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Organic aerosols in the Arctic are predominantly fuelled by anthropogenic sources in winter and natural sources in summer, according to observations from eight sites across the Arctic Aerosols play an important yet uncertain role in modulating the radiation balance of the sensitive Arctic atmosphere. Organic aerosol is one of the most abundant, yet least understood, fractions of the Arctic aerosol mass. Here we use data from eight observatories that represent the entire Arctic to reveal the annual cycles in anthropogenic and biogenic sources of organic aerosol. We show that during winter, the organic aerosol in the Arctic is dominated by anthropogenic emissions, mainly from Eurasia, which consist of both direct combustion emissions and long-range transported, aged pollution. In summer, the decreasing anthropogenic pollution is replaced by natural emissions. These include marine secondary, biogenic secondary and primary biological emissions, which have the potential to be important to Arctic climate by modifying the cloud condensation nuclei properties and acting as ice-nucleating particles. Their source strength or atmospheric processing is sensitive to nutrient availability, solar radiation, temperature and snow cover. Our results provide a comprehensive understanding of the current pan-Arctic organic aerosol, which can be used to support modelling efforts that aim to quantify the climate impacts of emissions in this sensitive region.

In recent years, the Indian capital city of Delhi has been impacted by very high levels of air pollution, especially during winters. Comprehensive knowledge of the composition and sources of the organic aerosol (OA), which constitutes a substantial fraction of total particulate mass (PM) in Delhi, is central to formulating effective public health policies. Previous source apportionment studies in Delhi identified key sources of primary OA (POA) and showed that secondary OA (SOA) played a major role, but were unable to resolve specific SOA sources. We address the latter through the first field deployment of an extractive electrospray ionization time-of-flight mass spectrometer (EESI-TOF) in Delhi, together with a high-resolution aerosol mass spectrometer (AMS). Measurements were conducted during the winter of 2018/2019, and positive matrix factorization (PMF) was used separately on AMS and EESI-TOF datasets to apportion the sources of OA. AMS PMF analysis yielded three primary and two secondary factors which were attributed to hydrocarbon-like OA (HOA), biomass burning OA (BBOA-1 and 2), more oxidized oxygenated OA (MO-OOA), and less oxidized oxygenated OA (LO-OOA). On average, 40 % of the total OA mass was apportioned to the secondary factors. The SOA contribution to total OA mass varied greatly between daytime (76.8 %, 10:00–16:00 local time (LT)) and nighttime (31.0 %, 21:00–04:00 local time). The higher chemical resolution of EESI-TOF data allowed identification of individual SOA sources. The EESI-TOF PMF analysis in total yielded six factors, two of which were primary factors (primary biomass burning and cooking-related OA). The remaining four factors were predominantly of secondary origin: aromatic SOA, biogenic SOA, aged biomass burning SOA, and mixed urban SOA. Due to the uncertainties in the EESI-TOF ion sensitivities, mass concentrations of EESI-TOF SOA dominated factors were related to the total AMS SOA (i.e., MO-OOA + LO-OOA) by multi-linear regression (MLR). Aromatic SOA was the major SOA component during the day-time, with 55.2 % contribution to total SOA mass (42.4 % contribution to total OA). Its contribution to total SOA, however, decreased to 25.4 % (7.9 % of total OA) during night-time. This factor was attributed to the oxidation of light aromatic compounds emitted mostly from traffic. Biogenic SOA accounted for 18.4 % of total SOA mass (14.2 % of total OA) during day-time and 36.1 % of total SOA mass (11.2 % of total OA) during night-time. Aged biomass burning and mixed urban SOA accounted for 15.2 % and 11.0 % of total SOA mass ( 11.7 % and 8.5 % of total OA mass) during day-time respectively and 15.4 % and 22.9 % of total SOA mass (4.8 % and 7.1 % of total OA mass) during night-time, respectively. A simple dilution/partitioning model was applied on all EESI-TOF factors to estimate the fraction of observed day-time concentrations resulting from local photochemical production (SOA) or emissions (POA). Aromatic SOA, aged biomass burning, and mixed urban SOA were all found to be dominated by local photochemical production, likely from the oxidation of locally emitted VOCs. In contrast, biogenic SOA was related to the oxidation of diffuse regional emissions of isoprene and monoterpenes. The findings of this study show that in Delhi, the night-time high concentrations are caused by POA emissions led by traffic and biomass burning, and the daytime OA is dominated by SOA, with aromatic SOA accounting for the largest fraction. Because aromatic SOA is possibly more toxic than biogenic SOA and primary OA, its dominance during the day-time suggests an increased OA toxicity and health-related consequences for the general public.

Two fractions of suspended particulate matter (PM1 and PM10) were collected on daily basis in the urban atmosphere of Krakow, southern Poland, during one-year period (April 2018 - April 2019). The following compounds were examined: elemental carbon (EC), organic carbon (OC), carbohydrates (among them levoglucosan – a recognized biomass tracer), as well as ions (Li+, Na+, NH4+, K+, Mg2+, Ca2+, F-, Cl-, NO2-, Br-. NO32-, PO43-, SO42-). Thermal-optical analysis with a Sunset carbon analyzer, (Sunset Lab. Inc.) was used to obtain information about organic and elemental carbon concentration, while HPAE-PAD Dionex ICS 3000 system was employed to determine the concentration of 14 carbohydrates. Concentration of ions was analysed using isocratic ion chromatography on an ICS-1100 instrument (Thermo Scientific).Distinct seasonality of chemical composition of PM1 and PM10 fraction was observed. Levoglucosan concentration ranged from 0.01 ug/m3 to 0.90 ug/m3 (PM1 fraction) and from 0.01 to ug/m3 to 1.95 ug/m3 (PM10 fraction) during the analysed period. Arabitol and Mannitol were detected only in PM10 fraction and ranged from 0.01 ug/m3 and 0.02 ug/m3, during winter season and to 0.15 ug/m3 and 0.10 ug/m3, respectively, during summer season. Significant seasonal differences were also found for ion concentrations: from 0.49 μg/m3 (SO42-), 0.15 μg/m3 (NO3-) and 0.05 μg/m3 (NH4+) during summer season, to be compared with 11.16 μg/m3 (SO42-), 9.30 μg/m3 (NO3-), 9.25 μg/m3 (NH4+) for winter season. The concentration of organic and elemental carbon in PM10 fraction ranged from 2.0 μg/m3 to 48.9 μg/m3 (OC) and from 0.3 μg/m3 to 10.0 μg/m3 (EC), to be compared with 1.4 μg/m3 to 18.1 μg/m3 (OC) and 0.2 μg/m3 to 4.4 μg/m3 (EC) for PM1 fraction. ACKNOWLEDGEMENTSThe presented work was funded by Polish National Science Centre (project No. 2019/33/N/ST10/02925) as well as COST Action COLOSSAL (CA16109) of EU. Work of AS and PF have been partly supported by the EU Project POWR.03.02.00-00-I004/16. Analytical infrastructure of AGH Center of Energy in Krakow was employed in analyses of selected ions.