Simulating indoor inorganic aerosols of outdoor origin with the inorganic aerosol thermodynamic equilibrium model ISORROPIA.

Outdoor aerosols can transform and have their composition altered upon transport indoors. Herein, IMAGES, a platform that simulates indoor organic aerosol with the 2-dimensional volatility basis set (2D-VBS), was extended to incorporate the inorganic aerosol thermodynamic equilibrium model, ISORROPIA. The model performance was evaluated by comparing aerosol component predictions to indoor measurements from an aerosol mass spectrometer taken during the summer and winter seasons. Since ammonia was not measured in the validation dataset, outdoor ammonia was estimated from aerosol measurements using a novel pH-based algorithm, while nitric acid was held constant. Modeled indoor ammonia sources included temperature-based occupant and surface emissions. Sensitivity to the nitric acid indoor surface deposition rate ß g , HNO 3 , g was explored by varying it in model runs, which did not affect modeled sulfate due to its non-volatile nature, though the fitting of a filter efficiency was required for good correlations of modeled sulfate with measurements in both seasons. Modeled summertime nitrate well-matched measured observations when ß g , HNO 3 , g = 2.75 h - 1 , but wintertime comparisons were poor, possibly due to missing thermodynamic processes within the heating, ventilating, and air-conditioning (HVAC) system. Ammonium was consistently overpredicted, potentially due to neglecting thirdhand smoke impacts observed in the field campaign, as well as HVAC impacts.

Modeling Emissions and Ozone Air Quality Impacts of Future Scenarios for Energy and Power Production in the Rocky Mountain States.

This study examines air quality impacts of scenarios for energy production and use in 2030 across Colorado, northern New Mexico, Utah, and Wyoming. Scenarios feature contrasting levels of oil and gas production and shares of electricity from coal, natural gas, and renewables. Hourly emissions are resolved for individual power plants; oil and gas emissions are basin-specific. Ozone decreased from 2011 to the 2030 baseline, with median and 90th percentile reductions in maximum daily 8 h average (MDA8) ozone across the four-state domain of 3.5 and 7.1 ppb, respectively, resulting in 200 fewer premature deaths annually. Relative to the 2030 baseline, MDA8 ozone increased in the "cheap gas" scenario, with median and 90th percentile increases of 0.1 and 1.0 ppb, and declined in a scenario with greenhouse gas (GHG) emissions fees, with median and 90th percentile reductions of 0.2 and 1.5 ppb. Reduced coal generation lowered SO2 emissions in all future scenarios compared to 2011. GHG emissions from electricity and oil and gas production declined by 4% (CO2-equivalent) from 2011 to the 2030 baseline, increased by 10% from the 2030 baseline to the cheap gas scenario, and declined by 28% from the 2030 baseline to the GHG fees scenario.

Decreasing Aerosol Water Is Consistent with OC Trends in the Southeast U.S.

Water is a ubiquitous and abundant component of atmospheric aerosols. It influences light scattering, the hydrological cycle, atmospheric chemistry, and secondary particulate matter (PM) formation. Despite the critical importance of aerosol liquid water, mass concentrations are not well-known. Using speciated ion and meteorological data from the Southeastern Aerosol Research and Characterization network, we employ the thermodynamic model ISORROPIAv2.1 to estimate water mass concentrations and evaluate trends from 2001 to 2012 in urban and rural locations. The purpose of this study is to better understand the historical trends of aerosol liquid water in the southeast U.S. in the context of improved air quality and recently noted reductions in particulate organic carbon (OC). Aerosol water mass concentrations decrease by ∼79% from 2001 to 2012 in the region. Decreases are more prominent in rural than in urban areas. Fractional contribution of water to PM also decreases during the same time period, and this is consistent with recently noted improvements in visibility. These findings agree with the hypotheses that aerosol liquid water facilitates formation of biogenic secondary organic aerosol (SOA) and that biogenically derived SOA is modulated in the presence of anthropogenic perturbations.

Differences between magnitudes and health impacts of BC emissions across the United States using 12 km scale seasonal source apportionment.

