pH is a fundamental aerosol property that affects ambient particle concentration and composition, linking pH to all aerosol environmental impacts. Here, PM1 and PM2. 5 pH are calculated based on data ...from measurements during the California Research at the Nexus of Air Quality and Climate Change (CalNex) study from 15 May to 15 June 2010 in Pasadena, CA. Particle pH and water were predicted with the ISORROPIA-II thermodynamic model and validated by comparing predicted to measured gas–particle partitioning of inorganic nitrate, ammonium, and chloride. The study mean ± standard deviation PM1 pH was 1.9 ± 0.5 for the SO42−–NO3−–NH4+–HNO3–NH3 system. For PM2. 5, internal mixing of sea salt components (SO42−–NO3−–NH4+–Na+–Cl−–K+–HNO3–NH3–HCl system) raised the bulk pH to 2.7 ± 0.3 and improved predicted nitric acid partitioning with PM2. 5 components. The results show little effect of sea salt on PM1 pH, but significant effects on PM2. 5 pH. A mean PM1 pH of 1.9 at Pasadena was approximately one unit higher than what we have reported in the southeastern US, despite similar temperature, relative humidity, and sulfate ranges, and is due to higher total nitrate concentrations (nitric acid plus nitrate) relative to sulfate, a situation where particle water is affected by semi-volatile nitrate concentrations. Under these conditions nitric acid partitioning can further promote nitrate formation by increasing aerosol water, which raises pH by dilution, further increasing nitric acid partitioning and resulting in a significant increase in fine particle nitrate and pH. This study provides insights into the complex interactions between particle pH and nitrate in a summertime coastal environment and a contrast to recently reported pH in the eastern US in summer and winter and the eastern Mediterranean. All studies have consistently found highly acidic PM1 with pH generally below 3.
Formation of cirrus clouds depends on the availability of ice nuclei to begin condensation of atmospheric water vapor. Although it is known that only a small fraction of atmospheric aerosols are ...efficient ice nuclei, the critical ingredients that make those aerosols so effective have not been established. We have determined in situ the composition of the residual particles within cirrus crystals after the ice was sublimated. Our results demonstrate that mineral dust and metallic particles are the dominant source of residual particles, whereas sulfate and organic particles are underrepresented, and elemental carbon and biological materials are essentially absent. Further, composition analysis combined with relative humidity measurements suggests that heterogeneous freezing was the dominant formation mechanism of these clouds.
The size of aerosol particles has fundamental effects on their chemistry and radiative effects. We explore those effects using aerosol size and
composition data in the lowermost stratosphere along ...with calculations of light scattering. In the size range between about 0.1 and
1.0 µm diameter (accumulation mode), there are at least two modes of particles in the lowermost stratosphere. The larger mode consists
mostly of particles produced in the stratosphere, and the smaller mode consists mostly of particles transported from the troposphere. The
stratospheric mode is similar in the Northern and Southern Hemisphere, whereas the tropospheric mode is much more abundant in the Northern
Hemisphere. The purity of sulfuric acid particles in the stratospheric mode shows that there is limited production of secondary organic aerosol in
the stratosphere, especially in the Southern Hemisphere. Out of eight sets of flights sampling the lowermost stratosphere (four seasons and two
hemispheres) there were three with large injections of specific materials: volcanic, biomass burning, or dust. The stratospheric and tropospheric
modes have very different roles for radiative effects on climate and for heterogeneous chemistry. Because the larger particles are more efficient at
scattering light, most of the radiative effect in the lowermost stratosphere is due to stratospheric particles. In contrast, the tropospheric
particles can have more surface area, at least in the Northern Hemisphere. The surface area of tropospheric particles could have significant
implications for heterogeneous chemistry because these particles, which are partially neutralized and contain organics, do not correspond to the
substances used for laboratory studies of stratospheric heterogeneous chemistry. We then extend the analysis of size-dependent properties to
particles injected into the stratosphere, either intentionally or from volcanoes. There is no single size that will simultaneously maximize the
climate impact relative to the injected mass, infrared heating, potential for heterogeneous chemistry, and undesired changes in direct sunlight. In
addition, light absorption in the far ultraviolet is identified as an issue requiring more study for both the existing and potentially modified
stratosphere.
Brown carbon (BrC) is an organic aerosol material that preferentially absorbs light of shorter wavelengths. Global‐scale radiative impacts of BrC have been difficult to assess due to the lack of BrC ...observational data. To address this, aerosol filters were continuously collected with near pole‐to‐pole latitudinal coverage over the Pacific and Atlantic basins in three seasons as part of the Atmospheric Tomography Mission. BrC chromophores in filter extracts were measured. We find that globally, BrC was highly spatially heterogeneous, mostly detected in air masses that had been transported from regions of extensive biomass burning. We calculate the average direct radiative effect due to BrC absorption accounted for approximately 7% to 48% of the top of the atmosphere clear‐sky instantaneous forcing by all absorbing carbonaceous aerosols in the remote atmosphere, indicating that BrC from biomass burning is an important component of the global radiative balance.
