Increase in secondary organic aerosol in an urban environment

Via, Marta; Minguillón, María Cruz; Reche, Cristina; Querol, Xavier; Alastuey, Andrés

The evolution of fine aerosol (PMinline-formula1) species as well as the contribution of potential sources to the total organic aerosol (OA) at an urban background site in Barcelona, in the western Mediterranean basin (WMB) was investigated. For this purpose, a quadrupole aerosol chemical speciation monitor (Q-ACSM) was deployed to acquire real-time measurements for two 1-year periods: May 2014–May 2015 (period A) and September 2017–October 2018 (period B). Total PMinline-formula1 concentrations showed a slight decrease (from 10.1 to 9.6 inline-formulaµg m−3 from A to B), although the relative contribution of inorganic and organic compounds varied significantly.

Regarding inorganic compounds, SOinline-formula M4inlinescrollmathml normal 4 normal 2 - 13pt17ptsvg-formulamathimg6734be199742c3e7a0dfe877974848e8 acp-21-8323-2021-ie00001.svg13pt17ptacp-21-8323-2021-ie00001.png , black carbon (BC) and NHinline-formula M5inlinescrollmathml normal 4 + 8pt15ptsvg-formulamathimgaa378b71f34a6c23384fc0eb7c6e7621 acp-21-8323-2021-ie00002.svg8pt15ptacp-21-8323-2021-ie00002.png showed a significant decrease from period A to B (inline-formula−21 %, inline-formula−18 % and inline-formula−9 %, respectively), whilst NOinline-formula M9inlinescrollmathml normal 3 - 9pt16ptsvg-formulamathimg1933cd4f78557ae19e1c84fa4d0b5473 acp-21-8323-2021-ie00003.svg9pt16ptacp-21-8323-2021-ie00003.png concentrations were higher in B (inline-formula+8 %). Source apportionment revealed OA contained 46 % and 70 % secondary OA (SOA) in periods A and B, respectively. Two secondary oxygenated OA sources (OOA) were differentiated by their oxidation status (i.e. ageing): less oxidized (LO-OOA) and more oxidized (MO-OOA). Disregarding winter periods, when LO-OOA production was not favoured, LO-OOA transformation into MO-OOA was found to be more effective in period B. The lowest LO-OOA-to-MO-OOA ratio, excluding winter, was in September–October 2018 (0.65), implying an accumulation of aged OA after the high temperature and solar radiation conditions in the summer season. In addition to temperature, SOA (sum of OOA factors) was enhanced by exposure to NOinline-formulax-polluted ambient and other pollutants, especially to Oinline-formula3 and during afternoon hours. The anthropogenic primary OA sources identified, cooking-related OA (COA), hydrocarbon-like OA (HOA), and biomass burning OA (BBOA), decreased from period A to B in both absolute concentrations and relative contribution (as a whole, 44 % and 30 %, respectively). However, their concentrations and proportion to OA grew rapidly during highly polluted episodes.

The influence of certain atmospheric episodes on OA sources was also assessed. Both SOA factors were boosted with long- and medium-range circulations, especially those coming from inland Europe and the Mediterranean (triggering mainly MO-OOA) and summer breeze-driven regional circulation (mainly LO-OOA). In contrast, POA was enhanced either during air-renewal episodes or stagnation anticyclonic events.

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Via, Marta / Minguillón, María Cruz / Reche, Cristina / et al: Increase in secondary organic aerosol in an urban environment. 2021. Copernicus Publications.

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