Downwind Regions Research Articles

Chemical composition observed over the mid‐Atlantic and the detection of pollution signatures far from source regions

The atmospheric composition of the central North Atlantic region has been sampled using the FAAM BAe146 instrumented aircraft during the Intercontinental Transport of Ozone and Precursors (ITOP) campaign, part of the wider International Consortium for Atmospheric Research on Transport and Transformation (ICARTT). This paper presents an overview of the ITOP campaign. Between late July and early August 2004, twelve flights comprising 72 hours of measurement were made in a region from approximately 20 to 40°W and 33 to 47°N centered on Faial Island, Azores, ranging in altitude from 50 to 9000 m. The vertical profiles of O3 and CO are consistent with previous observations made in this region during 1997 and our knowledge of the seasonal cycles within the region. A cluster analysis technique is used to partition the data set into air mass types with distinct chemical signatures. Six clusters provide a suitable balance between cluster generality and specificity. The clusters are labeled as biomass burning, low level outflow, upper level outflow, moist lower troposphere, marine and upper troposphere. During this summer, boreal forest fire emissions from Alaska and northern Canada were found to provide a major perturbation of tropospheric composition in CO, PAN, organic compounds and aerosol. Anthropogenic influenced air from the continental boundary layer of the USA was clearly observed running above the marine boundary layer right across the mid‐Atlantic, retaining high pollution levels in VOCs and sulfate aerosol. Upper level outflow events were found to have far lower sulfate aerosol, resulting from washout on ascent, but much higher PAN associated with the colder temperatures. Lagrangian links with flights of other aircraft over the USA and Europe show that such signatures are maintained many days downwind of emission regions. Some other features of the data set are highlighted, including the strong perturbations to many VOCs and OVOCs in this remote region.

A Theoretical and Numerical Study of Urban Heat Island-Induced Circulation and Convection

Urban heat island–induced circulation and convection in three dimensions are investigated theoretically and numerically in the context of the response of a stably stratified uniform flow to specified low-level heating that represents an urban heat island. In a linear, theoretical part of the investigation, an analytic solution for the perturbation vertical velocity in a three-dimensional, time-dependent, hydrostatic, nonrotating, inviscid, Boussinesq airflow system is obtained. The solution reveals a typical internal gravity wave field, including low-level upward motion downwind of the heating center. Precipitation enhancement observed downwind of urban areas may be partly due to this downwind upward motion. The comparison of two- and three-dimensional flow fields indicates that the dispersion of gravity wave energy into an additional dimension results in a faster approach to a quasi-steady state and a weaker quasi-steady flow well above the concentrated heating region in three dimensions. In a nonlinear, numerical modeling part of the investigation, extensive dry and moist simulations using a nonhydrostatic, compressible model with advanced physical parameterizations [Advanced Regional Prediction System (ARPS)] are performed. While the maximum perturbation vertical velocity in the linear internal gravity wave field exists in the downwind region close to the heating center, the maximum updraft in three-dimensional dry simulations propagates downwind and then becomes quasi stationary. In three-dimensional moist simulations, it is demonstrated that the downwind upward motion induced by an urban heat island can initiate moist convection and result in downwind precipitation. The cloud induced by the downwind upward motion grows rapidly to become deep convective clouds. Heavy rainfalls are localized in a region not far from the heating center by a convective precipitating system that is nearly stationary. The differences in results between two and three dimensions are explained by the presence of (moist) convergence in an additional dimension. The numerical simulation results indicate that the intensity and horizontal structure of the urban heat island affect those of circulation and convection and hence the distribution of surface precipitation.

SO2 modeling in İzmit Gulf, Turkey during the winter of 1997: 3 cases

In this study, SO2 dispersion over Izmit Gulf is simulated by California Puff (CALPUFF) model for three air pollution cases, which occurred on January 28, February 12, and February 26, 1997. These days are generally characterized by dominant high-pressure systems – pressure values reaching 1032 mb, low wind speeds and sometimes calm conditions, and low temperatures with a minimum of 0°C. Hourly simulations during those critical cases were carried out and results revealed very high concentrations of SO2 transported to the downwind regions of Tupras and Gebze, and values sometimes exceeded 1,000 μg/m3. Nighttime and morning simulations associated with inversion produced considerably higher values of SO2 than the afternoon simulations associated with breeze. Model verification was conducted by comparing the measured daily average values of eight stations with the model predicted values at the same receptor points. Results showed that the model well predicted the values at station Gebze in all three cases. The model sometimes underestimated and sometimes overestimated the concentrations at other receptor stations.

