Organic aerosol (OA) particles affect climate forcing and human health, but their sources and evolution remain poorly characterized. We present a unifying model framework describing the atmospheric evolution of OA that is constrained by high-time-resolution measurements of its composition, volatility, and oxidation state. OA and OA precursor gases evolve by becoming increasingly oxidized, less volatile, and more hygroscopic, leading to the formation of oxygenated organic aerosol (OOA), with concentrations comparable to those of sulfate aerosol throughout the Northern Hemisphere. Our model framework captures the dynamic aging behavior observed in both the atmosphere and laboratory: It can serve as a basis for improving parameterizations in regional and global models.

Aerosols of biological origin play a vital role in the Earth system, particularly in the interactions between atmosphere, biosphere, climate, and public health. Airborne bacteria, fungal spores, pollen, and other bioparticles are essential for the reproduction and spread of organisms across various ecosystems, and they can cause or enhance human, animal, and plant diseases. Moreover, they can serve as nuclei for cloud droplets, ice crystals, and precipitation, thus influencing the hydrological cycle and climate. The sources, abundance, composition, and effects of biological aerosols and the atmospheric microbiome are, however, not yet well characterized and constitute a large gap in the scientific understanding of the interaction and co-evolution of life and climate in the Earth system. This review presents an overview of the state of bioaerosol research, highlights recent advances, and outlines future perspectives in terms o fbioaerosolidentification, characterization, transport, and transforma- tion processes, as well as their interactions with climate, health, and ecosystems, focusing on the role bioaerosols play in the Earth system

Abstract. The volatilities of different chemical species in ambient aerosols are important but remain poorly characterized. The coupling of a recently developed rapid temperature-stepping thermodenuder (TD, operated in the range 54–230°C) with a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) during field studies in two polluted megacities has enabled the first direct characterization of chemically-resolved urban particle volatility. Measurements in Riverside, CA and Mexico City are generally consistent and show ambient nitrate as having the highest volatility of any AMS aerosol species while sulfate showed the lowest volatility. Total organic aerosol (OA) showed volatility intermediate between nitrate and sulfate, with an evaporation rate of 0.6% K−1 near ambient temperature, although OA dominates the residual species at the highest temperatures. Different types of OA were characterized with marker ions, diurnal cycles, and positive matrix factorization (PMF) and show significant differences in volatility. Reduced hydrocarbon-like OA (HOA, a surrogate for primary OA, POA), oxygenated OA (OOA, a surrogate for secondary OA, SOA), and biomass-burning OA (BBOA) separated with PMF were all determined to be semi-volatile. The most aged OOA-1 and its dominant ion, CO2+, consistently exhibited the lowest volatility, with HOA, BBOA, and associated ions for each among the highest. The similar or higher volatility of HOA/POA compared to OOA/SOA contradicts the current representations of OA volatility in most atmospheric models and has important implications for aerosol growth and lifetime. Our results strongly imply that all OA types should be considered semivolatile in models. The study in Riverside identified organosulfur species (e.g. CH3HSO3+ ion, likely from methanesulfonic acid), while both studies identified ions indicative of amines (e.g. C5H12N+) with very different volatility behaviors than inorganic-dominated ions. The oxygen-to-carbon ratio of OA in each ambient study was shown to increase both with TD temperature and from morning to afternoon, while the hydrogen-to-carbon ratio showed the opposite trend.

