A34B-01
Summertime State-Level Source-Receptor Relationships between NOx Emissions and Downwind Surface Ozone Concentrations over the Continental United States
Inter-state transport of ozone (O3) and its precursors can contribute substantially to state-level surface O3 concentrations, making it difficult for some states to meet the National Ambient Air Quality Standard (NAAQS) for O3 by limiting only their own emissions. This study uses the Community Multiscale Air Quality (CMAQ) model to analyze the effect of inter-state transport of O3 and its precursors on surface O3 in summer over each continental U.S. state. By examining the difference between a standard model simulation and perturbation simulations in which each state's NOx emissions are removed, we establish for the first time a summertime source-receptor matrix for all 48 continental states. The NAAQS requires surface O3 concentrations to not exceed 0.08ppm O3 averaged over an 8-hour period. We find that for 16 states at least one neighboring state's NOx emissions are responsible for a larger increase in peak 8-hour average surface O3 concentrations than the state's own emissions. For 80% of the contiguous states, inter-state transport is more important than local emissions for summertime 8-hour peak O3 concentrations. The large influence we find for out-of-state emissions on peak 8- hour O3 demonstrates the difficulty that some states found to be in violation of the 8-hour NAAQS for O3 may have in reaching compliance simply by controlling their own NOx emissions. To quantify the impact of NOx emissions from a single source state on surface O3 concentrations outside that state, we calculate an out-of-state influence ratio. This influence ratio is the percentage change in surface O3 outside the source state relative to the total change in surface O3 across the continental United States due to the NOx emissions from the source state. The influence ratio for peak 8-hour O3 is over 30% in all states except ME and VT, over 50% in 30 states, and over 70% in 11 states. EPA recognized the importance of inter-state transport when developing the Clean Air Interstate Rule (CAIR). Our source-receptor matrices indicate that the geographical range of the CAIR program is sufficient to address interstate transport of O3 in most of the states included in the program. However, the exclusion of TX, which has particularly large NOx emissions, from the CAIR O3 program leaves emission sources uncontrolled that contribute to over one third of peak O3 increases in four participating downwind states.
A34B-02 INVITED
Observations and Modeling of US Power Plant NOx Emission Reductions and Their Impact on Air Quality
Nitrogen oxide (NOx) emissions resulting from fossil fuel combustion lead to unhealthy levels of near-surface ozone (O3). One of the largest US sources, electric power generation, represented about 25% of US anthropogenic NOx emissions prior to the recent implementation of pollution controls by utility companies. Continuous emission monitoring data demonstrate that overall US power plant NOx emissions decreased about 50% during the summer ozone season since the late 1990's. Space-based instruments observed declining regional NOx levels between 1999 and 2005 in response to these emission reductions. Satellite-retrieved summertime nitrogen dioxide (NO2) columns and bottom-up emission estimates show larger decreases in the Ohio River Valley, where power plants dominate NOx emissions, than in the northeast US urban corridor. Model simulations predict lower O3 across much of the eastern US in response to these emission reductions.
A34B-03 INVITED
Day-of-week Effects in Ozone, Nitrogen Oxides, and VOC Reactivity Within and Downwind of Sacramento
Day-of-week patterns in human activities can be used to examine the ways in which differences in primary emissions result in changes in the rates of photochemical reactions, and the production of secondary pollutants. Data from twelve California Air Resources Board monitoring sites in Sacramento, CA, and the downwind Mountain Counties air basin are analyzed to reveal day of week patterns in ozone and its precursors in the summers of 1998-2002. On the weekends, NOx concentrations drop by 35%, whereas VOC reactivity changes by less than 15%, likely due to significant reductions in diesel truck traffic. The main driver for day-of- week differences in ozone abundance at urban sites is the reduced titration of O3 by fresh NO emissions on weekends. By analyzing day-of-week patterns in odd oxygen (O3+NO2), we can eliminate this complication and focus on the NOx-dependence of ozone production and the related photochemistry. In the downwind Mountain Counties, ozone production is found to be NOx-limited, whereas at the urban and suburban sites of Sacramento, ozone production is NOx-saturated. An analysis of day-of-week cycles in Sacramento reveals that boundary layer chemistry and mixing in of residual air from above both contribute to the observed accumulation of ozone at surface sites. This interpretation is consistent with the day-of-week patterns in a range of VOC with different emission sources and lifetimes and in higher nitrogen oxides that are co-products of ozone. We use isoprene as a probe of local oxidation rates, and infer that on the weekend OH is higher in the urban area and lower at one of the forested Mountain Counties sites, as expected from the NOx- dependence. Our analysis also demonstrates that in NOx-saturated environments, gas phase HNO3, and by extension particulate nitrate, concentrations are relatively insensitive to changes in NOx emissions.