Plain Language Summary
Combustion produces light‐absorbing aerosols that can affect the global radiation balance. Black carbon, which absorbs light over a broad wavelength range, has been extensively studied, but recent work shows that a significant component of the light‐absorbing aerosol is brown, absorbing mostly in the lower end of the visible and into the ultraviolet (UV). Incomplete combustion, such as in wild fires, is known to produce substantial levels of brown carbon. Here we report direct measurements of brown carbon determined from filter samples collected from aircraft flights that extended from pole to pole over three seasons. We observed brown carbon in aerosols that had been transported long distances from regions of wild fires at various locations across the globe. A radiative transfer model indicated that this brown carbon can substantially contribute to the overall radiative forcing by light‐absorbing aerosols.
Key Points
Globally, biomass burning is a large source of light‐absorbing carbonaceous aerosol that directly affect the planetary radiation balance
Transported over long distances, brown carbon is a significant component of these aerosols, but its contribution was highly variable
Brown carbon contributed up to 48% of average clear‐sky instantaneous forcing by light absorption by carbonaceous aerosols
We present the first data on the concentration of sea-salt aerosol throughout
most of the depth of the troposphere and over a wide range of latitudes,
which were obtained during the Atmospheric ...Tomography (ATom) mission.
Sea-salt concentrations in the upper troposphere are very small, usually less
than 10 ng per standard m3 (about 10 parts per trillion by mass) and
often less than 1 ng m−3. This puts stringent limits on the
contribution of sea-salt aerosol to halogen and nitric acid chemistry in the
upper troposphere. Within broad regions the concentration of sea-salt aerosol
is roughly proportional to water vapor, supporting a dominant role for wet
scavenging in removing sea-salt aerosol from the atmosphere. Concentrations
of sea-salt aerosol in the winter upper troposphere are not as low as in the
summer and the tropics. This is mostly a consequence of less wet scavenging
in the drier, colder winter atmosphere. There is also a source of sea-salt
aerosol over pack ice that is distinct from that over open water. With a
well-studied and widely distributed source, sea-salt aerosol provides an
excellent test of wet scavenging and vertical transport of aerosols in
chemical transport models.
Cirrus are high altitude clouds composed of ice crystals. They are the first tropospheric clouds that can scatter incoming solar radiation and the last which can trap outgoing terrestrial heat. ...Considering their extensive global coverage, estimated at between 25 and 33% of the Earth's surface, cirrus exert a measurable climate forcing. The global radiative influence depends on a number of properties including their altitude, ice crystal size and number density, and vertical extent. These properties in turn depend on the ability of upper tropospheric aerosol particles to initiate ice formation. Because aerosol populations, and therefore cirrus formation mechanisms, may change due to human activities, the sign of cirrus forcing (a net warming or cooling) due to anthropogenic effects is not universally agreed upon although most modeling studies suggest a positive effect. Cirrus also play a major role in the water cycle in the tropopause region, affecting not only redistribution in the troposphere but also the abundance of vapor entering the stratosphere. Both the current lack of understanding of cirrus properties and the need to improve our ability to project changes due to human activities in the future highlight the critical need to determine the aerosol particles on which cirrus form.
This review addresses what is currently known about the abundance, size and composition of cirrus-forming particles. We review aircraft-based field studies which have either collected cirrus ice residuals for off-line analysis or determined their size, composition and other properties in situ by capturing ice crystals and sublimating/removing the condensed phase water. This review is predominantly restricted to cirrus clouds. Limited comparisons are made to other ice-containing (e.g., mixed-phase) cloud types. The findings of recent reviews on laboratory measurements that mimic upper tropospheric cirrus formation are briefly summarized. The limitations of the current state of the art in cirrus ice residual studies are outlined. Important ancillary measurements and how they are integrated with ice residual data are also presented. Concluding statements focus on the need for specific instrumentation and future studies.
•We review the methods by which particles that form cirrus cloud ice crystals are identified.•We compare the particle types which form ice crystals depending on altitude and geographic region.•The mechanism by which cirrus clouds form is described based on ice nucleating particle composition.•Suggestions for instrument development to further our understanding of the climatic effect of cirrus clouds are made.
From 2016 to 2018 a DC-8 aircraft operated by the US National
Aeronautics and Space Administration (NASA) made four series of flights,
profiling the atmosphere from 180 m to ∼12 km above sea level ...(km a.s.l.)
from the Arctic to the Antarctic over both the Pacific and Atlantic oceans.
