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Biomass burning is the largest combustion-related source of volatile organic compounds (VOCs) to the atmosphere. We describe the development of a state-of-the-science model to simulate the photochemical formation of secondary organic aerosol (SOA) from biomass-burning emissions observed in dry (RH

Observations from a wintertime and summertime field campaign are used to assess the relationship between black and brown carbon (BC and BrC, respectively) optical properties and particle composition and coating state. The wintertime campaign, in Fresno, CA, was impacted by primary emissions from residential wood burning, secondary organic and inorganic particle formation, and BC from motor vehicles. Two major types of BrC were observed in wintertime. One occurred primarily at night—the result of primary biomass burning emissions. The second was enhanced in daytime and strongly associated with particulate nitrate and the occurrence of fog. The biomass-burning-derived BrC absorbed more strongly than the nitrate-associated BrC but had a weaker wavelength dependence. The wintertime BC-specific mass absorption coefficient (MAC BC ) exhibited limited dependence on the ensemble-average coating-to-BC mass ratio (R coat-rBC ) at all wavelengths, even up to R coat-rBC of ~5. For the summertime campaign, in Fontana, CA, BC dominated the light absorption, with negligible BrC contribution even after substantial photochemical processing. The summertime MAC BC exhibited limited dependence on R coat-rBC , even up to ratios of >10. Based on the four classes of BC-containing particles identified by Lee et al. (2017, https://doi.org/10.5194/acp-17-15055-2017) for the summertime measurements, the general lack of an absorption enhancement can be partly—although not entirely—attributed to an unequal distribution of coating materials between the BC-containing particle types. These observations demonstrate that in relatively near-source environments, even those impacted by strong secondary aerosol production, the ensemble-average, mixing-induced absorption enhancement for BC due to coatings can be quite small.

Inorganic and organic coatings on black carbon (BC) particles can enhance light absorption and affect atmospheric lifetimes of BC-containing particles and thus have significant implications for climate. To study the physical and chemical characteristics of atmospheric BC and BC-associated coatings, a soot particle-aerosol mass spectrometer was deployed during the winter of 2014–2015 in Fresno, a city located in the San Joaquin Valley of California, to selectively analyze BC-containing particles. Comparing soot particle-aerosol mass spectrometer measurements to those from the collocated single-particle soot photometer (SP2) and high-resolution aerosol mass spectrometer, we found that 17% of total submicrometer aerosol mass was associated with BC-containing particles, suggesting that a majority of the fine particles in Fresno contained no BC. Most BC-containing particles appeared to be associated with residential wood burning and vehicular traffic. These particles typically had a bulk-average mass ratio of coating to BC (Rcoat/rBC) less than 2. However, during periods of persistent fog larger Rcoat/rBC values were observed, with the coatings primarily composed of secondary inorganic and organic components that likely resulted from aqueous-phase processing. Specifically, compared to periods with less fog, the BC coating increased in concentration and contained a larger fraction of nitrate and oxidized organic matter. The size distributions of BC and associated organic coating were generally centered around 300nm in vacuum aerodynamic diameter. However, during foggy periods BC had an additional peak at ~400nm and organics and nitrate displayed a prominent mode in the accumulation size range.

We investigated the collection efficiency and effective ionization efficiency for secondary organic aerosol (SOA) particles made from α-pinene + O3 using the single-particle capabilities of the aerosol mass spectrometer (AMS). The mean count-based collection efficiency (CEp) for SOA across these experiments is 0.30 (±0.04 SD), ranging from 0.25 to 0.40. The mean mass-based collection efficiency (CEm) is 0.49 (±0.07 SD). This sub-unit collection efficiency and delayed vaporization is attributable to particle bounce in the vaporization region. Using the coupled optical and chemical detection of the light-scattering single-particle (LSSP) module of the AMS, we provide clear evidence that delayed vaporization is somewhat of a misnomer for these particles: SOA particles measured as a part of the AMS mass distribution do not vaporize at a slow rate; rather, they flash-vaporize, albeit often not on the initial impact with the vaporizer but instead upon a subsequent impact with a hot surface in the vaporization region. We also find that the effective ionization efficiency (defined as ions per particle, IPP) decreases with delayed arrival time. CEp is not a function of particle size (for the mobility diameter range investigated, 170–460 nm), but we did see a decrease in CEp with thermodenuder temperature, implying that oxidation state and/or volatility can affect CEp for SOA. By measuring the mean ions per particle produced for monodisperse particles as a function of signal delay time, we can separately determine CEp and CEm and thus more accurately measure the relative ionization efficiency (compared to ammonium nitrate) of different particle types.

An evaluation of the operation and performance of a Cavity Attenuated Phase-Shift Particle Extinction Monitor (CAPS PMex) was performed for use on board commercial aircraft as part of the research infrastructure IAGOS (In-service Aircraft for a Global Observing System, www.iagos.org). After extensive laboratory testing, a new flow system, using mass flow controllers, was installed to maintain constant purge and sample flows under low and varying pressure conditions. The instrument was then tested for pressures as low as 200 hPa and evaluated against particle-free compressed air and CO2. Extinction coefficients for the studied gases were in close agreement with literature values with differences between 2.2 and 8%, proving that the CAPS technology works at low pressures. The instrument's limit of detection, with respect to 3 times the variability of the background signal for the full pressure range, was 0.2 Mm−1 for 60 s integration time. During its first research aircraft operations, the IAGOS instrument prototype, composed of one CAPS PMex and one OPC, showed excellent results regarding the stability of the instruments and the potential for characterizing different aerosol types and for estimating the contribution of sub- and super-μm sized particles to aerosol light extinction.

