Black carbon (BC) and light-absorbing organic carbon (brown carbon, BrC) play key roles in warming the atmosphere, but the magnitude of their effects remains highly uncertain. Theoretical modelling ...and laboratory experiments demonstrate that coatings on BC can enhance BC's light absorption, therefore many climate models simply assume enhanced BC absorption by a factor of ∼1.5. However, recent field observations show negligible absorption enhancement, implying models may overestimate BC's warming. Here we report direct evidence of substantial field-measured BC absorption enhancement, with the magnitude strongly depending on BC coating amount. Increases in BC coating result from a combination of changing sources and photochemical aging processes. When the influence of BrC is accounted for, observationally constrained model calculations of the BC absorption enhancement can be reconciled with the observations. We conclude that the influence of coatings on BC absorption should be treated as a source and regionally specific parameter in climate models.
The reversible partitioning of glyoxal was studied in simulation chamber experiments for the first time by time-resolved measurements of gas-phase and particle-phase concentrations in ...sulfate-containing aerosols. Two complementary methods for the measurement of glyoxal particle-phase concentrations are compared: (1) an offline method utilizing filter sampling of chamber aerosols followed by HPLC-MS/MS analysis and (2) positive matrix factorization (PMF) analysis of aerosol mass spectrometer (AMS) data. Ammonium sulfate (AS) and internally mixed ammonium sulfate/fulvic acid (AS/FA) seed aerosols both show an exponential increase of effective Henry’s law coefficients (K H,eff) with AS concentration (c AS, in mol kg–1 aerosol liquid water, m = molality) and sulfate ionic strength, I(SO4 2–) (m). A modified Setschenow plot confirmed that “salting-in” of glyoxal is responsible for the increased partitioning. The salting constant for glyoxal in AS is K S CHOCHO = (−0.24 ± 0.02) m –1, and found to be independent of the presence of FA. The reversible glyoxal uptake can be described by two distinct reservoirs for monomers and higher molecular weight species filling up at characteristic time constants. These time constants are τ1 ≈ 102 s and τ2 ≈ 104 s at c AS < 12 m, and about 1–2 orders of magnitude slower at higher c AS, suggesting that glyoxal uptake is kinetically limited at high salt concentrations.
Gasoline direct injection (GDI) vehicles have recently been identified as a
significant source of carbonaceous aerosol, of both primary and secondary
origin. Here we investigated primary emissions ...and secondary organic aerosol
(SOA) from four GDI vehicles, two of which were also retrofitted with a
prototype gasoline particulate filter (GPF). We studied two driving test
cycles under cold- and hot-engine conditions. Emissions were characterized by
proton transfer reaction time-of-flight mass spectrometry (gaseous
non-methane organic compounds, NMOCs), aerosol mass spectrometry (sub-micron
non-refractory particles) and light attenuation measurements (equivalent
black carbon (eBC) determination using Aethalometers) together with
supporting instrumentation. Atmospheric processing was simulated using the
PSI mobile smog chamber (SC) and the potential aerosol mass oxidation flow
reactor (OFR). Overall, primary and secondary particulate matter (PM) and
NMOC emissions were dominated by the engine cold start, i.e., before thermal
activation of the catalytic after-treatment system. Trends in the
SOA oxygen to carbon ratio (O : C) for OFR and SC were
related to different OH exposures, but divergences in the H : C remained
unexplained. SOA yields agreed within experimental variability between the
two systems, with a tendency for higher values in the OFR than in the SC (or,
vice versa, lower values in the SC). A few aromatic compounds dominated the
NMOC emissions, primarily benzene, toluene, xylene isomers/ethylbenzene and
C3-benzene. A significant fraction of the SOA was explained by those
compounds, based on comparison of effective SOA yield curves with those of
toluene, o-xylene and 1,2,4-trimethylbenzene determined in our OFR, as well
as others from literature. Remaining discrepancies, which were smaller in the
SC and larger in the OFR, were up to a factor of 2 and may have resulted from
diverse reasons including unaccounted precursors and matrix effects. GPF
retrofitting significantly reduced primary PM through removal of refractory
eBC and partially removed the minor POA fraction. At cold-started conditions
it did not affect hydrocarbon emission factors, relative chemical composition
of NMOCs or SOA formation, and likewise SOA yields and bulk composition
remained unaffected. GPF-induced effects at hot-engine conditions
deserve attention in further studies.
