A21A-0012
Direct sensitivity analysis of ozone formation and transport in California's San Joaquin Valley
Spatial and temporal variations of ozone-limiting reagents and the importance of local vs upwind emission sources in the San Joaquin Valley of central California during a five-day episode are studied by first- and second- order sensitivity analysis with the Decoupled Direct Method. Despite considerable spatial variations, nitrogen oxides (NOx) emission reductions are overall more effective than volatile organic compound (VOC) control for attaining the 8-hr ozone standard in this region, in contrast to the VOC control that works better for attaining the prior 1-hr ozone standard. Inter-basin source contributions of NOx emissions are limited to the northern part of the SJV, while anthropogenic VOC (AVOC) emissions influence ozone formation in the SJV further downwind. Among model input parameters studied here, uncertainties in emissions of NOx and AVOC, and the rate coefficient of the OH+NO2 termination reaction, are found to have the greatest effects on first-order ozone responses to changes in NOx emissions. Uncertainties in biogenic VOC emissions only have a modest effect because they are generally not collocated with anthropogenic sources.
A21A-0013
Reduction on NOx emissions on urban areas by changing specific vehicle fleets: effects on NO2 and O3 concentration
The largest amount of NOx emissions in urban areas comes from on-road traffic, which is the largest contributor to urban air pollution (Colvile et al., 2001). Currently different strategies are being tested in order to reduce its effects; many of them oriented to the reduction of the unitary vehicles emissions, by alternative fuels use (such as biofuels, natural gas or hydrogen) or introduction of new technologies (such as hybrid electric vehicles or fuel cells). Atmospheric modelling permits to predict their consequences on tropospheric chemistry (Vautard et al., 2007). Hence, this work assesses the changes on NO2 and O3 concentrations when substituting a 10 per cent of the urban private cars fleets by petrol hybrid electric cars (HEC) or by natural gas cars (NGC) in Madrid and Barcelona urban areas (Spain). These two cities are selected in order to highlight the different patterns of pollutants transport (inland vs. coastal city) and the different responses to emissions reductions. The results focus on a typical summertime episode of air pollution, by means of the Eulerian air quality model ARW- WRF/HERMES/CMAQ, applied with high resolution (1-hr, 1km2) since of the complexity of both areas under study. The detailed emissions scenarios are implemented in the HERMES traffic emissions module, based on the Copert III-EEA/EMEP-CORINAIR (Nztiachristos and Samaras, 2000) methodology. The HEC introduction reduces NOx emissions from on-road traffic in a 10.8 per cent and 8.2 per cent; and the NGC introduction in a 10.3 per cent and 7.8 per cent, for Madrid and Barcelona areas, respectively. The scenarios also affect the NMVOCs reduction (ranging from -3.1 to -6.9 per cent), influencing the tropospheric photochemistry through the NOx/NMVOCs ratio. The abatement of the NO photooxidation but also to the reduction on primary NO2 involves a decrease on NO2 levels centred on urban areas. For example, the NO2 24-hr average concentration in downtown areas reduces up to 8 per cent (-6 μg m-3 on average). The urban areas are VOC-controlled, therefore the reduction on NOx emissions involves a minor increase on tropospheric O3 concentration (Jiménez and Baldasano, 2004), up to 1.5 per cent at some points. Nevertheless, the O3 precursors reduction has positive effects in the downwind areas affected by the urban plume, slightly reducing the O3 levels, but at the regional scale the reduction applied on urban traffic emissions has negligible effects. Both scenarios tested are very similar in terms of emissions reductions and air quality changes, which means that the NOx/NMVOCs ratio do not involve an O3-sensitivity regime variation among scenarios. The HEC scenario is more effective in reducing NO2 levels in urban areas than the NGC scenario (with maximum reductions affecting a larger area) and involves a larger increase in urban O3 concentration.