This program, the Atmospheric Tomography Mission (ATom), sought to sample
the troposphere in a representative manner, making measurements of
atmospheric composition in each season. This paper describes the
aerosol microphysical measurements and derived quantities obtained during
this mission. Dry size distributions from 2.7 nm to 4.8 µm in
diameter were measured in situ at 1 Hz using a battery of instruments: 10
condensation particle counters with different nucleation diameters, two
ultra-high-sensitivity aerosol size spectrometers (UHSASs), one of which
measured particles surviving heating to 300 ∘C, and a laser
aerosol spectrometer (LAS). The dry aerosol measurements were complemented
by size distribution measurements from 0.5 to 930 µm diameter at
near-ambient conditions using a cloud, aerosol, and precipitation
spectrometer (CAPS) mounted under the wing of the DC-8. Dry aerosol number,
surface area, and volume, and optical scattering and asymmetry parameters at
several wavelengths from the near-UV to the near-IR ranges were calculated from the
measured dry size distributions (2.7 nm to 4.8 µm). Dry aerosol mass
was estimated by combining the size distribution data with particle density
estimated from independent measurements of aerosol composition with a
high-resolution aerosol mass spectrometer and a single-particle soot
photometer. We describe the instrumentation and fully document the aircraft
inlet and flow distribution system, the derivation of uncertainties, and the
calculation of data products from combined size distributions. Comparisons
between the instruments and direct measurements of some aerosol properties
confirm that in-flight performance was consistent with calibrations and
within stated uncertainties for the two deployments analyzed. The unique
ATom dataset contains accurate, precise, high-resolution in situ
measurements of dry aerosol size distributions, and integral parameters, and
estimates and measurements of optical properties, for particles < 4.8 µm in diameter that can be used to evaluate aerosol abundance and
processes in global models.
Dust aerosols affect the radiative and energy balance at local and global
scales by scattering and absorbing sunlight and infrared light. A previous
study suggests that dust size distribution is one ...of the major sources of
uncertainty in modeling the dust global distribution. Climate models
overestimate the fine dust (≤5 µm) by an order of magnitude,
while underestimates of the coarse dust (≥5 µm) range between 0.5 to 1.5 orders of magnitude compared with the global observations. Here we improved the simulated size distribution of dust aerosol using a sectional aerosol model (Community Aerosol and Radiation Model for Atmospheres) coupled with the Community Earth System Model (CESM1/CARMA). Simulated dust mass size distributions peak at around 2–3 µm in diameter and increase by 4 orders of magnitude from 0.1 to 2 µm. Our model demonstrates that North African, Middle Eastern, and Asian dust accounts for ∼ 59.7 %, 12.5 %, and 13.3 % of the global annual mean dust emissions, with the remaining 14.5 % originating from scattered smaller dust sources. The model dust vertical distributions are validated against the NASA Atmospheric Tomography (ATom) field campaign datasets. Both simulations and ATom in situ measurements during the ATom field campaign suggest that dust mass concentrations over the remote ocean drop by 2 to 3 orders of magnitude from the surface to the upper troposphere (200 hPa). Our model suggests that Asian dust contributes to more than 40 % of annual mean dust mass abundances in the global upper troposphere and lower stratosphere (UTLS). The model suggests that Asian dust dominates the dust mass budget in the UTLS of the Asian summer monsoon (ASM) region, with a relative contribution 1–2 orders of magnitude higher than the dust originating from the North African and Middle Eastern deserts.
Measurements of primary biological aerosol particles (PBAP), especially at altitudes relevant to cloud formation, are scarce. Single-particle mass spectrometry (SPMS) has been used to probe aerosol ...chemical composition from ground and aircraft for over 20 years. Here we develop a method for identifying bioaerosols (PBAP and particles containing fragments of PBAP as part of an internal mixture) using SPMS. We show that identification of bioaerosol using SPMS is complicated because phosphorus-bearing mineral dust and phosphorus-rich combustion by-products such as fly ash produce mass spectra with peaks similar to those typically used as markers for bioaerosol. We have developed a methodology to differentiate and identify bioaerosol using machine learning statistical techniques applied to mass spectra of known particle types. This improved method provides far fewer false positives compared to approaches reported in the literature. The new method was then applied to two sets of ambient data collected at Storm Peak Laboratory and a forested site in Central Valley, California to show that 0.04–2 % of particles in the 200–3000 nm aerodynamic diameter range were identified as bioaerosol. In addition, 36–56 % of particles identified as biological also contained spectral features consistent with mineral dust, suggesting internal dust–biological mixtures.
Cloud condensation nuclei (CCN) can affect cloud properties and therefore the Earth’s radiative balance. New particle formation (NPF) from condensable vapours in the free troposphere has been ...suggested to contribute to CCN, especially in remote, pristine atmospheric regions, but direct evidence is sparse, and the magnitude of this contribution is uncertain. Here we use in-situ aircraft measurements of vertical profiles of aerosol size distributions to present a global-scale survey of NPF occurrence. We observed intense NPF occurring at high altitude in tropical convective regions over both the Pacific and Atlantic Oceans. Together with the results of chemical-transport models, our findings indicate that NPF persists at all longitudes as a global-scale band in the tropical upper troposphere, covering about 40% of the Earth’s surface. Furthermore, we find that this NPF in the tropical upper troposphere is a globally important source of CCN in the lower troposphere, where they can affect cloud properties. Our findings suggest that the production of CCN, as these new particles descend towards the surface, is currently not adequately captured in global models, because they tend to underestimate both the magnitude of tropical upper tropospheric NPF and the subsequent growth to CCN sizes. This has potential implications for cloud albedo and the global radiative balance.
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