ALBERT

Description: [1]  The emission of black carbon (BC) from East Asia and its long-range transport strongly influence the mass concentration of BC (M BC ) over the western Pacific. However, reliable and long-term BC data are still limited in this region, especially at elevated altitudes. In this study, we present accurate measurements of M BC using a continuous soot monitoring system (COSMOS) at Happo, a remote mountain site at an altitude of about 1.8 km in Japan, from August 2007 to August 2009. The annual average M BC at Happo was about 0.26 ± 0.18 (1 σ) µg m -3 . The monthly average M BC values exhibited similar seasonal variations during both years, with minimum values in winter. Around 40 % of the air sampled at the site was of free tropospheric (FT) origin, with about 10% originating in North China (NC) origin, respectively. The M BC values for FT (0.24 µg m -3 ) and NC (0.23 µg m -3 ) air were representative of the M BC values (0.26 µg m -3 ) at 1.8 km height in the western Pacific, which are strongly influenced by BC emissions in North China. The M BC values calculated using a regional scale model reproduced well the M BC observed at Happo. The model predicted that BC transported from northern China alone contributed ~53% to the measured M BC , consistent with trajectory analysis. The comparison of model-calculated and observed M BC values indicates that the minimum values of M BC in winter were caused by the suppressed upward transport of BC over the Asian continent. Biomass burning in Siberia substantially increased M BC in the spring of 2008.

Description: [1]  The new particle formation (NPF)-explicit version of the WRF-chem model, which we developed recently, can calculate the growth and sink of nucleated clusters explicitly with 20 aerosol size bins from 1 nm to 10 µm. In this study, the model is used to understand spatial and temporal variations of the frequency of NPF events and the concentrations of aerosols (condensation nuclei, CN) and cloud condensation nuclei (CCN) within the boundary layer in East Asia in spring 2009. Model simulations show distinct north–south contrast in the frequency and mechanism of NPF in East Asia. NPF mostly occurred over limited periods and regions between 30° and 45°N, such as northeast China, Korea, and Japan, including regions around active volcanoes (Miyakejima and Sakurajima). At these latitudes, NPF was considerably suppressed by high concentrations of preexisting particles under stagnant air conditions associated with high-pressure systems, while nucleation occurred more extensively on most days during the simulation period. Conversely, neither nucleation nor NPF occurred frequently south of 30°N because of lower SO 2 emissions and H 2 SO 4 concentrations. The period-averaged NPF frequency was 3 times higher at latitudes of 30° – 45°N than at latitudes of 20° – 30°N. The north–south contrast of NPF frequency is validated by surface measurements in outflow regions in East Asia. The period- and domain-averaged contribution of secondary particles is estimated to be 44% for CN (〉 10 nm) and 26% for CCN at a supersaturation of 1.0% in our simulation, though the contribution is highly sensitive to the magnitudes and size distributions of primary aerosol emissions and the coefficients in the nucleation parameterizations.

Description: 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_max correlates 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_max likely 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 Nc during A-FORCE. The mean Nc of 650 ± 240 cm−3 was 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.

Description: Size 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 Köhler 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.

Description: We evaluate the impact of transport from midlatitudes on aerosol number concentrations in the accumulation mode (light-scattering particles (LSP) with diameters 〉180 nm) in the Arctic during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) campaign. We focus on transport from the Asian continent. We find marked contrasts in the number concentration (NLSP), transport efficiency (TEN_LSP, the fraction transported from sources to the Arctic), size distribution, and the chemical composition of aerosols between air parcels from anthropogenic sources in East Asia (Asian AN) and biomass burning sources in Russia and Kazakhstan (Russian BB). Asian AN air had lower NLSP and TEN_LSP (25 cm−3 and 18% in spring and 6.2 cm−3 and 3.0% in summer) than Russian BB air (280 cm−3 and 97% in spring and 36 cm−3 and 7.6% in summer) due to more efficient wet scavenging during transport from East Asia. Russian BB in this spring is the most important source of accumulation-mode aerosols over the Arctic, and BB emissions are found to be the primary source of aerosols within all the data in spring during ARCTAS. On the other hand, the contribution of Asian AN transport had a negligible effect on the accumulation-mode aerosol number concentration in the Arctic during ARCTAS. Compared with background air, NLSP was 2.3–4.7 times greater for Russian BB air but 2.4–2.6 times less for Asian AN air in both spring and summer. This result shows that the transport of Asian AN air decreases aerosol number concentrations in the Arctic, despite the large emissions of aerosols in East Asia. The very low aerosol number concentrations in Asian AN air were caused by wet removal during vertical transport in association with warm conveyor belts (WCBs). Therefore, this cleansing effect will be prominent for air transported via WCBs from other midlatitude regions and seasons. The inflow of clean midlatitude air can potentially have an important impact on accumulation-mode aerosol number concentrations in the Arctic.

