Sanford Sillman

The relation between ozone, NOx and hydrocarbons in urban and polluted rural environments

Abstract Research over the past ten years has created a more detailed and coherent view of the relation between O3 and its major anthropogenic precursors, volatile organic compounds (VOC) and oxides of nitrogen (NOx). This article presents a review of insights derived from photochemical models and field measurements. The ozone–precursor relationship can be understood in terms of a fundamental split into a NOx-senstive and VOC-sensitive (or NOx-saturated) chemical regimes. These regimes are associated with the chemistry of odd hydrogen radicals and appear in different forms in studies of urbanized regions, power plant plumes and the remote troposphere. Factors that affect the split into NOx-sensitive and VOC-sensitive chemistry include: VOC/NOx ratios, VOC reactivity, biogenic hydrocarbons, photochemical aging, and rates of meteorological dispersion. Analyses of ozone–NOx–VOC sensitivity from 3D photochemical models show a consistent pattern, but predictions for the impact of reduced NOx and VOC in indivdual locations are often very uncertain. This uncertainty can be identified by comparing predictions from different model scenarios that reflect uncertainties in meteorology, anthropogenic and biogenic emissions. Several observation-based approaches have been proposed that seek to evaluate ozone–NOx–VOC sensitivity directly from ambient measurements (including ambient VOC, reactive nitrogen, and peroxides). Observation-based approaches have also been used to evaluate emission rates, ozone production efficiency, and removal rates of chemically active species. Use of these methods in combination with models can significantly reduce the uncertainty associated with model predictions.

The use of NO y , H2O2, and HNO3 as indicators for ozone‐NO x ‐hydrocarbon sensitivity in urban locations

Correlations are presented between model predictions for O3-NOx-hydrocarbon sensitivity and afternoon concentrations of four “indicator species”: NOy, O3/(NOy-NOx), HCHO/NOy, and H2O2/HNO3. The indicator species correlations are based on a series of photochemical simulations with varying rates of anthropogenic and biogenic emissions and meteorology. Hydrocarbon-sensitive chemistry in models is shown to be linked to afternoon NOy > 20 ppb, O3/(NOy - NOx) < 7, HCHO/NOy < 0.28, and H2O2/HNO3 < 0.4. Lower NOy and higher ratios correspond with NOx-sensitive ozone. The correlation between NOx-hydrocarbon sensitivity and indicator species remains, even when model emission rates and hydrocarbon/NOx ratios are changed by a factor of 2. Methods are developed for evaluating the goodness of fit between model NOx-hydrocarbon sensitivity and indicator values. Ozone chemistry is also analyzed in terms of fundamental properties of odd hydrogen, and theoretical criteria for the transition between NOx- and hydrocarbon-sensitive regimes are derived. A theoretical correlation between O3 and H2O2 + NOy - NOx is developed as a way to extend rural O3-NOy correlations into urban locations. Measured indicator values during pollution events in Los Angeles, Atlanta, and rural Virginia are used to illustrate the range of observed values under different environmental conditions.

Impact of temperature on oxidant photochemistry in urban, polluted rural and remote environments

The impact of temperature on formation of O3 and odd nitrogen photochemistry is investigated using urban-, regional- and global-scale simulations. Urban and polluted rural environments are explored with a regional simulation derived from a specific episode in the midwestern United States. The simulations predict that O3 increases with temperature in both urban and polluted rural environments. The O3-temperature relation is driven largely by chemistry of peroxyacetylnitrate (PAN) which represents an increased sink for both NOx and odd hydrogen at low temperatures. Isoprene emissions, H2O, and solar radiation also contribute to the O3-temperature relation. Possible correlations between temperature and anthropogenic emissions or stagnant meteorology were not included. Observations at urban and rural sites in the United States suggests that O3 increases with temperature at a faster rate than the models predict. Calculations with a one-dimensional global model suggest that increased temperature in the polluted boundary layer does not lead to increased O3 in the free troposphere, because increased export of O3 is balanced by decreased export of odd nitrogen species.

Factors regulating ozone over the United States and its export to the global atmosphere

