Model For Ozone And Related Chemical Tracers Research Articles

Evaluating the sensitivity of fine particulate matter (PM2.5) simulations to chemical mechanism in WRF-Chem over Delhi

Accurate prediction of PM2.5, its optical properties and dominant chemical components are essential for air quality studies. In this study, we investigated the effects of two gas phase chemical schemes coupled with three aerosol mechanisms on the simulated PM2.5 mass concentration in Delhi using the Weather Research and Forecasting model with Chemistry module (WRF-Chem). The model was employed to cover the entire northern region of India at 10 km horizontal spacing and results were compared with comprehensive field data set on dominant PM2.5 chemical compounds from the Winter Fog EXperiment (WiFEX) at the Indira-Gandhi International Airport, New Delhi, and surface PM2.5 observations in Delhi (17 sites), Punjab (3 sites), Haryana (4 sites), Uttar Pradesh (7 sites) and Rajasthan (17 sites). The Model for Ozone and related Chemical Tracers (MOZART) gas-phase chemical mechanism coupled with the Goddard Chemistry Aerosol Radiation and Transport (GOCART) aerosol scheme were selected in the first experiment as it is currently employed in the operational air quality forecasting system of Ministry of Earth Sciences (MoES), Government of India. Other two simulations were performed with the MOZART gas phase chemical mechanism coupled with the Model for Simulating Aerosol Interactions and Chemistry (MOZART-MOSAIC), and Carbon Bond 5 (CB-05) gas mechanism coupled with the Modal Aerosol Dynamics Model for Europe/Secondary Organic Aerosol Model (CB05 - MADE/SORGAM) aerosol mechanisms. The evaluation demonstrated that chemical mechanisms affect the evolution of gas-phase precursors and aerosol processes which in turn affect the optical depth and overall performance of the model for PM2.5. All the three chemical schemes underestimate the observed concentrations of major aerosol composition and precursor gases over Delhi. Comparison with observations suggests that, the simulations using MOZART gas-phase chemical mechanism with MOSAIC aerosol scheme performed better in simulating aerosols over Delhi and its optical depth over the IGP.

Simulations of winter ozone in the Upper Green River basin, Wyoming, using WRF-Chem

Abstract. In the Upper Green River basin (UGRB) of Wyoming and the Uintah Basin of Utah, strong wintertime ozone (O3) formation episodes leading to O3 mixing ratios occasionally exceeding 70 parts per billion (ppb) have been observed over the last 2 decades. Wintertime O3 events in the UGRB were first observed in 2005 and since then have continued to be observed intermittently when meteorological conditions are favorable, despite significant efforts to reduce emissions from oil and natural gas extraction and production. While O3 formation has been successfully simulated using observed volatile organic compound (VOC) and nitrogen oxide (NOx) mixing ratios, successful simulation of these wintertime episodes using emission inventories in a 3-D photochemical model has remained elusive. An accurate 3-D photochemical model driven by an emission inventory is critical to understanding the spatial extent of high-O3 events and which emission sources have the most impact on O3 formation. In the winter of 2016/17 (December 2016–March 2017) several high-O3 events were observed with 1 h mixing ratios exceeding 70 ppb. This study uses the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) to simulate one of the high-O3 events observed in the UGRB during March 2017. The WRF-Chem simulations were carried out using the 2014 edition of the Environmental Protection Agency National Emissions Inventory (EPA NEI2014v2), which, unlike previous versions, includes estimates of emissions from non-point oil and gas production sources. Simulations were carried out with two different chemical mechanisms: the Model for Ozone and Related Chemical Tracers (MOZART) and the Regional Atmospheric Chemistry Mechanism (RACM), and the results were compared with data from seven weather and air quality monitoring stations in the UGRB operated by the Wyoming Department of Environmental Quality (WYDEQ). The simulated meteorology compared favorably to observations with regard to temperature inversions, surface temperature, and wind speeds. Notably, because of snow cover present in the basin, the photolysis surface albedo had to be modified to predict O3 in excess of 70 ppb, although the models were relatively insensitive to the exact photolysis albedo if it was over 0.65. O3 precursors, i.e., NOx and VOCs, are predicted similarly in simulations with both chemical mechanisms, but simulated VOC mixing ratios are a factor of 6 or more lower than the observations, while NOx is also underpredicted but to a lesser degree. Sensitivity simulations revealed that increasing NOx and VOC emissions to match observations produced slightly more O3 compared to baseline simulations, but an additional sensitivity simulation with doubled NOx emissions resulted in a considerable increase in O3 formation. These results suggest that O3 formation in the basin is most sensitive to NOx emissions.

