ALBERT

Description: Multi-scale tracer and full-chemistry simulations with the STEM atmospheric chemistry model are used to analyze the effects of transported background ozone (O3) from the eastern Pacific on California air quality during the ARCTAS-CARB experiment conducted in June, 2008. Previous work has focused on the importance of long-range transport of O3 to North America air quality in springtime. However during this summer experiment the long-range transport of O3 is also shown to be important. Simulated and observed O3 transport patterns from the coast to inland northern California are shown to vary based on meteorological conditions and the O3 profiles over the oceans, which are strongly episodically affected by Asian inflows. Analysis of the correlations of O3 at various altitudes above the coastal site at Trinidad Head and at a downwind surface site in northern California, show that under long-range transport events, high O3 air-masses (O3〉60 ppb) at altitudes between about 2 and 4 km can be transported inland and can significantly influence surface O3 20–30 h later. These results show the importance of characterizing the vertical structure of the lateral boundary conditions (LBC) needed in air quality simulations. The importance of the LBC on O3 prediction during this period is further studied through a series of sensitivity studies using different forms of LBC. It is shown that the use of the LBC downscaled from RAQMS global model that assimilated MLS and OMI data improves the model performance. We also show that the predictions can be further improved through the use of LBC based on NASA DC-8 airborne observations during the ARCTAS-CARB experiment. These results indicate the need to develop observational strategies to provide information on the three-dimensional nature of pollutant distributions, in order to improve our capability to predict pollution levels and to better quantify the influence of these Asian inflows on the US west coast air quality.

Description: Chronic high surface ozone (O3) levels and the increasing sulfur oxides (SOx = SO2+SO4) ambient concentrations over South Coast (SC) and other areas of California (CA) are affected by both local emissions and long-range transport. In this paper, multi-scale tracer, full-chemistry and adjoint simulations using the STEM atmospheric chemistry model are conducted to assess the contribution of local emission sourcesto SC O3 and to evaluate the impacts of transported sulfur and local emissions on the SC sulfur budgetduring the ARCTAS-CARB experiment period in 2008. Sensitivity simulations quantify contributions of biogenic and fire emissions to SC O3 levels. California biogenic and fire emissions contribute 3–4 ppb to near-surface O3 over SC, with larger contributions to other regions in CA. During a long-range transport event from Asia starting from 22 June, high SOx levels (up to ~0.7 ppb of SO2 and ~1.3 ppb of SO4) is observed above ~6 km, but they did not affect CA surface air quality. The elevated SOx observed at 1–4 km is estimated to enhance surface SOx over SC by ~0.25 ppb (upper limit) on ~24 June. The near-surface SOx levels over SC during the flight week are attributed mostly to local emissions. Two anthropogenic SOx emission inventories (EIs) from the California Air Resources Board (CARB) and the US Environmental Protection Agency (EPA) are compared and applied in 60 km and 12 km chemical transport simulations, and the results are compared withobservations. The CARB EI shows improvements over the National Emission Inventory (NEI) by EPA, but generally underestimates surface SC SOx by about a factor of two. Adjoint sensitivity analysis indicated that SO2 levels at 00:00 UTC (17:00 local time) at six SC surface sites were influenced by previous day maritime emissions over the ocean, the terrestrial emissions over nearby urban areas, and by transported SO2 from the north through both terrestrial and maritime areas. Overall maritime emissions contribute 10–70% of SO2 and 20–60% fine SO4 on-shore and over the most terrestrial areas, with contributions decreasing with in-land distance from the coast. Maritime emissions also modify the photochemical environment, shifting O3 production over coastal SC to more VOC-limited conditions. These suggest an important role for shipping emission controls in reducing fine particle and O3 concentrations in SC.

