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Data from the Cumulus Humilis Aerosol Processing Study (CHAPS) are used to estimate the impact of both aerosol indirect effects and cloud dynamics on the microphysical and optical properties of shallow cumuli observed in the vicinity of Oklahoma City, Oklahoma. Not surprisingly, we find that the amount of light scattered by clouds is dominated by their liquid water content (LWC), which in turn is driven by buoyancy and cloud dynamics. However, removing the effect of cloud dynamics by examining the scattering normalized by LWC shows a statistically significant sensitivity of scattering to pollutant loading (increasing at a rate of 0.002 m2 g−1 ppbv−1). These results suggest that even moderately sized cities, like Oklahoma City, can have a measureable impact on the optical properties of shallow cumuli.

Nighttime chemical evolution of aerosol and trace gases in a coal‐fired power plant plume was monitored with the Department of Energy Grumman Gulfstream‐1 aircraft during the 2002 New England Air Quality Study field campaign. Quasi‐Lagrangian sampling in the plume at increasing downwind distances and processing times was guided by a constant‐volume balloon that was released near the power plant at sunset. While no evidence of fly ash particles was found, concentrations of particulate organics, sulfate, and nitrate were higher in the plume than in the background air. The enhanced sulfate concentrations were attributed to direct emissions of gaseous H2SO4, some of which had formed new particles as evidenced by enhanced concentrations of nucleation‐mode particles in the plume. The aerosol species were internally mixed and the particles were acidic, suggesting that particulate nitrate was in the form of organic nitrate. The enhanced particulate organic and nitrate masses in the plume were inferred as secondary organic aerosol, which was possibly formed from NO3 radical‐initiated oxidation of isoprene and other trace organic gases in the presence of acidic sulfate particles. Microspectroscopic analysis of particle samples suggested that some sulfate was in the form of organosulfates. Microspectroscopy also revealed the presence of sp2 hybridized C = C bonds, which decreased with increasing processing time in the plume, possibly because of heterogeneous chemistry on particulate organics. Constrained plume modeling analysis of the aircraft and tetroon observations showed that heterogeneous hydrolysis of N2O5 was negligibly slow. These results have significant implications for several issues related to the impacts of power plant emissions on air quality and climate.

The primary goal of the Cumulus Humilis Aerosol Processing Study (CHAPS) was to characterize and contrast freshly emitted aerosols below, within, and above fields of cumuli, and to study changes to the cloud microphysical structure within these same cloud fields in the vicinity of Oklahoma City during June 2007. CHAPS is one of few studies that have had an aerosol mass spectrometer (AMS) sampling downstream of a counterflow virtual impactor (CVI) inlet on an aircraft, allowing the examination of the chemical composition of activated aerosols within the cumuli. The results from CHAPS provide insights into changes in the aerosol chemical and optical properties as aerosols move through shallow cumuli downwind of a moderately sized city. Three instrument platforms were employed during CHAPS, including the U.S. Department of Energy Gulfstream-1 aircraft, which was equipped for in situ sampling of aerosol optical and chemical properties; the NASA Langley King Air B200, which carried the downward-looking NASA Langley High Spectral Resolution Lidar (HSRL) to measure profiles of aerosol backscatter, extinction, and depolarization between the King Air and the surface; and a surface site equipped for continuous in situ measurements of aerosol optical properties, profiles of aerosol backscatter, and meteorological conditions, including total sky cover and thermodynamic profiles of the atmosphere. In spite of record precipitation over central Oklahoma, a total of 8 research flights were made by the G-1 and 18 by the B200, including special satellite verification flights timed to coincide with NASA satellite A-Train overpasses.

Overnight atmospheric transport and chemical evolution of photochemically aged Houston urban and petrochemical industrial plume were investigated in July 2005. We report here on the 26 July episode in which the aged plume was tagged 1.5 h before sunset with a pair of free‐floating controlled meteorological balloons, which guided quasi‐Lagrangian aircraft sampling in the plume as it was advected 300 km to the north over 8 h. The aged plume around sunset was well mixed within a 1600 m residual layer, and was characterized by enhanced levels of aerosol, O3, CO, olefins, acetaldehyde, total odd nitrogen compounds (NOy), and relatively small amounts (<1 ppbv) of NOx. The plume experienced appreciable shearing overnight due to the development of a low‐altitude nocturnal jet between 300 and 500 m above mean sea level (MSL). However, the plume above 600 m MSL remained largely undiluted even after 8 h of transport due to lack of turbulent mixing above the jet. About 40–60% of the NOx present in the aged plume around sunset was found to be depleted over this 8 h period. A constrained plume modeling analysis of the quasi‐Lagrangian aircraft observations suggested that by dawn this NOx was converted to nitric acid, organic nitrates, and peroxy acyl nitrates via reactions of NO3 radicals with enhanced levels of olefins and aldehydes in the plume. Sensitivity of NOx depletion to heterogeneous hydrolysis of N2O5 on aerosols was examined. These results have significant implications for the impacts of urban and industrial pollution on far downwind regions.

