Increase in secondary organic aerosol in an urban environment

• Main Authors: M. Via, M. C. Minguillón, C. Reche, X. Querol, A. Alastuey
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-05-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://acp.copernicus.org/articles/21/8323/2021/acp-21-8323-2021.pdf

<p>The evolution of fine aerosol (PM<span class="inline-formula">₁</span>) 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 PM<span class="inline-formula">₁</span> concentrations showed a slight decrease (from 10.1 to 9.6 <span class="inline-formula">µg m⁻³</span> from A to B), although the relative contribution of inorganic and organic compounds varied significantly.</p> <p>Regarding inorganic compounds, SO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="13pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="6734be199742c3e7a0dfe877974848e8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-8323-2021-ie00001.svg" width="13pt" height="17pt" src="acp-21-8323-2021-ie00001.png"/></svg:svg></span></span>, black carbon (BC) and NH<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="aa378b71f34a6c23384fc0eb7c6e7621"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-8323-2021-ie00002.svg" width="8pt" height="15pt" src="acp-21-8323-2021-ie00002.png"/></svg:svg></span></span> showed a significant decrease from period A to B (<span class="inline-formula">−</span>21 %, <span class="inline-formula">−</span>18 % and <span class="inline-formula">−</span>9 %, respectively), whilst NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="9pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="1933cd4f78557ae19e1c84fa4d0b5473"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-8323-2021-ie00003.svg" width="9pt" height="16pt" src="acp-21-8323-2021-ie00003.png"/></svg:svg></span></span> concentrations were higher in B (<span class="inline-formula">+</span>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 NO<span class="inline-formula">_<i>x</i></span>-polluted ambient and other pollutants, especially to O<span class="inline-formula">₃</span> 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.</p> <p>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.</p>