W. L. Chameides

Ozone precursor relationships in the ambient atmosphere

The concentrations of ozone, nitrogen oxides, and nonmethane hydrocarbons measured near the surface in a variety of urban, suburban, rural, and remote locations are analyzed and compared in order to elucidate the relationships between ozone, its photochemical precursors, and the sources of these precursors. While a large gradient is found among remote, rural, and urban/suburban nitrogen oxide concentrations, the total hydrocarbon reactivity in all continental locations is found to be comparable. Apportionment of the observed hydrocarbon species to mobile and stationary anthropogenic sources and biogenic sources suggests that present-day emission inventories for the United States underestimate the size of mobile emissions. The analysis also suggests a significant role for biogenic hydrocarbon emissions in many urban/suburban locations and a dominant role for these sources in rural areas of the eastern United States. As one moves from remote locations to rural locations and then from rural to urban/suburban locations, ozone and nitrogen oxide concentrations tend to increase in a consistent manner while total hydrocarbon reactivity does not.

Growth of Continental-Scale Metro-Agro-Plexes, Regional Ozone Pollution, and World Food Production

Three regions of the northern mid-latitudes, the continental-scale metro-agro-plexes, presently dominate global industrial and agricultural productivity. Although these regions cover only 23 percent of the Earths continents, they account for most of the worlds commercial energy consumption, fertilizer use, food-crop production, and food exports. They also account for more than half of the worlds atmospheric nitrogen oxide (NOx,) emissions and, as a result, are prone to ground-level ozone (O3) pollution during the summer months. On the basis of a global simulation of atmospheric reactive nitrogen compounds, it is estimated that about 10 to 35 percent of the worlds grain production may occur in parts of these regions where ozone pollution may reduce crop yields. Exposure to yield-reducing ozone pollution may triple by 2025 if rising anthropogenic NOx emissions are not abated.

Fixing a critical climate accounting error

Rules for applying the Kyoto Protocol and national cap-and-trade laws contain a major, but fixable, carbon accounting flaw in assessing bioenergy. The accounting now used for assessing compliance with carbon limits in the Kyoto Protocol and in climate legislation contains a far-reaching but fixable flaw that will severely undermine greenhouse gas reduction goals (1). It does not count CO2 emitted from tailpipes and smokestacks when bioenergy is being used, but it also does not count changes in emissions from land use when biomass for energy is harvested or grown. This accounting erroneously treats all bioenergy as carbon neutral regardless of the source of the biomass, which may cause large differences in net emissions. For example, the clearing of long-established forests to burn wood or to grow energy crops is counted as a 100% reduction in energy emissions despite causing large releases of carbon.

Greater focus needed on methane leakage from natural gas infrastructure

Natural gas is seen by many as the future of American energy: a fuel that can provide energy independence and reduce greenhouse gas emissions in the process. However, there has also been confusion about the climate implications of increased use of natural gas for electric power and transportation. We propose and illustrate the use of technology warming potentials as a robust and transparent way to compare the cumulative radiative forcing created by alternative technologies fueled by natural gas and oil or coal by using the best available estimates of greenhouse gas emissions from each fuel cycle (i.e., production, transportation and use). We find that a shift to compressed natural gas vehicles from gasoline or diesel vehicles leads to greater radiative forcing of the climate for 80 or 280 yr, respectively, before beginning to produce benefits. Compressed natural gas vehicles could produce climate benefits on all time frames if the well-to-wheels CH4 leakage were capped at a level 45–70% below current estimates. By contrast, using natural gas instead of coal for electric power plants can reduce radiative forcing immediately, and reducing CH4 losses from the production and transportation of natural gas would produce even greater benefits. There is a need for the natural gas industry and science community to help obtain better emissions data and for increased efforts to reduce methane leakage in order to minimize the climate footprint of natural gas.

