Chemical evolution of primary and secondary biomass burning aerosols during daytime and nighttime

Primary emissions from wood and pellet stoves were aged in an atmospheric simulation chamber under daytime and nighttime conditions. The aerosol was analyzed with the online Aerosol Mass Spectrometer (AMS) and offline Fourier transform infrared spectroscopy (FTIR). Measurements using the two techniques agreed reasonably well in terms of the organic aerosol (OA) mass concentration, OA:OC trends, and concentrations of biomass burning markers – lignin-like compounds and anhydrosugars. Based on the AMS, around 15 % of the primary organic aerosol (POA) mass underwent some form of transformation during daytime oxidation conditions after 6–10 hours of atmospheric exposure. A lesser extent of transformation was observed during the nighttime oxidation. The decay of certain semi-volatile (e.g., levoglucosan) and less volatile (e.g., lignin-like) POA components was substantial during aging, highlighting the role of heterogeneous reactions and gas-particle partitioning. Lignin-like compounds were observed to degrade under both daytime and nighttime conditions, whereas anhydrosugars degraded only under daytime conditions. Among the marker mass fragments of primary biomass burning OA (bbPOA), heavy ones (higher m/z) were relatively more stable during aging. The biomass burning secondary OA (bbSOA) became more oxidized with continued aging and resembled those of aged atmospheric organic aerosols. The bbSOA formed during daytime oxidation was dominated by acids. Organonitrates were an important product of nighttime reactions in both humid and dry conditions. Our results underline the importance of changes to both the primary and secondary biomass burning aerosols during their atmospheric aging. Heavier AMS fragments seldomly used in atmospheric chemistry can be used as more stable tracers of bbPOA and in combination with the established levoglucosan marker, can provide an indication of the extent of bbPOA aging

Sources of water-soluble Brown Carbon at a South-Eastern European Site

Atmospheric brown carbon (BrC) is a highly uncertain, but potentially important contributor to light absorption in the atmosphere. Laboratory and field studies have shown that BrC can be produced from multiple sources, including primary emissions from fossil fuel combustion and biomass burning (BB), as well as secondary formation through a number of reaction pathways. It is currently thought that the dominant source of atmospheric BrC is primary emissions from BB, but relatively few studies demonstrate this in environments with complex source profiles. A field campaign was conducted during a month-long wintertime period in 2020 on the campus of the University of Peloponnese in the southwest of Patras, Greece which represents an urban site. During this time, ambient filter samples (a total of 35 filters) were collected from which the water-soluble BrC was determined using a semi-automated system similar to Hecobian et al. (2010), where absorption was measured over a 1 m path length. To measure the BrC, a UV-Vis Spectrophotometer was coupled to a Liquid Waveguide Capillary Cell and the light absorption intensity was recorded at 365 and 700 nm. The latter was used as a reference wavelength. We found that the average BrC absorption in Patras at a wavelength of 365 nm was 8.5 ± 3.9 Mm-1 suggesting that there was significant BrC in the organic aerosol during this period. Attribution of sources of BrC was done using simultaneous chemical composition data observations (primarily organic carbon, black carbon, and nitrate) combined with Positive Matrix Factorization analysis. This analysis showed that in addition to the important role of biomass burning (a contribution of about 20%) and other combustion emissions (also close to 20%), oxidized organic aerosol (approximately 40%) is also a significant contributor to BrC in the study area. Reference Hecobian, A., Zhang, X., Zheng, M., Frank, N., Edgerton, E.S., Weber, R.J., 2010. Water-soluble organic aerosol material and the light-absorption characteristics of aqueous extracts measured over the Southeastern United States. Atmos. Chem. Phys. 10, 5965–5977. https://doi.org/10.5194/acp-10-5965-201

