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We discuss the use of a two-dimensional volatility-oxidation space (2-D-VBS) to describe organic-aerosol chemical evolution. The space is built around two coordinates, volatility and the degree of oxidation, both of which can be constrained observationally or specified for known molecules. Earlier work presented the thermodynamics of organics forming the foundation of this 2-D-VBS, allowing us to define the average composition (C, H, and O) of organics, including organic aerosol (OA) based on volatility and oxidation state. Here we discuss how we can analyze experimental data, using the 2-D-VBS to gain fundamental insight into organic-aerosol chemistry. We first present a well-understood \"traditional\" secondary organic aerosol (SOA) system – SOA from α-pinene + ozone, and then turn to two examples of \"non-traditional\" SOA formation – SOA from wood smoke and dilute diesel-engine emissions. Finally, we discuss the broader implications of this analysis.

Experiments were conducted to investigate light absorption of organic aerosol (OA) in fresh and photo-chemically aged biomass-burning emissions. The experiments considered residential hardwood fuel (oak) and fuels commonly consumed in wild-land and prescribed fires in the United States (pocosin pine and gallberry). Photo-chemical aging was performed in an environmental chamber. We constrained the effective light-absorption properties of the OA using conservative limiting assumptions, and found that both primary organic aerosol (POA) in the fresh emissions and secondary organic aerosol (SOA) produced by photo-chemical aging contain brown carbon, and absorb light to a significant extent. This work presents the first direct evidence that SOA produced in aged biomass-burning emissions is absorptive. For the investigated fuels, SOA is less absorptive than POA in the long visible, but exhibits stronger wavelength-dependence and is more absorptive in the short visible and near-UV. Light absorption by SOA in biomass-burning emissions might be an important contributor to the global radiative forcing budget.

Carbonaceous particulate matter (PM), comprising black carbon (BC), primary organic aerosol (POA) and secondary organic aerosol (SOA, from atmospheric aging of precursors), is a highly toxic vehicle exhaust component. Therefore, understanding vehicle pollution requires knowledge of both primary emissions, and how these emissions age in the atmosphere. We provide a systematic examination of carbonaceous PM emissions and parameterisation of SOA formation from modern diesel and gasoline cars at different temperatures (22, −7 °C) during controlled laboratory experiments. Carbonaceous PM emission and SOA formation is markedly higher from gasoline than diesel particle filter (DPF) and catalyst-equipped diesel cars, more so at −7 °C, contrasting with nitrogen oxides (NO X ). Higher SOA formation from gasoline cars and primary emission reductions for diesels implies gasoline cars will increasingly dominate vehicular total carbonaceous PM, though older non-DPF-equipped diesels will continue to dominate the primary fraction for some time. Supported by state-of-the-art source apportionment of ambient fossil fuel derived PM, our results show that whether gasoline or diesel cars are more polluting depends on the pollutant in question, i.e. that diesel cars are not necessarily worse polluters than gasoline cars.

We develop the thermodynamic underpinnings of a two-dimensional volatility basis set (2D-VBS) employing saturation mass concentration (Co) and the oxygen content (O:C) to describe volatility, mixing thermodynamics, and chemical evolution of organic aerosol. The work addresses a simple question: \"Can we reasonably constrain organic-aerosol composition in the atmosphere based on only two measurable organic properties, volatility and the extent of oxygenation?\" This is an extension of our earlier one-dimensional approach employing volatility only (C* = γ Co, where γ is an activity coefficient). Using available constraints on bulk organic-aerosol composition, we argue that one can reasonably predict the composition of organics (carbon, oxygen and hydrogen numbers) given a location in the Co – O:C space. Further, we argue that we can constrain the activity coefficients at various locations in this space based on the O:C of the organic aerosol.

A new secondary organic aerosol (SOA) parameterization based on the volatility basis set is implemented in a regional air quality model WRF‐CHEM. Full meteorological and chemistry simulations are carried out for the United States for August–September 2006. Predicted organic aerosol (OA) concentrations are compared against surface measurements made by several networks and aircraft data from the TexAQS‐2006 field campaign. Elemental carbon simulations are also evaluated in order to evaluate the model's ability to capture their emissions, transport, and removal. Certain measurement limitations, such as daily averaged OA concentrations, impose some difficulties on the model evaluation, and hourly averaged OA measurements provide more informative constraints compared to daily concentrations. The updated model demonstrates a significant improvement in simulating the OA concentrations compared to the standard WRF‐CHEM, which predicts very little SOA. The improvement in organic carbon (OC) predictions is noticeable in correlations and model bias. The correlations of OC exceed that of the persistence forecasts for hourly concentrations in the southeast United States during daytime. The updated traditional SOA yields still lead to an underestimation of observed OA, while addition of the multigenerational volatile organic compound (VOC) oxidation drastically improves model performance. However, several key uncertainties remain in SOA formation and loss mechanisms, which are characterized through several perturbation simulations. Dry deposition of VOC oxidation products is an important factor in the atmospheric SOA budget. The combination of the biogenic VOC emissions, updated SOA yields, and aging mechanism result in biogenic SOA being the dominant OA component for much of the nonurban United States. Key Points VBS approach to secondary organic aerosol formation within WRF‐CHEM Evaluation using surface and aircraft data Organic aerosol modeling for the continental United States