Description: We report on total gaseous mercury (TGM) measurements made in Pudong, Shanghai in August/September 2009. The average TGM was 2.7 ± 1.7 ng m−3. This represents about 90% of the total atmospheric mercury. This is an underestimate for an annual-mean concentration because the meteorology in September favored predominantly easterly oceanic air, replaced in other seasons by airflow from industrial areas. The observed TGM follows a pattern seen in other cities around the world: a background elevated over mean hemispheric background (1.5 ng m−3), and pollution plumes of different magnitude and duration, interspersed with very sharp spikes of high concentration (60 ng m−3). The September 2009 Shanghai measurements are lower than those reported for most other Chinese cities and Mexico City, and similar to concentrations found in some Asian and in North American cities. Such comparisons are tenuous because of differences in season and year of the respective measurements. Our results should not be used for regulatory purposes. We find that the observed TGM are most likely coming from coal fired power plants, smelters and industrial sources, based on its high correlation with NOx, SO2, CO and wind directions.

Description: The MIRAGE-Shanghai experiment was designed to characterize the factors controlling regional air pollution near a Chinese megacity (Shanghai) and was conducted during September 2009. This paper provides information on the measurements conducted for this study. In order to have some deep analysis of the measurements, a regional chemical/dynamical model (version 3 of Weather Research and Forecasting Chemical model – WRF-Chemv3) is applied for this study. The model results are intensively compared with the measurements to evaluate the model capability for calculating air pollutants in the Shanghai region, especially the chemical species related to ozone formation. The results show that the model is able to calculate the general distributions (the level and the variability) of air pollutants in the Shanghai region, and the differences between the model calculation and the measurement are mostly smaller than 30%, except the calculations of HONO (nitrous acid) at PD (Pudong) and CO (carbon monoxide) at DT (Dongtan). The main scientific focus is the study of ozone chemical formation not only in the urban area, but also on a regional scale of the surrounding area of Shanghai. The results show that during the experiment period, the ozone photochemical formation was strongly under the VOC (volatile organic compound)-limited condition in the urban area of Shanghai. Moreover, the VOC-limited condition occurred not only in the city, but also in the larger regional area. There was a continuous enhancement of ozone concentrations in the downwind of the megacity of Shanghai, resulting in a significant enhancement of ozone concentrations in a very large regional area in the surrounding region of Shanghai. The sensitivity study of the model suggests that there is a threshold value for switching from VOC-limited condition to NOx (nitric oxide and nitrogen dioxide)-limited condition. The threshold value is strongly dependent on the emission ratio of NOx / VOCs. When the ratio is about 0.4, the Shanghai region is under a strong VOC-limited condition over the regional scale. In contrast, when the ratio is reduced to about 0.1, the Shanghai region is under a strong NOx-limited condition. The estimated threshold value (on the regional scale) for switching from VOC-limited to NOx-limited condition ranges from 0.1 to 0.2. This result has important implications for ozone production in this region and will facilitate the development of effective O3 control strategies in the Shanghai region.

Description: The impacts of transported background (TBG) pollutants on Western US ozone (O3) distributions in summer 2008 are studied using the multi-scale Sulfur Transport and dEposition Modeling system. Forward sensitivity simulations show that TBG extensively affect Western US surface O3, and can contribute to 〉50% of the total O3, varying among different geographical regions and land types. The stratospheric O3 impacts are weak. Ozone is the major contributor to surfaceO3 among the TBG pollutants, and TBG peroxyacetyl nitrate is the most important O3 precursor species. Compared to monthly mean daily maximum 8-h average O3, the secondary standard metric "W126 monthly index" shows larger responses to TBG perturbations and stronger non-linearity to the size of perturbations. Overall the model-estimated TBG impacts negatively correlate to the vertical resolution and positively correlate to the horizontal resolution. The estimated TBG impacts weakly depend on the uncertainties in US anthropogenic emissions. Ozone sources differ at three sites spanning ~10° in latitude. Mt. Bachelor (MBO) and Trinidad Head (THD) O3 are strongly affected by TBG, and occasionally by US emissions, while South Coast (SC) O3 is strongly affected by local emissions. The probabilities of airmasses originating from MBO (2.7 km) and THD (2.5 km) entraining into the boundary layer reach daily maxima of 66% and 34% at ~3:00 p.m. PDT, respectively, and stay above 50% during 9:00 a.m.–4:00 p.m. for those originating from SC (1.5 km). Receptor-based adjoint sensitivity analysis demonstrates the connection between the surface O3 and O3 aloft (at ~1–4 km) at these sites 1–2 days earlier. Assimilation of the surface in-situ measurements significantly reduced (~5 ppb in average, up to ~17 ppb) the modeled surface O3 errors during a long-range transport episode, and is useful for estimating the upper-limits of uncertainties in satellite retrievals (in this case 5–20% and 20–30% for Tropospheric Emission Spectrometer (TES) and Ozone Monitoring Instrument (OMI) O3 profiles, respectively). Satellite observations identified this transport event, but assimilation of the existing O3 vertical profiles from TES, OMI and THD sonde in this case did not efficiently improve the O3 distributions except near the sampling locations, due to their limited spatiotemporal resolution and possible uncertainties.