Long-term variations of ground-level NO2 concentrations along coastal areas in China

The long-term trends of the ground-level NO2 concentrations over coastal areas in China were analysed from 2007 to 2019 in this study, based on the measurements from Ozone Monitoring Instrument (OMI) and the simulations from the Model for Ozone and Related Chemical Tracers (MOZART)-4. Multiple Gaussian functions were used to simulate the atmospheric profiles of NO2 concentrations from the MOZART-4, and the ratio of the ground-level concentrations to the columns estimated from this Gaussian function was used to convert the OMI NO2 columns to that at the ground level. The estimation results showed that the ground-level NO2 concentrations were well consistent with those of ground measurements in 2017 (the correlation coefficients ranging from 0.84 to 0.87 in the four seasons). The NO2 concentrations presented an increasing rate of 0.6 μg/m3/yr during 2007–2011 and then a decreasing rate of 0.5 μg/m3/yr until 2019. Most areas in the China Sea suffered from the maximum NO2 concentrations during 2011–2013. The temporal variations of NO2 over coastal lands were consistent with those over the ocean during 2007–2016, but they presented different after the implementation of coastal emission control area (ECA) policies in 2016. The ECA policies significantly decreased the NO2 concentrations in the Yangtze River Delta (the YRD) and the Bohai Rim, and they inhibited the increase of NO2 in the Pearl River Delta (the PRD). These results might be helpful in understanding the current situation of ground-level NO2 pollution over oceans and coastal land in China and provide guidance for further development of ECA policies.

Time-dependent 3D simulations of tropospheric ozone depletion events in the Arctic spring using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem)

Abstract. Tropospheric bromine release and ozone depletion events (ODEs) as they commonly occur in the Arctic spring are studied using a regional model based on the open-source software package Weather Research and Forecasting model coupled with Chemistry (WRF-Chem). For this purpose, the MOZART (Model for Ozone and Related chemical Tracers)–MOSAIC (Model for Simulating Aerosol Interactions and Chemistry) chemical reaction mechanism is extended by bromine and chlorine reactions as well as an emission mechanism for reactive bromine via heterogeneous reactions on snow surfaces. The simulation domain covers an area of 5040 km×4960 km, centered north of Utqiaġvik (formerly Barrow), Alaska, and the time interval from February through May 2009. Several simulations for different strengths of the bromine emission are conducted and evaluated by comparison with in situ and ozone sonde measurements of ozone mixing ratios as well as by comparison with tropospheric BrO vertical column densities (VCDs) from the Global Ozone Monitoring Experiment-2 (GOME-2) satellite instrument. The base bromine emission scheme includes the direct emission of bromine due to bromide oxidation by ozone. Results of simulations with the base emission rate agree well with the observations; however, a simulation with 50 % faster emissions performs somewhat better. The bromine emission due to bromide oxidation by ozone is found to be important to provide an initial seed for the bromine explosion. Bromine release due to N2O5 was found to be important from February to mid March but irrelevant thereafter. A comparison of modeled BrO with in situ and multi-axis differential optical absorption spectroscopy (MAX-DOAS) data hints at missing bromine release and recycling mechanisms on land or near coasts. A consideration of halogen chemistry substantially improves the prediction of the ozone mixing ratio with respect to the observations. Meteorological nudging is essential for a good prediction of ODEs over the 3-month period.

Long-term ambient SO2 concentration and its exposure risk across China inferred from OMI observations from 2005 to 2018

Long-term trends in the ambient SO2 concentrations across China from 2005 to 2018 were estimated using Ozone Monitoring Instrument (OMI) SO2 columns and an atmospheric SO2 profile simulated by the Model for Ozone And Related Chemical Tracers (MOZART)-4. The estimated ground-level SO2 concentrations were in strong agreement with the ground observations (R = 0.86, root-mean-square error = 10.49 μg/m3, relative prediction error = 19%). From 2005 to 2018, the average ground-level SO2 concentration ranged from 0.97 to 106.41 μg/m3 across China, with higher values in eastern China than in western China. Considering the uneven distribution of SO2 concentration and population across China, the population-weighted SO2 concentration was estimated to be 20.30 μg/m3 on a national scale. The population-weighted SO2 levels showed decreasing trends, and the absolute values of the decreasing trend peaked in winter (1.41 μg/m3 per year), followed by those in autumn (0.92 μg/m3 per year), spring (0.74 μg/m3 per year) and summer (0.75 μg/m3 per year) from 2005 to 2018. Although the SO2 exposure risk in 2018 decreased significantly from 2005 on a national scale, 80% and 17% of people still faced a high SO2 exposure risk (higher than 20 μg/m3) in the North China Plain and Northwest China, respectively.

