Particle pH is a critical but poorly constrained quantity that affects many aerosol processes and properties, including aerosol composition, concentrations, and toxicity. We assess PM1 pH as a ...function of geographical location and altitude, focusing on the northeastern U.S., based on aircraft measurements from the Wintertime Investigation of Transport, Emissions, and Reactivity campaign (1 February to 15 March 2015). Particle pH and water were predicted with the ISORROPIA‐II thermodynamic model and validated by comparing predicted to observed partitioning of inorganic nitrate between the gas and particle phases. Good agreement was found for relative humidity (RH) above 40%; at lower RH observed particle nitrate was higher than predicted, possibly due to organic‐inorganic phase separations or nitrate measurement uncertainties associated with low concentrations (nitrate < 1 µg m−3). Including refractory ions in the pH calculations did not improve model predictions, suggesting they were externally mixed with PM1 sulfate, nitrate, and ammonium. Sample line volatilization artifacts were found to be minimal. Overall, particle pH for altitudes up to 5000 m ranged between −0.51 and 1.9 (10th and 90th percentiles) with a study mean of 0.77 ± 0.96, similar to those reported for the southeastern U.S. and eastern Mediterranean. This expansive aircraft data set is used to investigate causes in variability in pH and pH‐dependent aerosol components, such as PM1 nitrate, over a wide range of temperatures (−21 to 19°C), RH (20 to 95%), inorganic gas, and particle concentrations and also provides further evidence that particles with low pH are ubiquitous.
Key Points
Highly acidic aerosols (pH = 0.77 ± 0.96) for altitudes up to 5000 m in the northeastern U.S. in wintertime
Thermodynamically predicted HNO3−NO3− partitioning by ISORROPIA‐II agrees with observation above 40% RH
Particle pH should be explicitly determined to accurately assess properties impacted by aerosol acidity, such as HNO3−NO3− partitioning
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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.
Single-particle mass spectrometry (SPMS) instruments
characterize the composition of individual aerosol particles in real time.
Their fundamental ability to differentiate the externally mixed ...particle
types that constitute the atmospheric aerosol population enables a unique
perspective into sources and transformation. However, quantitative
measurements by SPMS systems are inherently problematic. We introduce a new
technique that combines collocated measurements of aerosol composition by
SPMS and size-resolved absolute particle concentrations on aircraft
platforms. Quantitative number, surface area, volume, and mass
concentrations are derived for climate-relevant particle types such as
mineral dust, sea salt, and biomass burning smoke. Additionally, relative
ion signals are calibrated to derive mass concentrations of internally mixed
sulfate and organic material that are distributed across multiple particle
types. The NOAA Particle Analysis by Laser Mass Spectrometry (PALMS) instrument
measures size-resolved aerosol chemical composition from aircraft. We
describe the identification and quantification of nine major atmospheric
particle classes, including sulfate–organic–nitrate mixtures, biomass
burning, elemental carbon, sea salt, mineral dust, meteoric material, alkali
salts, heavy fuel oil combustion, and a remainder class. Classes can be
sub-divided as necessary based on chemical heterogeneity, accumulated
secondary material during aging, or other atmospheric processing.
Concentrations are derived for sizes that encompass the accumulation and
coarse size modes. A statistical error analysis indicates that particle
class concentrations can be determined within a few minutes for abundances
above ∼10 ng m−3. Rare particle types require longer
sampling times. We explore the instrumentation requirements and the limitations of the
method for airborne measurements. Reducing the size resolution of the
particle data increases time resolution with only a modest increase in
uncertainty. The principal limiting factor to fast time response
concentration measurements is statistically relevant sampling across the
size range of interest, in particular, sizes D < 0.2 µm for
accumulation-mode studies and D > 2 µm for coarse-mode
analysis. Performance is compared to other airborne and ground-based
composition measurements, and examples of atmospheric mineral dust
concentrations are given. The wealth of information afforded by
composition-resolved size distributions for all major aerosol types
represents a new and powerful tool to characterize atmospheric aerosol
properties in a quantitative fashion.