Modeling reactive pollutant dispersion in an urban street canyon

Reactive pollutant dispersion in an urban street canyon with a street aspect ratio of one is numerically investigated using a computational fluid dynamics (CFD) model. The CFD model developed is a Reynolds-averaged Navier–Stokes equations (RANS) model with the renormalization group (RNG) k–ε turbulence model and includes transport equations for NO, NO 2, and O 3 with simple photochemistry. An area emission source of NO and NO 2 is considered in the presence of background O 3 and street bottom heating (Δ T=5 °C) with an ambient wind perpendicular to the along-canyon direction. A primary vortex is formed in the street canyon and the line connecting the centers of cross-sectional vortices meanders over time and in the canyon space. The cross-canyon-averaged temperature and reactive pollutant concentrations oscillate with a period of about 15 min. The averaged temperature is found to be in phase with NO and NO 2 concentrations but out of phase with O 3 concentration. The photostationary state defect is small in the street canyon except for near the roof level and the upper downwind region of the canyon and its local minimum is observed near the center of the primary vortex. The budget analysis of NO (NO 2) concentration shows that the magnitude of the advection or turbulent diffusion term is much larger (larger) than that of the chemical reaction term and that the advection term is largely balanced by the turbulent diffusion term. On the other hand, the budget analysis of O 3 concentration shows that the magnitude of the chemical reaction term is comparable to that of the advection or turbulent diffusion term. The inhomogeneous temperature distribution itself affects O 3 concentration to some extent due to the temperature-dependent photolysis rate and reaction rate constant.

Aerosol nucleation over oceans and the role of galactic cosmic rays

Abstract. We investigate formation of sulfate aerosol in the marine troposphere from neutral and charged nucleation of H2SO4 and H2O. A box model of neutral and charged aerosol processes is run on a grid covering the oceans. Input data are taken from a model of galactic cosmic rays in the atmosphere, and from global chemistry and transport models. We find a weak aerosol production over the tropical oceans in the lower and middle troposphere, and a stronger production at higher latitudes, most notably downwind of industrial regions. The strongest aerosol production however occurs in the upper troposphere over areas with frequent convective activity, in particular in the tropics. This finding supports the proposition by which non-sea salt marine boundary layer aerosol in tropical regions does not form in situ, but nucleates in the upper troposphere from convectively lifted and cloud processed boundary layer air rich in aerosol precursor gases, from where it descends in subsiding air masses compensating convection. Convection of boundary layer air also appears to drive the formation of condensation nuclei in the tropical upper troposphere which maintains the stratospheric aerosol layer in the absence of volcanic activity. Neutral nucleation contributes only marginally to aerosol production in our simulations. This highlights the importance of other mechanisms, including charged binary and ternary, and neutral ternary nucleation for aerosol formation. Our analysis indicates that the variation of ionization by galactic cosmic rays over the decadal solar cycle does not entail a response in aerosol production and cloud cover via the second indirect aerosol effect that would explain observed variations in global cloud cover. We estimate that the variation in radiative forcing resulting from a response of clouds to the change in galactic cosmic ray ionization and subsequent aerosol production over the decadal solar cycle is smaller than the concurrent variation of total solar irradiance.

Climatology of aerosol radiative properties in northern China

The climatology of aerosol radiative properties in northern China was examined from the ground-based solar and sky radiance measurements of six sites during 2001–2005. Compared with the scattering, the absorption ability of particles became relatively more remarkable in dusty days than in clear days over dust source and downwind regions. The aerosol optical thickness (AOT) of coarse particle ( r > 0.6 μm) absorption accounted for about 82.0% of that of total particles at 440, 670, 870 and 1020 nm over dust source regions, and 61.1% over downwind regions in winter and spring. According to extinction AOT, the corresponding percentages of coarse particles were 63.6% of total particle at the four wavelengths over dust source regions, and 37.7% over downwind regions in winter and spring. The single scattering albedo (SSA) showed a low sensitivity to AOT at the four wavelengths over both source and downwind regions, about 0.94 for fine particles ( r < 0.6 μm) and 0.85 for coarse particles in winter and spring, and 0.86 for fine particles and 0.73 for coarse particles in summer and autumn. The asymmetry factor also showed a low sensitivity to AOT at the four wavelengths in all seasons, about 0.58 for fine particles and 0.79 for coarse particles over source and downwind regions. The scattering phase functions of coarse particles at forward and backward angles were commonly larger than those of fine particles, with the minimum at 120° and 130° over source and downwind regions respectively.