23 This study addresses, through two types of experiments, the potential for the oceans to act as a 24 source of atmospheric ice-nucleating particles (INPs). The INP concentration via deposition 25 mode nucleation was measured in situ at a coastal site in British Columbia in August 2013. The 26 INP concentration at conditions relevant to cirrus clouds (i.e., -40°C and relative humidity with 27 respect to ice, RHice=139%) ranged from 0.2 L -1 to 3.3 L -1 . Correlations of the INP 28 concentrations with levels of anthropogenic tracers (i.e., CO, SO2, NOx, and black carbon) and 29 numbers of fluorescent particles do not indicate a significant influence from anthropogenic 30 sources or submicron bioaerosols, respectively. Additionally, the INPs measured in the 31 deposition mode showed a poor correlation with the concentration of particles with sizes larger 32 than 500 nm, which is in contrast with observations made in the immersion freezing mode. To 33 investigate the nature of particles that could have acted as deposition INP, laboratory 34 experiments with potential marine aerosol particles were conducted under the ice-nucleating 35 conditions used in the field. At -40°C, no deposition activity was observed with salt aerosol 36 particles (sodium chloride and two forms of commercial sea salt: Sigma-Aldrich and Instant 37 Ocean), particles composed of a commercial source of natural organic matter (Suwannee River 38 humic material), or particle mixtures of sea salt and humic material. In contrast, exudates from 39 three phytoplankton (Thalassiosira pseudonana, Nanochloris atomus, and Emiliania huxleyi) 40 and one marine bacterium (Vibrio harveyi) exhibited INP activity at low RHice values, down to 41 below 110%. This suggests that the INPs measured at the field site were of marine biological 42 origins, although we cannot rule out other sources, including mineral dust. 43 44 thank the University of Denver faculty start-up fund and PROF grant for 463 partial financial support. 464 465 References 466 Alpert, P.A., Aller, J.Y., and Knopf, D.A., 2011a. Ice nucleation from aqueous NaCl droplets with and 467 without marine diatoms, Atmos. Chem. Phys, 11, 5539-5555. 469Alpert, P. A., Aller, J. Y., and Knopf, D. A., 2011b. Initiation of the ice phase by marine biogenic surfaces 470 in supersaturated gas and supercooled aqueous phases, Phys.

Outdoor field measurements of bioaerosols are performed within a wide range of basic and applied scientific disciplines, each with its own goals, assumptions, and terminology. This article contains brief reviews of outdoor field bioaerosol research from these diverse interests, with emphasis on perspectives from the atmospheric sciences. The focus is on a high-level discussion of pressing scientific questions, grand challenges, and needs for cross-disciplinary collaboration. The research topics, in which bioaerosol field measurement is important, include (i) atmospheric physics, clouds, climate, and hydrological cycle; (ii) atmospheric chemistry; (iii) airborne allergencontaining particles; (iv) airborne human pathogens and national security; (v) airborne livestock and crop pathogens; and (vi) biogeography and biodiversity. We concisely review bioaerosol impacts and discuss properties that distinguish bioaerosols from abiological aerosols. We give extra focus to regions of specific interest, i.e., forests, polar regions, marine and coastal environments, deserts, urban and rural areas, and summarize key considerations related to bioaerosol measurements, such as of fluxes, of long-range transport, and of sampling from both stationary and vessel-driven platforms. Keeping in mind a series of key scientific questions posed within the diverse communities, we suggest that pressing scientific questions include the following: (i) emission sources and flux estimates; (ii) spatial distribution; (iii) changes in distribution; (iv) atmospheric aging; (v) metabolic activity; (vi) urbanization of allergies; (vii) transport of human pathogens; and (viii) climate-relevant properties.

Multiple state-of-the-art instruments sampled ambient aerosol in Riverside, California during the 2005 Study of Organic Aerosols at Riverside (SOAR) to investigate sources and chemical composition of fine particles (PM_f) in the inland region of Southern California. This paper briefly summarizes the spatial, meteorological and gas-phase conditions during SOAR-1 (15 July–15 August) and provides detailed intercomparisons of complementary measurements and average PM_f composition during this period. Daily meteorology and gas-phase species concentrations were highly repetitive with meteorological and gas-phase species concentrations displaying clear diurnal cycles and weekday/weekend contrast, with organic aerosol (OA) being the single largest component contributing approximately one-third of PM_f mass. In contrast with historical characterizations of OA in the region, several independent source apportionment efforts attributed the vast majority (~80%) of OA mass during SOAR-1 to secondary organic aerosol (SOA). Given the collocation of complementary aerosol measurements combined with a dominance of SOA during SOAR-1, this paper presents new results on intercomparisons among several complementary measurements and on PM_f composition during this period. Total non-refractory submicron (NR-PM₁) measurements from a high-resolution aerosol mass spectrometer (HR-AMS) are compared with measurements by tapered element oscillating microbalances (TEOM) including a filter dynamics measurement system (TEOM_FDMS). NR-PM₁ is highly correlated with PM_2.5 TEOM_FDMS measurements and accounts for the bulk of PM_2.5 mass with the remainder contributed primarily by refractory material. In contrast, measurements from a heated TEOM show substantial losses of semi-volatile material, including ammonium nitrate and semi-volatile organic material. Speciated HR-AMS measurements are also consistent and highly correlated with several complementary measurements, including those of a collocated compact AMS (C-AMS). Finally, elemental analysis (EA) of HR-AMS OA spectra allows direct comparison of HR-AMS organic carbon (OC) with measurements from two collocated Sunset thermal-optical semi-continuous monitors, and investigation of the elemental composition of OA in Riverside. While HR-AMS and base OC measurements from both Sunset instruments are similar within the combined uncertainties, a correction intended to account for the loss of semivolatile OC from the Sunset yields OC measurements ~30% higher than either HR-AMS or base Sunset measurements. Oxygen is the main heteroatom of ambient OA during SOAR-1 with a minimum atomic O/C of 0.28 during the morning rush hour and maximum of 0.42 during the afternoon. H/C is broadly anti-correlated with O/C, while N/C and S/C (excluding organonitrate (ON) and organosulfate (OS) functionaliti...