Full text
Available for:
IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
We use GEOS-Chem chemical transport model simulations of sulfate–ammonium aerosol data from the NASA ARCTAS and NOAA ARCPAC aircraft campaigns in the North American Arctic in April 2008, together ...with longer-term data from surface sites, to better understand aerosol sources in the Arctic in winter–spring and the implications for aerosol acidity. Arctic pollution is dominated by transport from mid-latitudes, and we test the relevant ammonia and sulfur dioxide emission inventories in the model by comparison with wet deposition flux data over the source continents. We find that a complicated mix of natural and anthropogenic sources with different vertical signatures is responsible for sulfate concentrations in the Arctic. East Asian pollution influence is weak in winter but becomes important in spring through transport in the free troposphere. European influence is important at all altitudes but never dominant. West Asia (non-Arctic Russia and Kazakhstan) is the largest contributor to Arctic sulfate in surface air in winter, reflecting a southward extension of the Arctic front over that region. Ammonium in Arctic spring mostly originates from anthropogenic sources in East Asia and Europe, with added contribution from boreal fires, resulting in a more neutralized aerosol in the free troposphere than at the surface. The ARCTAS and ARCPAC data indicate a median aerosol neutralization fraction NH
4
+/(2SO
4
2− + NO
3
−) of 0.5 mol mol
−1 below 2 km and 0.7 mol mol
−1 above. We find that East Asian and European aerosol transported to the Arctic is mostly neutralized, whereas West Asian and North American aerosol is highly acidic. Growth of sulfur emissions in West Asia may be responsible for the observed increase in aerosol acidity at Barrow over the past decade. As global sulfur emissions decline over the next decades, increasing aerosol neutralization in the Arctic is expected, potentially accelerating Arctic warming through indirect radiative forcing and feedbacks.
► We interpret Arctic sulfate–ammonium aerosol data using a chemical transport model. ► A mix of natural and anthropogenic sources contribute to Arctic sulfate burdens. ► West Asia is the largest source of sulfate to Arctic surface air in winter. ► Arctic ammonium is dominated by agricultural and open burning emissions. ► Spring aerosol is acidic throughout the troposphere and most acidic at the surface.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Remote sensing observations suggest Greenland ice sheet (GrIS) albedo has declined since 2001, even in the dry snow zone. We seek to explain the apparent dry snow albedo decline. We analyze samples ...representing 2012–2014 snowfall across NW Greenland for black carbon and dust light‐absorbing impurities (LAI) and model their impacts on snow albedo. Albedo reductions due to LAI are small, averaging 0.003, with episodic enhancements resulting in reductions of 0.01–0.02. No significant increase in black carbon or dust concentrations relative to recent decades is found. Enhanced deposition of LAI is not, therefore, causing significant dry snow albedo reduction or driving melt events. Analysis of Collection 5 Moderate Resolution Imaging Spectroradiometer (MODIS) surface reflectance data indicates that the decline and spectral shift in dry snow albedo contains important contributions from uncorrected Terra sensor degradation. Though discrepancies are mostly below the stated accuracy of MODIS products, they will require revisiting some prior conclusions with C6 data.
Key Points
No significant change in deposition of light‐absorbing impurities on Greenland found
Albedo decrease by light‐absorbing particles in snow is typically <0.005 in interior Greenland
MODIS‐observed albedo decline in Greenland dry snow partly due to instrument degradation
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
This study characterizes the partitioning behavior of a significant fraction of the ambient organic aerosol through simultaneous measurements of gas and particle water‐soluble organic carbon (WSOC). ...During the summer in Atlanta, WSOC gas/particle partitioning showed a strong RH dependence that was attributed to particulate liquid water. At elevated RH levels (>∼70%), a significant increase in WSOC partitioning to the particle phase was observed and followed the predicted water uptake by fine particles. The enhancement in particle‐phase partitioning translated to increased median particle WSOC concentrations ranging from 0.3–0.9 μg C m−3. The results provide a detailed overview of the WSOC partitioning behavior in the summertime in an urban region dominated by biogenic emissions, and indicate that secondary organic aerosol formation involving partitioning to liquid water may be a significant aerosol formation route that is generally not considered.
