Transport Of Anthropogenic Aerosols Research Articles

Size-resolved mixing state of black carbon in the Canadian high Arctic and implications for simulated direct radiative effect

Abstract. Transport of anthropogenic aerosol into the Arctic in the spring months has the potential to affect regional climate; however, modeling estimates of the aerosol direct radiative effect (DRE) are sensitive to uncertainties in the mixing state of black carbon (BC). A common approach in previous modeling studies is to assume an entirely external mixture (all primarily scattering species are in separate particles from BC) or internal mixture (all primarily scattering species are mixed in the same particles as BC). To provide constraints on the size-resolved mixing state of BC, we use airborne single-particle soot photometer (SP2) and ultrahigh-sensitivity aerosol spectrometer (UHSAS) measurements from the Alfred Wegener Institute (AWI) Polar 6 flights from the NETCARE/PAMARCMIP2015 campaign to estimate coating thickness as a function of refractory BC (rBC) core diameter and the fraction of particles containing rBC in the springtime Canadian high Arctic. For rBC core diameters in the range of 140 to 220 nm, we find average coating thicknesses of approximately 45 to 40 nm, respectively, resulting in ratios of total particle diameter to rBC core diameters ranging from 1.6 to 1.4. For total particle diameters ranging from 175 to 730 nm, rBC-containing particle number fractions range from 16 % to 3 %, respectively. We combine the observed mixing-state constraints with simulated size-resolved aerosol mass and number distributions from GEOS-Chem–TOMAS to estimate the DRE with observed bounds on mixing state as opposed to assuming an entirely external or internal mixture. We find that the pan-Arctic average springtime DRE ranges from −1.65 to −1.34 W m−2 when assuming entirely externally or internally mixed BC. This range in DRE is reduced by over a factor of 2 (−1.59 to −1.45 W m−2) when using the observed mixing-state constraints. The difference in DRE between the two observed mixing-state constraints is due to an underestimation of BC mass fraction in the springtime Arctic in GEOS-Chem–TOMAS compared to Polar 6 observations. Measurements of mixing state provide important constraints for model estimates of DRE.

High Sensitivity of Arctic Liquid Clouds to Long‐Range Anthropogenic Aerosol Transport

AbstractThe rate of warming in the Arctic depends upon the response of low‐level microphysical and radiative cloud properties to aerosols advected from distant anthropogenic and biomass‐burning sources. Cloud droplet cross‐section density increases with higher concentrations of cloud condensation nuclei, leading to an increase of cloud droplet absorption and scattering radiative cross sections. The challenge of assessing the magnitude of the effect has been decoupling the aerosol impacts on clouds from how clouds change solely due to natural meteorological variability. Here we address this issue with large, multi‐year satellite, meteorological, and tracer transport model data sets to show that the response of low‐level clouds in the Arctic to anthropogenic aerosols lies close to a theoretical maximum and is between 2 and 8 times higher than has been observed elsewhere. However, a previously described response of arctic clouds to biomass‐burning plumes appears to be overstated because the interactions are rare and modification of cloud radiative properties appears better explained by coincident changes in temperature, humidity, and atmospheric stability.

Influence of biomass burning from South Asia at a high-altitude mountain receptor site in China

Abstract. Highly time-resolved in situ measurements of airborne particles were conducted at Mt. Yulong (3410 m above sea level) on the southeastern edge of the Tibetan Plateau in China from 22 March to 14 April 2015. The detailed chemical composition was measured by a high-resolution time-of-flight aerosol mass spectrometer together with other online instruments. The average mass concentration of the submicron particles (PM1) was 5.7 ± 5.4 µg m−3 during the field campaign, ranging from 0.1 up to 33.3 µg m−3. Organic aerosol (OA) was the dominant component in PM1, with a fraction of 68 %. Three OA factors, i.e., biomass burning organic aerosol (BBOA), biomass-burning-influenced oxygenated organic aerosol (OOA-BB) and oxygenated organic aerosol (OOA), were resolved using positive matrix factorization analysis. The two oxygenated OA factors accounted for 87 % of the total OA mass. Three biomass burning events were identified by examining the enhancement of black carbon concentrations and the f60 (the ratio of the signal at m∕z 60 from the mass spectrum to the total signal of OA). Back trajectories of air masses and satellite fire map data were integrated to identify the biomass burning locations and pollutant transport. The western air masses from South Asia with active biomass burning activities transported large amounts of air pollutants, resulting in elevated organic concentrations up to 4-fold higher than those of the background conditions. This study at Mt. Yulong characterizes the tropospheric background aerosols of the Tibetan Plateau during pre-monsoon season and provides clear evidence that the southeastern edge of the Tibetan Plateau was affected by the transport of anthropogenic aerosols from South Asia.