Mesoscale modeling of Central American smoke transport to the United States: 2. Smoke radiative impact on regional surface energy budget and boundary layer evolution

During 20 April to 21 May 2003, large amounts of smoke aerosols from Central American Biomass Burning (CABB) fires were transported to southeastern United States. Using a coupled aerosol, radiation, and meteorology model built upon the heritage of the Regional Atmospheric Modeling System (RAMS) with new capabilities called the Assimilation and Radiation Online Modeling of Aerosols (AROMA), this paper, the second of a two‐part series, investigates smoke radiative impact on the regional surface energy budget, temperature and relevant boundary layer processes. Comparisons with limited ground‐based observations and MODIS aerosol optical thickness (AOT) showed that model consistently simulated the smoke AOT and smoke radiative impacts on the 2 m air temperature (2mT) and downward shortwave irradiance (DSWI). Over 30 days the 24‐hour mean smoke AOT was 0.18 (at 0.55 μm) near the smoke source region (Yucatan Peninsula and southern Mexico), and 0.09 in downwind region (e.g., southern Texas), both showing a diurnal variation of 24%. Maximum AOT occurred during late afternoon and minimum during early morning in smoke source region. The smoke radiative effects were dominant mostly during the daytime and resulted in the decrease of DSWI, sensible heat and latent heat by 22.5 Wm−2, 6.2 Wm−2, and 6.2 Wm−2, respectively, near the source region, in contrast to 15.8 Wm−2, 4.7 Wm−2, and 7.9 Wm−2, respectively, in downwind regions. Both maximum and minimum 2mT were decreased, and the overall diurnal temperature range (DTR) was reduced by 0.31°C and 0.26°C in the smoke source and downwind regions, respectively. The smoke absorption of solar radiation increased the lapse rate by 0.1–0.5 K/day in the planetary boundary layer (PBL), thus warming the air over the ocean surface. However, over the land surface where the coupling between the lower PBL and the cooler land surface is strong, such warming only occurred in the upper PBL and is amendable to the diurnal variation of smoke emission. The simulation numerically verifies the smoke self‐trapping feedback mechanism proposed by Robock (1988), where the increase of the atmospheric stability in the PBL caused by the smoke radiative effects further traps more smoke aerosols in the lower PBL. Such feedbacks, when coupled with favorable synoptic systems, may have important implications for air quality modeling and hydrological processes.

A comparison of dust properties between China continent and Korea, Japan in East Asia

The optical properties of Asian dust were examined from the Aerosol Robotic Network (AERONET) measurements at 10 sites during 2001–2005. A comparison of dust properties was performed between China continent as source regions and Korea, Japan as downwind regions where the dust properties were influenced by long-range transport. Higher aerosol optical thicknesses (AOT) appeared at Chinese sites than Korean and Japanese sites associated with lower Angström exponents ( α). The value of α decreased with increasing AOTs, and decreased to zero or negative when the heavy dust events occurred especially in source regions. The aerosol volume size distributions showed a tri-modal structure, one accumulation mode with radius 0.07 μm and two coarse modes with radii 1.3 and 2.9–3.8 μm in source regions, one accumulation mode with radius 0.09 μm and two coarse modes with radii 1.3–2.2 and 2.9–3.8 μm in downwind regions. In view of climate, the single scattering albedo (SSA) at 440, 670, 870 and 1020 nm was used as 0.95 in source regions of China, and in downwind regions such as 0.92 of Korea and 0.95 of Japan. The real parts of refractive index showed a behavior of low sensitivity to wavelengths, 1.51 in source regions of China, and in downwind regions 1.45 of Korea and 1.39 of Japan at the same four wavelengths. The imaginary parts also showed a low sensitivity to wavelengths, with values of 0.0015–0.0044 in source regions and 0.0018–0.0052 in downwind regions at the four wavelengths. The asymmetry factors at the four wavelengths were used as 0.67 over source and downwind regions in climate and radiation models. This difference in dust properties between source regions and downwind regions was mainly attributed to the impacts of dry depositing and wet scavenging during the long-range transport of dust plume.

