Exploring summertime organic aerosol formation in the eastern United States using a regional-scale budget approach and ambient measurements

[1] We present a new method for estimating the overall organic aerosol (OA) formation rate at the regional scale using a chemical transport model (CTM), PMCAMx-2008, and an extensive set of measurements (Speciation Trends Network, Interagency Monitoring of Protected Visual Environments, Pittsburgh Air Quality Study, Southeastern Aerosol Research and Characterization) for the eastern United States. PMCAMx-2008 takes into account up-to-date OA formation theory including primary OA evaporation, updated secondary OA (SOA) yields from traditional volatile organic precursor gases and multigenerational oxidation chemistry (aging) of vapors from anthropogenic sources, which lowers the volatility of the OA distribution over time. An overall OA formation rate of 22 ± 5 ktons d−1 is consistent with available measurements for this summer time period. We perform an extensive sensitivity analysis of uncertain OA model processes to demonstrate the relationship between the estimated total OA production rate and model performance. Perturbing, within reasonable limits, emissions of volatile precursors, SOA yields from isoprene oxidation, and the solubility of organic vapors produces model predictions for total OA that deviate little from the base case performance. The fractional error and fractional bias vary by less than 6% and 13%, respectively. These cases also result in total OA formation rates within 5 ktons d−1 of the base case. Neglecting chemical aging of anthropogenic OA components results in OA levels significantly lower than the observations everywhere, while aging biogenic SOA with the same parameters used for the base case anthropogenic SOA aging results in overpredictions in both the South and Midwest United States. Aging biogenic and anthropogenic SOA together with a reduced aging reaction rate results in reasonable model performance and an OA formation rate of ∼23 ktons d−1. This suggests that even though uncertainties in the OA aging mechanism and other important parameters may lead to uncertainties in the contributions of specific OA formation pathways, the proposed approach may be used to infer upper and lower limits on the total OA mass formation rate.</p

Atmospheric Aerosol Water-Soluble Organic Carbon Measurement: A Theoretical Analysis

The measurement of Water-Soluble Organic Carbon (WSOC) in atmospheric aerosol is usually carried out by sample collection on filters, extraction in ultrapure water, filtration, and measurement of the total organic carbon. This paper investigates the role of different conditions of sampling and extraction as well as the range of solubilities of the organic compounds that contribute to the WSOC. The sampling and extraction of WSOC can be described by a single parameter, <i>P</i>, expressing the ratio of water used per volume of air sampled on the analyzed filter. Two cases are examined in order to bound the range of interactions of the various organic aerosol components with each other. In the first we assume that the organic species form an ideal solution in the particle and in the second that the extraction of a single compound is independent of the presence of the other organics. The ideal organic solution model predicts that species with water solubility as low as 10^–4 g L^–1 contribute to the measured WSOC. In the other end, the independent compounds model predicts that low-solubility (as low as 10^–7 g L^–1) compounds are part of the WSOC. Studies of the WSOC composition are consistent with the predictions of the ideal organic solution model. A value of <i>P</i> = 0.1 cm³ m^–3 is proposed for the extraction of WSOC for typical organic aerosol concentrations (1–10 μg m^–3). WSOC measurements under high concentration conditions often used during source sampling will tend to give low WSOC values unless higher <i>P</i> values are used

Determination of the age distribution of primary and secondary aerosol species using a chemical transport model

[1] A computationally efficient scheme to allow tracking of aerosol species age as a function of space and time within a three-dimensional chemical transport model (CTM) has been developed. The aerosol age distribution is calculated by utilizing the Particulate Matter Source Apportionment Technology (PSAT) algorithm which allows the calculation of different source contributions to both primary and secondary particulate matter concentrations in the modeling domain. As an example, the aerosol age in the eastern United States, including both primary and secondary species, is examined using the regional CTM PMCAMx. The average calculated ages are on the order of a few days for particulate matter near the ground but are highly variable in space and time. Primary aerosol species had average ages of approximately 24 h over this polluted continental region while the average ages for secondary species were 48–72 h near the surface. As expected, the average age of all aerosol components increases vertically in the atmosphere. Age increases rapidly away from the sources of aerosol and its precursors, and for nonvolatile species it increases with particle size.</p

