The mixing state of black carbon (BC) aerosols, namely, the degree to which BC particles are coated with other aerosol components, has been recognized as important for evaluating aerosol radiative ...forcing. In order to resolve the BC mixing state explicitly in model simulations, a two‐dimensional aerosol representation, in which aerosols are given for individual particle diameters and BC mass fractions, is introduced. This representation was incorporated into an aerosol module, the Model of Aerosol Dynamics, Reaction, Ionization, and Dissolution (MADRID), and a new box model, MADRID‐BC, was developed. MADRID‐BC can accurately simulate changes in the entire BC mixing state resulting from condensation/evaporation processes. Aircraft observations conducted in March 2004 show that the mass fraction of thickly coated BC particles increased in air horizontally transported out from an urban area in Japan over the ocean. MADRID‐BC generally reproduces this feature well when observed bulk aerosol concentrations are used as constraints. The model simulations in this particular case show that for particles with BC core diameters of 100–200 nm, the particle diameters, including both core and coating materials, had already increased by a factor of 1.6 on average when they left the source region and by as large as a factor of 1.9 of the BC core diameters after their transport over the ocean for a half day. The model simulations also show that 58% of the total condensed mass was partitioned onto BC‐free particles during transport, indicating their importance for the BC mixing state. Although the model simulations are applied to a limited number of the observations in this study, they clearly show the time evolution of the coating thicknesses of BC‐containing particles, which is necessary for calculating aerosol optical properties and cloud condensation nuclei activities.
Black carbon (BC) aerosol deposited in and onto Arctic snow increases the snow's absorption of sunlight and accelerates snowmelt. Wet removal of BC from the atmosphere plays a key role in determining ...its abundance in the Arctic atmosphere and in Arctic snow. However, this process is poorly understood, mainly due to the scarcity of relevant measurements. To study wet deposition of BC, we made measurements of mass concentration of BC in snow and rain (CMBC) and of BC in air (MBC) with high accuracy (16% and 10%, respectively) at the Barrow Atmospheric Baseline Observatory, Alaska, from July 2013 to August 2017 and analyzed them along with routinely measured meteorological parameters from Barrow. Monthly mean MBC near the surface and CMBC were poorly correlated from midwinter to early spring, when CMBC was close to the annual median while MBC was at its annual peak. Seasonal variations in the altitude distribution of MBC may lead to these differences in seasonal variation of MBC near the surface and CMBC. About 50% of the annual wet deposition of BC occurred in the 3 months of summer, associated with high values of total precipitation and BC originating from biomass burning. Size distributions of BC in snow and rain were stable throughout the year, suggesting that the size distribution of BC in the lower troposphere was similarly stable. Calculations by two global models reproduced the observed seasonal variations of CMBC and showed that BC from biomass burning dominated CMBC in summer.
Plain Language Summary
Black carbon (BC) aerosol, a component of soot, efficiently absorbs solar radiation and heats the atmosphere. BC deposited in and onto Arctic snow increases the absorption of sunlight, accelerates snowmelt, and reduces the snow albedo. Wet deposition of BC greatly influences the concentrations of BC in the atmosphere and in fallen snow. Because measurements of BC wet deposition are scarce, we measured mass concentrations of BC in snow and rain (CMBC) and of BC in ambient air near the surface (MBC) with high accuracy (16% and 10%, respectively) at the Barrow Atmospheric Baseline Observatory, Alaska, from 2013 to 2017. MBC and CMBC showed different seasonal variations. Differences between BC concentrations near the surface and in the free troposphere may be one cause of this difference. About 50% of the annual wet deposition of BC occurred in the 3 months of summer, associated with high levels of BC emitted from biomass burning and high precipitation amounts. Calculations by two global models reproduced the observed seasonal variations of BC in snow and rain and showed that in summer, this BC was predominantly from biomass burning.
Key Points
We studied seasonal variations of black carbon (BC) concentrations in snow and rain in Arctic Alaska using measurements with 16% accuracy
About 50% of annual wet deposition occurred in summer, indicating the importance of wet deposition of BC emitted from biomass burning (BB)
Model calculations, which reproduce the observed BC in hydrometeors within a factor of 2, suggest large contributions of BB during summer
Quantitative evaluation of the performance of one of the most advanced mechanistic secondary organic aerosol (SOA) modules/models, the Model of Aerosol Dynamics, Reaction, Ionization, and Dissolution ...2 (MADRID2) in the three‐dimensional Models‐3/Community Multiscale Air Quality (CMAQ), in urban air is made. Model calculations are compared for the Tokyo, Japan, metropolitan area with measurements made using an Aerodyne quadrupole aerosol mass spectrometer (Q‐AMS) at an urban site for 9 days in July and August 2003. In general, model calculations reproduced absolute values and temporal variations of meteorological parameters, C2–C8 volatile organic compounds (VOCs), NOx (NO + NO2), inorganic aerosols, and O3 concentrations reasonably well at this site. However, model‐calculated SOA concentrations are a factor of 5 smaller than observed oxygenated organic aerosol (OOA) concentrations, and calculated total organic aerosol (OA = SOA + primary organic aerosol) concentrations are smaller by a factor of 2, indicating missing processes or sources in the current organic aerosol model calculations. On the other hand, observed features of diurnal and day‐to‐day variations of OOA were captured by our model calculations. Because of the large quantity of unidentified total nonmethane VOCs (NMVOCs) in urban air, a possible contribution of SOA formation from high‐molecular‐weight VOCs is examined through simple sensitivity studies, in which emissions are increased to account for unidentified NMVOCs. It is found that they are potentially one of the missing SOA sources, demonstrating the importance of reliable measurements of high‐molecular‐weight VOCs and total NMVOCs. Relationships between SOA and O3, including regional (∼150 × 150 km2) enhancements around the Tokyo metropolitan area, are also discussed.
