N. Oshima

Wet removal of black carbon in Asian outflow: Aerosol Radiative Forcing in East Asia (A‐FORCE) aircraft campaign

[1]xa0The Aerosol Radiative Forcing in East Asia (A-FORCE) aircraft campaign was conducted over East Asia in March–April 2009. During the A-FORCE campaign, 120 vertical profiles of black carbon (BC) and carbon monoxide (CO) were obtained in the planetary boundary layer (PBL) and the free troposphere. This study examines the wet removal of BC in Asian outflow using the A-FORCE data. The concentrations of BC and CO were greatly enhanced in air parcels sampled at 3–6 km in altitude over the Yellow Sea on 30 March 2009, associated with upward transport due to a cyclone with modest amounts of precipitation over northern China. In contrast, high CO concentrations without substantial enhancements of BC concentrations were observed in air parcels sampled at 5–6 km over the East China Sea on 23 April 2009, caused by uplifting due to cumulus convection with large amounts of precipitation over central China. The transport efficiency of BC (TEBC, namely the fraction of BC particles not removed during transport) in air parcels sampled above 2 km during the entire A-FORCE period decreased primarily with the increase in the precipitation amount that air parcels experienced during vertical transport, although their correlation was modest (r2 = 0.43). TEBC also depended on the altitude to which air parcels were transported from the PBL and the latitude where they were uplifted locally over source regions. The median values of TEBC for air parcels originating from northern China (north of 33°N) and sampled at 2–4 km and 4–9 km levels were 86% and 49%, respectively, during the A-FORCE period. These median values were systematically greater than the corresponding median values (69% and 32%, respectively) for air parcels originating from southern China (south of 33°N). Use of the A-FORCE data set will contribute to the reduction of large uncertainties in wet removal process of BC in global- and regional-scale models.

Emissions of black carbon in East Asia estimated from observations at a remote site in the East China Sea

[1]xa0East Asia, including China, is the largest source of anthropogenic black carbon (BC). In estimating the BC emissions from this region, it is advantageous to use BC mass concentrations measured at remote locations on the ocean appropriately distant from the large sources because of spatially uniform distributions through mixing during transport. We made continuous measurements of the BC mass concentration with an accuracy of about 10% at Cape Hedo on Okinawa Island, Japan, in the East China Sea, from February 2008 to May 2009, simultaneously with carbon monoxide (CO). The seasonal median BC concentrations at Hedo were highest (0.23–0.31 μg m−3 at standard temperature and pressure) in winter and spring when plumes from China, predominantly northern China north of 33°N, were often transported to the site. A three-dimensional chemical transport model is used to calculate the mass concentration of BC using the annual mean emission inventory of Zhang et al. (2009) for the base year 2006. The model results and the observed BC-CO correlation are used to exclude the BC data substantially influenced by wet deposition. The calculated BC mass concentrations agree with those observed to within about 30% in air strongly affected by emissions in China for winter and spring on average. We estimate the annually averaged BC emission flux over the whole of China to be 1.92 Tg yr−1 with an uncertainty of about 40%. This value is very close to the value of 1.81 Tg yr−1 estimated by Zhang et al. (2009). The overall uncertainty of 40% of the present estimate is a substantial improvement in the uncertainty (208%) of the bottom-up inventory.

Size dependence of wet removal of black carbon aerosols during transport from the boundary layer to the free troposphere

[1]xa0Size distributions of black carbon (BC) measured by aircraft over East Asia in spring 2009 were highly correlated with BC transport efficiency in air parcels uplifted from the planetary boundary layer to the free troposphere. The average single-particle BC mass decreased with decreasing transport efficiency, which suggests that aerosols containing larger BC mass were removed more efficiently. This is the first successful observation of the size-dependent wet removal of aerosols, qualitatively consistent with the Kohler theory. The size distribution of BC uplifted to the free troposphere with high efficiency was similar to the size distribution of BC in the planetary boundary layer. Conversely, the size distribution of BC uplifted with low efficiency was similar to that of background air in the free troposphere. We conclude that wet removal during upward transport is important in controlling the size distribution of BC in the free troposphere.