Aeolian dust experiment on climate impact: An overview of Japan–China joint project ADEC

The Aeolian Dust Experiment on Climate Impact (ADEC) was initiated in April 2000 as a joint five-year Japan–China project. The goal was to understand the impact of aeolian dust on climate via radiative forcing (RF). Field experiments and numerical simulations were conducted from the source regions in northwestern China to the downwind region in Japan in order to understand wind erosion processes temporal and spatial distribution of dust during their long-range transportation chemical, physical, and optical properties of dust and the direct effect of radiative forcing due to dust. For this, three intensive observation periods (IOP) were conducted from April 2002 to April 2004. The in situ and network observation results are summarized as follows: (1) In situ observations of the wind erosion process revealed that the vertical profile of moving sand has a clear size dependency with height and saltation flux and that threshold wind velocity is dependent on soil moisture. Results also demonstrated that saltation flux is strongly dependent on the parent soil size distribution of the desert surface. (2) Both lidar observations and model simulations revealed a multiple dust layer in East Asia. A numerical simulation of a chemical transport model, CFORS, illustrated the elevated dust layer from the Taklimakan Desert and the lower dust layer from the Gobi Desert. The global-scale dust model, MASINGAR, also simulated the dust layer in the middle to upper free troposphere in East Asia, which originated from North Africa and the Middle East during a dust storm in March 2003. Raman lidar observations at Tsukuba, Japan, found the ice cloud associated with the dust layer at an altitude of 6 to 9 km. Analysis from lidar and the radio-sonde observation suggested that the Asian dust acted as ice nuclei at the ice-saturated region. These results suggest the importance of dust's climate impact via the indirect effect of radiative forcing due to the activation of dust into ice nuclei. (3) Studies on the aerosol concentration indicated that size distributions of aerosols in downwind regions have bimodal peaks. One peak was in the submicron range and the other in the supermicron range. The main soluble components of the supermicron peak were Na +, Ca 2+, NO 3 −, and Cl −. In the downwind region in Japan, the dust, sea salt, and a mixture of the two were found to be dominant in coarse particles in the mixed boundary layer. (4) Observation of the optical properties of dust by sky-radiometer, particle shoot absorption photometer (PSAP), and Nephelometer indicated that unpolluted dust at source region has a weaker absorption than originally believed. A sensitivity experiment of direct RF by dust indicated that single scattering albedo is the most important of the optical properties of dust and that the sensitivity of instantaneous RF in the shortwave region at the top of the atmosphere to the refractive index strongly depends on surface albedo. A global scale dust model, MASINGAR, was used for evaluation of direct RF due to dust. The results indicated the global mean RF at the top and the bottom of the atmosphere were − 0.46 and − 2.13 W m − 2 with cloud and were almost half of the RF with cloud-free condition.

Mesoscale modeling of Central American smoke transport to the United States: 1. “Top‐down” assessment of emission strength and diurnal variation impacts

As is typical in the Northern Hemisphere spring, during 20 April to 21 May 2003, significant biomass burning smoke from Central America was transported to the southeastern United States (SEUS). A coupled aerosol, radiation, and meteorology model that is built upon the heritage of the Regional Atmospheric Modeling System (RAMS), having newly developed capabilities of Assimilation and Radiation Online Modeling of Aerosols (AROMA) algorithm, was used to simulate the smoke transport and quantify the smoke radiative impacts on surface energetics, boundary layer, and other atmospheric processes. This paper, the first of a two‐part series, describes the model and examines the ability of RAMS‐AROMA to simulate the smoke transport. Because biomass‐burning fire activities have distinct diurnal variations, the FLAMBE hourly smoke emission inventory that is derived from the geostationary satellite (GOES) fire products was assimilated into the model. In the “top‐down” analysis, ground‐based observations were used to evaluate the model performance, and the comparisons with model‐simulated results were used to estimate emission uncertainties. Qualitatively, a 30‐day simulation of smoke spatial distribution as well as the timing and location of the smoke fronts are consistent with those identified from the PM2.5 observation network, local air quality reports, and the measurements of aerosol optical thickness (AOT) and aerosol vertical profiles from the Southern Great Plains (SGP) Atmospheric Radiation Measurements (ARM) site in Oklahoma. Quantitatively, the model‐simulated daily mean near‐surface dry smoke mass correlates well with PM2.5 mass at 34 locations in Texas and with the total carbon mass and nonsoil potassium mass (KNON) at three IMPROVE sites along the smoke pathway (with linear correlation coefficients R = 0.77, 0.74, and 0.69 at the significance level larger than 0.99, respectively). The top‐down sensitivity analysis indicates that the total smoke particle emission during the study period is about 1.3 ± 0.2 Tg. The results further indicate that the simulation with a daily smoke emission inventory provides a slightly better correlation with measurements in the downwind region on daily scales but gives an unrealistic diurnal variation of AOT in the smoke source region. This study suggests that the assimilation of emission inventories from geostationary satellites is superior to that of polar orbiting satellites and has important implications for the modeling of air quality in areas influenced by fire‐related pollutants from distant sources.