Aerosol radiative, physical, and chemical properties in Beijing during June 1999

Beijing experiences air pollution such that the sky overhead is gray much of the time even on cloudless days. In order to understand the cause of this problem, the aerosol light scattering coefficient σ_(sp) and absorption coefficient σ_(ap) were measured under dry conditions (instrumental relative humidity 1.0 μm), the submicron aerosol was responsible for ∼80% of the light scattering at 530 nm. The largest contribution to the PM2.5 aerosol mass was due to organic compounds, which accounted for ∼30% of the mass. The contributions of sulfate, ammonium, and nitrate to the PM2.5 mass concentration were ∼15%, 5%, and 8%, respectively. Mineral aerosol contributed ∼16% to the PM2.5 aerosol mass. These data show that combustion-related particles rather than wind-blown dust dominated the light extinction budget during June 1999.

Airborne measurements of dimethylsulfide, sulfur dioxide, and aerosol ions over the southern Ocean South of Australia

Vertical distributions of dimethylsulfide (DMS), sulfur dioxide (SO2), aerosol methane-sulfonate (MSA), non-sea-salt sulfate (nss-SO42-), and other aerosol ions were measured in maritime air west of Tasmania (Australia) during December 1986. A few cloudwater and rainwater samples were also collected and analyzed for major anions and cations. DMS concentrations in the mixed layer (ML) were typically between 15–60 ppt (parts per trillion, 10−12; 24 ppt=1 nmol m−3 (20°C, 1013 hPa)) and decreased in the free troposphere (FT) to about <1–2.4 ppt at 3 km. One profile study showed elevated DMS concentrations at cloud level consistent with turbulent transport (‘cloud pumping’) of air below convective cloud cells. In another case, a diel variation of DMS was observed in the ML. Our data suggest that meteorological rather than photochemical processes were responsible for this behavior. Based on model calculations we estimate a DMS lifetime in the ML of 0.9 days and a DMS sea-to-air flux of 2–3 μmol m−2 d−1. These estimates pertain to early austral summer conditions and southern mid-ocean latitudes. Typical MSA concentrations were 11 ppt in the ML and 4.7–6.8 ppt in the FT. Sulfur-dioxide values were almost constant in the ML and the lower FT within a range of 4–22 ppt between individual flight days. A strong increase of the SO2 concentration in the middle FT (5.3 km) was observed. We estimate the residence time of SO2 in the ML to be about 1 day. Aqueous-phase oxidation in clouds is probably the major removal process for SO2. The corresponding removal rate is estimated to be a factor of 3 larger than the rate of homogeneous oxidation of SO2 by OH. Model calculations suggest that roughly two-thirds of DMS in the ML are converted to SO2 and one-third to MSA. On the other hand, MSA/nss-SO42- mole ratios were significantly higher compared to values previously reported for other ocean areas suggesting a relatively higher production of MSA from DMS oxidation over the Southern Ocean. Nss-SO42- profiles were mostly parallel to those of MSA, except when air was advected partially from continental areas (Africa, Australia). In contrast to SO2, nss-SO42- values decreased significantly in the middle FT. NH4+/nss-SO42- mole ratios indicate that most non-sea-salt sulfate particles in the ML were neutralized by ammonium.

NOx Production in Lightning

Abstract The rate of odd nitrogen (NOx) production by electrical discharge through air was theoretically and experimentally estimated to be ∼6 × 1016 NOx molecules per joule. The theoretical treatment employed a cylindrical shock-wave solution to calculate the rate of NOx production in high temperature reactions. The limits obtained were experimentally verified by subjecting a regulated air flow to electrical discharges followed by a measurement of NOx production using chemiluminescence. These measurements also indicated that water vapor content has no detectable effect on the NOx production rate. Our results imply that lightning is a significant source of NOx, producing about 30–40 megatons NOx-N per year and possibly accounting for as much as 50% of the total atmospheric NOx source.

