Carl M. Berkowitz

Unexpectedly high concentrations of molecular chlorine in coastal air

The fate of many atmospheric trace species, including pollutants such as nitrogen oxides and some volatile organic compounds, is controlled by oxidation reactions. In the daytime troposphere, these reactions are dominated by photochemically produced OH radicals; at night and in polluted environments, NO3 radicals are an important oxidant. Ozone can contribute to the oxidation of atmospheric species during both day and night. In recent years, laboratory investigations, modelling studies, measured Cl deficits in marine aerosols and species-nonspecific observations of gaseous inorganic chlorine compounds other than HCl have suggested that reactive halogen species may contribute significantly to—or even locally dominate—the oxidative capacity of the lower marine troposphere. Here we report night-time observations of molecular chlorine concentrations at a North American coastal site during onshore wind flow conditions that cannot be explained using known chlorine chemistry. The measured Cl2 mixing ratios range from <10 to 150 parts per 1012 (p.p.t.), exceeding those predicted for marine air by more than an order of magnitude. Using the observed chlorine concentrations and a simple photochemical box model, we estimate that a hitherto unrecognized chlorine source must exist that produces up to 330 p.p.t. Cl2 per day. The model also indicates that early-morning photolysis of molecular chlorine can yield sufficiently high concentrations of chlorine atoms to render the oxidation of common gaseous compounds by this species 100 times faster than the analogous oxidation reactions involving the OH radical, thus emphasizing the locally significant effect of chlorine atoms on the concentrations and lifetimes of atmospheric trace species in both the remote marine boundary layer and coastal urban areas.

The current state and future direction of Eulerian models in simulating the tropospheric chemistry and transport of trace species: a review

Abstract Limitations on comprehensive tropospheric chemistry/transport models are discussed within the context of a set of issues currently facing the environmental scientific and policy-making communities. A number of central improvements are discussed in a prioritized manner, with consideration of the key progress necessary to include feedback processes between meteorology and chemistry, aerosol formation, in cloud development with subsequent effects on wet removal, dry deposition and surface exchange processes, and impacts of chemical perturbations on radiation, climate, and weather. These improvements would result in a “third-generation model”. The computational framework for this code is outlined, and estimates of required computer resources presented.

Chemical and physical properties of plumes of anthropogenic pollutants transported over the North Atlantic during the North Atlantic Regional Experiment

Plumes of photochemical pollutants transported from the industrialized regions of the northeast United States and Canada were sampled over the North Atlantic Ocean at distances up to 1000 km from the coast. The plumes were found in well defined layers up to 1 km thick and were usually isolated from the surface by a low altitude inversion. Plume composition was consistent with the occurrence of extensive photochemical processing during transit from source regions as indicated by high 03 concentrations (03 maximum -150 parts per billion by volume (ppbv)), generally high fractional conversion (>85%) of NOx to its oxidation products, and high peroxide concentrations (median 3.6 ppbv; maximum 11 ppbv). These observations are in accord with processing times estimated from back trajectory analysis. CO and 03 concentrations were well correlated (r 2 = 0.64) with a slope (0.26) similar to previous measurements in photochemically aged air. Good correlations were also observed between CO and accumulation mode particle number densities (r 2 = 0.64), and CO and NOy (r 2 = 0.67). 03 was found to depend nonlinearly on the NOx oxidation product concentration. At low values of (NOy-NOx), the slope (14) was within the range of values measured previously in photochemically aged air masses, at higher concentrations the slope was much lower (4.6). The low slope at high concentrations is attributed to minimization of losses of NOx oxidation products in spatially well-defined plumes during transport. A strong linear correlation (r 2 = 0.73) was found between 03, and the concentration of radical sink species as represented by the quantity ((NOy-NOx) + 2H202).

Mesoscale meteorology of the New England coast, Gulf of Maine, and Nova Scotia: Overview

The North Atlantic Regional Experiment (NARE) domain includes the coastal and near-coastal areas of New England and Atlantic Canada, and the intervening Gulf of Maine. This area has a complex coastline on all scales. The meteorology of the region is affected by the contrasting properties of the land and ocean, especially the temperature contrast. This paper reviews the knowledge of coastal meteorology on scales from tens of meters to 100 km, emphasizing those processes that most strongly affect pollutant transport in the NARE area in summertime. Examples from the NARE 1993 Summer Intensive measurements and modeling studies are used to illustrate these processes. The understanding and interpretation of regional and global air quality measurements depend on meteorological information about such issues as the amount and location of vertical mixing, the three-dimensional trajectories of air parcels, the prevalence and scale of stratification, the speed of horizontal transport, and the scale of horizontal homogeneity or inhomogeneity. These issues are among those not fully understood in the complex coastal environment. From what is known, and from the NARE measurements, we can draw some conclusions. Polluted air from the U.S. East Coast transported over the Gulf of Maine becomes stably stratified, and vertical mixing is limited. The scale of vertical layering may be a few tens of meters or less. Layers separated by a few hundred meters in the vertical over Nova Scotia may have crossed the U.S. coast at points hundreds of kilometers apart. Wind speeds and directions, and therefore transport times, may differ substantially between the layers. Air at the surface may be isolated from, and have drastically different chemical contents from, air at only a few hundred meters above the surface.

Characterization of the sunset semi-continuous carbon aerosol analyzer.