The Chemistry Mechanism in the Community Earth System Model Version 2 (CESM2)

AbstractThe Community Earth System Model version 2 (CESM2) includes a detailed representation of chemistry throughout the atmosphere in the Community Atmosphere Model with chemistry and Whole Atmosphere Community Climate Model configurations. These model configurations use the Model for Ozone and Related chemical Tracers (MOZART) family of chemical mechanisms, covering the troposphere, stratosphere, mesosphere, and lower thermosphere. The new MOZART tropospheric chemistry scheme (T1) has a number of updates over the previous version (MOZART‐4) in CESM, including improvements to the oxidation of isoprene and terpenes, organic nitrate speciation, and aromatic speciation and oxidation and thus improved representation of ozone and secondary organic aerosol precursors. An evaluation of the present‐day simulations of CESM2 being provided for Climate Model Intercomparison Project round 6 (CMIP6) is presented. These simulations, using the anthropogenic and biomass burning emissions from the inventories specified for CMIP6, as well as online calculation of emissions of biogenic compounds, lightning NO, dust, and sea salt, indicate an underestimate of anthropogenic emissions of a variety of compounds, including carbon monoxide and hydrocarbons. The simulation of surface ozone in the southeast United States is improved over previous model versions, largely due to the improved representation of reactive nitrogen and organic nitrate compounds resulting in a lower ozone production rate than in CESM1 but still overestimates observations in summer. The simulation of tropospheric ozone agrees well with ozonesonde observations in many parts of the globe. The comparison of NOx and PAN to aircraft observations indicates the model simulates the nitrogen budget well.

Dust Storm Event of February 2019 in Central and East Coast of Australia and Evidence of Long-Range Transport to New Zealand and Antarctica

Between 11 and 15 February 2019, a dust storm originating in Central Australia with persistent westerly and south westerly winds caused high particle concentrations at many sites in the state of New South Wales (NSW); both inland and along the coast. The dust continued to be transported to New Zealand and to Antarctica in the south east. This study uses observed data and the WRF-Chem Weather Research Forecast model based on GOCART-AFWA (Goddard Chemistry Aerosol Radiation and Transport–Air Force and Weather Agency) dust scheme and GOCART aerosol and gas-phase MOZART (Model for Ozone And Related chemical Tracers) chemistry model to study the long-range transport of aerosols for the period 11 to 15 February 2019 across eastern Australia and onto New Zealand and Antarctica. Wildfires also happened in northern NSW at the same time, and their emissions are taken into account in the WRF-Chem model by using the Fire Inventory from NCAR (FINN) as the emission input. Modelling results using the WRF-Chem model show that for the Canterbury region of the South Island of New Zealand, peak concentration of PM10 (and PM2.5) as measured on 14 February 2019 at 05:00 UTC at the monitoring stations of Geraldine, Ashburton, Timaru and Woolston (Christchurch), and about 2 h later at Rangiora and Kaiapoi, correspond to the prediction of high PM10 due to the intrusion of dust to ground level from the transported dust layer above. The Aerosol Optical Depth (AOD) observation data from MODIS 3 km Terra/Aqua and CALIOP LiDAR measurements on board CALIPSO (Cloud-Aerosol LiDAR and Infrared Pathfinder Satellite Observations) satellite also indicate that high-altitude dust ranging from 2 km to 6 km, originating from this dust storm event in Australia, was located above Antarctica. This study suggests that the present dust storms in Australia can transport dust from sources in Central Australia to the Tasman sea, New Zealand and Antarctica.

Global and regional evaluation of a global model simulated AODs with AERONET and MODIS observations

