Aerosol particles cool the climate by scattering solar radiation and by acting as cloud condensation nuclei. Higher temperatures resulting from increased greenhouse gas levels have been suggested to ...lead to increased biogenic secondary organic aerosol and cloud condensation nuclei concentrations creating a negative climate feedback mechanism. Here, we present direct observations on this feedback mechanism utilizing collocated long term aerosol chemical composition measurements and remote sensing observations on aerosol and cloud properties. Summer time organic aerosol loadings showed a clear increase with temperature, with simultaneous increase in cloud condensation nuclei concentration in a boreal forest environment. Remote sensing observations revealed a change in cloud properties with an increase in cloud reflectivity in concert with increasing organic aerosol loadings in the area. The results provide direct observational evidence on the significance of this negative climate feedback mechanism.
Over Boreal regions, monoterpenes emitted from the forest are the main precursors for secondary organic aerosol (SOA) formation and the primary driver of the growth of new aerosol particles to ...climatically important cloud condensation nuclei (CCN). Autoxidation of monoterpenes leads to rapid formation of Highly Oxygenated organic Molecules (HOM). We have developed the first model with near-explicit representation of atmospheric new particle formation (NPF) and HOM formation. The model can reproduce the observed NPF, HOM gas-phase composition and SOA formation over the Boreal forest. During the spring, HOM SOA formation increases the CCN concentration by ~10 % and causes a direct aerosol radiative forcing of -0.10 W/m
. In contrast, NPF reduces the number of CCN at updraft velocities < 0.2 m/s, and causes a direct aerosol radiative forcing of +0.15 W/m
. Hence, while HOM SOA contributes to climate cooling, NPF can result in climate warming over the Boreal forest.
Atmospheric gas-to-particle conversion is a crucial or even dominant contributor to haze formation in Chinese megacities in terms of aerosol number, surface area and mass. Based on our comprehensive ...observations in Beijing during 15 January 2018-31 March 2019, we are able to show that 80-90% of the aerosol mass (PM
2.5
) was formed
via
atmospheric reactions during the haze days and over 65% of the number concentration of haze particles resulted from new particle formation (NPF). Furthermore, the haze formation was faster when the subsequent growth of newly formed particles was enhanced. Our findings suggest that in practice almost all present-day haze episodes originate from NPF, mainly since the direct emission of primary particles in Beijing has considerably decreased during recent years. We also show that reducing the subsequent growth rate of freshly formed particles by a factor of 3-5 would delay the buildup of haze episodes by 1-3 days. Actually, this delay would decrease the length of each haze episode, so that the number of annual haze days could be approximately halved. Such improvement in air quality can be achieved with targeted reduction of gas-phase precursors for NPF, mainly dimethyl amine and ammonia, and further reductions of SO
2
emissions. Furthermore, reduction of anthropogenic organic and inorganic precursor emissions would slow down the growth rate of newly-formed particles and consequently reduce the haze formation.
Based on our comprehensive observations in Beijing, we show that 80-90% of PM
2.5
was formed
via
atmospheric reactions during haze days and over 65% of the number concentration of haze particles resulted from new particle formation.
Although secondary particulate matter is reported to be the main
contributor of PM2.5 during haze in Chinese megacities,
primary particle emissions also affect particle concentrations. In
order to ...improve estimates of the contribution of primary sources to
the particle number and mass concentrations, we performed source
apportionment analyses using both chemical fingerprints and particle
size distributions measured at the same site in urban Beijing from
April to July 2018. Both methods resolved factors related to primary
emissions, including vehicular emissions and cooking emissions, which
together make up 76 % and 24 % of total particle number and
organic aerosol (OA) mass, respectively. Similar source types,
including particles related to vehicular emissions (1.6±1.1 µg m−3; 2.4±1.8×103 cm−3 and 5.5±2.8×103 cm−3
for two traffic-related components), cooking emissions (2.6±1.9 µg m−3 and 5.5±3.3×103 cm−3) and secondary aerosols (51±41 µg m−3 and 4.2±3.0×103 cm−3), were resolved by both methods. Converted mass
concentrations from particle size distributions components were
comparable with those from chemical fingerprints. Size distribution
source apportionment separated vehicular emissions into a component
with a mode diameter of 20 nm (“traffic-ultrafine”) and a
component with a mode diameter of 100 nm
(“traffic-fine”). Consistent with similar day- and nighttime diesel
vehicle PM2.5 emissions estimated for the Beijing area,
traffic-fine particles, hydrocarbon-like OA (HOA, traffic-related factor
resulting from source apportionment using chemical fingerprints) and
black carbon (BC) showed similar diurnal patterns, with higher
concentrations during the night and morning than during the afternoon
when the boundary layer is higher. Traffic-ultrafine particles showed
the highest concentrations during the rush-hour period, suggesting a
prominent role of local gasoline vehicle emissions. In the absence of
new particle formation, our results show that vehicular-related
emissions (14 % and 30 % for ultrafine and fine particles,
respectively) and cooking-activity-related emissions (32 %)
dominate the particle number concentration, while secondary particulate
matter (over 80 %) governs PM2.5 mass during the
non-heating season in Beijing.