Description: The MIRAGE-Shanghai experiment was designed to characterize the factors controlling regional air pollution near a Chinese Megacity (Shanghai) and was conducted during September 2009. This paper provides an overview of the measurements conducted for this study. In addition to the measurements, a regional chemical/dynamical model (version 3 of Weather Research and Forecasting Chemical model – WRF-Chemv3) is applied for this study. The model results are intensively compared with the measurements to evaluate the model capability for calculating air pollutants in the Shanghai region, especially the chemical species related to ozone formation. The results show that the model is able to calculate the general distributions (the level and the variability) of air pollutants in the Shanghai region, and the difference between the model calculation and the measurement are mostly smaller than 30%, except the calculations of HONO at PD (Pudong) and CO at DT (Dongtan). The main scientific focus is the study of ozone chemical formation not only in the urban area, but also on a regional scale of the surrounding area of Shanghai. The results show that during the experiment period, the ozone photochemical formation was strongly under the VOC-limited condition in the urban area of Shanghai. Moreover, the VOC-limited condition occurred not only in the city, but also in the larger regional area. There was a continuous enhancement of ozone concentrations in the downwind of the megacity of Shanghai, resulting in a significant enhancement of ozone concentrations in a very large regional area in the surrounding region of Shanghai. The sensitivity study of the model suggests that there is a threshold value for switching from VOC-limited condition to NOx-limited condition. The threshold value is strongly dependent on the emission ratio of NOx/VOCs. When the ratio is about 0.4, the Shanghai region is under a strong VOC-limited condition over the regional scale. In contrast, when the ratio is reduced to about 0.1, the Shanghai region is under a strong NOx-limited condition. The estimated threshold value (on the regional scale) for switching from VOC-limited to NOx-limited condition ranges from 0.1 to 0.2. This result has important implications for ozone production in this region and will facilitate the development of effective O3 control strategies in the Shanghai region.

Description: We report on total gaseous mercury (TGM) measurements made in Pudong, Shanghai in August/September 2009. The average TGM was 2.7 ± 1.7 ng m−3. This represents about 90% of the total atmospheric mercury. This is an underestimate for an annual-mean concentration because the meteorology in September favored predominantly easterly oceanic air, replaced in other seasons by airflow from industrial areas. The observed TGM follows a pattern seen in other cities around the world: a background elevated over mean hemispheric background (1.5 ng m−3), and pollution plumes of different magnitude and duration, interspersed with very sharp spikes of high concentration (60 ng m−3). The September 2009 Shanghai measurements are lower than those reported for most other Chinese cities and Mexico City, and similar to concentrations found in some Asian and in North American cities. Such comparisons are tenuous because of differences in season and year of the respective measurements. We find that the observed TGM are most likely coming from coal fired power plants, smelters and industrial sources, based on its high correlation with NOx, SO2, CO and wind directions.