ABSTRACTAerosol optical properties vary widely globally due to their short residence time and the large variability in their composition and size resulting from varying sources. In this study, we make use of the three‐dimensional global atmospheric chemistry transport model MOZART (Model for OZone And Related chemical Tracers), to simulate Aerosol Optical Depths (AODs) and compare with observations from MODIS (Moderate Resolution Imaging Spectroradiometer, level‐3) on board the Terra satellite and AERONET (AErosol RObotic NETwork level‐2), at regional and global scale for the period 2000–2007. The AOD values at a reference wavelength 550 nm are reported for all the data sets. Model and observations show intense aerosol burden over the parts of the world where developing countries are located. On a regional scale, varying features are noted for different regions depending upon topography, regional emissions and weather conditions. MOZART captures the general features observed in AERONET and MODIS fairly well; however, in biomass burning dominated regions (Central South America, South Africa, and North Australia) it underestimates the observed AOD. In industrial regions (North Europe and Mediterranean) MOZART overestimates the observations and over dust dominant regions (Saharan Africa, Middle East, and East Asia), the model AODs are comparable to AERONET and MODIS observations. In general, MOZART simulated AOD show similar seasonal and regional variability as observed in AERONET and MODIS. Our results are similar to the most commonly used models, i.e. GOCART (the GOddard Chemistry, Aerosol, Radiation, and Transport) and GEOS‐4 (Goddard Earth Observing System‐version 4).

WRF-Chem simulated surface ozone over south Asia during the pre-monsoon: effects of emission inventories and chemical mechanisms

Abstract. We evaluate numerical simulations of surface ozone mixing ratios over the south Asian region during the pre-monsoon season, employing three different emission inventories in the Weather Research and Forecasting model with Chemistry (WRF-Chem) with the second-generation Regional Acid Deposition Model (RADM2) chemical mechanism: the Emissions Database for Global Atmospheric Research – Hemispheric Transport of Air Pollution (EDGAR-HTAP), the Intercontinental Chemical Transport Experiment phase B (INTEX-B) and the Southeast Asia Composition, Cloud, Climate Coupling Regional Study (SEAC4RS). Evaluation of diurnal variability in modelled ozone compared to observational data from 15 monitoring stations across south Asia shows the model ability to reproduce the clean, rural and polluted urban conditions over this region. In contrast to the diurnal average, the modelled ozone mixing ratios during noontime, i.e. hours of intense photochemistry (11:30–16:30 IST – Indian Standard Time – UTC +5:30), are found to differ among the three inventories. This suggests that evaluations of the modelled ozone limited to 24 h average are insufficient to assess uncertainties associated with ozone buildup. HTAP generally shows 10–30 ppbv higher noontime ozone mixing ratios than SEAC4RS and INTEX-B, especially over the north-west Indo-Gangetic Plain (IGP), central India and southern India. The HTAP simulation repeated with the alternative Model for Ozone and Related Chemical Tracers (MOZART) chemical mechanism showed even more strongly enhanced surface ozone mixing ratios due to vertical mixing of enhanced ozone that has been produced aloft. Our study indicates the need to also evaluate the O3 precursors across a network of stations and the development of high-resolution regional inventories for the anthropogenic emissions over south Asia accounting for year-to-year changes to further reduce uncertainties in modelled ozone over this region.

Effect of different emission inventories on modeled ozone and carbon monoxide in Southeast Asia

Abstract. In order to improve our understanding of air quality in Southeast Asia, the anthropogenic emissions inventory must be well represented. In this work, we apply different anthropogenic emission inventories in the Weather Research and Forecasting Model with Chemistry (WRF-Chem) version 3.3 using Model for Ozone and Related Chemical Tracers (MOZART) gas-phase chemistry and Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) aerosols to examine the differences in predicted carbon monoxide (CO) and ozone (O3) surface mixing ratios for Southeast Asia in March and December 2008. The anthropogenic emission inventories include the Reanalysis of the TROpospheric chemical composition (RETRO), the Intercontinental Chemical Transport Experiment-Phase B (INTEX-B), the MACCity emissions (adapted from the Monitoring Atmospheric Composition and Climate and megacity Zoom for the Environment projects), the Southeast Asia Composition, Cloud, Climate Coupling Regional Study (SEAC4RS) emissions, and a combination of MACCity and SEAC4RS emissions. Biomass-burning emissions are from the Fire Inventory from the National Center for Atmospheric Research (NCAR) (FINNv1) model. WRF-Chem reasonably predicts the 2 m temperature, 10 m wind, and precipitation. In general, surface CO is underpredicted by WRF-Chem while surface O3 is overpredicted. The NO2 tropospheric column predicted by WRF-Chem has the same magnitude as observations, but tends to underpredict the NO2 column over the equatorial ocean and near Indonesia. Simulations using different anthropogenic emissions produce only a slight variability of O3 and CO mixing ratios, while biomass-burning emissions add more variability. The different anthropogenic emissions differ by up to 30% in CO emissions, but O3 and CO mixing ratios averaged over the land areas of the model domain differ by ~4.5% and ~8%, respectively, among the simulations. Biomass-burning emissions create a substantial increase for both O3 and CO by ~29% and ~16%, respectively, when comparing the March biomass-burning period to the December period with low biomass-burning emissions. The simulations show that none of the anthropogenic emission inventories are better than the others for predicting O3 surface mixing ratios. However, the simulations with different anthropogenic emission inventories do differ in their predictions of CO surface mixing ratios producing variations of ~30% for March and 10–20% for December at Thai surface monitoring sites.

