Sub-micrometer particle size distributions measured during four summer cruises of the Swedish icebreaker Oden 1991, 1996, 2001, and 2008 were combined with dimethyl sulfide gas data, back ...trajectories, and daily maps of pack ice cover in order to investigate source areas and aerosol formation processes of the boundary layer aerosol in the central Arctic. With a clustering algorithm, potential aerosol source areas were explored. Clustering of particle size distributions together with back trajectories delineated five potential source regions and three different aerosol types that covered most of the Arctic Basin: marine, newly formed and aged particles over the pack ice. Most of the pack ice area with < 15% of open water under the trajectories exhibited the aged aerosol type with only one major mode around 40 nm. For newly formed particles to occur, two conditions had to be fulfilled over the pack ice: the air had spent 10 days while traveling over ever more contiguous ice and had traveled over less than 30% open water during the last 5 days. Additionally, the air had experienced more open water (at least twice as much as in the cases of aged aerosol) during the last 4 days before arrival in heavy ice conditions at Oden. Thus we hypothesize that these two conditions were essential factors for the formation of ultrafine particles over the central Arctic pack ice. In a comparison the Oden data with summer size distribution data from Alert, Nunavut, and Mt. Zeppelin, Spitsbergen, we confirmed the Oden findings with respect to particle sources over the central Arctic. Future more frequent broken-ice or open water patches in summer will spur biological activity in surface water promoting the formation of biological particles. Thereby low clouds and fogs and subsequently the surface energy balance and ice melt may be affected.
Nucleation experiments starting from the reaction of OH radicals with SO2 have been performed in the IfT-LFT flow tube under atmospheric conditions at 293±0.5 K for a relative humidity of 13-61%. The ...presence of different additives (H2 , CO, 1,3,5-trimethylbenzene) for adjusting the OH radical concentration and resulting OH levels in the range (4-300) ×105 molecule cm-3 did not influence the nucleation process itself. The number of detected particles as well as the threshold H2 SO4 concentration needed for nucleation was found to be strongly dependent on the counting efficiency of the used counting devices. High-sensitivity particle counters allowed the measurement of freshly nucleated particles with diameters down to about 1.5 nm. A parameterization of the experimental data was developed using power law equations for H2 SO4 and H2 O vapour. The exponent for H2 SO4 from different measurement series was in the range of 1.7-2.1 being in good agreement with those arising from analysis of nucleation events in the atmosphere. For increasing relative humidity, an increase of the particle number was observed. The exponent for H2 O vapour was found to be 3.1 representing an upper limit. Addition of 1.2×1011 molecule cm-3 or 1.2×1012 molecule cm-3 of NH3 (range of atmospheric NH3 peak concentrations) revealed that NH3 has a measureable, promoting effect on the nucleation rate under these conditions. The promoting effect was found to be more pronounced for relatively dry conditions, i.e. a rise of the particle number by 1-2 orders of magnitude at RH = 13% and only by a factor of 2-5 at RH = 47% (NH3 addition: 1.2×1012 molecule cm-3 ). Using the amine tert-butylamine instead of NH3 , the enhancing impact of the base for nucleation and particle growth appears to be stronger. Tert-butylamine addition of about 1010 molecule cm-3 at RH = 13% enhances particle formation by about two orders of magnitude, while for NH3 only a small or negligible effect on nucleation in this range of concentration appeared. This suggests that amines can strongly influence atmospheric H2 SO4 -H2 O nucleation and are probably promising candidates for explaining existing discrepancies between theory and observations.
In the course of global warming dramatic changes are taking place in the Arctic and boreal environments. However, physical aerosol data in from the central summer Arctic taken over the course of 18 ...yr from 1991 to 2008 do not show systematic year-to-year changes, albeit substantial interannual variations. Besides the limited extent of the data several causes may be responsible for these findings. The processes controlling concentrations and particle size distribution of the aerosol over the central Arctic perennial pack ice area, north of 80°, may not have changed substantially during this time. Environmental changes are still mainly effective in the marginal ice zone, the ice-free waters and continental rims and have not propagated significantly into the central Arctic yet where they could affect the local aerosol and its sources. The analysis of meteorological conditions of the four expedition summers reveal substantial variations which we see as main causes of the measured variations in aerosol parameters. With combined lognormal fits of the hourly number size distributions of the four expeditions representative mode parameters for the summer aerosol in the central Arctic have been calculated. The combined aerosol statistics discussed in the present paper provide comprehensive physical data on the summer aerosol in the central Arctic. These data are the only surface aerosol information from this region.