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.

During the fourth Fire Lab at Missoula Experiment (FLAME-4, October–November 2012) a large variety of regionally and globally significant biomass fuels was burned at the US Forest Service Fire Sciences Laboratory in Missoula, Montana. The particle emissions were characterized by an extensive suite of instrumentation that measured aerosol chemistry, size distribution, optical properties, and cloud-nucleating properties. The trace gas measurements included high-resolution mass spectrometry, one- and two-dimensional gas chromatography, and open-path Fourier transform infrared (OP-FTIR) spectroscopy. This paper summarizes the overall experimental design for FLAME-4 – including the fuel properties, the nature of the burn simulations, and the instrumentation employed – and then focuses on the OP-FTIR results. The OP-FTIR was used to measure the initial emissions of 20 trace gases: CO2, CO, CH4, C2H2, C2H4, C3H6, HCHO, HCOOH, CH3OH, CH3COOH, glycolaldehyde, furan, H2O, NO, NO2, HONO, NH3, HCN, HCl, and SO2. These species include most of the major trace gases emitted by biomass burning, and for several of these compounds, this is the first time their emissions are reported for important fuel types. The main fire types included African grasses, Asian rice straw, cooking fires (open (three-stone), rocket, and gasifier stoves), Indonesian and extratropical peat, temperate and boreal coniferous canopy fuels, US crop residue, shredded tires, and trash. Comparisons of the OP-FTIR emission factors (EFs) and emission ratios (ERs) to field measurements of biomass burning verify that the large body of FLAME-4 results can be used to enhance the understanding of global biomass burning and its representation in atmospheric chemistry models. Crop residue fires are widespread globally and account for the most burned area in the US, but their emissions were previously poorly characterized. Extensive results are presented for burning rice and wheat straw: two major global crop residues. Burning alfalfa produced the highest average NH3 EF observed in the study (6.63 ± 2.47 g kg−1), while sugar cane fires produced the highest EF for glycolaldehyde (6.92 g kg−1) and other reactive oxygenated organic gases such as HCHO, HCOOH, and CH3COOH. Due to the high sulfur and nitrogen content of tires, they produced the highest average SO2 emissions (26.2 ± 2.2 g kg−1) and high NOx and HONO emissions. High variability was observed for peat fire emissions, but they were consistently characterized by large EFs for NH3 (1.82 ± 0.60 g kg−1) and CH4 (10.8 ± 5.6 g kg−1). The variability observed in peat fire emissions, the fact that only one peat fire had previously been subject to detailed emissions characterization, and the abundant emissions from tropical peatlands all impart high value to our detailed measurements of the emissions from burning three Indonesian peat samples. This study also provides the first EFs for HONO and NO2 for Indonesian peat fires. Open cooking fire emissions of HONO and HCN are reported for the first time, and the first emissions data for HCN, NO, NO2, HONO, glycolaldehyde, furan, and SO2 are reported for \"rocket\" stoves: a common type of improved cookstove. The HCN / CO emission ratios for cooking fires (1.72 × 10−3 ± 4.08 × 10−4) and peat fires (1.45 × 10−2 ± 5.47 × 10−3) are well below and above the typical values for other types of biomass burning, respectively. This would affect the use of HCN / CO observations for source apportionment in some regions. Biomass burning EFs for HCl are rare and are reported for the first time for burning African savanna grasses. High emissions of HCl were also produced by burning many crop residues and two grasses from coastal ecosystems. HCl could be the main chlorine-containing gas in very fresh smoke, but rapid partitioning to aerosol followed by slower outgassing probably occurs.