Coupling dry deposition to vegetation phenology in the Community Earth System Model: Implications for the simulation of surface O3

Dry deposition is an important removal process controlling surface ozone. We examine the representation of this ozone loss mechanism in the Community Earth System Model. We first correct the dry deposition parameterization by coupling the leaf and stomatal vegetation resistances to the leaf area index, an omission which has adversely impacted over a decade of ozone simulations using both the Model for Ozone and Related chemical Tracers (MOZART) and Community Atmospheric Model‐Chem (CAM‐Chem) global models. We show that this correction increases O3 dry deposition velocities over vegetated regions and improves the simulated seasonality in this loss process. This enhanced removal reduces the previously reported bias in summertime surface O3 simulated over eastern U.S. and Europe. We further optimize the parameterization by scaling down the stomatal resistance used in the Community Land Model to observed values. This in turn further improves the simulation of dry deposition velocity of O3, particularly over broadleaf forested regions. The summertime surface O3 bias is reduced from 30 ppb to 14 ppb over eastern U.S. and 13 ppb to 5 ppb over Europe from the standard to the optimized scheme, respectively. O3 deposition processes must therefore be accurately coupled to vegetation phenology within 3‐D atmospheric models, as a first step toward improving surface O3 and simulating O3 responses to future and past vegetation changes.

Spatio-temporal variation of biomass burning sources over South and Southeast Asia

In this study, we have investigated the seasonality and long-term trends of major biomass burning (BB) sources over South and Southeast Asia (S-SE Asia). The activities of BB and related emissions show bi-modal seasonality in S-SE Asia. From January to May period, the BB dominates in the northern hemisphere parts of S-SE Asia. From July to September, the activities shift to the southern hemisphere where the emissions from Indonesian and Malaysian islands make largest contributions. Overall, the activities of BB are lowest during October–December period in S-SE Asia. The seasonality of BB intensity and rain are just opposite in the phase over India. The climatological (1997–2008) emissions of carbon monoxide (CO), oxides of nitrogen (NOx) and non-methane hydrocarbons (NMHCs) show strong spatio-temporal variation. The trends show large inter-annual variations with highest and lowest values during years 1997 and 2000, respectively. In the southern hemisphere parts of S-SE Asia mainly in Indonesia, the intensity of biomass fires has been modulated by the large scale climatic phenomena like El Nino and Southern Oscillation (ENSO). The annual emissions of trace gases in southern hemisphere region during the El Nino years exceed to those for the normal years. The estimates for northern hemisphere region during the La Nina years were significantly higher than those for the normal years. The Model for Ozone And Related Chemical Tracers (MOZART) simulations of columnar CO and NOx tend to capture the prominent features of BB emissions in S-SE Asia. The impacts of extensive fires in Indonesia during El Nino year of 2006 compared to a normal year of 2005 were clearly seen in the MOZART-4 simulations of both CO and NOx.

Characteristics of tropospheric ozone variability over an urban site in Southeast Asia: A study based on MOZAIC and MOZART vertical profiles

AbstractThe Measurement of Ozone and Water Vapor by Airbus In‐Service Aircraft (MOZAIC) profiles of O3 and CO were analyzed to study their variation in the troposphere over Bangkok. Mixing ratios of O3 and CO were enhanced in planetary boundary layer (PBL) being highest in winter followed by summer and wet seasons. The daytime profiles of O3 show higher values compared to nighttime observations in PBL region, but little differences were observed in the free troposphere. The decreasing mixing ratios of O3 in the lower and upper troposphere were associated with shallow and deep convections, respectively. Back trajectory and fire count data indicate that the seasonal variations in trace gases were caused mainly by the regional shift in long‐range transport and biomass‐burning patterns. In wet season, flow of oceanic air and negligible presence of local biomass burning resulted in lowest O3 and CO, while their high levels in dry season were due to extensive biomass burning and transport of continental air masses. The Model for Ozone and Related Chemical Tracers (MOZART) underestimated both O3 and CO in the PBL region but overestimated these in the free troposphere. Simulations of O3 and CO also show the daytime/nighttime differences but do not capture several key features observed in the vertical distributions. The observed and simulated values of O3 and CO during September–November 2006 were significantly higher than the same period of 2005. The year‐to‐year differences were mainly due to El Niño‐led extensive fires in Indonesia during 2006 but normal condition during 2005.