Climatology of aerosol optical properties in northern China

The climatology of aerosol optical properties in northern China were examined from the ground-based sun and sky radiance measurements at six sites during 2001–2005. Cases of high aerosol optical thickness (AOT) mostly attributed to the occurrence of dust events in spring and the impacts of pollutant particles in summer. The Alfa parameter decreased with AOT even zero or negative in appearance of dust events. The relationship between Alfa and AOT at sites of downwind regions was characterized by one integrated exponent during summer and autumn and by two integrated exponents during spring and winter. But for the sites near dust source regions, only one integrated exponent was enough in any season. This is a possible way to identify and estimate the effects of different sources on aerosol loading and particle size. The aerosol volume size distributions had three modes, one accumulation mode with radius 0.07–0.15 μm and two coarse modes with radii 1.3–1.7 and 3.8–5.0 μm. As AOT larger than 0.5, a pseudo mode appeared between the accumulation mode and two coarse modes with radius 0.3–0.5 μm at sites of dust source regions. The average effective radii during winter and spring were commonly larger than that of summer and autumn. Real parts of the refractive index at 440–1020 nm varied with AOT in range of 1.40–1.54 at sites of dust source regions and in range of 1.45–1.55 at sites of downwind regions. Imaginary parts of the refractive index appeared low sensitivity to wavelengths from 440 to 1020 nm. The averages of imaginary parts during winter and spring were 0.0015–0.0018 for 440–1020 nm at DH and IM sites, and 0.009–0.014 during winter and spring and 0.011–0.058 during summer and autumn at other sites.

Influence of reduced carbon emissions and oxidation on the distribution of atmospheric CO2: Implications for inversion analyses

Recent inverse analyses constraining carbon fluxes using atmospheric CO2 observations have assumed that the CO2 source from atmospheric oxidation of reduced carbon is released at the surface rather than distributed globally in the atmosphere. This produces a bias in the estimates of surface fluxes. We used a three‐dimensional (3D) atmospheric chemistry model (GEOS‐CHEM) to evaluate the magnitude of this effect on modeled concentrations and flux estimates. We find that resolving the 3D structure of the atmospheric CO2 source, as opposed to emitting this reduced carbon as CO2 at the surface, yields a decrease in the modeled annual mean interhemispheric gradient (N‐S) of 0.21 ppm. Larger adjustments (up to −0.6 ppm) are apparent on a regional basis in and downwind of regions of high reduced carbon emissions. We used TransCom3 annual mean simulations from three transport models to evaluate the implications for inversion estimates. The main impacts are systematic decreases in estimates of northern continental land uptake (i.e., by 0.22 to 0.26 Pg C yr−1), and reductions in tropical land carbon efflux with smaller changes over oceans and in the Southern Hemisphere. These adjustments represent a systematic bias in flux estimates, accounting for changes of 9 to 27% in the estimated northern land CO2 sink for the three models evaluated here. Our results highlight the need for a realistic description of reduced carbon emission and oxidation processes in deriving inversion estimates of CO2 surface fluxes.

Aircraft Observations of Air-mass Modification Over the Sea of Okhotsk during Sea-ice Growth