A five-year observation of atmospheric metals on Ulleung Island in the East/Japan Sea: Temporal variability and source identification

We investigated seasonal characteristics (concentration, enrichment factors, and correlation coefficients) of chemical constituents, including sources for sea salt-corrected metals (Al, Fe, K, Ca, Mg, S, As, Cd, Co, Cu, Li, Mn, Mo, Ni, Pb, Sn, Sr, Ti, V, and Zn), in 334 aerosol samples collected during October 2003–October 2008 at Ulleung Island in the southern East/Japan Sea. High Al concentrations were found in spring (geometric means of 1.23 and 1.28 μg m−3 in March and April, respectively) due to Asian dust (yellow sand) originating from northeastern China. The dust mineral transport by strong winds resulted in a change of metal composition showing soil-dominated condition. In the rainy period (June–September), the aerosol metal concentrations and composition were influenced largely by wet deposition and the summer monsoon, together with anthropogenic aerosol transport. The correlation coefficients in the summer period (July and August) showed a positive correlation of Al with K (r = 0.74) and As (r = 0.63), probably reflecting anthropogenic-originated Al coming from coal burning. In autumn (October and November) and winter (December and January), mixed sources of soil and anthropogenic-originated aerosols were pronounced by the development of northwesterly winds from the Asian continent. Especially, the soil-originated proportion was relatively higher in autumn than in winter when the anthropogenic-originated aerosols from heating fuels predominated. Anthropogenic As, Sn, Mo, Zn, Pb, and Cd between low- and high-dust periods that had similar air–mass movements in spring were at similar concentrations, but higher in the air masses including eastern China trajectories and/or slow movement at lower height during high-dust periods.

Chemical composition of PM<sub>10</sub> and PM<sub>1</sub> at the high-altitude Himalayan station Nepal Climate Observatory-Pyramid (NCO-P) (5079 m a.s.l.)

Abstract. We report chemical composition data for PM10 and PM1 from the Nepal Climate Observatory-Pyramid (NCO-P), the world's highest aerosol observatory, located at 5079 m a.s.l. at the foothills of Mt. Everest. Despite its high altitude, the average PM10 mass apportioned by the chemical analyses is of the order of 6 μg m−3 (i.e., 10 μg/scm), with almost a half of this mass accounted for by organic matter, elemental carbon (EC) and inorganic ions, the rest being mineral dust. Organic matter, in particular, accounted for by 2.0 μg m−3 (i.e., 3.6 μg/scm) on a yearly basis, and it is by far the major PM10 component beside mineral oxides. Non-negligible concentrations of EC were also observed (0.36 μg/scm), confirming that light-absorbing aerosol produced from combustion sources can be efficiently transported up the altitudes of Himalayan glaciers. The concentrations of carbonaceous and ionic aerosols follow a common time trend with a maximum in the premonsoon season, a minimum during the monsoon and a slow recovery during the postmonsoon and dry seasons, which is the same phenomenology observed for other Nepalese Himalayan sites in previous studies. Such seasonal cycle can be explained by the seasonal variations of dry and moist convection and of wet scavenging processes characterizing the climate of north Indian subcontinent. We document the effect of orographic transport of carbonaceous and sulphate particles upslope the Himalayas, showing that the valley breeze circulation, which is almost permanently active during the out-of-monsoon season, greatly impacts the chemical composition of PM10 and PM1 in the high Himalayas and provides an efficient mechanism for bringing anthropogenic aerosols into the Asian upper troposphere (>5000 m a.s.l.). The concentrations of mineral dust are impacted to a smaller extent by valley breezes and follow a unique seasonal cycle which suggest multiple source areas in central and south-west Asia. Our findings, based on two years of observations of the aerosol chemical composition, provide clear evidence that the southern side of the high Himalayas is impacted by transport of anthropogenic aerosols which constitute the Asian brown cloud.

