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Detailed characterization of the aerosol content of wildfire smoke plumes is typically performed through in situ aircraft observations, which have limited temporal and spatial coverage. Extending such observations to regional or global scales requires new remote sensing approaches, such as retrievals that make use of spectropolarimetric, multiangle imaging. In this work measurements made during the Imaging Polarimetric Assessment and Characterization of Tropospheric Particulate Matter (ImPACT‐PM) field campaign in a smoke plume near the town of Lebec in Southern California by the Navy Center for Interdisciplinary Remotely Piloted Aircraft Studies Twin Otter aircraft on 8 July 2016 are used in conjunction with near‐coincident measurements from the Airborne Multiangle SpectroPolarimetric Imager (AirMSPI) on the National Aeronautics and Space Administration ER‐2 high‐altitude research aircraft to assess the sensitivity of spectropolarimetric measurements to the black carbon content of the plume. Tracking visible features in the smoke through the sequence of AirMSPI observations allowed the height of the plume to be estimated through geometric techniques. Then, by constraining the fractional amounts of the aerosol constituents with the in situ data, radiative closure was obtained through simulations performed with a polarimetric radiative transfer code, demonstrating the ability to constrain the black carbon mass fraction to approximately 5%, given the uncertainties in the AirMSPI measurements and the assumption of external mixing of aerosol components. The AirMSPI retrieval, made using a limited set of observations from the 470 nm polarimetric spectral band alone, was also generally consistent with operational retrievals of aerosol optical depth and surface reflectance made by the Multi‐Angle Implementation of Atmospheric Correction algorithm at 1 km resolution. The sensitivity of polarimetric remote sensing imagery to the black carbon content of wildfire smoke was demonstrated using the results of a NASA field campaign that took place over central California in July 2016. The chemical contents of the smoke plume were measured by an aircraft flying inside the smoke. This information was used to build a model of the smoke that was then used to estimate what would be observed by the remote sensing instrument. Good agreement was achieved in terms of the total amount of aerosol, the brightness of the underlying surface, and the fraction of black carbon in the plume. Polarimetric observations are shown to be sensitive to the amount of black carbon in a wildfire smoke plumeRadiative closure was achieved between multiangle polarimetric remote sensing and in situ airborne measurements in the smoke plumeWe demonstrated the ability of polarimetric data to constrain the black carbon mass fraction assuming external mixing of chemical components

Meteorology and microphysics affect cloud formation, cloud droplet distributions, and shortwave reflectance. The Eastern Pacific Emitted Aerosol Cloud Experiment and the Stratocumulus Observations of Los‐Angeles Emissions Derived Aerosol‐Droplets studies provided measurements in six case studies of cloud thermodynamic properties, initial particle number distribution and composition, and cloud drop distribution. In this study, we use simulations from a chemical and microphysical aerosol‐cloud parcel (ACP) model with explicit kinetic drop activation to reproduce observed cloud droplet distributions of the case studies. Four cases had subadiabatic lapse rates, resulting in fewer activated droplets, lower liquid water content, and higher cloud base height than an adiabatic lapse rate. A weighted ensemble of simulations that reflect measured variation in updraft velocity and cloud base height was used to reproduce observed droplet distributions. Simulations show that organic hygroscopicity in internally mixed cases causes small effects on cloud reflectivity (CR) (<0.01), except for cargo ship and smoke plumes, which increased CR by 0.02 and 0.07, respectively, owing to their high organic mass fraction. Organic hygroscopicity had larger effects on droplet concentrations for cases with higher aerosol concentrations near the critical diameter (namely, polluted cases with a modal peak near 0.1 µm). Differences in simulated droplet spectral widths (k) caused larger differences in CR than organic hygroscopicity in cases with organic mass fractions of 60% or less for the cases shown. Finally, simulations from a numerical parameterization of cloud droplet activation suitable for general circulation models compared well with the ACP model, except under high organic mass fraction. Using distributions of updraft velocity and lapse rate increases droplet spectral widthOrganic hygroscopicity changes cloud drop only for high concentrations near critical diameterChanges in drop spectral width cause larger reflectivity differences than organic hygroscopicity