Siberia is one of few continental regions in the Northern Hemisphere where the atmosphere may sometimes approach pristine background conditions. We present the time series of aerosol and carbon ...monoxide (CO) measurements between September 2006 and December 2011 at the Zotino Tall Tower Observatory (ZOTTO) in Central Siberia (61° N; 89° E). We investigate the seasonal, weekly and diurnal variations of aerosol properties (including absorption and scattering coefficients and derived parameters, such as equivalent black carbon (BCe), Ångström exponent, single scattering albedo, and backscattering ratio) and the CO mixing ratios. Criteria were established to distinguish polluted from near-pristine air masses, providing quantitative characteristics for each type. Depending on the season, 23–36% of the sampling time at ZOTTO was found to be representative of a clean atmosphere. The summer pristine data indicate that primary biogenic and secondary organic aerosol formation are quite strong particle sources in the Siberian taiga. The summer seasons 2007–2008 were dominated by an Aitken mode around 80 nm size, whereas the summer 2009 with prevailing easterly winds produced particles in the accumulation mode around 200 nm size. We found these differences to be mainly related to air temperature, through its effect on the production rates of biogenic volatile organic compounds (VOC) precursor gases. In winter, the particle size distribution peaked at 160 nm, and the footprint of clean background air was characteristic for aged particles from anthropogenic sources at great distances from ZOTTO and diluted biofuel burning emissions from domestic heating. The wintertime polluted air originates mainly from large cities south and southwest of the site; these particles have a dominant mode around 100 nm, and the ΔBCe / ΔCO ratio of 7–11 ng m−3 ppb−1 suggests dominant contributions from coal and biofuel burning for heating. During summer, anthropogenic emissions are the dominant contributor to the pollution particles at ZOTTO, while only 12% of the polluted events are classified as biomass-burning-dominated, but then often associated with extremely high CO concentrations and aerosol absorption coefficients. Two biomass-burning case studies revealed different ΔBCe / ΔCO ratios from different fire types, with the agricultural fires in April~2008 yielding a very high ratio of 21 ng m−3 ppb−1. Overall, we find that anthropogenic sources dominate the aerosol population at ZOTTO most of the time, even during nominally clean episodes in winter, and that near-pristine conditions are encountered only in the growing season and then only episodically.
In a numerical study, influences of relative humidity (RH) on aerosol optical properties and direct radiative forcing in the surface boundary layer at Xinken (XK) in Pearl River Delta of China are ...investigated based on the observed particle hygroscopic growth (
f
g
(
D
p
,
RH
)
) and the relevant aerosol physical and chemical properties. The model is validated by comparing the simulated ambient extinction coefficients with those measured by a Raman LIDAR. At 550
nm, the scattering coefficient increases with a factor of 1.54 and 2.31 at a RH increase from 30% to 80% and 90%, respectively. This ratio is mainly controlled by
f
g
, and generally increases with increasing wavelength and decreases with increasing particle effective diameter. From RH 30% to 90%, the average single scattering albedo (
ω
0
) changes from 0.77 to 0.88, 0.91 to 0.97, and 0.86 to 0.94 for internal, respectively, external mixtures, and the retrieved mixing state of the elemental carbon (EC) at XK. The assumption that the absorption coefficient does not change upon humidification alone can introduce an overestimation of
ω
0
of 0.02 (
∼
5
%
) at RH 90% for an internal mixture. However, accounting for the EC mixing state at XK, this error is only about 0.3%. The mean upscatter fraction (
β
¯
) is evaluated as 0.233 at RH 30% with a 30% decrease to 0.191 at RH 90%. The estimation of aerosol direct radiative forcing (
Δ
F
R
) in the surface boundary layer at XK strongly depends on RH and the EC mixing state. The critical
ω
0
at XK is estimated to be about 0.77–0.80 at 550
nm. Assuming an internal or coated mixture of EC, no pronounced
Δ
F
R
is observed when
RH
<
60
%
, whereas a cooling effect arises while
RH
>
60
%
. Under the actual EC mixing conditions at XK, the effect of
Δ
F
R
is cooling. Over 40% of this cooling effect is contributed by water at RH 80% and
Δ
F
R
at RH 90% exceeds that at RH 30% by about a factor of 2.7.
Many of the next generation of global climate models will include aerosol schemes which explicitly simulate the microphysical processes that determine the particle size distribution. These models ...enable aerosol optical properties and cloud condensation nuclei (CCN) concentrations to be determined by fundamental aerosol processes, which should lead to a more physically based simulation of aerosol direct and indirect radiative forcings. This study examines the global variation in particle size distribution simulated by 12 global aerosol microphysics models to quantify model diversity and to identify any common biases against observations. Evaluation against size distribution measurements from a new European network of aerosol supersites shows that the mean model agrees quite well with the observations at many sites on the annual mean, but there are some seasonal biases common to many sites. In particular, at many of these European sites, the accumulation mode number concentration is biased low during winter and Aitken mode concentrations tend to be overestimated in winter and underestimated in summer. At high northern latitudes, the models strongly underpredict Aitken and accumulation particle concentrations compared to the measurements, consistent with previous studies that have highlighted the poor performance of global aerosol models in the Arctic. In the marine boundary layer, the models capture the observed meridional variation in the size distribution, which is dominated by the Aitken mode at high latitudes, with an increasing concentration of accumulation particles with decreasing latitude. Considering vertical profiles, the models reproduce the observed peak in total particle concentrations in the upper troposphere due to new particle formation, although modelled peak concentrations tend to be biased high over Europe. Overall, the multi-model-mean data set simulates the global variation of the particle size distribution with a good degree of skill, suggesting that most of the individual global aerosol microphysics models are performing well, although the large model diversity indicates that some models are in poor agreement with the observations. Further work is required to better constrain size-resolved primary and secondary particle number sources, and an improved understanding of nucleation and growth (e.g. the role of nitrate and secondary organics) will improve the fidelity of simulated particle size distributions.
