Molecular insights into new particle formation in Barcelona, Spain
Authors: Brean James , Beddows David C. S. , Shi Zongbo , Temime-Roussel Brice , Marchand Nicolas , Querol Xavier , Alastuey Andrés , Minguillón María Cruz , Harrison Roy M. 
Published: August 2020

Abstract. Atmospheric aerosols contribute some of the greatest uncertainties to
estimates of global radiative forcing and have significant effects on human health. New particle formation (NPF) is the process by which new aerosols of
sub-2 nm diameter form from gas-phase precursors and contributes
significantly to particle numbers in the atmosphere, accounting for
approximately 50 % of cloud condensation nuclei globally. Here, we study
summertime NPF in urban Barcelona in north-eastern Spain utilising particle counting instruments down to 1.9 nm and a Nitrate Chemical Ionisation Atmospheric Pressure interface Time
of Flight Mass Spectrometer (CI-APi-ToF). The rate of formation of new particles is seen to increase linearly with sulfuric acid concentration, although particle formation rates fall short of chamber
studies of H2SO4–DMA–H2O while exceeding those of H2SO4–BioOxOrg–H2O nucleation, although a role of highly oxygenated molecules (HOMs) cannot be ruled out. The sulfuric acid dimer : monomer ratio is significantly lower than that seen in experiments involving sulfuric acid and dimethylamine (DMA) in chambers, indicating that stabilisation of sulfuric acid clusters by bases is weaker in this dataset than in
chambers, either due to rapid evaporation due to high summertime
temperatures or limited pools of stabilising amines. Such a mechanism cannot be verified in these data, as no higher-order H2SO4–amine clusters nor H2SO4–HOM clusters were measured. The high
concentrations of HOMs arise from isoprene, alkylbenzene, monoterpene and
polycyclic aromatic hydrocarbon (PAH) oxidation, with alkylbenzenes providing greater concentrations of HOMs due to significant local sources. The concentration of these HOMs shows a
dependence on temperature. The organic compounds measured primarily fall
into the semivolatile organic compound (SVOC) volatility class arising from alkylbenzene and isoprene oxidation. Low-volatility organic compounds (LVOCs) largely arise from oxidation of alkylbenzenes, PAHs and monoterpenes, whereas extremely low-volatility organic compounds (ELVOCs) arise from primarily PAH and monoterpene oxidation. New particle formation without growth past 10 nm is also
observed, and on these days oxygenated organic concentrations are lower than
on days with growth by a factor of 1.6, and thus high concentrations of low-volatility oxygenated organics which primarily derive from traffic-emitted
volatile organic compounds (VOCs) appear to be a necessary condition for the growth of newly formed
particles in Barcelona. These results are consistent with prior observations
of new particle formation from sulfuric acid–amine reactions in both chambers and the real atmosphere and are likely representative of the urban background of many European Mediterranean cities. A role for HOMs in the
nucleation process cannot be confirmed or ruled out, and there is strong
circumstantial evidence of the participation of HOMs across multiple volatility classes in particle growth.