Natural aerosol feedbacks are expected to become more important in the future, as anthropogenic aerosol emissions decrease due to air quality policy. One such feedback is initiated by the increase in ...biogenic volatile organic compound (BVOC) emissions with higher temperatures, leading to higher secondary organic aerosol (SOA) production and a cooling of the surface via impacts on cloud radiative properties. Motivated by the considerable spread in feedback strength in Earth System Models (ESMs), we here use two long-term observational datasets from boreal and tropical forests, together with satellite data, for a process-based evaluation of the BVOC-aerosol-cloud feedback in four ESMs. The model evaluation shows that the weakest modelled feedback estimates can likely be excluded, but highlights compensating errors making it difficult to draw conclusions of the strongest estimates. Overall, the method of evaluating along process chains shows promise in pin-pointing sources of uncertainty and constraining modelled aerosol feedbacks.
Various parameterizations of organic aerosol (OA) formation and its subsequent evolution in the two-dimensional Volatility Basis Set (2D-VBS) framework are evaluated using ground measurements ...collected in the 2013 PEGASOS field campaign in the boreal forest station of Hyytiälä in southern Finland. A number of chemical aging schemes that performed well in the polluted environment of the Po Valley in Italy during the PEGASOS 2012 campaign are examined, taking into account various functionalization and fragmentation pathways for biogenic and anthropogenic OA components. All seven aging schemes considered have satisfactory results, consistent with the ground measurements. Despite their differences, these schemes predict similar contributions of the various OA sources and formation pathways for the periods examined. The highest contribution comes from biogenic secondary OA (bSOA), as expected, contributing 40–63% depending on the modeling scheme. Anthropogenic secondary OA (aSOA) is predicted to contribute 11–18% of the total OA, while SOA from intermediate-volatility compounds (SOA-iv) oxidation contributes another 18–27%. The fresh primary OA (POA) contributes 4%, while the SOA resulting from the oxidation of the evaporated semivolatile POA (SOA-sv) varies between 4 and 6%. Finally, 5–6% is predicted to be due to long-range transport from outside the modeling domain.
Using chemical ionization mass spectrometry to detect particle‐phase acids and aerosol mass spectrometry (AMS) measurements from Colorado, USA, and two studies in Hyytiälä, Finland, we quantify the ...fraction of organic aerosol (OA) mass that is composed of molecules with acid functional groups (facid). Molecules containing one or more carboxylic acid functionality contributed approximately 29% (45–51%) of the OA mass in Colorado (Finland). Organic acid mass concentration correlates well with AMS m/z 44 (primarily CO2+), a commonly used marker for highly oxidized aerosol. Using the average empirical relationship between AMS m/z 44 and organic acids in these three studies, together with m/z 44 data from 29 continental northern hemispheric (NH) AMS data sets, we estimate that molecules containing carboxylic acid functionality constitute on average 28% (range 10–50%) of NH continental OA mass with typically higher values at rural/remote sites and during summer and lower values at urban sites and during winter.