A21A-0014
Integrating Kalman Filter Inverse Modeling and Direct Sensitivities to Evaluate NOx Emission Inventory Biases Based on Satellite-Derived NO2 columns
Regional air quality models are used to develop control strategies for reducing the ambient concentrations of harmful pollutants such as ozone and fine particulate matter. Regional models rely on detailed emission inventories, and these inventories still have a substantial amount of uncertainty despite continuing efforts for improvement. It is important to reduce nitrogen oxide (NOx) emission uncertainties, because these compounds regulate the levels of ozone in the troposphere, lead to formation of nitric acid, and impact the levels of hydroxyl radicals. A method was developed to constrain ground-level NOx emissions using an iterative Kalman filter inverse modeling technique and SCIAMACHY satellite observations of NO2. For the inverse modeling calculations, the relationship between emissions and modeled ambient concentrations was developed using sensitivities provided by the decoupled direct method in three dimensions (DDM-3D). The method was successfully tested using a controlled emissions scenario with a known synthetic solution (i.e., pseudodata test), and then applied to a summer 2004 episode where emissions of NOx were examined over a region covering the southeastern United States. The results indicate that while ground level NOx emission estimates in urban areas appear to be only slightly too high, rural emissions need to increase by a factor of two in order to produce the NO2 column densities observed by the satellite. However, over rural areas, the inverse is highly sensitive to NO2 concentrations in the upper troposphere where its origins are likely to be from lightning emissions of NO. Regional models often ignore lightning emissions, because these have been shown to have negligible impacts on boundary layer pollutant concentrations. But if satellite emissions are to be used to any extent in the context of regional inverse modeling or data assimilation, the upper level sources need to be better quantified. Disclaimer: The research presented here was performed under the Memorandum of Understanding between the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Commerce's National Oceanic and Atmospheric Administration (NOAA) and under agreement number DW13921548. This work constitutes a contribution to the NOAA Air Quality Program. Although it has been reviewed by EPA and NOAA and approved for publication, it does not necessarily reflect their policies or views.
A21A-0015
Long-Term Changes in Gas- and Particle-Phase Emissions From On-Road Diesel and Gasoline Vehicles
Gas- and particle-phase pollutants were measured separately for a) light-duty (LD) vehicles and b) medium- (MD) and heavy-duty (HD) diesel trucks. Measurements were made during summer 2006 at the Caldecott Tunnel in the San Francisco Bay area as part of a continuing campaign to track changes in vehicle emissions over time. When normalized to fuel consumption, NOx emission factors were found to be 3.0+-0.2 and 39+-3 g/kg for LD vehicles and MD/HD diesel trucks, respectively. Corresponding PM2.5 emission factors were 0.07+-0.02 and 1.4+-0.3 g/kg. Results from 2006 are compared to similar measurements made at the same site in 1997. NOx emission factors have decreased by 67+-3 and 25+-12% for LD vehicles and MD/HD diesel trucks, respectively. The ratio of HD to LD emission factor for NOx increased from 6+-1 to 12+-1 between 1997 and 2006, which indicates an increase in the relative importance of diesel trucks as a source of NOx emissions. This is compounded by the fact that in the U.S., diesel fuel sales have been increasing 3 times faster than gasoline sales. High time-resolution (1 Hz) measurements of NOx and CO2 were used to calculate NOx emission factors from individual HD truck exhaust plumes. MD/HD diesel trucks were also found to be a significant source of direct aldehyde emissions, which react in the atmosphere to form peroxyacyl nitrates.
A21A-0016
Temperature Dependent Alkyl Nitrate Formation
In studying the effects of climate change on air quality, it is important to describe correctly the temperature dependence of the relevant atmospheric chemistry. The SAPRC99 chemical mechanism was modified to include missing temperature dependence of alkyl nitrate (RONO2) formation. Alkyl nitrate formation competes with NO to NO2 conversion and hence inhibits production of ozone. We use the Community Multiscale Air Quality model (CMAQ), a three-dimensional regional air quality model, to compare the modified chemical mechanism that includes temperature-dependent RONO2 formation with the base mechanism in Central California. We consider both present day meteorology and a future scenario with perturbed temperatures that are equivalent to a doubling of global background CO2 relative to pre-industrial levels (see Steiner et al., JGR 2006). Changes in predicted alkyl nitrate concentrations (both up and down) follow as expected. NOX and ozone concentrations also change throughout the Central California modeling domain.