Atmospheric volatile organic compound (VOC) oxidation mechanisms under pristine (rural/remote) and urban (anthropogenically‐influenced) conditions follow distinct pathways due to large differences in nitrogen oxide (NOx) concentrations. These two pathways lead to products that have different chemical and physical properties and reactivity. Under pristine conditions, isoprene hydroxy hydroperoxides (ISOPOOHs) are the dominant first‐generation isoprene oxidation products. Utilizing authentic ISOPOOH standards, we demonstrate that two of the most commonly used methods of measuring VOC oxidation products (i.e., gas chromatography and proton transfer reaction mass spectrometry) observe these hydroperoxides as their equivalent high‐NO isoprene oxidation products – methyl vinyl ketone (MVK) and methacrolein (MACR). This interference has led to an observational bias affecting our understanding of global atmospheric processes. Considering these artifacts will help close the gap on discrepancies regarding the identity and fate of reactive organic carbon, revise our understanding of surface‐atmosphere exchange of reactive carbon and SOA formation, and improve our understanding of atmospheric oxidative capacity. Hydroperoxides are being observed as their carbonyl analogues in GC and PTR‐MSInterference results in observational bias towards urban conditionsThe implications of this interference are discussed

The California Research at the Nexus of Air Quality and Climate Change (CalNex) field study was conducted throughout California in May, June, and July of 2010. The study was organized to address issues simultaneously relevant to atmospheric pollution and climate change, including (1) emission inventory assessment, (2) atmospheric transport and dispersion, (3) atmospheric chemical processing, and (4) cloud‐aerosol interactions and aerosol radiative effects. Measurements from networks of ground sites, a research ship, tall towers, balloon‐borne ozonesondes, multiple aircraft, and satellites provided in situand remotely sensed data on trace pollutant and greenhouse gas concentrations, aerosol chemical composition and microphysical properties, cloud microphysics, and meteorological parameters. This overview report provides operational information for the variety of sites, platforms, and measurements, their joint deployment strategy, and summarizes findings that have resulted from the collaborative analyses of the CalNex field study. Climate‐relevant findings from CalNex include that leakage from natural gas infrastructure may account for the excess of observed methane over emission estimates in Los Angeles. Air‐quality relevant findings include the following: mobile fleet VOC significantly declines, and NOxemissions continue to have an impact on ozone in the Los Angeles basin; the relative contributions of diesel and gasoline emission to secondary organic aerosol are not fully understood; and nighttime NO3chemistry contributes significantly to secondary organic aerosol mass in the San Joaquin Valley. Findings simultaneously relevant to climate and air quality include the following: marine vessel emissions changes due to fuel sulfur and speed controls result in a net warming effect but have substantial positive impacts on local air quality. An overview of the multi‐platform, multi‐agency CalNex field study in 2010A synthesis of CalNex research to dateEmphasis on policy‐relevant conclusions linking climate and air quality issues

The vapor−liquid equilibrium of organic species and water determines the partitioning of volatile organic species between the gas and aerosol phases in the atmosphere. Essential properties that govern this partitioning are the compounds' vapor pressures and the liquid-phase activity coefficients of the organic species. Key issues include (1) the amount of water absorbed by organic particles, both single-component and multicomponent, as a function of relative humidity and (2) how organic and water partitioning is altered as the relative humidity changes, for a fixed total (gas plus particle) quantity of each organic component. We present here calculations relating to each of these issues for organic molecules that are characteristic of those that have been identified in ambient aerosols. Uncertainties exist in knowledge of vapor pressures of atmospheric organics and in activity coefficients, which are calculated by the UNIFAC method. These uncertainties, the level of which is difficult to estimate because of a lack of appropriate thermodynamic data, can lead to significant uncertainty in predicted gas−aerosol partitioning. Current theories for estimating both vapor pressures and activity coefficients are likely to be improved eventually, but the methods discussed here will probably remain the procedures of choice for the immediate future.

We report on results from a World Climate Research Program workshop on representations of scavenging and deposition processes in global transport models of the atmosphere. 15 models were evaluated by comparing simulations of radon, lead, sulfur dioxide, and sulfate against each other, and against observations of these constituents. This paper provides a survey on the simulation differences between models. It identifies circumstances where models are consistent with observations or with each other, and where they differ from observations or with each other. The comparison shows that most models are able to simulate seasonal species concentrations near the surface over continental sites to within a factor of 2 over many regions of the globe. Models tend to agree more closely over source (continental) regions than for remote (polar and oceanic) regions. Model simulations differ most strongly in the upper troposphere for species undergoing wet scavenging processes. There are not a sufficient number of observations to characterize the climatology (long‐term average) of species undergoing wet scavenging in the upper troposphere. This highlights the need for either a different strategy for model evaluation (e.g., comparisons on an event by event basis) or many more observations of a few carefully chosen constituents.