Particulate matter (PM) pollution is a severe environmental problem in the Beijing–Tianjin–Hebei (BTH) region in North
China. PM studies have been conducted extensively in Beijing, but the
chemical ...composition, sources, and atmospheric processes of PM are still
relatively less known in nearby Tianjin and Hebei. In this study, fine PM
in urban Shijiazhuang (the capital of Hebei Province) was characterized using
an Aerodyne quadrupole aerosol chemical speciation monitor (Q-ACSM) from
11 January to 18 February in 2014. The average mass concentration of
non-refractory submicron PM (diameter <1 µm, NR-PM1) was
178±101 µg m−3, and it was composed of 50 % organic aerosol
(OA), 21 % sulfate, 12 % nitrate, 11 % ammonium, and 6 % chloride.
Using the multilinear engine (ME-2) receptor model, five OA sources were
identified and quantified, including hydrocarbon-like OA from vehicle
emissions (HOA, 13 %), cooking OA (COA, 16 %), biomass burning OA (BBOA,
17 %), coal combustion OA (CCOA, 27 %), and oxygenated OA (OOA, 27 %).
We found that secondary formation contributed substantially to PM in episodic
events, whereas primary emissions were dominant (most significant) on average.
The episodic events with the highest NR-PM1 mass range of
300–360 µg m−3 were comprised of 55 % of secondary species. On the
contrary, a campaign-average low OOA fraction (27 %) in OA indicated the
importance of primary emissions, and a low sulfur oxidation degree
(FSO4) of 0.18 even at RH >90 % hinted at insufficient
oxidation. These results suggested that in Shijiazhuang in wintertime fine PM
was mostly from primary emissions without sufficient atmospheric aging,
indicating opportunities for air quality improvement by mitigating direct
emissions. In addition, secondary inorganic and organic (OOA) species
dominated in pollution events with high-RH conditions, most likely due to
enhanced aqueous-phase chemistry, whereas primary organic aerosol (POA)
dominated in pollution events with low-RH and stagnant conditions. These
results also highlighted the importance of meteorological conditions for PM
pollution in this highly polluted city in North China.
Highly oxidized multifunctional compounds (HOMs) have been demonstrated to be important for atmospheric secondary organic aerosols (SOA) and new-particle formation (NPF), yet it remains unclear which ...the main atmospheric HOM formation pathways are. In this study, a nitrate-ion-based chemical ionization atmospheric-pressure-interface time-of-flight mass spectrometer (CI-APi-TOF) was deployed to measure HOMs in the boreal forest in Hyytiälä, southern Finland. Positive matrix factorization (PMF) was applied to separate the detected HOM species into several factors, relating these “factors” to plausible formation pathways. PMF was performed with a revised error estimation derived from laboratory data, which agrees well with an estimate based on ambient data. Three factors explained the majority (> 95 %) of the data variation, but the optimal solution found six factors, including two nighttime factors, three daytime factors, and a transport factor. One nighttime factor is almost identical to laboratory spectra generated from monoterpene ozonolysis, while the second likely represents monoterpene oxidation initiated by NO3. The exact chemical processes forming the different daytime factors remain unclear, but they all have clearly distinct diurnal profiles, very likely related to monoterpene oxidation with a strong influence from NO, presumably through its effect on peroxy radical (RO2) chemistry. Apart from these five “local” factors, the sixth factor is interpreted as a transport related factor. These findings improve our understanding of HOM production by confirming current knowledge and inspiring future research directions and provide new perspectives on using factorization methods to understand short-lived atmospheric species.
Water-soluble organic carbon (WSOC) is a large fraction of
organic aerosols (OA) globally and has significant impacts on climate and
human health. The sources of WSOC remain very uncertain in ...polluted regions.
Here we present a quantitative source apportionment of WSOC, isolated from
aerosols in China using radiocarbon (14C) and offline high-resolution
time-of-flight aerosol mass spectrometer measurements. Fossil emissions on
average accounted for 32–47 % of WSOC. Secondary organic carbon (SOC)
dominated both the non-fossil and fossil derived WSOC, highlighting the
importance of secondary formation to WSOC in severe winter haze episodes.