Abstract. Submicron aerosol was analyzed during the MILAGRO field campaign in March 2006 at the T0 urban supersite in Mexico City with a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) and complementary instrumentation. Mass concentrations, diurnal cycles, and size distributions of inorganic and organic species are similar to results from the CENICA supersite in April 2003 with organic aerosol (OA) comprising about half of the fine PM mass. Positive Matrix Factorization (PMF) analysis of the high resolution OA spectra identified three major components: chemically-reduced urban primary emissions (hydrocarbon-like OA, HOA), oxygenated OA (OOA, mostly secondary OA or SOA), and biomass burning OA (BBOA) that correlates with levoglucosan and acetonitrile. BBOA includes several very large plumes from regional fires and likely also some refuse burning. A fourth OA component is a small local nitrogen-containing reduced OA component (LOA) which accounts for 9% of the OA mass but one third of the organic nitrogen, likely as amines. OOA accounts for almost half of the OA on average, consistent with previous observations. OA apportionment results from PMF-AMS are compared to the PM2.5 chemical mass balance of organic molecular markers (CMB-OMM, from GC/MS analysis of filters). Results from both methods are overall consistent. Both assign the major components of OA to primary urban, biomass burning/woodsmoke, and secondary sources at similar magnitudes. The 2006 Mexico City emissions inventory underestimates the urban primary PM2.5 emissions by a factor of ~4, and it is ~16 times lower than afternoon concentrations when secondary species are included. Additionally, the forest fire contribution is underestimated by at least an order-of-magnitude in the inventory.

Abstract. Smoke particle emissions from the combustion of biomass fuels typical for the western and southeastern United States were studied and compared under high humidity and ambient conditions in the laboratory. The fuels used are Montana ponderosa pine (Pinus ponderosa), southern California chamise (Adenostoma fasciculatum), and Florida saw palmetto (Serenoa repens). Information on the non-refractory chemical composition of biomass burning aerosol from each fuel was obtained with an aerosol mass spectrometer and through estimation of the black carbon concentration from light absorption measurements at 870 nm. Changes in the optical and physical particle properties under high humidity conditions were observed for hygroscopic smoke particles containing substantial inorganic mass fractions that were emitted from combustion of chamise and palmetto fuels. Light scattering cross sections increased under high humidity for these particles, consistent with the hygroscopic growth measured for 100 nm particles in HTDMA measurements. Photoacoustic measurements of aerosol light absorption coefficients reveal a 20% reduction with increasing relative humidity, contrary to the expectation of light absorption enhancement by the liquid coating taken up by hygroscopic particles. This reduction is hypothesized to arise from two mechanisms: 1. Shielding of inner monomers after particle consolidation or collapse with water uptake; 2. The contribution of mass transfer through evaporation and condensation at high relative humidity to the usual heat transfer pathway for energy release by laser-heated particles in the photoacoustic measurement of aerosol light absorption. The mass transfer contribution is used to evaluate the fraction of aerosol surface covered with liquid water solution as a function of RH.