Recent assessments have analyzed the health impacts of PM2.5 from emissions from different locations and sectors using simplified or reduced-form air quality models. Here we present an alternative approach using the adjoint of the Community Multiscale Air Quality (CMAQ) model, which provides source-receptor relationships at highly resolved sectoral, spatial, and temporal scales. While damage resulting from anthropogenic emissions of BC is strongly correlated with population and premature death, we found little correlation between damage and emission magnitude, suggesting that controls on the largest emissions may not be the most efficient means of reducing damage resulting from anthropogenic BC emissions. Rather, the best proxy for locations with damaging BC emissions is locations where premature deaths occur. Onroad diesel and nonroad vehicle emissions are the largest contributors to premature deaths attributed to exposure to BC, while onroad gasoline emissions cause the highest deaths per amount emitted. Emissions in fall and winter contribute to more premature deaths (and more per amount emitted) than emissions in spring and summer. Overall, these results show the value of the high-resolution source attribution for determining the locations, seasons, and sectors for which BC emission controls have the most effective health benefits.

Epoxide pathways improve model predictions of isoprene markers and reveal key role of acidity in aerosol formation.

Isoprene significantly contributes to organic aerosol in the southeastern United States where biogenic hydrocarbons mix with anthropogenic emissions. In this work, the Community Multiscale Air Quality model is updated to predict isoprene aerosol from epoxides produced under both high- and low-NOx conditions. The new aqueous aerosol pathways allow for explicit predictions of two key isoprene-derived species, 2-methyltetrols and 2-methylglyceric acid, that are more consistent with observations than estimates based on semivolatile partitioning. The new mechanism represents a significant source of organic carbon in the lower 2 km of the atmosphere and captures the abundance of 2-methyltetrols relative to organosulfates during the simulation period. For the parametrization considered here, a 25% reduction in SOx emissions effectively reduces isoprene aerosol, while a similar reduction in NOx leads to small increases in isoprene aerosol.

4D-Var Inversion of European NH3 Emissions Using CrIS NH3 Measurements and GEOS-Chem Adjoint With Bi-Directional and Uni-Directional Flux Schemes.

We conduct the first 4D-Var inversion of NH3 accounting for NH3 bi-directional flux, using CrIS satellite NH3 observations over Europe in 2016. We find posterior NH3 emissions peak more in springtime than prior emissions at continental to national scales, and annually they are generally smaller than the prior emissions over central Europe, but larger over most of the rest of Europe. Annual posterior anthropogenic NH3 emissions for 25 European Union members (EU25) are 25% higher than the prior emissions and very close (<2% difference) to other inventories. Our posterior annual anthropogenic emissions for EU25, the UK, the Netherlands, and Switzerland are generally 10%-20% smaller than when treating NH3 fluxes as uni-directional emissions, while the monthly regional difference can be up to 34% (Switzerland in July). Compared to monthly mean in-situ observations, our posterior NH3 emissions from both schemes generally improve the magnitude and seasonality of simulated surface NH3 and bulk NH x wet deposition throughout most of Europe, whereas evaluation against hourly measurements at a background site shows the bi-directional scheme better captures observed diurnal variability of surface NH3. This contrast highlights the need for accurately simulating diurnal variability of NH3 in assimilation of sun-synchronous observations and also the potential value of future geostationary satellite observations. Overall, our top-down ammonia emissions can help to examine the effectiveness of air pollution control policies to facilitate future air pollution management, as well as helping us understand the uncertainty in top-down NH3 emissions estimates associated with treatment of NH3 surface exchange.

Assessing near-field and downwind impacts of reactivity-based substitutions.

Three-dimensional chemical transport modeling of six different solvent substitution test scenarios was used to investigate possible transport effects of using volatile organic compound (VOC) reactivity scales for air quality management purposes with a particular focus on the northeastern United States. The primary issues analyzed are whether uses of reactivity-based substitutions adversely affect ozone concentrations downwind of the area in which they are applied and which reactivity scales appear most appropriate for areas where ozone transport between multiple cities is significant. VOC substitution scenarios were designed to assess biases in ozone metrics associated with substituting relatively highly reactive VOCs (as defined by the Maximum Incremental Reactivity [MIR] scale) associated with solvent use with less reactive VOCs that might be considered as possible substitutes. Aiming to balance industrially realistic and scientifically relevant constraints, the set of solvent emissions to be substituted included toluene, isomers of xylene, 2-butoxyethanol by the surrogate, and lower reactivity compounds 2-methylheptane and n-butyl acetate. For a 14-day episode in August 2002, seven scenarios were modeled including base-case emissions, removal of the selected higher reactivity solvent compounds, and substitution tests using the equivalent mass or equivalent reactivity-adjusted emissions based on the MIR and Maximum Ozone Incremental Reactivity (MOIR) reactivity scales. Results show that downwind increases in ozone concentrations are noticeable for the MIR-based substitution test scenarios although sensitivities demonstrate that these could be used to complement oxides of nitrogen (NOx) controls. However, using the MOIR-scaled substitution test scenario led to results that were less biased, and the population-weighted metric showed little bias compared with the base case. Temporally and spatially extensive decreases are evident with the solvent mass substitution and the selected emissions removal test scenarios, supporting the conclusion that reactivity-based control can be used to regionally reduce ozone.