High formation of secondary organic aerosol from the photo-oxidation of toluene

Toluene and other aromatics have long been viewed as the dominant anthropogenic secondary organic aerosol (SOA) precursors, but the SOA mass yields from toluene reported in previous studies vary widely. Experiments conducted in the Carnegie Mellon University environmental chamber to study SOA formation from the photo-oxidation of toluene show significantly larger SOA production than parameterizations employed in current air-quality models. Aerosol mass yields depend on experimental conditions: yields are higher under higher UV intensity, under low-NOx conditions and at lower temperatures. The extent of oxidation of the aerosol also varies with experimental conditions, consistent with ongoing, progressive photochemical aging of the toluene SOA. Measurements using a thermodenuder system suggest that the aerosol formed under high- and low-NOx conditions is semi-volatile. These results suggest that SOA formation from toluene depends strongly on ambient conditions. An approximate parameterization is proposed for use in air-quality models until a more thorough treatment accounting for the dynamic nature of this system becomes available.</p

Modeling global secondary organic aerosol formation and processing with the volatility basis set: Implications for anthropogenic secondary organic aerosol

[1] The volatility basis set, a computationally efficient framework for the description of organic aerosol partitioning and chemical aging, is implemented in the Goddard Institute for Space Studies General Circulation Model II′ for a coupled global circulation and chemical transport model to simulate secondary organic aerosol (SOA) formation. The latest smog chamber information about the yields of anthropogenic and biogenic precursors is incorporated in the model. SOA formation from monoterpenes, sesquiterpenes, isoprene, and anthropogenic precursors is estimated as 17.2, 3.9, 6.5, and 1.6 Tg yr−1, respectively. Reducing water solubility of secondary organic gas from 105 to 103 mol L−1 atm−1 (1 atm = 1.01325 × 105 N m−2) leads to a 90% increase in SOA production and an increase of over 340% in total atmospheric burden, from 0.54 to 2.4 Tg. Increasing the temperature sensitivity of SOA leads to a 30% increase in production, to 38.2 Tg yr−1. Since the additional SOA is formed at high altitude, where deposition time scales are longer, the average lifetime is doubled from 6.8 to 14.3 days, resulting in a near tripling of atmospheric burden to 1.5 Tg. Chemical aging of anthropogenic SOA by gas-phase reaction of the SOA components with the hydroxyl radical adds an additional 2.7–9.3 Tg yr−1 of anthropogenic SOA to the above production rates and nearly doubles annual average total SOA burdens. The possibility of such high anthropogenic SOA production rates challenges the assumption that anthropogenic volatile organic compounds are not important SOA precursors on a global scale. Model predictions with and without SOA aging are compared with data from two surface observation networks: the Interagency Monitoring of Protected Visual Environments for the United States and the European Monitoring and Evaluation Programme.</p

Simulating the oxygen content of ambient organic aerosol with the 2D volatility basis set

A module predicting the oxidation state of organic aerosol (OA) has been developed using the two-dimensional volatility basis set (2D-VBS) framework. This model is an extension of the 1D-VBS framework and tracks saturation concentration and oxygen content of organic species during their atmospheric lifetime. The host model, a one-dimensional Lagrangian transport model, is used to simulate air parcels arriving at Finokalia, Greece during the Finokalia Aerosol Measurement Experiment in May 2008 (FAME-08). Extensive observations were collected during this campaign using an aerosol mass spectrometer (AMS) and a thermodenuder to determine the chemical composition and volatility, respectively, of the ambient OA. Although there are several uncertain model parameters, the consistently high oxygen content of OA measured during FAME-08 (O:C = 0.8) can help constrain these parameters and elucidate OA formation and aging processes that are necessary for achieving the high degree of oxygenation observed. The base-case model reproduces observed OA mass concentrations (measured mean = 3.1 μg m−3, predicted mean = 3.3 μg m−3) and O:C (predicted O:C = 0.78) accurately. A suite of sensitivity studies explore uncertainties due to (1) the anthropogenic secondary OA (SOA) aging rate constant, (2) assumed enthalpies of vaporization, (3) the volatility change and number of oxygen atoms added for each generation of aging, (4) heterogeneous chemistry, (5) the oxidation state of the first generation of compounds formed from SOA precursor oxidation, and (6) biogenic SOA aging. Perturbations in most of these parameters do impact the ability of the model to predict O:C well throughout the simulation period. By comparing measurements of the O:C from FAME-08, several sensitivity cases including a high oxygenation case, a low oxygenation case, and biogenic SOA aging case are found to unreasonably depict OA aging, keeping in mind that this study does not consider possibly important processes like fragmentation that may offset mass gains and affect the prediction bias. On the other hand, many of the cases chosen for this study predict average O:C estimates that are consistent with the observations, illustrating the need for more thorough experimental characterizations of OA parameters including the enthalpy of vaporization and oxidation state of the first generation of SOA products. The ability of the model to predict OA concentrations is less sensitive to perturbations in the model parameters than its ability to predict O:C. In this sense, quantifying O:C with a predictive model and constraining it with AMS measurements can reduce uncertainty in our understanding of OA formation and aging.</p