New measurements of water diffusion in secondary organic aerosol (SOA) material produced by oxidation of α-pinene and in a number of organic/inorganic model mixtures (3-methylbutane-1,2,3-tricarboxylic acid (3-MBTCA), levoglucosan, levoglucosan/NH4HSO4, raffinose) are presented. These indicate that water diffusion coefficients are determined by several properties of the aerosol substance and cannot be inferred from the glass transition temperature or bouncing properties. Our results suggest that water diffusion in SOA particles is faster than often assumed and imposes no significant kinetic limitation on water uptake and release at temperatures above 220 K. The fast diffusion of water suggests that heterogeneous ice nucleation on a glassy core is very unlikely in these systems. At temperatures below 220 K, model simulations of SOA particles suggest that heterogeneous ice nucleation may occur in the immersion mode on glassy cores which remain embedded in a liquid shell when experiencing fast updraft velocities. The particles absorb significant quantities of water during these updrafts which plasticize their outer layers such that these layers equilibrate readily with the gas phase humidity before the homogeneous ice nucleation threshold is reached. Glass formation is thus unlikely to restrict homogeneous ice nucleation. Only under most extreme conditions near the very high tropical tropopause may the homogeneous ice nucleation rate coefficient be reduced as a consequence of slow condensed-phase water diffusion. Since the differences between the behavior limited or non limited by diffusion are small even at the very high tropical tropopause, condensed-phase water diffusivity is unlikely to have significant consequences on the direct climatic effects of SOA particles under tropospheric conditions., Atmospheric Chemistry and Physics, 15 (23), ISSN:1680-7375, ISSN:1680-7367

We discuss the major mass spectral features of different types of refractory carbona-ceous particles, ionized after laser vapourization with an Aerodyne High-ResolutionSoot-Particle Aerosol Mass Spectrometer (SP-AMS). The SP-AMS was operated witha switchable 1064 nm laser and a 600◦C thermal vapourizer, yielding respective measurements of the refractory and non-refractory particle components. Six samples wereinvestigated, all of which were composed primarily of refractory material: fuel-rich andfuel-lean propane/air diffusion-flame combustion particles; graphite-spark-generatedparticles; a commercial Fullerene-enriched Soot; Regal Black, a commercial carbonblack; and nascent aircraft-turbine combustion particles.All samples exhibited a spectrum of carbon-cluster ions Cn+xin their refractory massspectrum. Smaller clusters (x1). In all six cases, the ions C+1and C+3contributed over 60% to the total15C+15 were present. When such signalswere present, C+1/C+3was close to 1. When absent, C+1/C+3was, Atmospheric Chemistry and Physics, 13, ISSN:1680-7375, ISSN:1680-7367

We discuss the major mass spectral features of different types of refractory carbonaceous particles, ionized after laser vaporization with an Aerodyne high-resolution soot-particle aerosol mass spectrometer (SP-AMS). The SP-AMS was operated with a switchable 1064 nm laser and a 600 °C thermal vaporizer, yielding respective measurements of the refractory and non-refractory particle components. Six samples were investigated, all of which were composed primarily of refractory material: fuel-rich and fuel-lean propane/air diffusion-flame combustion particles; graphite-spark-generated particles; a commercial fullerene-enriched soot; Regal Black, a commercial carbon black; and nascent aircraft-turbine combustion particles. All samples exhibited a spectrum of carbon-cluster ions Cxn+ in their refractory mass spectrum. Smaller clusters (x < 6) were found to dominate the Cxn+ distribution. For fullerene soot, fuel-rich-flame particles and spark-generated particles, significant Cxn+ clusters at x ≫ 6 were present, with significant contributions from multiply charged ions (n > 1). In all six cases, the ions C1+ and C3+ contributed over 60% to the total C1+ intensity. Furthermore, the ratio of these major ions C1+ / C3+ could be used to predict whether significant Cxn+ signals with x > 5 were present. When such signals were present, C1+ / C3+ was close to 1. When absent, C1+ / C3+ was < 0.8. This ratio may therefore serve as a proxy to distinguish between the two types of spectra in atmospheric SP-AMS measurements. Significant refractory oxygenated ions such as CO+ and CO2+ were also observed for all samples. We discuss these signals in detail for Regal Black, and describe their formation via decomposition of oxygenated moieties incorporated into the refractory carbon structure. These species may be of importance in atmospheric processes such as water uptake and heterogeneous chemistry. If atmospherically stable, these oxidized species may be useful for distinguishing between different combustion sources. If unstable, they may provide a means to estimate the atmospheric age of an rBC sample. Future studies should attempt to establish which of these scenarios is more realistic., Atmospheric Chemistry and Physics, 14 (5), ISSN:1680-7375, ISSN:1680-7367