Impact of a future H<sub>2</sub>-based road transportation sector on the composition and chemistry of the atmosphere – Part 2: Stratospheric ozone

Abstract. The prospective future adoption of molecular hydrogen (H2) to power the road transportation sector could greatly improve tropospheric air quality but also raises the question of whether the adoption would have adverse effects on the stratospheric ozone. The possibility of undesirable impacts must be fully evaluated to guide future policy decisions. Here we evaluate the possible impact of a future (2050) H2-based road transportation sector on stratospheric composition and chemistry, especially on the stratospheric ozone, with the MOZART (Model for OZone And Related chemical Tracers) model. Since future growth is highly uncertain, we evaluate the impact of two world evolution scenarios, one based on an IPCC (Intergovernmental Panel on Climate Change) high-emitting scenario (A1FI) and the other on an IPCC low-emitting scenario (B1), as well as two technological options: H2 fuel cells and H2 internal combustion engines. We assume a H2 leakage rate of 2.5% and a complete market penetration of H2 vehicles in 2050. The model simulations show that a H2-based road transportation sector would reduce stratospheric ozone concentrations as a result of perturbed catalytic ozone destruction cycles. The magnitude of the impact depends on which growth scenario evolves and which H2 technology option is applied. For the evolution growth scenario, stratospheric ozone decreases more in the H2 fuel cell scenarios than in the H2 internal combustion engine scenarios because of the NOx emissions in the latter case. If the same technological option is applied, the impact is larger in the A1FI emission scenario. The largest impact, a 0.54% decrease in annual average global mean stratospheric column ozone, is found with a H2 fuel cell type road transportation sector in the A1FI scenario; whereas the smallest impact, a 0.04% increase in stratospheric ozone, is found with applications of H2 internal combustion engine vehicles in the B1 scenario. The impacts of the other two scenarios fall between the above two boundary scenarios. However, the magnitude of these changes is much smaller than the increases in 2050 stratospheric ozone projected, as stratospheric ozone is expected to recover due to the limits in ozone depleting substance emissions imposed in the Montreal Protocol.

On the potential high acid deposition in northeastern China

AbstractThere is an acid deposition conundrum in China: contrary to conventional wisdom, extremely high ambient sulfate concentrations in northeastern China are not always accompanied by correspondingly high acidities. To investigate this discrepancy, data from two independent sets of in situ field measurements were analyzed along with Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) satellite observations and Model for Ozone and Related chemical Tracers (MOZART) chemical transport model calculations. The field measurements included soluble aerosol ion concentrations and pH and particulate data from 11 cities, as well as pH measurement data from 74 sites in China. This study explores the basis for and the impacts of the large discrepancy in northeastern China between the major acidity precursors (SO2 and NOx) and measured acidity levels as indicated by pH values. There are extremely high SO2 emissions and ambient concentrations in northeastern China, while the corresponding acidity is unusually low (high pH) in this region. This is inconsistent with the usual situation where high‐acidity precursor pollutants result in low pH (high acidity) values and acid rain conditions. In other regions, such as southern China and the United States, high SO2 concentrations are typically well correlated with high acidities. Using measured soluble particle measurements (including both positively and negatively charged ions), it is seen that there are high values of alkaline ions in northeastern China that play an important role in neutralizing acidity in this region. This result strongly suggests that the high alkaline concentrations, especially Ca2+, increase warm season pH values by about 0.5 in northern China, partially explaining the inconsistency between sulfate concentrations and acidity. This has a very important implication for acid rain mitigation—especially in northeastern China. However, there are additional issues pertaining to the precursor‐acidity relationship that need further investigation. Why is it that the reduction in acidity due to the alkaline ions is only significant in summer? During winter, the measured alkaline ions play a much smaller role in explaining the discrepancy. The measured alkaline ions in this study were mostly obtained from particles in the PM2.5 range. However, the size of calcium particles is typically much larger—extending well beyond 2.5 µm—and so a significant amount of calcium may be underestimated by PM2.5 measurements alone. The under‐sampling of calcium particles is further exacerbated in that the sampling protocol excluded particle (and soluble ion and pH) measurements during dust storms. This all leads to the need for an improved understanding of pollutant‐ion‐particulate interactions in China, and their role in explaining the counter‐intuitive conclusion that dust mitigation strategies in China could have the unintended consequence of exacerbating acid rain conditions.

