Abstract. The spatial distribution and properties of submicron organic aerosol (OA) are among the key sources of uncertainty in our understanding of aerosol effects on climate. Uncertainties are particularly large over remote regions of the free troposphere and Southern Ocean, where very few data have been available and where OA predictions from AeroCom Phase II global models span 2 to 3 orders of magnitude, greatly exceeding the model spread over source regions. The (nearly) pole-to-pole vertical distribution of non-refractory aerosols was measured with an aerosol mass spectrometer onboard the NASA DC-8 aircraft as part of the Atmospheric Tomography (ATom) mission during the Northern Hemisphere summer (August 2016) and winter (February 2017). This study presents the first extensive characterization of OA mass concentrations and their level of oxidation in the remote atmosphere. OA and sulfate are the major contributors by mass to submicron aerosols in the remote troposphere, together with sea salt in the marine boundary layer. Sulfate was dominant in the lower stratosphere. OA concentrations have a strong seasonal and zonal variability, with the highest levels measured in the lower troposphere in the summer and over the regions influenced by biomass burning from Africa (up to 10 µg sm−3). Lower concentrations (∼0.1–0.3 µg sm−3) are observed in the northern middle and high latitudes and very low concentrations (<0.1 µg sm−3) in the southern middle and high latitudes. The ATom dataset is used to evaluate predictions of eight current global chemistry models that implement a variety of commonly used representations of OA sources and chemistry, as well as of the AeroCom-II ensemble. The current model ensemble captures the average vertical and spatial distribution of measured OA concentrations, and the spread of the individual models remains within a factor of 5. These results are significantly improved over the AeroCom-II model ensemble, which shows large overestimations over these regions. However, some of the improved agreement with observations occurs for the wrong reasons, as models have the tendency to greatly overestimate the primary OA fraction and underestimate the secondary fraction. Measured OA in the remote free troposphere is highly oxygenated, with organic aerosol to organic carbon (OA ∕ OC) ratios of ∼2.2–2.8, and is 30 %–60 % more oxygenated than in current models, which can lead to significant errors in OA concentrations. The model–measurement comparisons presented here support the concept of a more dynamic OA system as proposed by Hodzic et al. (2016), with enhanced removal of primary OA and a stronger production of secondary OA in global models needed to provide better agreement with observations.
Abstract. New-particle formation (NPF) is a significant source of aerosol particles into the atmosphere. However, these particles are initially too small to have climatic importance and must grow, primarily through net uptake of low-volatility species, from diameters ∼ 1 to 30–100 nm in order to potentially impact climate. There are currently uncertainties in the physical and chemical processes associated with the growth of these freshly formed particles that lead to uncertainties in aerosol-climate modeling. Four main pathways for new-particle growth have been identified: condensation of sulfuric-acid vapor (and associated bases when available), condensation of organic vapors, uptake of organic acids through acid–base chemistry in the particle phase, and accretion of organic molecules in the particle phase to create a lower-volatility compound that then contributes to the aerosol mass. The relative importance of each pathway is uncertain and is the focus of this work. The 2013 New Particle Formation Study (NPFS) measurement campaign took place at the DOE Southern Great Plains (SGP) facility in Lamont, Oklahoma, during spring 2013. Measured gas- and particle-phase compositions during these new-particle growth events suggest three distinct growth pathways: (1) growth by primarily organics, (2) growth by primarily sulfuric acid and ammonia, and (3) growth by primarily sulfuric acid and associated bases and organics. To supplement the measurements, we used the particle growth model MABNAG (Model for Acid–Base chemistry in NAnoparticle Growth) to gain further insight into the growth processes on these 3 days at SGP. MABNAG simulates growth from (1) sulfuric-acid condensation (and subsequent salt formation with ammonia or amines), (2) near-irreversible condensation from nonreactive extremely low-volatility organic compounds (ELVOCs), and (3) organic-acid condensation and subsequent salt formation with ammonia or amines. MABNAG is able to corroborate the observed differing growth pathways, while also predicting that ELVOCs contribute more to growth than organic salt formation. However, most MABNAG model simulations tend to underpredict the observed growth rates between 10 and 20 nm in diameter; this underprediction may come from neglecting the contributions to growth from semi-to-low-volatility species or accretion reactions. Our results suggest that in addition to sulfuric acid, ELVOCs are also very important for growth in this rural setting. We discuss the limitations of our study that arise from not accounting for semi- and low-volatility organics, as well as nitrogen-containing species beyond ammonia and amines in the model. Quantitatively understanding the overall budget, evolution, and thermodynamic properties of lower-volatility organics in the atmosphere will be essential for improving global aerosol models.
