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Brown carbon (BrC) is an organic aerosol (OA) component that possesses light-absorbing properties in the UV-Vis spectrum, impacting climate. However, the current understanding of climate repercussions stemming from BrC emissions remains insufficient due to a lack of comprehensive knowledge regarding its chemical makeup, light-absorption, and the role of atmospheric aging in shaping BrC properties. This study investigates BrC in PM 1 (particulate matter <1μm) during winter in Helsinki, Finland, in a street canyon and a residential area with wood combustion. The aim was to ascertain BrC sources, chemical composition, and contribution to UV-Vis light absorption. The study utilized a seven-wavelength aethalometer (AE33) to measure black carbon (BC) and BrC light absorptions, and a soot particle aerosol mass spectrometer (SP-AMS) to determine OA composition. An OA source apportionment using positive matrix factorization followed by a multiple regression analysis between BrC absorption and each factor was performed to determine the mass absorption coefficients of BrC (MAC BrC ) and light absorption contributions of distinct sources across 370-660 nm wavelengths. The BrC UV-Vis absorption relative to the one of BC was higher at 370 nm, with a median contribution of 20.1 % in the residential area and 18.2 % at the traffic site. Residential BrC absorption showed sporadic peaks, while street canyon absorption was lower but consistent. MAC BrC was higher for biomass burning organic aerosol but still significant for long-range transported (LRT) and traffic-related aerosols. Hydrocarbon-like organic aerosol exhibited higher MAC BrC at 470 nm than at 370 nm. Combined with particulate mass concentrations, biomass burning and LRT contributed the most to light absorption. Uncertainties regarding MAC BrC were evaluated. The chemical composition analysis revealed stronger correlations between BrC absorption and SP-AMS-measured ions, especially in residential areas and for polycyclic aromatic hydrocarbons and oxidized aromatics. The study emphasizes the importance of anthropogenic sources in BrC light absorption., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

This study aims to picture the phenomenology of urban ambient total lung deposited surface area (LDSA) (including head/throat (HA), tracheobronchial (TB), and alveolar (ALV) regions) based on multiple path particle dosimetry (MPPD) model during 2017-2019 period collected from urban background (UB, n = 15), traffic (TR, n = 6), suburban background (SUB, n = 4), and regional background (RB, n = 1) monitoring sites in Europe (25) and USA (1). Briefly, the spatial-temporal distribution characteristics of the deposition of LDSA, including diel, weekly, and seasonal patterns, were analyzed. Then, the relationship between LDSA and other air quality metrics at each monitoring site was investigated. The result showed that the peak concentrations of LDSA at UB and TR sites are commonly observed in the morning (06:00-8:00 UTC) and late evening (19:00-22:00 UTC), coinciding with traffic rush hours, biomass burning, and atmospheric stagnation periods. The only LDSA night-time peaks are observed on weekends. Due to the variability of emission sources and meteorology, the seasonal variability of the LDSA concentration revealed significant differences (p = 0.01) between the four seasons at all monitoring sites. Meanwhile, the correlations of LDSA with other pollutant metrics suggested that Aitken and accumulation mode particles play a significant role in the total LDSA concentration. The results also indicated that the main proportion of total LDSA is attributed to the ALV fraction (50 %), followed by the TB (34 %) and HA (16 %). Overall, this study provides valuable information of LDSA as a predictor in epidemiological studies and for the first time presenting total LDSA in a variety of European urban environments., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier B.V. All rights reserved.)

Urban air pollutant concentrations are highly variable both in space and time. In order to understand these variabilities high-resolution measurements of air pollutants are needed. Here we present results of a mobile laboratory and a drone measurements made within a street-canyon network in Helsinki, Finland, in summer and winter 2017. The mobile laboratory measured the total number concentration (N) and lung-deposited surface area (LDSA) of aerosol particles, and the concentrations of black carbon, nitric oxide (NO x ) and ozone (O 3 ). The drone measured the vertical profile of LDSA. The main aims were to examine the spatial variability of air pollutants in a wide street canyon and its immediate surroundings, and find the controlling environmental variables for the observed variability's. The highest concentrations with the most temporal variability were measured at the main street canyon when the mobile laboratory was moving with the traffic fleet for all air pollutants except O 3 . The street canyon concentration levels were more affected by traffic rates whereas on surrounding areas, meteorological conditions dominated. Both the mean flow and turbulence were important, the latter particularly for smaller aerosol particles through LDSA and N. The formation of concentration hotspots in the street network were mostly controlled by mechanical processes but in winter thermal processes became also important for aerosol particles. LDSA showed large variability in the profile shape, and surface and background concentrations. The expected exponential decay functions worked better in well-mixed conditions in summer compared to winter. We derived equation for the vertical decay which was mostly controlled by the air temperature. Mean wind dominated the profile shape over both thermal and mechanical turbulence. This study is among the first experimental studies to demonstrate the importance of high-resolution measurements in understanding urban pollutant variability in detail., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 The Authors. Published by Elsevier B.V. All rights reserved.)

To convey the severity of ambient air pollution level to the public, air quality index (AQI) is used as a communication tool to reflect the concentrations of individual pollutants on a common scale. However, due to the enhanced air pollution control in recent years, air quality has improved, and the roles of some air pollutant species included in the existing AQI as urban air pollutants have diminished. In this study, we suggest the current AQI should be revised in a way that new air pollution indicators would be considered so that it would better represent the health effects caused by local combustion processes from traffic and residential burning. Based on the air quality data of 2017-2019 in three different sites in Helsinki metropolitan area, we assumed the statistical distributions of the current indicators (NO 2 and PM 2.5 ) and the proposed particulate indicators (BC, LDSA and PNC) were related as they have similar sources in urban regions despite the varying correlations between the current and proposed indicators (NO 2 : r = 0.5-0.85, PM 2.5 : r = 0.28-0.72). By fitting the data to an optimal distribution function, together with expert opinions, we improved the current Finnish AQI and determined the AQI breakpoints for the proposed indicators where this robust statistical approach is transferrable to other cities. The addition of the three proposed indicators to the current AQI would decrease the number of good air quality hours in all three environments (largest decrease in urban traffic site, ~22 %). The deterioration of air quality class appeared more severe during peak hours in the urban traffic site due to vehicular emission and evenings in the detached housing site where domestic wood combustion often takes place. The introduction of the AQI breakpoints of the three new indicators serve as a first step of improving the current AQI before further air quality guideline levels are updated., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Erkka Saukko works at Pegasor Oy, the manufacturer of Pegasor AQ Urban, which measured lung deposited surface area (LDSA) described in the manuscript., (Copyright © 2022 The Authors. Published by Elsevier B.V. All rights reserved.)