A multiphase CMAQ version 5.0 adjoint.

We present the development of a multiphase adjoint for the Community Multiscale Air Quality (CMAQ) model, a widely used chemical transport model. The adjoint model provides location- and time-specific gradients that can be used in various applications such as backward sensitivity analysis, source attribution, optimal pollution control, data assimilation, and inverse modeling. The science processes of the CMAQ model include gas-phase chemistry, aerosol dynamics and thermodynamics, cloud chemistry and dynamics, diffusion, and advection. Discrete adjoints are implemented for all the science processes, with an additional continuous adjoint for advection. The development of discrete adjoints is assisted with algorithmic differentiation (AD) tools. Particularly, the Kinetic PreProcessor (KPP) is implemented for gas-phase and aqueous chemistry, and two different automatic differentiation tools are used for other processes such as clouds, aerosols, diffusion, and advection. The continuous adjoint of advection is developed manually. For adjoint validation, the brute-force or finite-difference method (FDM) is implemented process by process with box- or column-model simulations. Due to the inherent limitations of the FDM caused by numerical round-off errors, the complex variable method (CVM) is adopted where necessary. The adjoint model often shows better agreement with the CVM than with the FDM. The adjoints of all science processes compare favorably with the FDM and CVM. In an example application of the full multiphase adjoint model, we provide the first estimates of how emissions of particulate matter (PM2.5) affect public health across the US.

Coupling of organic and inorganic aerosol systems and the effect on gas-particle partitioning in the southeastern US.

Several models were used to describe the partitioning of ammonia, water, and organic compounds between the gas and particle phases for conditions in the southeastern US during summer 2013. Existing equilibrium models and frameworks were found to be sufficient, although additional improvements in terms of estimating pure-species vapor pressures are needed. Thermodynamic model predictions were consistent, to first order, with a molar ratio of ammonium to sulfate of approximately 1.6 to 1.8 (ratio of ammonium to 2× sulfate, RN/2S ≈ 0.8 to 0.9) with approximately 70% of total ammonia and ammonium (NH x ) in the particle. Southeastern Aerosol Research and Characterization Network (SEARCH) gas and aerosol and Southern Oxidant and Aerosol Study (SOAS) Monitor for AeRosols and Gases in Ambient air (MARGA) aerosol measurements were consistent with these conditions. CMAQv5.2 regional chemical transport model predictions did not reflect these conditions due to a factor of 3 overestimate of the nonvolatile cations. In addition, gas-phase ammonia was overestimated in the CMAQ model leading to an even lower fraction of total ammonia in the particle. Chemical Speciation Network (CSN) and aerosol mass spectrometer (AMS) measurements indicated less ammonium per sulfate than SEARCH and MARGA measurements and were inconsistent with thermodynamic model predictions. Organic compounds were predicted to be present to some extent in the same phase as inorganic constituents, modifying their activity and resulting in a decrease in [H+]air (H+ in µgm-3 air), increase in ammonia partitioning to the gas phase, and increase in pH compared to complete organic vs. inorganic liquid-liquid phase separation. In addition, accounting for nonideal mixing modified the pH such that a fully interactive inorganic-organic system had a pH roughly 0.7 units higher than predicted using traditional methods (pH = 1.5 vs. 0.7). Particle-phase interactions of organic and inorganic compounds were found to increase partitioning towards the particle phase (vs. gas phase) for highly oxygenated (O : C≥0.6) compounds including several isoprene-derived tracers as well as levoglu-cosan but decrease particle-phase partitioning for low O: C, monoterpene-derived species.