Seasonal variations of the transport of black carbon and carbon monoxide from the Asian continent to the western Pacific in the boundary layer

[1]xa0Continuous in situ measurements of the mass concentration of black carbon (BC) aerosols and mixing ratio of carbon monoxide (CO) were made at Cape Hedo on Okinawa Island, Japan, a remote site located in the East China Sea, from March 2008 to May 2009. For the first time, we show temporal variations of BC and CO at Hedo in Asian outflows throughout the year. Annual average concentrations of BC and CO were 0.29 μg m−3 and 150 ppbv, respectively. The origins of the observed air masses were determined by using 5-day back trajectories, suggesting that about 51% of the air masses arriving at Hedo were from the Chinese region during spring and winter, while about 78% of air masses were of maritime origin during summer. Because of the more frequent transport of Chinese air to Hedo in spring and winter, the average and background concentrations of BC and CO in these seasons were higher by about a factor of 2 than those in summer and fall. Air masses from north China made the largest contributions to elevating the BC levels at Hedo because of the high BC emission rate and frequency of transport. The observed ΔBC/ΔCO ratio systematically decreased with the decrease in model-calculated transport efficiency (TEBCcal). On the basis of this result, we derive region-specific ΔBC/ΔCO ratios by selecting data with TEBCcal > 80%. The annually averaged ΔBC/ΔCO ratios for air originated from north and south China were 7.0 ± 3.3 and 7.5 ± 4.6 ng m−3 ppbv−1, respectively, about half the annual BC/CO emission ratio derived from the emission inventory of Zhang et al. (2009). We evaluate the CO emission inventory of Zhang et al. (2009) for China by comparing observed (ground-based and aircraft) and model-calculated CO values. The comparison indicates that the CO emissions from China were underestimated by about a factor of 2.

Measurements of regional‐scale aerosol impacts on cloud microphysics over the East China Sea: Possible influences of warm sea surface temperature over the Kuroshio ocean current

[1]xa0Cloud 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 500–900xa0km 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, Dxa0>xa010xa0nm) but also larger aerosol mode diameters likely contributed to high Ncduring A-FORCE. The mean Nc of 650xa0±xa0240xa0cm−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.

Vertical transport mechanisms of black carbon over East Asia in spring during the A‐FORCE aircraft campaign

[1]xa0Mechanisms of vertical transport of black carbon (BC) aerosols and their three-dimensional transport pathways over East Asia in spring were examined through numerical simulations for the Aerosol Radiative Forcing in East Asia (A-FORCE) aircraft campaign in March–April 2009 using a modified version of the Community Multiscale Air Quality (CMAQ) modeling system. The simulations reproduced the spatial distributions of mass concentration of BC and its transport efficiency observed by the A-FORCE campaign reasonably well, including its vertical and latitudinal gradients and dependency on precipitation amount that air parcels experienced during the transport. During the A-FORCE period, two types of pronounced upward BC mass fluxes from the planetary boundary layer (PBL) to the free troposphere were found over northeastern and inland-southern China. Over northeastern China, cyclones with modest precipitation were the primary uplifting mechanism of BC. Over inland-southern China, both cumulus convection and orographic uplifting along the slopes of the Tibetan Plateau played important roles in the upward transport of BC, despite its efficient wet deposition due to a large amount of precipitation supported by an abundant moisture supply by the low-level southerlies. In addition to the midlatitude (35–45°N) eastward outflow within the PBL (21% BC removal by precipitation during transport), the uplifting of BC over northeastern and inland-southern China and the subsequent BC transport by the midlatitude lower tropospheric (50% BC removal) and subtropical (25–35°N) midtropospheric westerlies (67% BC removal), respectively, provided the major transport pathways for BC export from continental East Asia to the Pacific.

Global climate change driven by soot at the K-Pg boundary as the cause of the mass extinction

The mass extinction of life 66 million years ago at the Cretaceous/Paleogene boundary, marked by the extinctions of dinosaurs and shallow marine organisms, is important because it led to the macroevolution of mammals and appearance of humans. The current hypothesis for the extinction is that an asteroid impact in present-day Mexico formed condensed aerosols in the stratosphere, which caused the cessation of photosynthesis and global near-freezing conditions. Here, we show that the stratospheric aerosols did not induce darkness that resulted in milder cooling than previously thought. We propose a new hypothesis that latitude-dependent climate changes caused by massive stratospheric soot explain the known mortality and survival on land and in oceans at the Cretaceous/Paleogene boundary. The stratospheric soot was ejected from the oil-rich area by the asteroid impact and was spread globally. The soot aerosols caused sufficiently colder climates at mid–high latitudes and drought with milder cooling at low latitudes on land, in addition to causing limited cessation of photosynthesis in global oceans within a few months to two years after the impact, followed by surface-water cooling in global oceans in a few years. The rapid climate change induced terrestrial extinctions followed by marine extinctions over several years.