Variability in tropospheric carbon monoxide over an urban site in Southeast Asia

This paper analyses MOZAIC (Measurements of Ozone aboard Airbus in-service airCraft) measurements of carbon monoxide (CO) profiles over Bangkok to discuss the seasonality in vertical distribution during year 2005–2006. The mixing ratios of CO were enhanced in the lower troposphere being highest in winter followed by summer and wet seasons. During all the seasons, the mixing ratio of CO decreased rapidly and remained low in the middle troposphere. At higher altitudes (6–12 km), CO shows enhanced values particularly during wet and early winter seasons. The strong seasonality in CO was caused by the seasonal shift in the patterns of the long-range transport and biomass burning (BB) in South and Southeast Asia (S–SE Asia). Flow of cleaner air and negligible BB resulted in the lowest mixing ratio of CO in the wet season. In addition to anthropogenic influence, the long-range transport and BB caused the higher CO in the winter and summer seasons, respectively. Despite extensive local BB activities in Thailand during the summer season, the moderate levels of CO were attributed to the dilution due to flow of cleaner marine from the Indian and Pacific Oceans. We have also compared the observations with the Model for Ozone And Related Chemical Tracers (MOZART) simulations. Mostly the observations lie between the MOZART-2 and MOZART-4 simulations as they underestimate and overestimate the observed CO, respectively. In the middle and upper troposphere, both the observed and simulated mixing ratios of CO during September–November of year 2006 were higher by 15–30 ppbv compared to the same period of year 2005. Our analysis indicates the impact of El Niño induced extensive BB in Indonesia during the year 2006.

Study of tropospheric CO and O3 enhancement episode over Indonesia during Autumn 2006 using the Model for Ozone and Related chemical Tracers (MOZART-4)

An intense biomass burning event occurred over Indonesia in Autumn of 2006. We study the impact of this event on the free tropospheric abundances of carbon monoxide (CO) and ozone (O3) using MOPITT (Measurements of Pollution In The Troposphere) observations, ozonesonde measurements and 3D chemistry transport model MOZART (Model for Ozone and Related chemical Tracers). MOPITT observations showed an episode of enhanced CO in the free troposphere over the Indonesian region during October–November 2006. This feature is reproduced well by MOZART. The model mass diagnostics identifies the source of enhanced CO mixing ratio in the free troposphere (100–250 ppbv) as due to convective processes. The implication of the fire plume on the vertical distribution of O3 over Kuala Lumpur has been studied. The tropospheric O3 increased over this location by 10–25 ppbv during Autumn 2006 as compared to Autumn 2005 and 2007. The MOZART model simulation significantly underestimated this tropospheric O3 enhancement. The model is run both with and without Indonesian biomass burning emissions to estimate the contribution of fire emission in CO and O3 enhancement. Biomass burning emission is found to be responsible for an average increase in CO by 104 ± 56 ppbv and O3 by 5 ± 1 ppbv from surface to 100 hPa range. The model results also showed that biomass burning and El Niño related dynamical changes both contributed (∼4 ppbv–12 ppbv) to the observed increase in tropospheric O3 over the Indonesian region during Autumn 2006.

Enhancement of Ozone and Carbon Monoxide Associated with Upper Cut-off Low during Springtime in East Asia

In order to verify the enhancement of ozone and carbon monoxide (CO) during springtime in East Asia, we investigated weather conditions and data from remote sensors, air quality models, and air quality monitors. These include the geopotential height archived from the final (FNL) meteorological field, the potential vorticity and the wind velocity simulated by the Meteorological Mesoscale Model 5 (MM5), the back trajectory estimated by the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model, the total column amount of ozone and the aerosol index retrieved from the Total Ozone Mapping Spectrometer (TOMS), the total column density of CO retrieved from the Measurement of Pollution in the Troposphere (MOPITT), and the concentration of ozone and CO simulated by the Model for Ozone and Related Chemical Tracers (MOZART). In particular, the total column density of CO, which mightoriginate from the combustion of fossil fuels and the burning of biomass in China, increased in East Asia during spring 2000. In addition, the enhancement of total column amounts of ozone and CO appeared to be associated with both the upper cut-off low near 500 hPa and the frontogenesis of a surface cyclone during a weak Asian dust event. At the same time, high concentrations of ozone and CO on the Earth’s surface were shown at the Seoul air quality monitoring site, located at the surface frontogenesis in Korea. It was clear that the ozone was invaded by the downward stretched vortex anomalies, which included the ozone-rich airflow, during movement and development of the cut-off low, and then there was the catalytic photochemical reaction of ozone precursors on the Earth’s surface during the day. In addition, air pollutants such as CO and aerosol were tracked along both the cyclone vortex and the strong westerly as shown at the back trajectory in Seoul and Busan, respectively. Consequently, the maxima of ozone and CO between the two areas showed up differently because of the time lag between those gases, including their catalytic photochemical reactions together with the invasion from the upper troposphere, as well as the path of their transport from China during the weak Asian dust event.