Absorption aerosol optical depth (AAOD) measurements made by sun‐sky photometers are currently the only constraint available for estimates of the global radiative forcing of black carbon (BC), but ...their validation studies are limited. In this paper, we report the first attempt to compare AAODs derived from single‐particle soot photometer (SP2) and ground‐based sun‐sky photometer (sky radiometer, SKYNET) measurements. During the Aerosol Radiative Forcing in East Asia (A‐FORCE) experiments, BC size distribution and mixing state vertical profiles were measured using an SP2 on board a research aircraft near the Fukue Observatory (32.8°N, 128.7°E) over the East China Sea in spring 2009 and late winter 2013. The aerosol extinction coefficients (bext) and single scattering albedo (SSA) at 500 nm were calculated based on aerosol size distribution and detailed BC mixing state information. The calculated aerosol optical depth (AOD) agreed well with the sky radiometer measurements (2 ± 6%) when dust loadings were low (lidar‐derived nonspherical particle contribution to AOD less than 20%). However, under these low‐dust conditions, the AAODs obtained from sky radiometer measurements were only half of the in situ estimates. When dust loadings were high, the sky radiometer measurements showed systematically higher AAODs even when all coarse particles were assumed to be dust for in situ measurements. These results indicate that there are considerable uncertainties in AAOD measurements. Uncertainties in the BC refractive index, optical calculations from in situ data, and sky radiometer retrieval analyses are discussed.
Key Points
First attempt to compare absorption aerosol optical depth with SP2 measurements
Systematic discrepancy in the BC column absorption was found
Uncertainties in BC column absorption are examined
We have developed a quasi-monochromatic hard X-ray source via laser Compton scattering (LCS) based on an S-band compact electron linac at AIST. The number of total photons and the maximum X-ray ...energy was 10
7
photons/sec and about 40
keV, respectively, in 15-degree crossing angle between a 42
MeV electron beam and a 800
nm Ti:Sa laser. The biological observation of the human bone with fractures has been successfully demonstrated using the LCS X-ray of 26.4
keV with the in-line phase contrast scheme. Good contrast enhancement is clearly observed between the absorption and the phase contrast images.
Regional aerosol model calculations were made using the Weather Research and Forecasting (WRF)‐Community Multiscale Air Quality (CMAQ) and WRF‐chem models to study spatial and temporal variations of ...aerosols around Beijing, China, in the summer of 2006, when the Campaigns of Air Quality Research in Beijing and Surrounding Region 2006 (CAREBeijing) intensive campaign was conducted. Model calculations captured temporal variations of primary (such as elemental carbon (EC)) and secondary (such as sulfate) aerosols observed in and around Beijing. The spatial distributions of aerosol optical depth observed by the MODIS satellite sensors were also reproduced over northeast China. Model calculations showed distinct differences in spatial distributions between primary and secondary aerosols in association with synoptic‐scale meteorology. Secondary aerosols increased in air around Beijing on a scale of about 1000 × 1000 km2 under an anticyclonic pressure system. This air mass was transported northward from the high anthropogenic emission area extending south of Beijing with continuous photochemical production. Subsequent cold front passage brought clean air from the north, and polluted air around Beijing was swept to the south of Beijing. This cycle was repeated about once a week and was found to be responsible for observed enhancements/reductions of aerosols at the intensive measurement sites. In contrast to secondary aerosols, the spatial distributions of primary aerosols (EC) reflected those of emissions, resulting in only slight variability despite the changes in synoptic‐scale meteorology. In accordance with these results, source apportionment simulations revealed that primary aerosols around Beijing were controlled by emissions within 100 km around Beijing within the preceding 24 h, while emissions as far as 500 km and within the preceding 3 days were found to affect secondary aerosols.