Contributions from fossil emissions to SOC were 61 ± 4 and
50 ± 9 % in Shanghai and Beijing, respectively, significantly
larger than those in Guangzhou (36 ± 9 %) and Xi'an
(26 ± 9 %). The most important primary sources were biomass burning
emissions, contributing 17–26 % of WSOC. The remaining primary sources
such as coal combustion, cooking and traffic were generally very small but
not negligible contributors, as coal combustion contribution could exceed
10 %. Taken together with earlier 14C source apportionment studies
in urban, rural, semi-urban and background regions in Asia, Europe and the USA,
we demonstrated a dominant contribution of non-fossil emissions (i.e.,
75 ± 11 %) to WSOC aerosols in the Northern Hemisphere; however, the
fossil fraction is substantially larger in aerosols from East Asia and the
eastern Asian pollution outflow, especially during winter, due to increasing coal
combustion. Inclusion of our findings can improve a modelling of effects of
WSOC aerosols on climate, atmospheric chemistry and public health.
Air quality measures that were implemented in Europe in the 1990s resulted in reductions of ozone precursor concentrations.
In this study, the effect of these reductions on ozone is investigated by ...analyzing surface measurements of this pollutant for the time period between 2000 and 2015.
Using a nonparametric timescale decomposition methodology, the long-term, seasonal and short-term variation in ozone observations were extracted.
A clustering algorithm was applied to the different timescale variations, leading to a classification of sites across Europe based on the temporal characteristics of ozone.
The clustering based on the long-term variation resulted in a site-type classification, while a regional classification was obtained based on the seasonal and short-term variations.
Long-term trends of deseasonalized mean and meteo-adjusted peak ozone concentrations were calculated across large parts of Europe for the time period 2000–2015.
A multidimensional scheme was used for a detailed trend analysis, based on the identified clusters, which reflect precursor emissions and meteorological influence either on the inter-annual or the short-term timescale.
Decreasing mean ozone concentrations at rural sites and increasing or stabilizing at urban sites were observed.
At the same time, downward trends for peak ozone concentrations were detected for all site types.
In addition, a reduction of the amplitude in the seasonal cycle of ozone and a shift in the occurrence of the seasonal maximum towards earlier time of the year were observed.
Finally, a reduced sensitivity of ozone to temperature was identified.
It was concluded that long-term trends of mean and peak ozone concentrations are mostly controlled by precursor emissions changes, while seasonal cycle trends and changes in the sensitivity of ozone to temperature are among other factors driven by regional climatic conditions.
Understanding the sources of light-absorbing organic (brown) carbon (BrC) and its interaction with black carbon (BC) and other non-refractory particulate matter (NR-PM) fractions is important for ...reducing uncertainties in the aerosol direct radiative forcing. In this study, we combine multiple filter-based techniques to achieve long-term, spectrally resolved, source- and species-specific atmospheric absorption closure. We determine the mass absorption efficiency (MAE) in dilute bulk solutions at 370 nm to be equal to 1.4 m2 g−1 for fresh biomass smoke, 0.7 m2 g−1 for winter-oxygenated organic aerosol (OA), and 0.13 m2 g−1 for other less absorbing OA. We apply Mie calculations to estimate the contributions of these fractions to total aerosol absorption. While enhanced absorption in the near-UV has been traditionally attributed to primary biomass smoke, here we show that anthropogenic oxygenated OA may be equally important for BrC absorption during winter, especially at an urban background site. We demonstrate that insoluble tar balls are negligible in residential biomass burning atmospheric samples of this study and thus could attribute the totality of the NR-PM absorption at shorter wavelengths to methanol-extractable BrC. As for BC, we show that the mass absorption cross-section (MAC) of this fraction is independent of its source, while we observe evidence for a filter-based lensing effect associated with the presence of NR-PM components. We find that bare BC has a MAC of 6.3 m2 g−1 at 660 nm and an absorption Ångström exponent of 0.93 ± 0.16, while in the presence of coatings its absorption is enhanced by a factor of ∼ 1.4. Based on Mie calculations of closure between observed and predicted total light absorption, we provide an indication for a suppression of the filter-based lensing effect by BrC. The total absorption reduction remains modest, ∼ 10 %–20 % at 370 nm, and is restricted to shorter wavelengths, where BrC absorption is significant. Overall, our results allow an assessment of the relative importance of the different aerosol fractions to the total absorption for aerosols from a wide range of sources and atmospheric ages. When integrated with the solar spectrum at 300–900 nm, bare BC is found to contribute around two-thirds of the solar radiation absorption by total carbonaceous aerosols, amplified by the filter-based lensing effect (with an interquartile range, IQR, of 8 %–27 %), while the IQR of the contributions by particulate BrC is 6 %–13 % (13 %–20 % at the rural site during winter). Future studies that will directly benefit from these results include (a) optical modelling aiming at understanding the absorption profiles of a complex aerosol composed of BrC, BC and lensing-inducing coatings; (b) source apportionment aiming at understanding the sources of BC and BrC from the aerosol absorption profiles; (c) global modelling aiming at quantifying the most important aerosol absorbers.