We quantify the hygroscopic properties of particles freshly emitted from biomass burning and after several hours of photochemical aging in a smog chamber. Values of the hygroscopicity parameter, κ, were calculated from cloud condensation nuclei (CCN) measurements of emissions from combustion of 12 biomass fuels commonly burned in North American wildfires. Prior to photochemical aging, the κ of the fresh primary aerosol varied widely, between 0.06 (weakly hygroscopic) and 0.6 (highly hygroscopic). The hygroscopicity of the primary aerosol was positively correlated with the inorganic mass fraction of the particles. Photochemical processing reduced the range of κ values to between 0.08 and 0.3. The changes in κ were driven by the photochemical production of secondary organic aerosol (SOA). SOA also contributed to growth of particles formed during nucleation events. Analysis of the nucleation mode particles enabled the first direct quantification of the hygroscopicity parameter κ for biomass burning SOA, which was on average 0.11, similar to values observed for biogenic SOA. Although initial CCN activity of biomass burning aerosol emissions are highly variable, after a few hours of photochemical processing κ converges to a value of 0.2 ± 0.1. Therefore, photochemical aging reduces the variability of biomass burning CCN κ, which should simplify analysis of the potential effects of biomass burning aerosol on climate.

Experimental simulations of galaxy-forming conditions using lasers show that the Biermann battery generates seed magnetic fields, which turbulence can amplify to affect galaxy evolution. Experimental astrophysical magnetic fields In an example of the relatively young field of laboratory astrophysics, researchers at the LULI 2000 laser facility in Palaiseau, near Paris, have tackled the question of how magnetic fields are produced in an astrophysical environment. The standard model assumes that galactic magnetic fields arise from the amplification of 'seed' fields through dynamo or turbulent processes. It is thought that the tiny seed fields are induced by a thermally generated electronic field through the Biermann battery effect. The feasibility of this mechanism was demonstrated in a system mimicking the conditions associated with astrophysical shocks in the intergalactic medium. Seed fields were indeed produced by the Biermann effect. When the results are scaled to the intergalactic medium, where turbulence can act on timescales in the region of 700 million years, amplification to the observed levels of galactic magnetic fields becomes possible. The standard model for the origin of galactic magnetic fields is through the amplification of seed fields via dynamo or turbulent processes to the level consistent with present observations 1 , 2 , 3 . Although other mechanisms may also operate 4 , 5 , currents from misaligned pressure and temperature gradients (the Biermann battery process) inevitably accompany the formation of galaxies in the absence of a primordial field. Driven by geometrical asymmetries in shocks 6 associated with the collapse of protogalactic structures, the Biermann battery is believed to generate tiny seed fields to a level of about 10 −21  gauss (refs 7 , 8 ). With the advent of high-power laser systems in the past two decades, a new area of research has opened in which, using simple scaling relations 9 , 10 , astrophysical environments can effectively be reproduced in the laboratory 11 , 12 . Here we report the results of an experiment that produced seed magnetic fields by the Biermann battery effect. We show that these results can be scaled to the intergalactic medium, where turbulence, acting on timescales of around 700 million years, can amplify the seed fields 13 , 14 sufficiently to affect galaxy evolution.

Restaurants and vehicles are important urban sources of particulate matter (PM). Due to the ubiquitous presence of these sources within cities, large variabilities in PM concentrations occur in source-rich environments (e.g. downtown), especially during times of peak activity such as meal times and rush hour. Due to intracity variations in factors such as racial-ethnic composition and economic status, we hypothesized that certain socio-economic groups living closer to sources are exposed to higher PM concentrations. To test this hypothesis, we coupled mobile PM measurements with census data in two midsize US cities: Oakland, CA, and Pittsburgh, PA. A novel aspect of our study is that our measurements are performed at a high (block-level) spatial resolution, which enables us to assess the direct relationship between PM concentrations and socio-economic metrics across different neighborhoods of these two cities. We find that restaurants cause long-term average PM enhancements of 0.1 to 0.3 µg m−3 over length scales between 50 and 450 m. We also find that this PM pollution from restaurants is unevenly distributed amongst different socio-economic groups. On average, areas near restaurant emissions have about 1.5× people of color (African American, Hispanic, Asian, etc), 2.5× poverty, and 0.8× household income, compared to areas far from restaurant emissions. Our findings imply that there are socio-economic disparities in long-term exposure to PM emissions from restaurants. Further, these socio-economic groups also frequently experience acutely high levels of cooking PM (tens to hundreds of µg m−3 in mass concentrations) and co-emitted pollutants. While there are large variations in socio-economic metrics with respect to restaurant proximity, we find that these metrics are spatially invariant with respect to highway proximity. Thus, any socio-economic disparities in exposure to highway emissions are, at most, mild, and certainly small compared to disparities in exposure to restaurant emissions.