Description: Extensive measurements of black carbon (BC) aerosol were conducted in and near the North American Arctic during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) aircraft campaign in April and June–July 2008. We identify the pathways and mechanisms of transport of BC to the Arctic from the Asian continent using these data. The concentration, transport efficiency, and measured altitude of BC over the North American Arctic were highly dependent on season and origin of air parcels, e.g., biomass burning (BB) in Russia (Russian BB) and anthropogenic (AN) in East Asia (Asian AN). Russian BB air was mainly measured in the middle troposphere and caused maximum BC concentrations at this altitude in spring. The median BC concentration and transport efficiency of the Russian BB air were 270 ng m−3 (at STP) and 80% in spring and 20 ng m−3 and 4% in summer, respectively. Asian AN air was measured most frequently in the upper troposphere, with median values of 20 ng m−3 and 13% in spring and 5 ng m−3 and 0.8% in summer. These distinct differences are explained by differences in the transport mechanisms and accumulated precipitation along trajectories (APT), which is a measure of wet removal processes during transport. The transport of Russian BB air to the Arctic was nearly isentropic with slow ascent (low APT), while Asian AN air underwent strong uplift associated with warm conveyor belts (high APT). The APT values in summer were much larger than those in spring due to the increase in humidity in summer. These results show that the impact of BC emitted from AN sources in East Asia on the Arctic was very limited in both spring and summer. The BB emissions in Russia in spring are demonstrated to be the most important sources of BC transported to the North American Arctic.

Description: [1]  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 onboard 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 (b ext ) 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 non-spherical 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, and optical calculations from in situ data, and sky radiometer retrieval analyses are discussed.

Description: New particle formation (NPF) is one of the most important processes in controlling the concentrations of aerosols (condensation nuclei, CN) and cloud condensation nuclei (CCN) in the atmosphere. In this study, we introduce a new aerosol model representation with 20 size bins between 1 nm and 10 μm and activation-type and kinetic nucleation parameterizations into the WRF-chem model (called NPF-explicit WRF-chem). Model calculations were conducted in the Beijing region in China for the periods during the Campaign of Air Quality Research in Beijing and Surrounding Region 2006 (CARE-Beijing 2006) campaign conducted in August and September 2006. Model calculations successfully reproduced the timing of NPF and no-NPF days in the measurements (21 of 26 days). Model calculations also reproduced the subsequent rapid growth of new particles with a time scale of half a day. These results suggest that once a reasonable nucleation rate at a diameter of 1 nm is given, explicit calculations of condensation and coagulation processes can reproduce the clear contrast between NPF and no-NPF days as well as further growth up to several tens of nanometers. With this reasonable representation of the NPF process, we show that NPF contributed 20%–30% of the CN concentrations (〉10 nm in diameter) in and around Beijing on average. We also show that NPF increases CCN concentrations at higher supersaturations (S 〉 0.2%), while it decreases them at lower supersaturations (S 〈 0.1%). This is likely because NPF suppresses the increases in both the size and hygroscopicity of preexisting particles through the competition of condensable gases between new particles and preexisting particles. Sensitivity calculations show that a reduction of primary aerosol emissions, such as black carbon (BC), would not necessarily decrease CCN concentrations because of an increase in NPF. Sensitivity calculations also suggest that the reduction ratio of primary aerosol and SO2 emissions will be key in enhancing or damping the BC mitigation effect.

Description: The 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.

Description: 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 (CPCs) 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 ( d p ) larger than 10 nm (total CN 10 ), PM 0.17 -CN 10 ( d p ~10–130 nm), and SP2 black carbon ( N BC ; d p ~75–850 nm). Large increases in total CN 10 that were not associated with N BC were observed in the free troposphere, suggesting influences from new particle formation (NPF). Statistical characteristics of total CN 10 , PM 0.17 -CN 10 , and N BC 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 CN 10 and N BC , along with the ratio of PM 0.17 to total CN 10 and carbon monoxide mixing ratio (CO), is used to interpret the observed variability. The median concentrations of total CN 10 and PM 0.17 -CN 10 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 CN 10 in the UT to NPF. Possible mechanisms affecting NPF in the free troposphere are discussed.