In order to quantitatively investigate the role of leads and sea-ice in air-mass modification, aircraft observations were conducted over the partially ice-covered Sea of Okhotsk. We investigated two cold-air outbreak events with different sea-ice concentrations. In both cases, the difference between the temperatures of surface air and the sea surface (ΔT) dropped rapidly with the accumulated fetch-width of leads up to about 35-40 km, and then decreased very slowly. The surface sensible heat flux originating from open water was about 300 W m−2 within a few kilometres from the coast and decreased with increasing accumulated fetch-width. The sensible heat flux was about 100 W m−2 on average. These results indicate that the downwind air-mass modification depends mainly on the total (accumulated) extent of open water. The total buoyancy flux \(\left(\overline {w'T \, '_v}\right)\) calculated by the joint frequency distribution method correlated very well with ice concentration. Such a relationship was not clear in the case of the moisture flux \((\overline {w'q'})\). The ratio between rising thermals \(\left(\overline {w^+T^+_v}\right) \) and cold downdrafts \(\left(\overline {w^-T^-_v}\right)\) differed significantly between upwind and downwind regions; that is, the buoyancy flux was dominated by \(\overline {w^+T^+_v} \) in the developing stage of the boundary layer, while \(\overline {w^-T^-_v} \) also became important after the development of the boundary layer.

Environmental quality and its impact on the local economy: A case study from Dongguan, South China

Industries that pollute the environment developed in the early 1990s in Dongguan City, especially those in the middle and lower valley of the Dongjiang River (waterside area) have seriously affected water and air quality and hence have affected soil quality through polluted water irrigation and precipitation. Investigation of the economic growth and environmental pollution in the 32 township units in the City from 1990 to 2000 also reveals that environmental pollution has affected the history of the development of the towns or townships in the waterside region in the delta of the River Dongjiang, especially the towns near the river mouth, and has prevented their industry from upgrading and hence has slowed down their economic growth. The impact of environmental pollution on economic growth is most severe in the towns along the lower valley, or in down-wind regions. In contrast, economic development in the protected area of the upper valley of the waterside region indicates that environmental protection that strictly controls the discharge of pollutants does not restrict the development of the local economy.

Impacts of chemistry‐aerosol coupling on tropospheric ozone and sulfate simulations in a general circulation model

We have implemented fully interactive tropospheric gas‐phase chemistry and sulfate aerosol modules into the new generation state‐of the‐art Goddard Institute for Space Studies (GISS) modelE general circulation model (GCM). The code has been developed with a unique flexibility to perform simulations in coupled or off‐line (decoupled) mode. Both modes use identical chemical calculations, but the decoupled simulation relies on previously saved off‐line oxidant and aerosol concentration fields whereas the coupled simulation is fully interactive. Here we describe the application of the model to isolate the impacts of the two‐way chemistry‐aerosol coupling on the predictions of sulfate aerosol and ozone pollution and to provide insights into the mechanisms that drive the different predictions between coupled and off‐line models. On annual and global scales, the differences between the coupled and off‐line simulations are small, but larger deviations do occur on regional and seasonal scales. The chemistry‐aerosol coupling leads to ∼20% increases in surface sulfate over SO2 source regions in the northern hemisphere summer due to higher H2O2 levels and aqueous‐phase oxidation rates in the coupled model. Compensating sulfate decreases occur in downwind regions and in the upper troposphere due to depleted SO2. At middle to high tropospheric altitudes in the northern hemisphere, ozone and OH are increased in the coupled model relative to the off‐line model by ∼10% due to reductions in sulfate loading and subsequent suppression of N2O5 heterogeneous hydrolysis. The use of off‐line versus coupled models has implications for the simulation of the intercontinental transport of pollutants and their precursors.

Aerosol properties and their spatial and temporal variations over North China in spring 2001