A two-dimensional volatility basis set: 1. organic-aerosol mixing thermodynamics

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.</p

Simulation of in situ ultrafine particle formation in the eastern United States using PMCAMx-UF

[1] A three-dimensional chemical transport model has been developed incorporating the Dynamic Model for Aerosol Nucleation for the simulation of aerosol dynamics into the regional model PMCAMx. Using a scaled version of the ternary H2SO4-NH3-H2O nucleation theory and the Two Moment Aerosol Sectional algorithm, the new model (PMCAMx-UF) is used to simulate a summertime period in the eastern United States. The model predicts, in agreement with observations, frequent nucleation events that take place over hundreds to thousands of kilometers, especially in the northeastern United States. Detailed comparison with the observations of the Pittsburgh Air Quality Study suggests that the model reproduces reasonably well the details of the events in this sulfur rich area but has a tendency to overpredict the frequency of the events. Regional nucleation is predicted to increase the total number concentrations by roughly a factor of 2.5 over the whole domain. The corresponding increases for particles larger than 10 nm (N10) and 100 nm (N100) were 75% and 15%, respectively. In the Ohio River Valley the increases are as much as a factor of 10 for total particle number and 40% for N100. Contrary to the total particle concentration, increases of N100 take place often in areas different than those of the nucleation events. Nucleation is predicted to decrease the N100 in some areas even if it increases the total number concentration. The sensitivity of the model to the nucleation rate scaling parameter and the ammonia levels is discussed.</p

A two-dimensional volatility basis set – Part 2: Diagnostics of organic-aerosol evolution

<p>We discuss the use of a two-dimensional volatility-oxidation space (2-D-VBS) to describe organicaerosol 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 wellunderstood “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.</p

Particle number concentrations over Europe in 2030: the role of emissions and new particle formation

<p>The aerosol particle number concentration is a key parameter when estimating impacts of aerosol particles on climate and human health. We use a three-dimensional chemical transport model with detailed microphysics, PMCAMx-UF, to simulate particle number concentrations over Europe in the year 2030, by applying emission scenarios for trace gases and primary aerosols. The scenarios are based on expected changes in anthropogenic emissions of sulfur dioxide, ammonia, nitrogen oxides, and primary aerosol particles with a diameter less than 2.5 μm (PM_2.5) focusing on a photochemically active period, and the implications for other seasons are discussed. <br><br>For the baseline scenario, which represents a best estimate of the evolution of anthropogenic emissions in Europe, PMCAMx-UF predicts that the total particle number concentration (<em>N</em>_tot) will decrease by 30–70% between 2008 and 2030. The number concentration of particles larger than 100 nm (<em>N</em>₁₀₀), a proxy for cloud condensation nuclei (CCN) concentration, is predicted to decrease by 40–70% during the same period. The predicted decrease in <em>N</em>_tot is mainly a result of reduced new particle formation due to the expected reduction in SO₂ emissions, whereas the predicted decrease in <em>N</em>₁₀₀ is a result of both decreasing condensational growth and reduced primary aerosol emissions. For larger emission reductions, PMCAMx-UF predicts reductions of 60–80% in both <em>N</em>_totand <em>N</em>₁₀₀ over Europe. <br><br>Sensitivity tests reveal that a reduction in SO₂ emissions is far more efficient than any other emission reduction investigated, in reducing <em>N</em>_tot. For <em>N</em>₁₀₀, emission reductions of both SO₂ and PM_2.5 contribute significantly to the reduced concentration, even though SO₂ plays the dominant role once more. The impact of SO₂ for both new particle formation and growth over Europe may be expected to be somewhat higher during the simulated period with high photochemical activity than during times of the year with less incoming solar radiation. <br><br>The predicted reductions in both <em>N</em>_tot and <em>N</em>₁₀₀ between 2008 and 2030 in this study will likely reduce both the aerosol direct and indirect effects, and limit the damaging effects of aerosol particles on human health in Europe.</p