Chemical evolution of volatile organic compounds in the outflow of the Mexico City Metropolitan area

Abstract. The volatile organic compound (VOC) distribution in the Mexico City Metropolitan Area (MCMA) and its evolution as it is uplifted and transported out of the MCMA basin was studied during the 2006 MILAGRO/MIRAGE-Mex field campaign. The results show that in the morning hours in the city center, the VOC distribution is dominated by non-methane hydrocarbons (NMHCs) but with a substantial contribution from oxygenated volatile organic compounds (OVOCs), predominantly from primary emissions. Alkanes account for a large part of the NMHC distribution in terms of mixing ratios. In terms of reactivity, NMHCs also dominate overall, especially in the morning hours. However, in the afternoon, as the boundary layer lifts and air is mixed and aged within the basin, the distribution changes as secondary products are formed. The WRF-Chem (Weather Research and Forecasting with Chemistry) model and MOZART (Model for Ozone and Related chemical Tracers) were able to approximate the observed MCMA daytime patterns and absolute values of the VOC OH reactivity. The MOZART model is also in agreement with observations showing that NMHCs dominate the reactivity distribution except in the afternoon hours. The WRF-Chem and MOZART models showed higher reactivity than the experimental data during the nighttime cycle, perhaps indicating problems with the modeled nighttime boundary layer height. A northeast transport event was studied in which air originating in the MCMA was intercepted aloft with the Department of Energy (DOE) G1 on 18 March and downwind with the National Center for Atmospheric Research (NCAR) C130 one day later on 19 March. A number of identical species measured aboard each aircraft gave insight into the chemical evolution of the plume as it aged and was transported as far as 1000 km downwind; ozone was shown to be photochemically produced in the plume. The WRF-Chem and MOZART models were used to examine the spatial extent and temporal evolution of the plume and to help interpret the observed OH reactivity. The model results generally showed good agreement with experimental results for the total VOC OH reactivity downwind and gave insight into the distributions of VOC chemical classes. A box model with detailed gas phase chemistry (NCAR Master Mechanism), initialized with concentrations observed at one of the ground sites in the MCMA, was used to examine the expected evolution of specific VOCs over a 1–2 day period. The models clearly supported the experimental evidence for NMHC oxidation leading to the formation of OVOCs downwind, which then become the primary fuel for ozone production far away from the MCMA.

Impacts of long‐range transport of global pollutants and precursor gases on U.S. air quality under future climatic conditions

The U.S. air quality is impacted by emissions both within and outside the United States. The latter impact is manifested as long‐range transport (LRT) of pollutants across the U.S. borders, which can be simulated by lateral boundary conditions (LBC) into a regional modeling system. This system consists of a regional air quality model (RAQM) that integrates local‐regional source emissions and chemical processes with remote forcing from the LBC predicted by a nesting global chemical transport model (model for ozone and related chemical tracers (MOZART)). The present‐day simulations revealed important LRT effects, varying among the five major regions with ozone problems, i.e., northeast United States, midwest United States, Texas, California, and southeast United States. To determine the responses of the LRT impacts to projected global climate and emissions changes, the MOZART and RAQM simulations were repeated for future periods (2048–2052 and 2095–2099) under two emissions scenarios (IPCC A1Fi and B1). The future U.S. air quality projected by the MOZART is less sensitive to the emissions scenarios than that simulated by the RAQM with or without incorporating the LRT effects via the LBC from the MOZART. The result of RAQM with the LRT effects showed that the southeast United States has the largest sensitivity of surface ozone mixing ratio to the emissions changes in the 2095–2099 climate (−24% to +25%) followed by the northeast and midwest United States. The net increase due to the LRT effects in 2095–2099 ranges from +4% to +13% in daily mean surface ozone mixing ratio and +4% to +11% in mean daily maximum 8‐h average ozone mixing ratios. Correspondingly, the LRT effects in 2095–2099 cause total column O3 mixing ratio increases, ranging from +7% to +16%, and also 2 to 3 more days with the surface ozone exceeding the national standard. The results indicate that future U.S. air quality changes will be substantially affected by global emissions.