Aerosol properties and their spatial and temporal variations over North China were analysed based on ground-based radiometer data for spring 2001. On the basis of the retrievals from sun/sky radiance and broad-band radiation measurements at four AERONET stations and eight first-class radiation stations over North China, the analysis comprised a detailed description of aerosol loading, size and absorption in this period. The impact of dust events on aerosol properties over the downwind region was emphasized.Heavy aerosol loading and notable temporal variation over North China were revealed by both datasets. The average aerosol optical depth at 750 nm at the eight radiation stations ranged from 0.32 in Ejinaqi to 0.68 in Beijing, with the averaged coefficient of variation being 70%. Aerosol optical depth was dominantly contributed to by dust over western China, with 68% (±5%) of aerosol optical depth at 550 nm being attributed to large dust aerosols. A dramatic increase in aerosol optical depth associated with the remarkable decrease in the Ångstrom wavelength exponent was observed in Beijing and Xianghe during the dust episode. This indicated that a huge number of large particles were emitted into the atmosphere during the dust period, with the result that the contribution to aerosol optical depth from coarse particles approached the value observed in the dust source region.Anthropogenic pollution also frequently contributed to the high aerosol optical depth in Beijing and Xianghe, but this was characterized by fine particles, with more than 70% of aerosol optical depth at 550 nm being attributed to fine particles. Pure desert aerosol in Chinese dust source regions absorbs much less solar radiation than predicted by known aerosol models. The retrieved single-scattering albedo was around 0.98 (±0.01) and had little wavelength dependence, which is in agreement with the ground-based and satellite retrievals in other dust source regions. Contrarily, anthropogenic aerosol exhibits much stronger absorption in the urban region, with the single-scattering albedo ranging from 0.89 ± 0.04 (at 440 nm) to 0.83 ± 0.05 (at 1020 nm). Due to the large difference in the absorption between dust and anthropogenic aerosol, the consequence of the input of a large volume of dust aerosols is not only to enhance the aerosol loading but also to reduce the aerosol absorption. Retrievals in Beijing showed that the single-scattering albedo increased to about 0.90 and had little spectral dependence when anthropogenic pollution and a dust event together affected Beijing; as for the pure dust period, it ranged from 0.92 at 440 nm to 0.97 at 1020 nm. The significant decrease in the aerosol absorption due to the dust outbreak can result in a decrease in aerosol atmospheric heating efficiency; this warrants further research since the increasing trend of aerosol loading with strong absorption in China is supposed to play an active role in regional and global climate change and the hydrological cycle.

Aerosol properties and their spatial and temporal variations over North China in spring 2001

Aerosol properties and their spatial and temporal variations over North China were analysed based on ground-based radiometer data for spring 2001. On the basis of the retrievals from sun/sky radiance and broad-band radiation measurements at four AERONET stations and eight first-class radiation stations over North China, the analysis comprised a detailed description of aerosol loading, size and absorption in this period. The impact of dust events on aerosol properties over the downwind region was emphasized.Heavy aerosol loading and notable temporal variation over North China were revealed by both datasets. The average aerosol optical depth at 750 nm at the eight radiation stations ranged from 0.32 in Ejinaqi to 0.68 in Beijing, with the averaged coefficient of variation being 70%. Aerosol optical depth was dominantly contributed to by dust over western China, with 68% (±5%) of aerosol optical depth at 550 nm being attributed to large dust aerosols. A dramatic increase in aerosol optical depth associated with the remarkable decrease in the Ångstrom wavelength exponent was observed in Beijing and Xianghe during the dust episode. This indicated that a huge number of large particles were emitted into the atmosphere during the dust period, with the result that the contribution to aerosol optical depth from coarse particles approached the value observed in the dust source region.Anthropogenic pollution also frequently contributed to the high aerosol optical depth in Beijing and Xianghe, but this was characterized by fine particles, with more than 70% of aerosol optical depth at 550 nm being attributed to fine particles. Pure desert aerosol in Chinese dust source regions absorbs much less solar radiation than predicted by known aerosol models. The retrieved single-scattering albedo was around 0.98 (±0.01) and had little wavelength dependence, which is in agreement with the ground-based and satellite retrievals in other dust source regions. Contrarily, anthropogenic aerosol exhibits much stronger absorption in the urban region, with the single-scattering albedo ranging from 0.89 ± 0.04 (at 440 nm) to 0.83 ± 0.05 (at 1020 nm). Due to the large difference in the absorption between dust and anthropogenic aerosol, the consequence of the input of a large volume of dust aerosols is not only to enhance the aerosol loading but also to reduce the aerosol absorption. Retrievals in Beijing showed that the single-scattering albedo increased to about 0.90 and had little spectral dependence when anthropogenic pollution and a dust event together affected Beijing; as for the pure dust period, it ranged from 0.92 at 440 nm to 0.97 at 1020 nm. The significant decrease in the aerosol absorption due to the dust outbreak can result in a decrease in aerosol atmospheric heating efficiency; this warrants further research since the increasing trend of aerosol loading with strong absorption in China is supposed to play an active role in regional and global climate change and the hydrological cycle.