A34B-04
Relating Satellite Data to NOx Source Estimates Through Inverse Modelling : a 10-year Emission Assessment
A one decade record (1997-2006) of tropospheric NO2 columns retrieved by the GOME and SCIAMACHY satellite instruments is used to derive a top-down inventory for anthropogenic, pyrogenic and natural NOx emissions. This is realized by combining the satellite observations together with a chemistry-transport model and its adjoint, which enables to estimate the influence of the emission updates on the chemical lifetime of NOx by taking chemical feedbacks into account. The optimization is performed at the spatial resolution of the model, which allows to improve for the fine scale distribution, the seasonality and the interannual variability of the emissions. The long-term data record used allows inferring anthropogenic emission trends over the main industrialized regions of the Northern Hemisphere. The NOx emission changes inferred by the inversion imply changes on OH and O3 concentrations. These changes as well as potential trends will be quantified in this paper, especially over regions bearing rapid economic changes or emission abatement strategies.
A34B-05 INVITED
Modelling of long-term trends and variability of tropospheric ozone and its precursors and their sensitivity to NOx emission changes
The tropospheric chemical composition has changed markedly over time. Particularly large changes have been observed during recent decades associated largely with economic growth and increased energy consumption. In the European RETRO project we compiled new global emission data sets for NOx, CO and NMVOC species and performed multi-decadal simulations using three state-of-the-art chemistry transport and chemistry climate models. The models were evaluated with available observations from satellite, ground based networks and aircraft sampling. Some historic data were re-analyzed specifically for the project. In spite of some biases found in the model simulations, all three models are generally consistent with respect to the overall trend in ozone and precursor concentrations. Longer-term changes are mostly related to changing emissions, while shorter term fluctuations are predominantly attributed to meteorological variability. Additional sensitivity calculations provide some insight into the modeled response to projected NOx emission changes. http://retro.enes.org
A34B-06
Effect of NOx emission controls from world regions on the long-range transport of ozone air pollution and human mortality
We model the influences of 10% reductions in anthropogenic nitrogen oxide (NOx) emissions from each of nine world regions on surface ozone air quality in that region and all other regions, using the MOZART-2 model of tropospheric chemistry and transport. In doing so, we quantify the relative importance of long-range transport between different world regions for ozone. We find that the strongest inter-regional influences are for Europe to the Former Soviet Union (FSU), East Asia to Southeast Asia, and Europe to Africa. The largest influences per unit of NOx reduced, however, are seen for tropical source regions, due to greater sensitivity of ozone production to NOx emissions. Results show, for example, that NOx reductions in North America are about 20% as effective per ton at reducing ozone in Europe, as NOx reductions from Europe itself. In estimating the changes in cases of premature mortality associated with ozone, we find that NOx reductions in North America, Europe, and FSU reduce more mortalities outside of the source regions than within. Among world regions, an average ton of NOx reduced in India causes the greatest number of avoided mortalities (mainly in India itself). We also assess the long-term increases in global ozone resulting from methane increases due to the regional NOx reductions. For many of the more distant source-receptor pairs, the long-term increase in ozone roughly negates the direct short-term ozone decrease. The increase in methane and long-term ozone per unit of NOx reduced is greatest in tropical source regions and varies among different regions by a factor of ten.
A34B-07
Laboratory Measurement of the Gas-Phase Rate Constant for Formation of Nitric Acid from the Reaction of OH and NO2
The rate constant for the reaction OH + NO2 + M → HONO2 + M is among the most influential parameters affecting air pollution levels. There remains significant uncertainty about this rate, due to lack of laboratory data at 1 atm and to the unknown yield of a secondary channel forming peroxynitrous acid (HOONO). New experimental measurements of both the kinetics and HOONO/HONO2 branching ratios at 760 Torr are presented. The results are compared with current recommendations; when incorporated in models, the new parameters lead to significantly higher modeled ozone levels and reduced formation of nitric acid.
A34B-08 INVITED
Nitrogen oxide emissions from lightning: Present knowledge, uncertainties, and strategies for improvements
Lightning-induced nitrogen oxides (LNOx) have several important implications for atmospheric chemistry and climate. The global LNOx source is one of the largest natural sources of NOx in the atmosphere and certainly the largest source of NOx in the upper troposphere, in particular in the tropics. The LNOx source rate has been often cited to be the least known one within the total atmospheric NOx budget. Considerable progress has been made recently which allows reducing the uncertainty of the global LNOx value. This includes ground-based and satellite observations of various lightning signals, satellite observations of NO2 column and profile distributions, airborne in-situ measurements of NOx abundance near thunderstorms at midlatitudes and over tropical continents, where most lightning occurs, detailed cloud-resolving model studies, and improved global models. This paper reviews the present knowledge on the global LNOx source rate and its still large uncertainties (Schumann and Huntrieser, ACP, 2007). Mainly as a result of global model studies for various LNOx parameterisations tested with related observations, the best estimate of the annual global LNOx nitrogen mass source and its uncertainty range is (5+-3) Tg/a in this study. The paper estimates the LNOx accuracy required for various applications and lays out strategies for improving estimates in the future. An accuracy of about 1 Tg/a or 20%, as necessary in particular for understanding tropical tropospheric chemistry, is still a challenging goal. Finally we point out to some recent findings from analysis of field experiments comparing LNOx efficiency of mid- latitude and tropical thunderstorms.