Abstract 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/m3 (0.1 μgC/cm2) for TC, optical OC, and thermal OC, and 0.01 μgC/m3 (0.01 μgC/cm2) for optical EC. Similarly, Unit B had an LOD of approximately 0.3 μgC/m3 (0.2 μgC/cm2) for TC, optical OC, and thermal OC, and 0.02 μgC/m3 (0.01 μgC/cm2) for optical EC. The LOD for thermal EC is estimated to be 0.2 μgC/m3 (0.1 μgC/cm2) for both units. The pooled RSDs were 4.9% for TC (carbon mass loadings 0.6–6.0 μgC/cm2), 5.6% for optical OC (carbon mass loadings 0.6–5.4 μgC/cm2), 5.3% for thermal OC (carbon mass loadings 0.6–5.3 μgC/cm2), and 9.6% for optical EC (carbon mass loadings 0–1.4 μgC/cm2), which indicates good precision between the instruments. The RSD for thermal EC is higher at 24.3% (carbon mass loadings 0–1.2 μgC/cm2). 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.

Multisensor Estimation of Mixing Heights over a Coastal City

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.

Ozone loss in soot aerosols

The fractal-like structure of atmospheric soot (e.g., elemental carbon) provides a large surface area available for heterogeneous chemistry in the upper troposphere and lower stratosphere [Blake and Kato, 1995]. One potentially important reaction is ozone decomposition on soot. Although extensively studied in the laboratory, a wide range of reaction probabilities have been observed (γ∼10−3 to γ∼10−7) which have been attributed to differences in reactivity between fresh (i.e., nonoxidized) versus aged (i.e., oxidized) soot [Schurath and Naumann, 1998]. The importance in understanding soot-ozone chemistry is particularly important in light of recent nighttime field measurements [Berkowitz et al., 2000] made over Portland, Oregon. The data revealed episodes of an anticorrelation between ozone mixing ratio and aerosol surface area density. During these episodes a single scattering albedo in the range 0.8–0.9 was measured, indicating an increased absorptive component of the aerosol, perhaps due to elemental carbon. In addition, an increase in the concentration of aerosols contained in the small size range of the fine mode (<0.1–0.15 μm) was observed, suggestive of new aerosol formation. In this article we attempt to explain these field observations. One explanation of the field observations is ozone loss occurring on atmospheric soot aerosol. Here we present laboratory results obtained using a static aerosol reactor that indicate that direct ozone loss on soot aerosol is unlikely under ambient conditions in the troposphere. An alternative and more likely explanation of the field data is based on ozone-mediated organic aerosol production. This could occur by either nighttime nitrate radical oxidation or direct ozone oxidation of hydrocarbons as suggested previously [Starn et al., 1998; Griffin et al., 1999; Kamens et al., 1999; Yu et al., 1999; De Gouw and Lovejoy, 1998].

Overview of the Cumulus Humilis Aerosol Processing Study.

Abstract 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-lookin...

Synoptic patterns associated with the flux of excess ozone to the western North Atlantic

Observations of O3, CO, and aerosols taken in the vicinity of Halifax, Nova Scotia, during the summers of 1992 and 1993 indicate long-range transport of O3 that was photochemically generated within urban areas of North America. We identify eastward moving cyclonic systems as the main synoptic-scale transport mechanism into this region with the consequence that transport events are highly episodic. The potential for further well-defined long-range transport was found to be slight as a result of subsequent mixing by these cyclonic systems. The ozone flux associated with these cyclones into the lowest 1.5 km of this region is estimated to be on the order of 1 Gmol per 24-hour cyclonic event. We conclude that the circulation patterns associated with these fronts are a major pathway for the export of pollutants from North America.

A modeling study of boundary layer processes associated with ozone layers observed during the 1993 North Atlantic regional experiment

Boundary layer processes associated with three pollution events during the North Atlantic Regional Experiment (NARE) 1993 field campaign are examined using airborne measurements and a Lagrangian particle dispersion model in conjunction with a mesoscale model employing four-dimensional data assimilation. This nonphotochemical modeling system was able to qualitatively reproduce many of the observed features during a 15-day period of the NARE field campaign. Simulated particle releases from urban regions within the daytime convective boundary layer were transported to heights up to 2 km above ground level. Particles located in the upper part of the residual layer in the early evening hours were quickly advected by higher wind speeds aloft to the sampling domain; however, particles released within the nocturnal stable boundary layer or the marine boundary layer remained within 200 m of the surface and often exhibited complex circulation patterns. As a consequence of the diurnal boundary layer characteristics, emissions released within a relatively close time interval were often found to have significantly different trajectories. Mixing of particles from various source regions results in a plume that does not have a unique age but is better characterized by a distribution of ages which vary with altitude. It is shown that much of the layering over Yarmouth is well established by convective boundary layer processes and vertical wind shears prior to the air masses leaving land. As expected, sea surface temperatures were found to play an important role in defining the vertical gradient of potential temperature and hence the amount of vertical mixing over the Gulf of Maine. Peak particle concentrations within 1 km of the ocean were often associated with a low-level jet over the Gulf of Maine. A common feature during the analysis period is synoptic-scale lifting in advance of low-pressure systems which appears to be partially responsible for lifting particles to the heights observed over Yarmouth.