Particulate matter from biomass burning emissions affects air quality, ecosystems and climate; however, quantifying these effects requires that the connection between primary emissions and secondary aerosol production is firmly established....
<p>The inorganic composition of aerosol impacts numerous chemical and physical processes and properties. However, many chemical transport models show large variability in both the concentration of the inorganic aerosols and their precursors (up to 3 orders of magnitude differences) and the inorganic aerosol composition. Different models predict very different properties (e.g., aerosol liquid water concentration and aerosol acidity) and outcomes (e.g., heterogeneous uptake of gases or aerosols&#8217; direct and indirect impacts on climate). Here, we use airborne observations from campaigns conducted around the world to investigate how the inorganic fine aerosol (PM<sub>1</sub>) composition, and one of its key parameters, aerosol acidity, changes from polluted regions (Mexico City, Los Angeles, Northeastern US, and Seoul) to remote ocean basins (the Atmospheric Tomography campaigns 1 and 2) in order to provide constraints for the chemical transport models. I find that the empirical ammonium balance with major ions (ammonium balance = mol NH<sub>4</sub> / (2&#215;mol SO<sub>4</sub> + mol NO<sub>3</sub>)) rapidly decreases from ~1 at the highest inorganic PM<sub>1</sub> concentration to 0 at the lowest inorganic PM<sub>1</sub>. The data indicate a robust trend for ammonium balance vs inorganic PM<sub>1</sub> at all altitude levels in the troposphere, suggesting that NH<sub>3</sub> emissions and subsequent neutralization of H<sub>2</sub>SO<sub>4</sub> becomes negligible in the most remote (lowest inorganic PM<sub>1</sub>) regions. Further, a robust trend for PM<sub>1</sub> pH (calculated with E-AIM) vs inorganic PM<sub>1</sub> is observed at all levels for these campaigns, as well, decreasing from a pH of ~3 to a pH of ~ &#8211;1 from the highest to lowest inorganic PM<sub>1</sub>. The data overall implies very low NH<sub>3</sub> (and NH<sub>4</sub><sup>+</sup>) throughout most of the atmosphere, contrary to predictions of some models, implying different physical properties than predicted in models. We compare these trends of ammonium balance and pH vs inorganic PM<sub>1</sub> against 9 chemical transport models (CTMs), and we find that the CTMs show large variability for both the ammonium balance and pH vs inorganic PM<sub>&#173;1</sub>, compared to observations. Generally, we find a high bias in the ammonium balance and pH, likely due to too much NH<sub>&#173;3</sub> in model (possibly too high NH<sub>3</sub> emissions over oceans or too long lifetime) and inclusion of externally mixed seasalt into the submicron pH calculation. These results overall would imply different aerosol properties in the models than observed, impacting the chemistry, optical properties, and cloud properties.</p>
<p>Organic aerosol (OA) is one of the major contributors to the PM2.5 burden both in the continental Northern Hemisphere and globally. Understanding its sources and aging is central to current air quality control strategies. For the remote troposphere, sparse in-situ data to date results in highly under constrained OA prediction models, with model diversity of up to three orders of magnitude in the recent AEROCOM-II comparison.&#160;&#160;</p><p>In the course of the recent NASA Atmospheric Tomography (ATom) set of aircraft missions, we have acquired four unique global datasets of submicron aerosol concentration and composition over the remote Atlantic and Pacific Oceans. In the remote FT OA and sulfate are the main components (about 0.3 &#181;g sm-3 in total, fairly constant outside of continental outflow. However, OA in the remote FT exhibits a much higher average carbon oxidation state than in continental airmasses (OSc up to +1 compared to -1 over the continents), much higher than assumed in most models. This also suggests a fairly hygroscopic OA. Nevertheless, in the cleanest/most remote parts of the global free troposphere (FT), sulfate predominates. This is not captured by current global models and suggests an additional chemical removal of OA (and possibly continuing formation of sulfate).</p><p>Using several different hydrocarbon-ratio based photochemical clocks in combination with back trajectories to infer the age of the airmasses sampled during ATom, we estimate that the lifetime of OA in the remote UT (after most of the convective removal has happened) is of the order of 4 days. In contrast, for chemically inert black carbon, the estimated removal timescale using the same method is significantly longer (about a week), in general agreement with previous estimates of physical removal that are used in models. The significantly shorter OA lifetime suggests an additional, chemical removal mechanism. This provides a key constraint for modeling of OA in the FT, based solely on measurements.&#160; Both heterogeneous oxidation by OH and aerosol photolysis are possible pathways for OA removal that have been suggested previously. Sensitivity studies in CESM2 AND GEOS-Chem with updated chemistry and aerosol sources are used to constrain the relative importance of each pathway for OA removal during ATom.</p>
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