Assessment of ozone photochemistry in the western North Pacific as inferred from PEM-West A observations during the fall 1991

This study examines the influence of photochemical processes on ozone distributions in the western North Pacific. The analysis is based on data generated during NASAs western Pacific Exploratory Mission (PEM-West A) during the fall of 1991. Ozone trends were best described in terms of two geographical domains: the western North Pacific rim (WNPR) and the western tropical North Pacific (WTNP). For both geographical regions, ozone photochemical destruction, D(O3), decreased more rapidly with altitude than did photochemical formation, F(O3). Thus the ozone tendency, P(O3), was typically found to be negative for z 6–8 km. For nearly all altitudes and latitudes, observed nonmethane hydrocarbon (NMHC) levels were shown to be of minor importance as ozone precursor species. Air parcel types producing the largest positive values of P(O3) included fresh continental boundary layer (BL) air and high-altitude (z > 7 km) parcels influenced by deep convection/lightning. Significant negative P(O3) values were found when encountering clean marine BL air or relatively clean lower free-tropospheric air. Photochemical destruction and formation fluxes for the Pacific rim region were found to exceed average values cited for marine dry deposition and stratospheric injection in the northern hemisphere by nearly a factor of 6. This region was also found to be in near balance with respect to column-integrated O3 photochemical production and destruction. By contrast, for the tropical regime column-integrated O3 showed photochemical destruction exceeding production by nearly 80%. Both transport of O3 rich midlatitude air into the tropics as well as very high-altitude (10–17 km) photochemical O3 production were proposed as possible additional sources that might explain this estimated deficit. Results from this study further suggest that during the fall time period, deep convection over Asia and Malaysia/Indonesia provided a significant source of high-altitude NOx to the western Pacific. Given that the high-altitude NOx lifetime is estimated at between 3 and 9 days, one would predict that this source added significantly to high altitude photochemical O3 formation over large areas of the western Pacific. When viewed in terms of strong seasonal westerly flow, its influence would potentially span a large part of the Pacific.

Is ozone pollution affecting crop yields in China

Newly available data from non-urban locations in China along with regional model simulations suggest that ground-level ozone may be sufficiently high to affect Chinas winter wheat production. As non-urban ozone increases with industrialization, its effects on crops could hinder efforts to meet increasing food demands in the coming decades, in China.

A three-dimensional global model investigation of seasonal variations in the atmospheric burden of anthropogenic sulfate aerosols

A global three-dimensional chemical transport model is used to investigate seasonal variations of anthropogenic sulfur in the troposphere. Particular emphasis is placed on detailed comparisons of the modeled surface sulfur dioxide (SO2) and sulfate (SO4) concentrations and sulfate wet deposition fluxes with measurements from the Eulerian Model Evaluation Field Study (EMEFS) and Cooperative Program for Monitoring and Evaluation of the Long Range Transmission of Air Pollutants in Europe (EMEP) field programs in North America and Europe, respectively. Initial comparisons of model results with measurements reveal a systematic tendency of the model to overestimate SO2 concentrations and underestimate SO4 concentrations while producing a reasonable fit to measured wet deposition fluxes. Through a series of sensitivity tests we find that the addition of a nonphotochemical pathway for converting SO2 to SO4 in the boundary layer with a pseudo first-order rate of constant of 1–2×10−6 s−1 provides the most reasonable method of bringing the model results into better agreement with the EMEFS and EMEP data sets. We propose that this additional pathway may be related to heterogeneous reactions between SO2 and atmospheric aerosols that typically are not included in models of the atmospheric sulfur cycle. Despite the vastly improved simulation of surface SO2 and SO4 when this hypothetical heterogeneous oxidation pathway is included, the model is unable to simultaneously simulate the large seasonal cycle in surface SO4 observed over eastern North America and the almost total absence of a seasonal cycle in surface SO4 over Europe. The seasonal cycles in model-predicted column SO4 burdens are similar, but not identical, to those for surface SO4 because of regional differences in transport, free tropospheric oxidation, and in-cloud removal. We find that the summer-to-winter ratio in column SO4 is larger over eastern North America than it is over Europe; however, both are larger than that for eastern Asia, where wintertime column SO4 is predicted to exceed summertime column SO4.