ALBERT

Description: 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.

Description: Particle number size distributions have been measured simultaneously by scanning mobility particle sizers (SMPSs) at five sites in central London for a 1 month campaign in January–February 2017. These measurements were accompanied by condensation particle counters (CPCs) to measure total particle number count at four of the sites and Aethalometers measuring black carbon (BC) at five sites. The spatial distribution and inter-relationships of the particle size distribution and SMPS total number counts with CPC total number counts and black carbon measurements have been analysed in detail as well as variations in the size distributions. One site (Marylebone Road) was in a street canyon with heavy traffic, one site (Westminster University) was on a rooftop adjacent to the Marylebone Road sampler, and a further sampler was located at Regent's University within a major park to the north of Marylebone Road. A fourth sampler was located nearby at 160 m above ground level on the BT tower and a fifth sampler was located 4 km to the west of the main sampling region at North Kensington. Consistent with earlier studies it was found that the mode in the size distribution had shifted to smaller sizes at the Regent's University (park) site, the mean particle shrinkage rate being 0.04 nm s−1 with slightly lower values at low wind speeds and some larger values at higher wind speeds. There was evidence of complete evaporation of the semi-volatile nucleation mode under certain conditions at the elevated BT Tower site. While the SMPS total count and black carbon showed typical traffic-dominated diurnal profiles, the CPC count data typically peaked during night-time as did CPC∕SMPS and CPC∕BC ratios. This is thought to be due to the presence of high concentrations of small particles (2.5–15 nm diameter) probably arising from condensational growth from traffic emissions during the cooler night-time conditions. Such behaviour was most marked at the Regent's University and Westminster University sites and less so at Marylebone Road, while at the elevated BT Tower site the ratio of particle number (CPC) to black carbon peaked during the morning rush hour and not at night-time, unlike the other sites. An elevation in nucleation mode particles associated with winds from the west and WSW sector was concluded to result from emissions from London Heathrow Airport, despite a distance of 22 km from the central London sites.

Description: Particle nucleation is one of the main sources of atmospheric particulate matter by number, with new particles having great relevance for human health and climate. Highly oxidized multifunctional organic molecules (HOMs) have been recently identified as key constituents in the growth and, sometimes, in initial formation of new particles. While there have been many studies of HOMs in atmospheric chambers, flow tubes, and clean environments, analyses of data from polluted environments are scarce. Here, measurements of HOMs and particle size distributions down to small molecular clusters are presented alongside volatile organic compounds (VOCs) and trace-gas data from a campaign in June 2017, in Beijing. Many gas-phase HOMs have been characterized and their temporal trends and behaviours analysed in the context of new particle formation. The HOMs identified have a degree of oxidation comparable to that seen in other, cleaner, environments, likely due to an interplay between the higher temperatures facilitating rapid hydrogen abstractions and the higher concentrations of NOx and other RO2⚫ terminators ending the autoxidation sequence more rapidly. Our data indicate that alkylbenzenes, monoterpenes, and isoprene are important precursor VOCs for HOMs in Beijing. Many of the C5 and C10 compounds derived from isoprene and monoterpenes have a slightly greater degree of average oxidation state of carbon compared to those from other precursors. Most HOMs except for large dimers have daytime peak concentrations, indicating the importance of OH⚫ chemistry in the formation of HOMs, as O3 tends to be lower on days with higher HOM concentrations; similarly, VOC concentrations are lower on the days with higher HOM concentrations. The daytime peaks of HOMs coincide with the growth of freshly formed new particles, and their initial formation coincides with the peak in sulfuric acid vapours, suggesting that the nucleation process is sulfuric-acid-dependent, with HOMs contributing to subsequent particle growth.

Description: Particle nucleation is one of the main sources of atmospheric particulate matter by number, with new particles having great relevance for human health and climate. Highly oxidised multifunctional organic molecules (HOMs) have been recently identified as key constituents in the growth, and, sometimes, in initial formation of new particles. While there have been many studies of HOMs in atmospheric chambers, flow tubes and clean environments, analyses of data from polluted environments are scarce. Here, measurements of HOMs and particle size distributions down to small molecular clusters are presented alongside VOC and trace gas data from a campaign in Beijing. Many gas phase HOMs have been characterised and their temporal trends and behaviours analysed in the context of new particle formation. The HOMs identified have a comparable degree of oxidation to those seen in other, cleaner, environments, likely due to an interplay between the higher temperatures facilitating rapid hydrogen abstractions and the higher concentrations of NOx and other RO2. terminators ending the autoxidation sequence more rapidly. Our data indicate that alkylbenzenes, monoterpenes, and isoprene are important precursor VOCs for HOMs in Beijing. Many of the C5 and C10 compounds derived from isoprene and monoterpenes have a slightly greater degree of average oxidation state of carbon compared to those from other precursors. Most HOMs except for large dimers have daytime peak concentrations, indicating the importance of OH. chemistry in the formation of HOMs, as O3 is lower on the days with higher HOM concentrations; similarly, VOC concentrations are lower on the days with higher HOM concentrations. The daytime peaks of HOMs coincide with the growth of freshly formed new particles, and their initial formation coincides with the peak in sulphuric acid vapours, suggesting that the nucleation process is sulphuric acid-dependent, with HOMs contributing to subsequent particle growth.

Description: Particle number size distributions have been measured simultaneously by Scanning Mobility Particle Sizers (SMPS) at five sites in Central London for a one month campaign in January–February 2017. These measurements were accompanied by condensation particle counters (CPC) to measure total particle number count at four of the sites and aethalometers measuring Black Carbon (BC) at five sites. The spatial distribution and inter-relationships of the particle size distribution and SMPS total number counts with CPC total number counts and Black Carbon measurements have been analysed in detail as well as variations in the size distributions. One site (Marylebone Road) was in a heavily-trafficked street canyon, one site (Westminster University) was on a rooftop adjacent to the Marylebone Road sampler, a further sampler was located at Regent's University within a major park to the north of Marylebone Road. A fourth sampler was located nearby at 160m above ground level on the BT tower and a fifth sampler was located 4km to the west of the main sampling region at North Kensington. Consistent with earlier studies it was found that the mode in the size distribution had shifted to smaller sizes at the Regent's University (park) site, the mean particle shrinkage rate being 0.04nms−1 with slightly lower values at low wind speeds and some larger values at higher wind speeds. There was evidence of complete evaporation of the semi-volatile nucleation mode under certain conditions at the elevated BT Tower site. Whereas SMPS total count and Black Carbon showed typical traffic-dominated diurnal profiles, the CPC count data typically peaked during nighttime as did CPC/SMPS and CPC/BC ratios. This is thought to be due to the presence of high concentrations of small particles (2.5–15nm diameter) probably arising from condensational growth from traffic emissions during the cooler nighttime conditions. Such behaviour was most marked at the Regent's University and Westminster University sites and less so at Marylebone Road, while at the elevated BT Tower site the ratio of particle number (CPC) to Black Carbon peaked during the morning rush hour and not at nighttime, unlike the other sites. An elevation in nucleation mode particles associated with winds from the West and WSW sector was concluded to result from emissions from London Heathrow Airport, despite a distance of 22km from the Central London sites.