The field-deployable Sunset Semi-Continuous Organic Carbon/Elemental Carbon (Sunset OCEC) aerosol analyzer utilizes the modified National Institute for Occupational Safety and Health thermal-optical method to determine total carbon (TC), organic carbon (OC), and elemental carbon (EC) at near real-time. Two sets of OC and EC are available: thermal OC and EC, and optical OC and EC. The former is obtained by the thermal-optical approach, and the latter is obtained by directly determining EC optically and deriving optical OC from TC. However, the performance of the Sunset OCEC is not yet fully characterized. Two collocated Sunset OCEC analyzers, Unit A and Unit B, were used to determine the pooled relative standard deviation (RSD) and limit of detection (LOD) between September 18 and November 6, 2007 in Richland, WA. The LOD of Unit A was approximately 0.2 μgC/m 3 (0.1 μgC/cm 2 ) for TC, optical OC, and thermal OC, and 0.01 μgC/m 3 (0.01 μgC/cm 2 ) for optical EC. Similarly, Unit B had an LOD of approximately 0.3 μgC/m 3 (0.2 μgC/cm 2 ) for TC, optical OC, and thermal OC, and 0.02 μgC/m 3 (0.01 μgC/cm 2 ) for optical EC. The LOD for thermal EC is estimated to be 0.2 μgC/m 3 (0.1 μgC/cm 2 ) for both units. The pooled RSDs were 4.9% for TC (carbon mass loadings 0.6-6.0 μgC/cm 2 ), 5.6% for optical OC (carbon mass loadings 0.6-5.4 μgC/cm 2 ), 5.3% for thermal OC (carbon mass loadings 0.6-5.3 μgC/cm 2 ), and 9.6% for optical EC (carbon mass loadings 0-1.4 μgC/cm 2 ), which indicates good precision between the instruments. The RSD for thermal EC is higher at 24.3% (carbon mass loadings 0-1.2 μgC/cm 2 ). Low EC mass loadings in Richland contributed to the poor RSD of EC. The authors found that excessive noise from the nondispersive infrared (NDIR) laser in the Sunset OCEC analyzer could result in a worsened determination of OC and EC. It is recommended that a \"quieter\" NDIR laser and detector be used in the Sunset OCEC analyzer to improve quantification. Future work should re-evaluate the precision of the EC parameters in an environment favorable for EC collection. Investigation among quantification differences using various thermal-optical protocols to determine OC and EC is also in need.

Nighttime chemical evolution of aerosol and trace gases in a coal‐fired power plant plume was monitored with the Department of Energy Grumman Gulfstream‐1 aircraft during the 2002 New England Air Quality Study field campaign. Quasi‐Lagrangian sampling in the plume at increasing downwind distances and processing times was guided by a constant‐volume balloon that was released near the power plant at sunset. While no evidence of fly ash particles was found, concentrations of particulate organics, sulfate, and nitrate were higher in the plume than in the background air. The enhanced sulfate concentrations were attributed to direct emissions of gaseous H 2 SO 4 , some of which had formed new particles as evidenced by enhanced concentrations of nucleation‐mode particles in the plume. The aerosol species were internally mixed and the particles were acidic, suggesting that particulate nitrate was in the form of organic nitrate. The enhanced particulate organic and nitrate masses in the plume were inferred as secondary organic aerosol, which was possibly formed from NO 3 radical‐initiated oxidation of isoprene and other trace organic gases in the presence of acidic sulfate particles. Microspectroscopic analysis of particle samples suggested that some sulfate was in the form of organosulfates. Microspectroscopy also revealed the presence of sp 2 hybridized C = C bonds, which decreased with increasing processing time in the plume, possibly because of heterogeneous chemistry on particulate organics. Constrained plume modeling analysis of the aircraft and tetroon observations showed that heterogeneous hydrolysis of N 2 O 5 was negligibly slow. These results have significant implications for several issues related to the impacts of power plant emissions on air quality and climate.

Predicted concentrations from the Regional Air Pollutant Transport (RAPT) model are compared with the corresponding observed values of sulfate, and the results used to define strengths and weaknesses in the model formulation. RAPT was developed to provide long-term (i.e., monthly) average values of pollutants. It has hourly time steps, and incorporates a number of simplifying assumptions on mixing heights, horizontal diffusion and emission averaging. Daily predicted values were analyzed for diagnostic use only, rather than for verification of prediction ability. The analysis indicates that the model performed reasonably well with regard to short-term temporal predictions of spatially averaged concentrations. Less confidence can be placed in site-specific predictions. Spatial patterns in the analysis highlight the sensitivity of the model's short-term simulations to several features including input data, boundary conditions and the assumption of horizontal dispersion being wholly defined by trajectory variations.

An airborne microwave temperature profiler (MTP) was deployed during the Texas 2000 Air Quality Study (TexAQS-2000) to make measurements of boundary layer thermal structure. An objective technique was developed and tested for estimating the mixed layer (ML) height from the MTP vertical temperature profiles. The technique identifies the ML height as a threshold increase of potential temperature from its minimum value within the boundary layer. To calibrate the technique and evaluate the usefulness of this approach, coincident estimates from radiosondes, radar wind profilers, an aerosol backscatter lidar, and in situ aircraft measurements were compared with each other and with the MTP. Relative biases among all instruments were generally less than 50 m, and the agreement between MTP ML height estimates and other estimates was at least as good as the agreement among the other estimates. The ML height estimates from the MTP and other instruments are utilized to determine the spatial and temporal evolution of ML height in the Houston, Texas, area on 1 September 2000. An elevated temperature inversion was present, so ML growth was inhibited until early afternoon. In the afternoon, large spatial variations in ML height developed across the Houston area. The highest ML heights, well over 2 km, were observed to the north of Houston, while downwind of Galveston Bay and within the late afternoon sea breeze ML heights were much lower. The spatial variations that were found away from the immediate influence of coastal circulations were unexpected, and multiple independent ML height estimates were essential for documenting this feature.