The EURODELTA III exercise has facilitated a comprehensive intercomparison and evaluation of chemistry transport model performances. Participating models performed calculations for four 1-month ...periods in different seasons in the years 2006 to 2009, allowing the influence of different meteorological conditions on model performances to be evaluated. The exercise was performed with strict requirements for the input data, with few exceptions. As a consequence, most of differences in the outputs will be attributed to the differences in model formulations of chemical and physical processes. The models were evaluated mainly for background rural stations in Europe. The performance was assessed in terms of bias, root mean square error and correlation with respect to the concentrations of air pollutants (NO2, O3, SO2, PM10 and PM2.5), as well as key meteorological variables. Though most of meteorological parameters were prescribed, some variables like the planetary boundary layer (PBL) height and the vertical diffusion coefficient were derived in the model preprocessors and can partly explain the spread in model results. In general, the daytime PBL height is underestimated by all models. The largest variability of predicted PBL is observed over the ocean and seas. For ozone, this study shows the importance of proper boundary conditions for accurate model calculations and then on the regime of the gas and particle chemistry. The models show similar and quite good performance for nitrogen dioxide, whereas they struggle to accurately reproduce measured sulfur dioxide concentrations (for which the agreement with observations is the poorest). In general, the models provide a close-to-observations map of particulate matter (PM2.5 and PM10) concentrations over Europe rather with correlations in the range 0.4–0.7 and a systematic underestimation reaching −10 µg m−3 for PM10. The highest concentrations are much more underestimated, particularly in wintertime. Further evaluation of the mean diurnal cycles of PM reveals a general model tendency to overestimate the effect of the PBL height rise on PM levels in the morning, while the intensity of afternoon chemistry leads formation of secondary species to be underestimated. This results in larger modelled PM diurnal variations than the observations for all seasons. The models tend to be too sensitive to the daily variation of the PBL. All in all, in most cases model performances are more influenced by the model setup than the season. The good representation of temporal evolution of wind speed is the most responsible for models' skillfulness in reproducing the daily variability of pollutant concentrations (e.g. the development of peak episodes), while the reconstruction of the PBL diurnal cycle seems to play a larger role in driving the corresponding pollutant diurnal cycle and hence determines the presence of systematic positive and negative biases detectable on daily basis.
Organic gases undergoing conversion to form secondary organic aerosol (SOA) during atmospheric aging are largely unidentified, particularly in regions influenced by anthropogenic emissions. SOA ...dominates the atmospheric organic aerosol burden and this knowledge gap contributes to uncertainties in aerosol effects on climate and human health. Here we characterize primary and aged emissions from residential wood combustion using high resolution mass spectrometry to identify SOA precursors. We determine that SOA precursors traditionally included in models account for only ~3-27% of the observed SOA, whereas for the first time we explain ~84-116% of the SOA by inclusion of non-traditional precursors. Although hundreds of organic gases are emitted during wood combustion, SOA is dominated by the aging products of only 22 compounds. In some cases, oxidation products of phenol, naphthalene and benzene alone comprise up to ~80% of the observed SOA. Identifying the main precursors responsible for SOA formation enables improved model parameterizations and SOA mitigation strategies in regions impacted by residential wood combustion, more productive targets for ambient monitoring programs and future laboratories studies, and links between direct emissions and SOA impacts on climate and health in these regions.
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