Oxidative Potential of Atmospheric Particles at an Eastern Mediterranean Site

Aerosol oxidative potential (OP; the inherent ability of ambient particles to generate reactive oxygen species in vivo) may be linked to the health effects of population exposure to aerosol and is a metric of their toxicity. The goal of this work was to quantify the water-soluble OP of particles in an urban area in Patras, Greece and to investigate its links with source emissions or components of this particulate matter (PM). A field campaign was conducted during a monthlong wintertime period in 2020 (January 10 to February 13) on the campus of the University of Peloponnese in the southwest of Patras. During this time, ambient filter samples (a total of 35 filters) were collected. To measure the water-soluble OP we used a semiautomated system similar to Fang et al. (2015) based on the dithiothreitol (DTT) assay. The accuracy of our system was validated by measuring the DTT activity of 11 phenanthrequinone (PQN) solutions on both our system and the identical semi-automated validated system at the National Observatory of Athens (NOA). These two sets of analysed DTT activities (current vs. NOA system) were significantly correlated (R2=0.99) with a slope of 1.15 ± 0.04 and an intercept close to zero. We found that the average water-soluble OP in Patras was 1.5 ± 0.3 nmol min-1 m-3, ranging from 0.7 to 2 nmol min-1 m-3. The OP measured in Patras during the campaign is higher than reported values from similar wintertime studies in other urban areas such as Athens (Paraskevopoulou et al., 2019). The average watersoluble OP during a summer study for Patras was significantly lower and equal to 0.18 ± 0.02 nmol min-1 m- 3. Taking into account the average PM1 mass concentrations for these two periods (summer: 6 μg m-3 and winter: 23 μg m-3) it is clear that the increase in OP was two times the increase in PM mass making the wintertime aerosol more toxic. Additionally, the water-soluble brown carbon (BrC) was determined using an offline semi-automated system, where absorption was measured over a 1 m path length. The average BrC absorption in Patras at a wavelength of 365 nm was 8.6 ± 3.9 Mm-1 suggesting that there was significant BrC in the organic aerosol during this period. The coefficients of determination, R2, in Table 1 are used as a metric of the potential relationships between the various carbonaceous aerosol components and the DTT activity. The results suggest that the OP is not dominated by a single source or component, but that there are multiple components contributing to it during the study period. Interestingly, the highest correlation coefficient (R2 = 0.46) was found between the OP and Brown Carbon. This is consistent with recently published results for an urban site in Atlanta where the oxidative potential measured with the DTT method also had stronger correlations with BrC during the winter (Gao et al., 2020)

Particle wall-loss correction methods in smog chamber experiments

The interaction of particles with the chamber walls has been a significant source of uncertainty when analyzing results of secondary organic aerosol (SOA) formation experiments performed in Teflon chambers. A number of particle wall-loss correction methods have been proposed including the use of a size-independent loss rate constant, the ratio of suspended organic mass to that of a conserved tracer (e.g., sulfate seeds), and a size-dependent loss rate constant, etc. For complex experiments such as the chemical aging of SOA, the results of the SOA quantification analysis can be quite sensitive to the adopted correction method due to the evolution of the particle size distribution and the duration of these experiments. We evaluated the performance of several particle wall-loss correction methods for aging experiments of α-pinene ozonolysis products. Determining the loss rates from seed loss periods is necessary for this system because it is not clear when chemical reactions have been completed. Results from the OA&thinsp;∕&thinsp;sulfate ratio and the size-independent correction methods can be influenced significantly by the size dependence of the particle wall-loss process. Coagulation can also affect the particle size distribution, especially for particles with diameter less than 100&thinsp;nm, thus introducing errors in the results of the wall-loss correction. The corresponding loss rate constants may vary from experiment to experiment, and even during a specific experiment. Friction between the Teflon chamber walls and non-conductive surfaces can significantly increase particle wall-loss rates and the chamber may require weeks to recover to its original condition. Experimental procedures are proposed for the characterization of particle losses during different stages of these experiments and the evaluation of corresponding particle wall-loss correction.</p

A portable dual-smog-chamber system for atmospheric aerosol field studies

Smog chamber experiments using ambient air as a starting point can improve our understanding of the evolution of atmospheric particulate matter at timescales longer than those achieved by traditional laboratory experiments. These types of studies can take place under more realistic environmental conditions addressing the interactions among multiple pollutants. The use of two identical smog chambers, with the first serving as the baseline chamber and the second as the perturbation chamber (in which addition or removal of pollutants, addition of oxidants, change in the relative humidity, etc.), can facilitate the interpretation of the results in such inherently complex experiments. The differences of the measurements in the two chambers can be used as the basis for the analysis of the corresponding chemical or physical processes of ambient air. A portable dual-smog-chamber system was developed using two identical pillow-shaped smog chambers (1.5&thinsp;m3 each). The two chambers are surrounded by UV lamps in a hexagonal arrangement yielding a total JNO2 of 0.1&thinsp;min−1. The system can be easily disassembled and transported, enabling the study of various atmospheric environments. Moreover, it can be used with natural sunlight. The results of test experiments using ambient air as the starting point are discussed as examples of applications of this system.</p