As part of the second Aerosol Characterization Experiment (ACE‐2) during June and July of 1997, aerosol‐size distributions were measured on board the CIRPAS Pelicanaircraft through the use of a DMA and 2 OPCs. During the campaign, the boundary‐layer aerosol typically possessed characteristics representative of a background marine aerosol or a continentally influenced aerosol, while the free‐tropospheric aerosol was characterized by the presence or absence of a Saharan dust layer. A range of radiative closure comparisons were made using the data obtained during vertical profiles flown on 4 missions. Of particular interest here are the comparisons made between the optical properties as determined through the use of measured aerosol‐size distributions and those measured directly by an airborne 14‐wavelength sunphotometer and 3 nephelometers. Variations in the relative humidity associated with each of the direct measurements required consideration of the hygroscopic properties of the aerosol for size‐distribution‐based calculations. Simultaneous comparison with such a wide range of directly‐measured optical parameters not only offers evidence of the validity of the physicochemical description of the aerosol when closure is achieved, but also provides insight into potential sources of error when some or all of the comparisons result in disagreement. Agreement between the derived and directly‐measured optical properties varied for different measurements and for different cases. Averaged over the 4 case studies, the derived extinction coefficient at 525 nm exceeded that measured by the sunphotometer by 2.5% in the clean boundary layer, but underestimated measurements by 13% during pollution events. For measurements within the free troposphere, the mean derived extinction coefficient was 3.3% and 17% less than that measured by the sunphotometer during dusty and non‐dusty conditions, respectively. Likewise, averaged discrepancies between the derived and measured scattering coefficient were −9.6%, +4.7%, +17%, and −41% for measurements within the clean boundary layer, polluted boundary layer, free troposphere with a dust layer, and free troposphere without a dust layer, respectively. Each of these quantities, as well as the majority of the >100 individual comparisons from which they were averaged, were within estimated uncertainties.

Analysis of the aerosol properties during 3 recent international field campaigns (ACE‐1, TARFOX and ACE‐2) are described using satellite retrievals from NOAA AVHRR data. Validation of the satellite retrieval procedure is performed with airborne, shipboard, and land‐based sunphotometry during ACE‐2. The intercomparison between satellite and surface optical depths has a correlation coefficient of 0.93 for 630 nm wavelength and 0.92 for 860 nm wavelength. The standard error of estimate is 0.025 for 630 nm wavelength and 0.023 for 860 nm wavelength. Regional aerosol properties are examined in composite analysis of aerosol optical properties from the ACE‐1, TARFOX and ACE‐2 regions. ACE‐1 and ACE‐2 regions have strong modes in the distribution of optical depth around 0.1, but the ACE‐2 tails toward higher values yielding an average of 0.16 consistent with pollution and dust aerosol intrusions. The TARFOX region has a noticeable mode of 0.2, but has significant spread of aerosol optical depth values consistent with the varied continental aerosol constituents off the eastern North American Coast.

We report on results from a World Climate Research Program workshop on representations ofscavenging and deposition processes in global transport models of the atmosphere. 15 modelswere evaluated by comparing simulations of radon, lead, sulfur dioxide, and sulfate against eachother, and against observations of these constituents. This paper provides a survey on the simulationdifferences between models. It identifies circumstances where models are consistent withobservations or with each other, and where they differ from observations or with each other. Thecomparison shows that most models are able to simulate seasonal species concentrations nearthe surface over continental sites to within a factor of 2 over many regions of the globe. Modelstend to agree more closely over source (continental) regions than for remote (polar and oceanic)regions. Model simulations differ most strongly in the upper troposphere for species undergoingwet scavenging processes. There are not a sufficient number of observations to characterize theclimatology (long-term average) of species undergoing wet scavenging in the upper troposphere.This highlights the need for either a different strategy for model evaluation (e.g., comparisons onan event by event basis) or many more observations of a few carefully chosen constituents.

As part of the second Aerosol Characterization Experiment (ACE-2) during June and July of 1997, aerosol-size distributions were measured on board the CIRPAS Pelican aircraft through the use of a DMA and 2 OPCs. During the campaign, the boundary-layer aerosol typically possessed characteristics representative of a background marine aerosol or a continentally influenced aerosol, while the free-tropospheric aerosol was characterized by the presence or absence of a Saharan dust layer. A range of radiative closure comparisons were made using the data obtained during vertical profiles flown on 4 missions. Of particular interest here are the comparisons made between the optical properties as determined through the use of measured aerosol-size distributions and those measured directly by an airborne 14-wavelength sunphotometer and 3 nephelometers. Variations in the relative humidity associated with each of the direct measurements required consideration of the hygroscopic properties of the aerosol for size-distribution-based calculations. Simultaneous comparison with such a wide range of directly-measured optical parameters not only offers evidence of the validity of the physicochemical description of the aerosol when closure is achieved, but also provides insight into potential sources of error when some or all of the comparisons result in disagreement. Agreement between the derived and directly-measured optical properties varied for different measurements and for different cases. Averaged over the 4 case studies, the derived extinction coefficient at 525 nm exceeded that measured by the sunphotometer by 2.5% in the clean boundary layer, but underestimated measurements by 13% during pollution events. For measurements within the free troposphere, the mean derived extinction coefficient was 3.3% and 17% less than that measured by the sunphotometer during dusty and non-dusty conditions, respectively. Likewise, averaged discrepancies between the derived and measured scattering coefficient were −9.6%, +4.7%, +17%, and −41% for measurements within the clean boundary layer, polluted boundary layer, free troposphere with a dust layer, and free troposphere without a dust layer, respectively. Each of these quantities, as well as the majority of the >100 individual comparisons from which they were averaged, were within estimated uncertainties.