Author(s): Jacob, Daniel J; Logan, Jennifer A; Gardner, Geraldine M; Yevich, Rose M; Spivakovsky, Clarisa M; Wofsy, Steven C; Sillman, Sanford; Prather, Michael J | Abstract: The factors regulating summertime O3 over the United States and its export to the global atmosphere are examined with a 3-month simulation using a continental scale, three-dimensional photochemical model. It is found that reducing NOx emissions by 50% from 1985 levels would decrease rural O3 concentrations over the eastern United States by about 15% under almost all meteorological conditions, while reducing anthropogenic hydrocarbon emissions by 50% would have less than a 4% effect except in the largest urban plumes. The strongly NOx-limited conditions in the model reflect the dominance of rural areas as sources of O3 on the regional scale. The correlation between O3 concentrations and temperature observed at eastern U.S. sites is attributed in part to the association of high temperatures with regional stagnation, and in part to an actual dependence of O3 production on temperature driven primarily by conversion of NOx to peroxyacetylnitrate (PAN). The net number of O3 molecules produced per molecule of NOx consumed (net O3 production efficiency, accounting for both chemical production and chemical loss of O3) has a mean value of 6.3 in the U.S. boundary layer; it is 3 times higher in the western United States than in the east because of lower NOx concentrations in the west. Approximately 70% of the net chemical production of O3 in the U.S. boundary layer is exported (the rest is deposited). Only 6% of the NOx emitted in the United States is exported out of the U.S. boundary layer as NOx or PAN, but this export contributes disproportionately to total U.S. influence on global tropospheric O3because of the high O3 production efficiency per unit NOx in the remote troposphere. It is estimated that export of U.S. pollution supplies 8 Gmol O3 d−1 to the global troposphere in summer, including 4 Gmol d−1 from direct export of O3 out of the U.S. boundary layer and 4 Gmol d−1 from production of O3 downwind of the United States due to exported NOx. This U.S. pollution source can be compared to estimates of 18–28 Gmol d−1 for the cross-tropopause transport of O3 over the entire northern hemisphere in summer.

Total reactive nitrogen (NO y ) as an indicator of the sensitivity of ozone to reductions in hydrocarbon and NO x emissions

For areas in the United States not meeting the federal air quality standard for ozone, an issue of continuing controversy is the emphasis to be placed on controlling nitrogen oxides (NOx) in addition to emissions of reactive organic gases (ROG). To assess conditions under which ROG or NOx controls would be most effective, we have analyzed predictions from four studies that represent different locations and meteorological conditions, distinct chemical inputs, e.g., with or without significant biogenic emissions, and different air quality models. A consistent association is found between the sensitivity of ozone to reductions in ROG versus NOx emissions and the simulated total reactive nitrogen (NOy) at the time and place of peak ozone. In the studies examined, ozone was predicted to be reduced most effectively by ROG controls at locations where NOy concentrations exceeded a threshhold value falling in the range of 10 to 25 ppb, whereas NOx controls were predicted to be more effective where NOy concentrations were below that threshhold. The NOy level explains much of the difference in ozone sensitivity at different locations and provides a basis for comparison of predicted sensitivity from different models. In contrast, the morning concentration ratio of ROG to NOx that has been used in the past is a less reliable indicator of O3 sensitivity. Measurement of NOy concentrations along with ozone would assist in empirical testing of model predictions of responses to emission reductions.

Dependence of ozone production on NO and hydrocarbons in the troposphere

An expression for the production rate of 03, P(O 3), is derived based on a radical budget equation applicable to low and high NOx conditions. Differentiation of this equation with respect to NO or hydrocarbons (HC) gives an approximate analytic formula in which the relative sensitivity of P(O3) to changes in NO or HC depends only on the fraction of radicals which are removed by reactions with NOx. This formula is tested by comparison with results from a photochemical calculation driven by trace gas observations from the 1995 Southern Oxidants Study (SOS) campaign in Nashville, Tennessee.

Climatologies of NOx and NOy: A comparison of data and models

Abstract Climatologies of tropospheric NOx (NO + NO2) and NOy (total reactive nitrogen: NOx + N03 + 2 × N2O5 + HNO2 + HNO3 + HNO4 + ClONO2 + PAN (peroxyacetylnitrate) + other organic ni trates) have been compiled from data previously published and, in most cases, publicly archived. Emphasis has been on non-urban measurements, including rural and remote ground sites, as well as aircraft data. Although the distribution of data is sparse, a compilation in this manner can begin to provide an understanding of the spatial and temporal distributions of these reactive nitrogen species. The cleanest measurements in the boundary layer are in Alaska, northern Canada and the eastern Pacific, with median NO mixing ratios below 10 pptv, NOx below 50 pptv, and NOy below 300 pptv. The highest NO values (greater than 1 ppbv) were found in eastern North America and Europe, with correspondingly high NOy (∼ 5 ppbv). A significantly narrower range of concentrations is seen in the free troposphere, particularly at 3–6 km, with NO typically about 10 pptv in the boreal summer. NO increases with altitude to ∼ 100 pptv at 9–12 km, whereas NOy does not show a trend with altitude, but varies between 100 and 1000 pptv. Decreasing mixing ratios eastward of the Asian and North American continents are seen in all three species at all altitudes. Model-generated climatologies of NOx and NOy from six chemical transport models are also presented and are compared with observations in the boundary layer and the middle troposphere for summer and winter. These comparisons test our understanding of the chemical and transport processes responsible for these species distributions. Although the model results show differences between them, and disagreement with observations, none are systematically different for all seasons and altitudes. Some of the differences between the observations and model results may likely be attributed to the specific meteorological conditions at the time that measurements were made differing from the model meteorology, which is either climatological flow from GCMs or actual meteorology for an arbitrary year. Differences in emission inventories, and convection and washout schemes in the models will also affect the calculated NOα and NOy distributions.