Condensation Particle Counters Combined with a Low-Pressure Impactor for Fast Measurement of Mode-Segregated Aerosol Number Concentration

An integrated airborne system comprising two condensation particle counters (CPCs, models 3771 and 3772, TSI Inc.) and a low-pressure impactor (LPI) has been developed (LPI-CPCs) for fast measurement of mode-segregated aerosol number concentration. The CPC 3771 is connected to the LPI to measure aerosol number concentrations below 0.17 μm in aerodynamic diameter, while the CPC 3772 directly measures the total aerosol number concentration. The former approximately corresponds to the Aitken mode fraction of the aerosol number concentration. The key concept is that the cutoff diameter of the LPI (aerodynamic diameter at which the transmission efficiency is 50%) is controlled by simultaneously modifying the pressure and flow rate through the LPI. The instrument was deployed onboard the King Air B200T aircraft during the Aerosol Radiative Forcing in East Asia (A-FORCE) conducted over the western Pacific in the spring of 2009. The results from the aircraft measurements, together with those from laboratory experiments, are used to demonstrate the in-flight performance of the instrument. We propose that fast airborne measurement of mode-segregated aerosols by LPI-CPCs is useful for a better understanding of the variability of aerosol number concentration in the troposphere. Copyright 2013 American Association for Aerosol Research

Effects of wet deposition on the abundance and size distribution of black carbon in East Asia

An improved understanding of the variations in the mass concentration and size distribution of black carbon (BC) in the free troposphere (FT) over East Asia, where BC emissions are very high, is needed to reliably estimate the radiative forcing of BC in climate models. We measured these parameters and the carbon monoxide (CO) concentration by conducting the Aerosol Radiative Forcing in East Asia (A-FORCE) 2013W aircraft campaign in East Asia in winter 2013 and compared these data with measurements made in the same region in spring 2009. The median BC concentrations in the FT originating from North China (NC) and South China (SC) showed different seasonal variations, which were primarily caused by variations in meteorological conditions. CO concentrations above the background were much higher in SC than in NC in both seasons, suggesting a more active upward transport of CO. In SC, precipitation greatly increased from winter to spring, leading to an increased wet deposition of BC. As a result, the median BC concentration in the FT was highest in SC air in winter. This season and region were optimal for the effective transport of BC from the planetary boundary layer to the FT. The count median diameters of the BC size distributions generally decreased with altitude via wet removal during upward transport. The altitude dependence of the BC size distributions was similar in winter and spring, in accord with the similarity in the BC mixing state. The observed BC concentrations and microphysical properties will be useful for evaluating the performance of climate models.

Wet deposition of black carbon at a remote site in the East China Sea

Mass concentrations of black carbon (BC) in air (MBC) and rainwater (CBC) in the East China Sea were measured at Hedo on Okinawa Island, Japan, from April 2010 to March 2013. The monthly averaged MBC and CBC showed marked seasonal variations, being highest in spring (0.32u2009±u20090.13u2009µgu2009m−3 and 92u2009±u200976u2009µgu2009L−1, respectively) and lowest in summer (0.06u2009±u20090.03u2009µgu2009m−3 and 8.0u2009±u20094.1u2009µgu2009L−1, respectively). The high MBC and CBC in spring were associated with transport of air masses from the Asian continent by northwesterly winds. The BC wet deposition flux (FBC), estimated as the product of CBC and precipitation amount, also showed a distinct seasonal variation. The monthly average FBC during the four spring seasons (16.8u2009±u20096.7u2009mgu2009m−2u2009month−1) was about 3 times higher than the annual average FBC (5.5u2009±u20099.9u2009mgu2009m−2u2009month−1) owing to the high CBC and precipitation amount in spring. As a result, about 76% of the annual BC deposition occurred in spring on average. The FBC in spring is comparable to the average BC net flux in North China, indicating the importance of precipitation over the East China Sea as a sink of BC transported from North China. In summer, CBC values were correlated with MBC for rain events associated with local convective activity, as identified by the convective available potential energy. A one-dimensional thermodynamic model successfully explained the relation between CBC and MBC.