Key Points
Organic acids in aerosols strongly correlate with AMS m/z 44
Estimated contribution of organic acids to continental NH OA ranges from 10 to 50%
facid higher at rural/remote sites and during summer and lower at urban sites
The volatility distribution of secondary organic aerosols that formed and had undergone aging – i.e., the particle mass fractions of semi-volatile, low-volatility and extremely low volatility organic ...compounds in the particle phase – was characterized in a boreal forest environment of Hyytiälä, southern Finland. This was done by interpreting field measurements using a volatility tandem differential mobility analyzer (VTDMA) with a kinetic evaporation model. The field measurements were performed during April and May 2014. On average, 40 % of the organics in particles were semi-volatile, 34 % were low-volatility organics and 26 % were extremely low volatility organics. The model was, however, very sensitive to the vaporization enthalpies assumed for the organics (ΔHVAP). The best agreement between the observed and modeled temperature dependence of the evaporation was obtained when effective vaporization enthalpy values of 80 kJ mol−1 were assumed. There are several potential reasons for the low effective enthalpy value, including molecular decomposition or dissociation that might occur in the particle phase upon heating, mixture effects and compound-dependent uncertainties in the mass accommodation coefficient. In addition to the VTDMA-based analysis, semi-volatile and low-volatility organic mass fractions were independently determined by applying positive matrix factorization (PMF) to high-resolution aerosol mass spectrometer (HR-AMS) data. The factor separation was based on the oxygenation levels of organics, specifically the relative abundance of mass ions at m∕z 43 (f43) and m∕z 44 (f44). The mass fractions of these two organic groups were compared against the VTDMA-based results. In general, the best agreement between the VTDMA results and the PMF-derived mass fractions of organics was obtained when ΔHVAP = 80 kJ mol−1 was set for all organic groups in the model, with a linear correlation coefficient of around 0.4. However, this still indicates that only about 16 % (R2) of the variation can be explained by the linear regression between the results from these two methods. The prospect of determining of extremely low volatility organic aerosols (ELVOAs) from AMS data using the PMF analysis should be assessed in future studies.
Relatively high concentrations of preexisting particles, acting as a condensation sink (CS) of gaseous precursors, have been thought to suppress the occurrence of new particle formation (NPF) in ...urban environments, yet NPF still occurs frequently. Here, we aim to understand the factors promoting and inhibiting NPF events in urban Beijing by combining one-year-long measurements of particle number size distributions and PM2.5 chemical composition. Our results show that indeed the CS is an important factor controlling the occurrence of NPF events, with its chemical composition affecting the efficiency of the background particles in removing gaseous H2SO4 (effectiveness of the CS) driving NPF. During our observation period, the CS was found to be more effective for ammonium nitrate-rich (NH4NO3-rich) fine particles. On non-NPF event days, particles acting as CS contained a larger fraction of NH4NO3 compared to NPF event days under comparable CS levels. In particular, in the CS range from 0.02 to 0.03 s–1, the nitrate fraction was 17% on NPF event days and 26% on non-NPF event days. Overall, our results highlight the importance of considering the chemical composition of preexisting particles when estimating the CS and their role in inhibiting NPF events, especially in urban environments.
Secondary
organic aerosol (SOA) forms a major part of the tropospheric submicron
aerosol. Still, the exact formation mechanisms of SOA have remained elusive.
Recently, a newly discovered group of ...oxidation products of volatile organic
compounds (VOCs), highly oxygenated organic molecules (HOMs), have been
proposed to be responsible for a large fraction of SOA formation. To assess
the potential of HOMs to form SOA and to even take part in new particle
formation, knowledge of their exact volatilities is essential. However, due
to their exotic, and partially unknown, structures, estimating their
volatility is challenging. In this study, we performed a set of continuous
flow chamber experiments, supported by box modelling, to study the
volatilities of HOMs, along with some less oxygenated compounds, formed in the
ozonolysis of α-pinene, an abundant VOC emitted by boreal forests.
Along with gaseous precursors, we periodically injected inorganic seed
aerosol into the chamber to vary the condensation sink (CS) of low-volatility
vapours. We monitored the decrease of oxidation products in the gas phase in
response to increasing CS, and were able to relate the responses to the
volatilities of the compounds. We found that HOM monomers are mainly of low
volatility, with a small fraction being semi-volatile. HOM dimers were all at
least low volatility, but probably extremely low volatility; however, our
method is not directly able to distinguish between the two. We were able to
model the volatility of the oxidation products in terms of their carbon,
hydrogen, oxygen and nitrogen numbers. We found that increasing levels of
oxygenation correspond to lower volatilities, as expected, but that the
decrease is less steep than would be expected based on many existing models
for volatility, such as SIMPOL. The hydrogen number of a compound also
predicted its volatility, independently of the carbon number, with higher
hydrogen numbers corresponding to lower volatilities. This can be explained
in terms of the functional groups making up a molecule: high hydrogen numbers
are associated with, e.g. hydroxy groups, which lower volatility more than,
e.g. carbonyls, which are associated with a lower hydrogen number. The method
presented should be applicable to systems other than α-pinene
ozonolysis, and with different organic loadings, in order to study different
volatility ranges.
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