Nighttime observations of anomalously high levels of hydroxyl radicals above a deciduous forest canopy

Diurnal measurements of hydroxyl and hydroperoxy radicals (OH and HO2) made during the Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET) summer intensive of 1998 indicate that these key components of gas phase atmospheric oxidation are sustained in significant amounts throughout the night in this northern forested region. Typical overnight levels of OH observed were 0.04 parts per trillion (pptv) (1.1 × 106 molecules/cm3), while HO2 concentrations ranged from 1 to 4 pptv. Results of diagnostic testing performed before, after, and during the deployment suggest little possibility of interferences in the measurements. Collocated measurements of the reactive biogenic hydrocarbon isoprene corroborate the observed levels of OH by exhibiting significant decays overnight above the forest canopy. The observed isoprene lifetimes ranged from 1.5 to 12 hours in the dark, and they correlate well to those expected from chemical oxidation by the measured OH abundances. Possible dark reactions that could generate such elevated levels of OH include the ozonolysis of extremely reactive biogenic terpenoids. However, in steady state models, which include this hypothetical production mechanism, HO2 radicals are generated in greater quantities than were measured. Nonetheless, if the measurements are representative of the nocturnal boundary layer in midlatitude temperate forests, this observed nocturnal phenomenon might considerably alter our understanding of the diurnal pattern of atmospheric oxidation in such pristine, low-NOx environments.

Simulation of summertime ozone over North America

The concentrations of O3 and its precursors over North America are simulated for three summer months with a three-dimensional, continental-scale photochemical model using meteorological input from the Goddard Institute for Space Studies (GISS) general circulation model (GCM). The model has 4°×5° grid resolution and represents non linear chemistry in urban and industrial plumes with a subgrid nested scheme. Simulated median afternoon O3 concentrations at rural U.S. sites are within 5 ppb of observations in most cases, except in the south central United States where concentrations are overpredicted by 15–20 ppb. The model captures successfully the development of regional high-O3 episodes over the northeastern United States on the back side of weak, warm, stagnant anticyclones. Simulated concentrations of CO and nonmethane hydrocarbons are generally in good agreement with observations, concentrations of NOx are underpredicted by 10–30%, and concentrations of peroxyacylnitrates (PANs) are overpredicted by a factor of 2 to 3. The overprediction of PANs is attributed to flaws in the photochemical mechanism, including excessive production from oxidation of isoprene, and may also reflect an underestimate of PANs deposition. Subgrid nonlinear chemistry as captured by the nested plumes scheme decreases the net O3 production computed in the United States boundary layer by 8% on average.

Effects of additional nonmethane volatile organic compounds, organic nitrates, and direct emissions of oxygenated organic species on global tropospheric chemistry

[1] This work evaluates the sensitivity of tropospheric ozone and its precursors to the representation of nonmethane volatile organic compounds (NMVOCs) and organic nitrates. A global 3-D tropospheric chemistry/transport model (IMPACT) has been exercised initially using the GEOS-Chem chemical reaction mechanism. The model was then extended by adding emissions and photochemical reactions for aromatic and terpenoid hydrocarbons, and by adding explicit representation of hydroxy alkyl nitrates produced from isoprene. Emissions of methanol, phenol, acetic acid and formic acid associated with biomass burning were also added. Results show that O3 increases by 20% in most of the troposphere, peroxyacetyl nitrate (PAN) increases by 30% over much of the troposphere and OH increases by 10%. NOx (NO + NO2) decreases near source regions and increases in remote locations, reflecting increased transport of NOx away from source regions by organic nitrates. The increase in O3 was driven largely by the increased role of PAN as a transporter of NOx and by the rerelease of NOx from isoprene nitrates. The increased PAN production was associated with increases in methyl glyoxal and hydroxyacetone. Comparison with measured values show reasonable agreement for O3 and PAN, but model measurement agreement does not either improve or degrade in the extended model. The extended model shows improved agreement with measurements for methanol, acetic acid and peroxypropional nitrate (PPN). Results from the extended model were consistent with measured alkyl nitrates and glycolaldehyde, but hydroxyacetone and methyl glyoxal were overestimated. The latter suggests that the effect of the isoprene nitrates is somewhat smaller than estimated here. Although the model measurement comparison does not show specific improvements with the extended model, it provides a more complete description of tropospheric chemistry that we believe is important to include.