Description: Chronic ozone (O3) problems and the increasing sulfur oxides (SOx=SO2+SO4) ambient concentrations over South Coast (SC) and other areas of California (CA) are affected by both local emissions and long-range transport. In this paper, multi-scale tracer and full-chemistry simulations with the STEM atmospheric chemistry model are used to assess the contribution of local emission sources to SC O3 and evaluate the impacts of transported sulfur and local emissions on the SC sulfur budget during the ARCTAS-CARB experiment period in 2008. Sensitivity simulations quantify contributions of biogenic and fire emissions to SC O3 levels. California biogenic and fire emissions contribute 3–4 ppb to near-surface O3 over SC, with larger contributions to other regions in CA. Long-range transport from Asia is estimated to enhance surface SO4 over SC by ~0.5 μg/sm3, and the higher SOx levels (up to ~0.7 ppb of SO2 and ~6 μg/sm3 of SO4) observed above ~6 km did not affect surface air quality in the study region. Enhanced near-surface SOx levels over SC during the flight week were attributed mostly to local emissions. Two anthropogenic SOx emission inventories (EIs) from the California Air Resources Board (CARB) and the US Environmental Protection Agency (EPA) are compared and applied in 60 km and 12 km chemical transport simulations, and the results are compared with observations. The CARB EI shows improvements over the National Emission Inventory (NEI) by EPA, but generally underestimates surface SC SOx by about a factor of two. Maritime (mostly shipping) emissions contribute to the high SO2 levels over the ocean and on-shore, and fine SO4 over the downwind areas is impacted by maritime sources. Maritime emissions also modify the NOx-VOC limitations over coastal areas. These suggest an important role for shipping emission controls in reducing fine particle and O3 concentrations in SC.

Description: Traditionally, balloon-based radiosonde soundings are used to study the spatial distribution of atmospheric water vapour. However, this approach cannot be frequently employed due to its high cost. In contrast, GPS tomography technique can obtain water vapour in a high temporal resolution. In the tomography technique, an iterative or non-iterative reconstruction algorithm is usually utilised to overcome rank deficiency of observation equations for water vapour inversion. However, the single iterative or non-iterative reconstruction algorithm has their limitations. For instance, the iterative reconstruction algorithm requires accurate initial values of water vapour while the non-iterative reconstruction algorithm needs proper constraint conditions. To overcome these drawbacks, we present a combined iterative and non-iterative reconstruction approach for the three-dimensional (3-D) water vapour inversion using GPS observations and COSMIC profiles. In this approach, the non-iterative reconstruction algorithm is first used to estimate water vapour density based on a priori water vapour information derived from COSMIC radio occultation data. The estimates are then employed as initial values in the iterative reconstruction algorithm. The largest advantage of this approach is that precise initial values of water vapour density that are essential in the iterative reconstruction algorithm can be obtained. This combined reconstruction algorithm (CRA) is evaluated using 10-day GPS observations in Hong Kong and COSMIC profiles. The test results indicate that the water vapor accuracy from CRA is 16 and 14% higher than that of iterative and non-iterative reconstruction approaches, respectively. In addition, the tomography results obtained from the CRA are further validated using radiosonde data. Results indicate that water vapour densities derived from the CRA agree with radiosonde results very well at altitudes above 2.5 km. The average RMS value of their differences above 2.5 km is 0.44 g m−3.

Description: Multi-scale tracer and full-chemistry simulations with the STEM atmospheric chemistry model are used to analyze the effects of transported background ozone (O3) from the eastern Pacific on California air quality during the ARCTAS-CARB experiment conducted in June 2008. Previous work has focused on the importance of long-range transport of O3 to North America air quality in springtime. However during this summer experiment the long-range transport of O3 is also shown to be important. Simulated and observed O3 transport patterns from the coast to inland northern California are shown to vary based on meteorological conditions and the oceanic O3 profiles, which are strongly episodically affected by Asian inflows. Analysis of the correlations of O3 at various altitudes above the coastal site at Trinidad Head and at a downwind surface site in northern California, show that under long-range transport events, high O3 air-masses (O3〉60 ppb) at altitudes between about 2 and 4 km can be transported inland and can significantly influence surface O3 20–30 h later. These results show the importance of characterizing the vertical structure of the lateral boundary conditions (LBC) needed in air quality simulations. The importance of the LBC on O3 prediction during this period is further studied through a series of sensitivity studies using different forms of LBC. It is shown that the use of the LBC downscaled from RAQMS global model that assimilated MLS and OMI data improves the model performance. We also show that the predictions can be further improved through the use of LBC based on NASA DC-8 airborne observations during the ARCTAS-CARB experiment. These results indicate the need to develop observational strategies to improve the representation of the vertical and temporal variations in the air over the eastern Pacific.