Changes in the atmospheric circulation and environmental pollution in Siberia from the industrial regions of Norilsk and the Urals in the early 21st century

Changes in the anthropogenic impact from two large Russian industrial regions (Norilsk and the Urals) on the environment of Siberia due to the transformation of the atmospheric circulation in the early 2000s are assessed. Five-day trajectories of air mass transport for every day of January, April, July, and October in 1981–2007 calculated by the HYSPLIT-4 model with the NOAA reanalysis database (NCEP/NCAR Reanalysis Data Files) are used to analyze the transport of anthropogenic aerosols. Rosgidromet (Russian Hydrology and Meteorology Center) information on atmospheric emissions from the regions under study is used. Seasonal and long-term variations in the average concentrations of anthropogenic heavy metals (Pb, Ni, and Cu) from those sources in near-surface air and the average flows of pollutants onto the surface were estimated for different Siberian sites. Differences between the 2000s and the 1990s are characterized by various mosaic spatial patterns in different seasons. From the viewpoint of pollution of surface environments and objects, the effect from the transformation of the atmospheric circulation may be comparable (but opposite) with that from changes in source emissions, which decreased in the studied decades. Annual depositions of heavy metals (only from the two sources) onto the catchment area of every large Siberian river (Ob, Yenisei, and Lena) are comparable with the flows of these metals in the river run-off.

Transport of anthropogenic aerosols from Asia and subsequent chemical transformation

Aerosol chemical composition and gaseous species were measured at Cape Hedo, Okinawa, Japan (CHO), during the ABC/EAREX project period in March 2005, using high‐time‐resolution instruments including an Aerodyne quadrupole aerosol mass spectrometer (Q‐AMS), a tapered element oscillating microbalance (TEOM), and gas monitors in order to investigate the transport and subsequent chemical transformation of aerosol in the east Asian region. Sulfate was the dominant species in fine aerosol mode and the average concentration of ammonium, sulfate and organics was 1.25, 6.37 and 2.16 μg m−3, respectively. The sulfate concentration observed at CHO in 2005 was about 1.5–2 times higher than that in 1994. For the majority of high‐sulfate observations, the air mass was transported from the central east Chinese region (between Shanghai and the Shandong Peninsula). Sulfate transport was intermittent and strongly correlated with the passage of synoptic‐scale high‐/low‐pressure systems. Chemical components and their concentration showed significant change on 17–18 March, which is accounted for by the change in air mass origin and the synoptic‐scale weather system. In addition, it is suggested that the difference of air quality at the air mass origin reflects the chemical composition at CHO. The high sulfate concentration required heterogeneous conversion of SO2 to sulfate at a rate of 2.0% h−1. A pronounced signal at m/z = 44 confirmed that organic compounds were oxidized.

Atmospheric aerosol over Alaska: 1. Spatial and seasonal variability

Aerosol chemical composition data for seven National Park Service locations in Alaska for the period from 1986 to 1995 were analyzed. Data for elements, black carbon, and total mass were obtained for samples of atmospheric particles less than 2.5 μm in diameter. Assuming that all elemental aerosol S was from SO4=, concentrations of excess (nonmarine) sulfate (XSO4=) were calculated by subtraction of the sea salt SO4= mass fraction to Na and observed Na concentrations. Mass concentration of XSO4= ranged from 0.01 to 3.9 μg m−3, and mass concentration of black carbon (BC) ranged from 0.01 to 5 μg m−3. Seasonal variations with maxima during the winter/spring season and minima in the summer for most of the sites in Alaska were observed for the XSO4= but not for the BC and the fine mass concentration. The seasonal variations of XSO4= concentration were always higher in northwestern Alaska. BC concentration in central Alaska peaked during the summertime because of the high BC emission from forest fires. The Pb concentration in central Alaska shows a negative trend over the timescale of measurements. It is concluded that all of the sites experience aerosol pollution from long distance, regional, and local sources. Seasonal variations of the XSO4= concentration are related to the seasonal variations in long‐range transport of midlatitudinal S emissions, to the varying rate of photochemical SO4= fine aerosol production and to fine aerosol removal processes in the troposphere. The spatial distribution of XSO4= shows a strong northwest to southeast negative gradient that can be associated with the long‐range transport of anthropogenic aerosol from pollution sources located to the north or northwest of Alaska.