months (spring + summer) was 10 μg m −3 , while during the cold period (autumn + winter) it raised to 28 μg m −3 .In the case of PM 1 the seasonal differences are correspondingly smaller, the total NaCl concentration amounts to 6.1 μg m −3 (warm months) and 13 μg m −3 (cold months).The elevated concentrations of NaCl during cold season can be linked to the use of road salt to prevent icing on the streets.The percentage of trace elements varies slightly, depending on the fraction, from 0.2 to 0.4%.On the other hand, the percentage of crustal matter is higher in the warm season (spring + summer), compared to the cold season (autumn + winter).In the case of PM 1 , it is on average 2.1% (spring + summer) and 1.6% (autumn + winter), and in the case of PM 10 : 6.2% and 4.5%, respectively.This is probably related to soil resuspension processes active during spring and summer.Unidentified matter constituted from 1.5 to 27% in case of PM 1 , and from 8.0 to 52% in case of PM 10 .It is noticeable that for both fractions the biggest share of unidentified matter was observed during warm period (spring and summer).

Conclusions
This is the first 1-year study dedicated to the fine (PM 1 ) and coarse (PM 10 ) particulate matter, collected simultaneously from April 21, 2018 to March 19, 2019 in Krakow, southern Poland.The chemical analyses carried out (incl.ion chromatography (IC), anion exchange chromatography with pulsed amperometric detection (HPAE-PAD), thermal-optical OC/EC method, energy dispersive X-ray fluorescence (EDXRF) and gas chromatography-mass spectrometry (GC/MS) allowed to explore the chemical characteristics of both PM fractions.Other authors also tried to analyse these two fractions (PM 1 and PM 10 ) of particulate matter in the urban environment [75][76][77][78] .However, the number of analytical methods used was smaller.In addition, this is the first time when PM 1 fraction was used to provide isotope mass-balance in Poland.Application of the mass closure method, based on performed analyses, allowed to obtain seasonal variability of the chemical composition of two particulate matter fractions.The dominant revealed component was the organic matter (from 27 to 64% depending on the season of the year and the analysed fraction).The results from HPAE-PAD and GC/MS demonstrated that carbohydrates and PAHs had no significant share in organic carbon reservoir.The HPAE-PAD technique showed high concentrations of levoglucosan (marker of biomass combustion) during the heating-season.Its average concentrations for the entire measurement period were 0.16 µg•m −3 and 0.33 µg•m −3 for PM 1 and PM 10 fractions, respectively.Both of analyses (HPAE-PAD and GC/MS) provided valuable data on the presence of sugars and PAHs in the samples.
In this work, we focused on the analysis of the carbonaceous fraction, but further work on the obtained data is highly recommended and planned in the future.They may provide valuable information in aspects other than those discussed in this work (e.g. the use of levoglucosan as a marker of biomass combustion using other data processing methods, or health aspects in the case of PAHs).The application of the carbon isotope-mass balance method to identify the sources of PM 1 and PM 10 fractions of suspended particulate matter emissions allowed to distinguish three main emission sources: (1) the emissions from coal combustion, (2) the emissions related to road transport, and (3) the biogenic emissions.The results presented here clearly show that during the heating season emissions from coal combustion had the biggest contribution to the PM 10 carbonaceous aerosol reservoir (44%), and together with the biogenic emissions they were the biggest contributors to PM 1 (41% and 44%, respectively).In the non-heating season, the dominant source of carbon were biogenic emissions (48% of PM 10 , 54% of PM 1 ).Since the research was conducted just before the Krakow City Council introduced a total ban on combustion of solid fuels (starting from September 1, 2019), it would be recommended to conduct similar studies in the coming years to check whether the introduced regulations resulted in any noticeable improvement in air quality in Krakow.

Figure 1 .
Figure 1.Sampling site.The red arrows inside the figure indicate the exact location of the high-volume samplers in each picture included (sources: www.google.com/ maps, www.geopo rtal.gov.pl, authors' photographs, the picture was edited using MS Office tools).

Figure 2 .
Figure 2. (a-d) Seasonal variability of PAHs concentrations in PM 10 (a, b) and PM 1 (c, d) fractions during the study period.

Figure 3 .
Figure 3. Seasonal variability of levoglucosan concentrations in PM 10 and PM 1 fractions, with ambient air temperature during the study period shown in the background.

Figure 4 .
Figure 4. (a, b).Relationships between radiocarbon contents expressed in pMC (a) and δ 13 C TC values (b) measured in aggregated monthly PM 1 and PM 10 samples.Measurement uncertainties of the data points in (a) are of the size of symbols used.

Figure 5 .
Figure 5. (a, b).Seasonal variability of the mass fractions of carbon originating from biogenic emissions, from coal combustion and emissions related to road transport (F bio , F coal , F traff , respectively), present in the carbonaceous fraction of the analysed PM 10 (a) and PM 1 (b) samples collected in Krakow in the period April 21, 2018-March 19, 2019, derived from the isotope-mass balance approach (Eqs.2-4).

Figure 6 .
Figure 6.(a, b) Chemical mass closure results of PM 10 and PM 1 fractions for 4 different seasons, expressed in concentration units and percentages by weight (OM-organic matter, EC-elemental carbon, CM-soil/crustal matter, SIA-secondary inorganic ions, NaCl-salt, TE-trace elements, U-unidentified matter).
referred to as soil dust), salt (road salt or sea salt, depending on the location), trace elements and the last group generally referred to as not-identified matter (other, etc.).The emission categories adopted in this work are presented in Table4S(Supplementary Information).

Table 1 .
Monthly average results for PAHs and sugars identified in PM 10 and PM 1 followed by PM weight average and percentage share of carbon from PAHs and sugars in the organic carbon reservoir.

Table 2 .
38mparison of the percentage shares of emission sources obtained in this work with the shares of the same sources obtained in other studies for the atmosphere of Krakow.aSameketal.37.bZimnoch et al.38.c This work.

Table 3 .
39,39rison of values of the signatures used in the isotope-mass balance in different studies conducted in Krakow.*Thestudydoesnotprovideisotopicsignatures, there is a reference to the publications38,39, so to not duplicate the results from38, we present39.**No exact value in the provided range is given.