Aerosol optical properties (AOPs) of sub-
10
μ
m
particles under dry conditions (relative humidity (RH)
<
20
%
) were investigated at Xinken in Pearl River Delta of China from 4 October to 5 November ...2004. Severe aerosol optical pollution has been found characterized by strongly light-absorbing particles. At 550
nm, the magnitude of the light scattering (
333
±
138
Mm
-
1
) and absorption (
70
±
42
Mm
-
1
), limited visual range (
5.3
±
2.5
km
), and low single scattering albedo (
0.83
±
0.05
) show Xinken to be comparable to the most polluted urban cores rather than even a polluted rural site. Other presented AOPs include hemispheric backscattering fraction (
11
±
1
%
) and asymmetry parameter (
0.67
±
0.01
) at 550
nm, and Ångström exponent (
a
˚
450
/
700
=
1.6
±
0.15
). Systematic relationships exist among the various AOPs. Their diurnal variations cannot be solely explained with the development of atmospheric boundary layer, but also obviously influenced by local wind patterns, variation of sources, photochemistry and nighttime heterogeneous chemistry. The fractional contributions to extinction, scattering and absorption (
f
ep
;
sp
;
ap
) due to sub-aerosol-populations and individual chemical compounds are derived by the Mie simulations. The sub-
1
μ
m
particles contribute more than 90% of the particle extinction (
σ
ep
,
550
nm
). Under dry conditions,
f
ep
of non-sea-salt sulfate, particulate organic matter, elemental carbon (EC) and residual are about 44%, 17%, 17% and 15%, respectively. However, the water uptake of particles can contribute 50–60% to
σ
ep
,
550
nm
at RH 90%. So in the ambient atmosphere, sulfate, carbonaceous material and water all play very important roles, concerning the visibility impairment at Xinken. The respective mass extinction, scattering and absorption efficiencies (
α
ep
;
sp
;
ap
) are mostly consistent with previously reported values for the polluted area.
α
ap
,
EC
,
550
nm
for sub-
1
μ
m
and sub-
10
μ
m
ranges are
9.3
±
1.4
and
7.2
±
1.0
m
2
g
-
1
, respectively. Our results imply a higher
α
ap
,
EC
for more internally mixed EC under the atmospheric conditions present at Xinken.
Every year, from December to April, anthropogenic haze spreads over most of the North Indian Ocean, and South and Southeast Asia. The Indian Ocean Experiment (INDOEX) documented this Indo‐Asian haze ...at scales ranging from individual particles to its contribution to the regional climate forcing. This study integrates the multiplatform observations (satellites, aircraft, ships, surface stations, and balloons) with one‐ and four‐dimensional models to derive the regional aerosol forcing resulting from the direct, the semidirect and the two indirect effects. The haze particles consisted of several inorganic and carbonaceous species, including absorbing black carbon clusters, fly ash, and mineral dust. The most striking result was the large loading of aerosols over most of the South Asian region and the North Indian Ocean. The January to March 1999 visible optical depths were about 0.5 over most of the continent and reached values as large as 0.2 over the equatorial Indian ocean due to long‐range transport. The aerosol layer extended as high as 3 km. Black carbon contributed about 14% to the fine particle mass and 11% to the visible optical depth. The single‐scattering albedo estimated by several independent methods was consistently around 0.9 both inland and over the open ocean. Anthropogenic sources contributed as much as 80% (±10%) to the aerosol loading and the optical depth. The in situ data, which clearly support the existence of the first indirect effect (increased aerosol concentration producing more cloud drops with smaller effective radii), are used to develop a composite indirect effect scheme. The Indo‐Asian aerosols impact the radiative forcing through a complex set of heating (positive forcing) and cooling (negative forcing) processes. Clouds and black carbon emerge as the major players. The dominant factor, however, is the large negative forcing (‐20±4 W m−2) at the surface and the comparably large atmospheric heating. Regionally, the absorbing haze decreased the surface solar radiation by an amount comparable to 50% of the total ocean heat flux and nearly doubled the lower tropospheric solar heating. We demonstrate with a general circulation model how this additional heating significantly perturbs the tropical rainfall patterns and the hydrological cycle with implications to global climate.
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