Analysis of the aerosol properties during 3 recent international field campaigns (ACE-1, TARFOX and ACE-2) are described using satellite retrievals from NOAA AVHRR data. Validation of the satellite retrieval procedure is performed with airborne, shipboard, and land-based sunphotometry during ACE-2. The intercomparison between satellite and surface optical depths has a correlation coefficient of 0.93 for 630 nm wavelength and 0.92 for 860 nm wavelength. The standard error of estimate is 0.025 for 630 nm wavelength and 0.023 for 860 nm wavelength. Regional aerosol properties are examined in composite analysis of aerosol optical properties from the ACE-1, TARFOX and ACE-2 regions. ACE-1 and ACE-2 regions have strong modes in the distribution of optical depth around 0.1, but the ACE-2 tails toward higher values yielding an average of 0.16 consistent with pollution and dust aerosol intrusions. The TARFOX region has a noticeable mode of 0.2, but has significant spread of aerosol optical depth values consistent with the varied continental aerosol constituents off the eastern North American Coast.

The atmospheric oxidation mechanism of n-butane is investigated by means of density functional theory and ab initio calculations. Calculation of energies of reactants, transition states, and stable intermediates predicts the detailed pathways leading to experimentally observed products of n-butane oxidation. Also serving as a model system for the oxidation of larger alkanes, quantitative information is obtained for elementary reaction steps that heretofore have been subject to speculation. Complete basis set model chemistries CBS-4 and CBS-q were used with B3LYP/6-31G(d,p) optimized geometries to calculate energies of over 70 stable species and transition states. Energies based on density functional theory were obtained at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d,p) level of theory. The principal pathway following formation of the 1-butyl radical from hydroxyl (OH) attack on n-butane is found to be 1,5-H shift of the 1-butoxy radical. After conversion to the δ-hydroxy-1-butoxy radical, another 1,5-H shift is expected to be the primary route to 4-hydroxy-1-butanal. 4-Hydroperoxy-1-butanal can be formed after 1,6-H shift in chemically activated 4-hydroxy-1-butylperoxy radicals. Whereas β-scission in 1-butoxy is an endothermic process, fragmentation of 2-butoxy into C2H5 and CH3CHO is predicted to be the major degradation pathway of the secondary butyl radicals.

An approximate non-linear filter is applied to systems described by coupled ordinary and partial differential equations with both volume and boundary disturbances and measurement errors. Measurements are assumed to be carried out at u discrete number of spatial positions in the distributed portion of the system. The filter is applicable to cases in which the data are available continuously in time and at discrete sampling instants. Both versions of the filter are applied to an ingot heating problem of some interest in steel-making. The numerical results show that the filter is rapidly convergent and quite robust.

In this paper, we discuss the partial differential equation of Riccati type that describes the optimal filtering error covariance function for a linear distributed-parameter system with pointwise observations. Since this equation contains the Dirac delta function, it is impossible to apply directly the usual methods of functional analysis to prove existence and uniqueness of a bounded solution. By using properties of the fundamental solution and the classical technique of successive approximation, we prove the existence and uniqueness theorem. We then prove the comparison theorem for partial differential equations of Riccati type. Finally, we consider some applications of these theorems to the distributed-parameter optimal sensor location problem.

A four-parameter model is presented for the triple velocity correlation which appears in the mean Reynolds stress tensor equations of turbulence. A comparison with the closure models of Hanjalic and Launder [1], Daly and Harlow [2], Wyngaard, Cote´, and Rao [3], and Shir [4], in the context of existing experimental data, indicates that the proposed model is superior. It is also demonstrated that, of the one-parameter models, the Hanjalic-Launder model best approximates the data. Due to the complexity of the four-parameter representation, it is recommended that the Hanjalic-Launder model be used for computational purposes.