Airborne measurements of aerosols were conducted over the western Pacific in the spring of 2009 during the Aerosol Radiative Forcing in East Asia (A‐FORCE) aircraft campaign. The A‐FORCE flights ...intensively covered an important vertical‐latitudinal range in the outflow region of East Asia (0–9 km altitude; 27°N–38°N). This paper presents the variability of aerosol particle number concentrations obtained by condensation particle counters and a Single‐Particle Soot Photometer (SP2), with the focus on those in the free troposphere. The number concentration data include total condensation nuclei with particle diameters (dp) larger than 10 nm (total CN10), PM0.17‐CN10 (dp ~10–130 nm), and SP2 black carbon (NBC; dp ~75–850 nm). Large increases in total CN10 that were not associated with NBC were observed in the free troposphere, suggesting influences from new particle formation (NPF). Statistical characteristics of total CN10, PM0.17‐CN10, and NBC in the lower troposphere (LT; 0–3 km), middle troposphere (MT; 3–6 km), and upper troposphere (UT; 6–9 km) are investigated. The correlation between total CN10 and NBC, along with the ratio of PM0.17 to total CN10 and carbon monoxide mixing ratio (CO), is used to interpret the observed variability. The median concentrations of total CN10 and PM0.17‐CN10 in the UT were higher than those in the MT by a factor of ~1.4 and ~1.6, respectively. We attribute the enhancements of CN10 in the UT to NPF. Possible mechanisms affecting NPF in the free troposphere are discussed.
Key Points
The aerosol number concentrations over East Asia are intensively discussedThe data suggest the importance of free‐tropospheric new particle formation
Cloud microphysical properties and aerosol concentrations were measured aboard an aircraft over the East China Sea and Yellow Sea in April 2009 during the Aerosol Radiative Forcing in East Asia ...(A‐FORCE) experiment. We sampled stratocumulus and shallow cumulus clouds over the ocean in 9 cases during 7 flights 500–900 km off the east coast of Mainland China. In this study we report aerosol impacts on cloud microphysical properties by focusing on regional characteristics of two key parameters, namely updraft velocity and aerosol size distribution. First, we show that the cloud droplet number concentration (highest 5%, Nc_max) correlates well with the accumulation‐mode aerosol number concentration (Na) below the clouds. We then show that Nc_maxcorrelates partly with near‐surface stratification evaluated as the difference between the sea surface temperature (SST) and 950‐hPa temperature (SST − T950). Cold air advection from China to the East China Sea was found to bring not only a large number of aerosols but also a dry and cold air mass that destabilized the atmospheric boundary layer, especially over the warm Kuroshio ocean current. Over this high‐SST region, greater updraft velocities and hence greater Nc_maxlikely resulted. We hypothesize that the low‐level static stability determined by SST and regional‐scale airflow modulates both the cloud microphysics (aerosol impact on clouds) and macro‐structure of clouds (cloud base and top altitudes, hence cloud liquid water path). Second, we show that not only higher aerosol loading in terms of total aerosol number concentration (NCN, D > 10 nm) but also larger aerosol mode diameters likely contributed to high Ncduring A‐FORCE. The mean Nc of 650 ± 240 cm−3was more than a factor of 2 larger than the global average for clouds influenced by continental sources. A crude estimate of the aerosol‐induced cloud albedo radiative forcing is also given.
Key Points
Cloud droplet concentration was quite high over East Asia due to aerosol
Cloud droplet concentration partly correlates with near‐surface stratification
Both aerosol concentration and mode diameter are large over East Asia
We examine the ozone production efficiency in transpacific Asian pollution plumes, and the implications for ozone air quality in California, by using aircraft and surface observations in April–May ...2002 from the Intercontinental Transport and Chemical Transformation 2002 (ITCT 2K2) campaign off the California coast and the Pacific Exploration of Asian Continental Emission–B (PEACE‐B) campaign over the northwest Pacific. The observations are interpreted with a global three‐dimensional chemical transport model (GEOS‐CHEM). The model reproduces the mean features observed for CO, reactive nitrogen oxides (NOy), and ozone but underestimates the strong (∼20 ppbv) stratospheric contribution to ozone in the middle troposphere. The ITCT 2K2 aircraft sampled two major transpacific Asian pollution plumes, one on 5 May at 5–8 km altitude with CO up to 275 ppbv but no elevated ozone and one on 17 May at 2.5–4 km altitude with CO up to 225 ppbv and ozone up to 90 ppbv. We show that the elevated ozone in the latter plume is consistent with production from peroxyacetylnitrate (PAN) decomposition during subsidence of the plume over the northeast Pacific. This production is particularly efficient because of the strong radiation and low humidity of the subsiding environment. We argue that such PAN decomposition represents a major and possibly dominant component of the ozone enhancement in transpacific Asian pollution plumes. Strong dilution of Asian pollution plumes takes place during entrainment in the U.S. boundary layer, greatly reducing their impact at U.S. surface sites. California mountain sites are more sensitive to Asian pollution because of their exposure to the free troposphere. Model results indicate a mean Asian pollution enhancement of 7 ppbv ozone at Sequoia National Park in May 2002 on those days when the 8‐hour average ozone concentration exceeded 80 ppbv.
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