Mass transport in the heliosphere by energetic neutral atoms

Energetic neutral atoms (ENAs) are recognized as a powerful tool for remote probing of distant hot space plasmas, in particular the plasmas abundant at the heliospheric boundary where the expanding solar wind meets the surrounding local interstellar medium. We show here for the first time that the heliospheric ENAs originating in the heliospheric sheath between the termination shock and the heliopause and reaching the inner heliosphere also provide an important and heretofore unaccounted source of atomic hydrogen in the Sun's vicinity. These ENAs are a major contributor to the density of interplanetary hydrogen at heliocentric distances &lt;1 AU and could dominate in the downwind (interstellar wind) region under typical solar and interstellar conditions. Mass transport by heliospheric ENAs may become especially important for determining the origin of the pickup ions attributed to the inner source of neutral particles in the Sun's vicinity and for characterization of the three‐dimensional solar wind flow by imaging in extreme ultraviolet.

A study of long‐term trends in mineral dust aerosol distributions in Asia using a general circulation model

Dust events have been observed in Japan with high frequency since 2000. On the other hand, the frequency of dust storms is said to have decreased in the desert regions of China since about the middle of the 1970s. This study simulates dust storms and transportation of mineral dust aerosols in the east Asia region from 1981 to 2001 using an aerosol transport model, Spectral Radiation‐Transport Model for Aerosol Species (SPRINTARS), implemented in the Center for Climate System Research/National Institute for Environmental Studies atmospheric global circulation model, in order to investigate the main factors that control a dust event and its long‐term variation. The model was forced to simulate a real atmospheric condition by a nudging technique using European Centre for Medium‐Range Weather Forecasts reanalysis data on wind velocities, temperature, specific humidity, soil wetness, and snow depth. From a comparison between the long‐term change in the dust emission and model parameters, it is found that the wind speed near the surface level had a significant influence on the dust emission, and snow is also an important factor in the early spring dust emission. The simulated results suggested that dust emissions from northeast China have a great impact on dust mass concentration in downwind regions, such as the cities of northeastern China, Korea, and Japan. When the frequency of dust events was high in Japan, a low‐pressure system tended to develop over the northeast China region that caused strong winds. From 2000 to 2001 the simulated dust emission flux decreased in the Taklimakan desert and the northwestern part of China, while it increased in the Gobi desert and the northeastern part of China. Consequently, dust particles seem to be transported more from the latter region by prevailing westerlies in the springtime to downwind areas as actually observed. In spite of the similarity, however, there is still a large disagreement between observed and simulated dust frequencies and concentrations. A more realistic land surface and uplift mechanism of dust particles should be modeled to improve the model simulation. Desertification of the northeastern China region may be another reason for this disagreement.

Aerosol optical properties measured on board the Ronald H. Brown during ACE‐Asia as a function of aerosol chemical composition and source region

During the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) intensive field campaign conducted in the spring of 2001, aerosol properties were measured on board the R/V Ronald H. Brown to study the effects of the Asian aerosol on atmospheric chemistry and climate in downwind regions. Aerosol properties measured in the marine boundary layer included chemical composition; number size distribution; and light scattering, hemispheric backscattering, and absorption coefficients. In addition, optical depth and vertical profiles of aerosol 180° backscatter were measured. Aerosol within the ACE‐Asia study region was found to be a complex mixture resulting from marine, pollution, volcanic, and dust sources. Presented here as a function of air mass source region are the mass fractions of the dominant aerosol chemical components, the fraction of the scattering measured at the surface due to each component, mass scattering efficiencies of the individual components, aerosol scattering and absorption coefficients, single scattering albedo, Ångström exponents, optical depth, and vertical profiles of aerosol extinction. All results, except aerosol optical depth and the vertical profiles of aerosol extinction, are reported at a relative humidity of 55 ± 5%. An overdetermined data set was collected so that measured and calculated aerosol properties could be compared, internal consistency in the data set could be assessed, and sources of uncertainty could be identified. By adjusting the measured size distribution to take into account nonsphericity of the dust aerosol, calculated and measured aerosol mass and scattering coefficients agreed within overall experimental uncertainties. Differences between measured and calculated aerosol absorption coefficients were not within reasonable uncertainty limits, however, and may indicate the inability of Mie theory and the assumption of internally mixed homogeneous spheres to predict absorption by the ACE‐Asia aerosol. Mass scattering efficiencies of non‐sea‐salt sulfate aerosol, sea salt, submicron particulate organic matter, and dust found for the ACE‐Asia aerosol are comparable to values estimated for ACE 1, Aerosols99, and the Indian Ocean Experiment (INDOEX). Unique to the ACE‐Asia aerosol were the large mass fractions of dust, the dominance of dust in controlling the aerosol optical properties, and the interaction of dust with soot aerosol.