A21A-0017
Transport of NOx and its reaction products in a global model and its impact on ozone
Intercontinental transport of O3 is often attributed to the direct transport of O3 following its formation in source regions. However, transport of O3 and other reactive tropospheric gases and aerosols are also influenced by the transport of NOx and NOx reaction products, which have lifetimes ranging from hours to a week or longer. Here, we show results from a global model that includes tracers for the source regions of NOx and each of its major reaction products (PAN, HNO3, alkyl nitrates). The model (IMPACT) includes detailed representation of organic nitrates formed from isoprene and there role as precursors of O3. Sources of O3 are also traced based on the traced source of NOx associated with ozone formation. This results in a different view of the sources and transport of O3 than can be obtained when only transport of O3 is included. Tracers of NOx and NOx reaction products will also be used to identify correlation patterns associated with intercontinental transport.
A21A-0018
High NO2 Observed From OMI AURA over the Indo-Gangetic plains
The formation of dense haze, fog and smog, a winter season phenomena, affects millions of people in the Indian sub-continent and especially in the Indo-Gangetic (IG) plains. Recent studies have shown the effect of increasing aerosol loading and pollution on agricultural crops, vegetation, hydrological cycle and climatic conditions in the IG plains. In the present study, we present spatial distribution of nitrogen dioxide (NO2) observed from the Ozone Monitoring Instrument (OMI), a key sensor for atmospheric chemistry on NASA's Earth Observing System (EOS) Aura satellite, over the Indian subcontinent. High NO2 concentrations are observed in the IG plains and surroundings, originating from coal based thermal power plants (including vehicular transport sector in the mega cities) situated in the urban and rural areas. These plants are likely responsible for the formation of smog as well as high surface ozone affecting species of plants and vegetation of the IG plains. The present results show complex atmospheric chemistry over the IG plains that need to be further investigated through an international cooperation.
A21A-0019
The "weekend effect" in tropospheric NO2 seen from the Ozone Monitoring Instrument
The Ozone Monitoring Instrument has gathered daily global data on NO2 and other atmospheric trace gases since its launch on the EOS Aura satellite in 2004. The large accumulated data set makes it possible to monitor changes of both meteorological and anthropogenic origin in tropospheric NO2 amounts. In particular, averages on time scales on the order of a year show a distinct "weekend effect" in NO2 variation, with smaller NO2 amounts seen on Saturday and/or Sunday than on the remaining weekdays. Using the OMI NO2 Standard Product (SP), we examine this effect in relation to geopolitical boundaries and investigate implications for identifying sources. We also use the SP data to find evidence for other short-term anthropogenic changes in NO2 emissions over heavily polluted regions including the United States, Europe and China.
A21A-0020
Nitrogen Oxides from Biogenic Alkyl Nitrates: A Natural Source of Tropospheric Ozone
Observations indicate that the tropical and southern oceans are source regions for biogenic emissions of alkyl nitrates. These compounds have lifetimes of several days to a month and are a significant source of reactive odd nitrogen (NOx) in remote regions of the atmosphere. These biogenically produced NOx precursors represent a natural control on atmospheric composition, including the important greenhouse gases methane (CH4) and tropospheric ozone (O3). We present simulations from the UCI global chemical transport model (CTM) using measurement-based fluxes of methyl and ethyl nitrate from their oceanic source regions and examine the contribution of these gases to global atmospheric composition. We also discuss the sensitivity of our results to our representation of two sub-gridscale processes: wet scavenging and photolysis in the presence of broken cloud fields. Quantification of the transport and chemistry of these compounds improves our understanding of natural tropospheric ozone production as well as hydroxyl radical (OH) chemistry in both the remote regions of the modern atmosphere and the pre-industrial atmosphere.