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Nitrous acid (HONO) is an important precursor to hydroxyl radical
(OH) that determines atmospheric oxidative capacity and thus impacts climate
and air quality. Wildfire is not only a major direct ...source of HONO, it also
results in highly polluted conditions that favor the heterogeneous formation of
HONO from nitrogen oxides (NOx= NO + NO2) and nitrate on both
ground and particle surfaces. However, these processes remain poorly
constrained. To quantitatively constrain the HONO budget under various
fire and/or smoke conditions, we combine a unique dataset of field concentrations
and isotopic ratios (15N / 14N and 18O / 16O) of NOx
and HONO with an isotopic box model. Here we report the first isotopic
evidence of secondary HONO production in near-ground wildfire plumes (over a
sample integration time of hours) and the subsequent quantification of the
relative importance of each pathway to total HONO production. Most
importantly, our results reveal that nitrate photolysis plays a minor role
(<5 %) in HONO formation in daytime aged smoke, while
NO2-to-HONO heterogeneous conversion contributes 85 %–95 % to total
HONO production, followed by OH + NO (5 %–15 %). At nighttime, heterogeneous
reduction of NO2 catalyzed by redox active species (e.g., iron oxide
and/or quinone) is essential (≥ 75 %) for HONO production in addition
to surface NO2 hydrolysis. Additionally, the 18O / 16O of HONO
is used for the first time to constrain the NO-to-NO2 oxidation
branching ratio between ozone and peroxy radicals. Our approach provides a
new and critical way to mechanistically constrain atmospheric chemistry and/or air
quality models on a diurnal timescale.
High levels of fine particulate matter (PM2.5) pollution in East Asia often exceed local air quality standards. Observations from the Korea–United States Air Quality (KORUS-AQ) field campaign in May ...and June 2016 showed that development of extreme pollution (haze) occurred through a combination of long-range transport and favorable meteorological conditions that enhanced local production of PM2.5. Atmospheric models often have difficulty simulating PM2.5 chemical composition during haze, which is of concern for the development of successful control measures. We use observations from KORUS-AQ to examine the ability of the GEOS-Chem chemical transport model to simulate PM2.5 composition throughout the campaign and identify the mechanisms driving the pollution event. At the surface, the model underestimates sulfate by -64 % but overestimates nitrate by +36 %. The largest underestimate in sulfate occurs during the pollution event, for which models typically struggle to generate elevated sulfate concentrations due to missing heterogeneous chemistry in aerosol liquid water in the polluted boundary layer. Hourly surface observations show that the model nitrate bias is driven by an overestimation of the nighttime peak. In the model, nitrate formation is limited by the supply of nitric acid, which is biased by +100 % against aircraft observations. We hypothesize that this is due to a large missing sink, which we implement here as a factor of 5 increase in dry deposition. We show that the resulting increased deposition velocity is consistent with observations of total nitrate as a function of photochemical age. The model does not account for factors such as the urban heat island effect or the heterogeneity of the built-up urban landscape, resulting in insufficient model turbulence and surface area over the study area that likely results in insufficient dry deposition. Other species such as NH3 could be similarly affected but were not measured during the campaign. Nighttime production of nitrate is driven by NO2 hydrolysis in the model, while observations show that unexpectedly elevated nighttime ozone (not present in the model) should result in N2O5 hydrolysis as the primary pathway. The model is unable to represent nighttime ozone due to an overly rapid collapse of the afternoon mixed layer and excessive titration by NO. We attribute this to missing nighttime heating driving deeper nocturnal mixing that would be expected to occur in a city like Seoul. This urban heating is not considered in air quality models run at large enough scales to treat both local chemistry and long-range transport. Key model failures in simulating nitrate, mainly overestimated daytime nitric acid, incorrect representation of nighttime chemistry, and an overly shallow and insufficiently turbulent nighttime mixed layer, exacerbate the model's inability to simulate the buildup of PM2.5 during haze pollution. To address the underestimate in sulfate most evident during the haze event, heterogeneous aerosol uptake of SO2 is added to the model, which previously only considered aqueous production of sulfate from SO2 in cloud water. Implementing a simple parameterization of this chemistry improves the model abundance of sulfate but degrades the SO2 simulation, implying that emissions are underestimated. We find that improving model simulations of sulfate has direct relevance to determining local vs. transboundary contributions to PM2.5. During the haze pollution event, the inclusion of heterogeneous aerosol uptake of SO2 decreases the fraction of PM2.5 attributable to long-range transport from 66 % to 54 %. Locally produced sulfate increased from 1 % to 25 % of locally produced PM2.5, implying that local emissions controls could have a larger effect than previously thought. However, this additional uptake of SO2 is coupled to the model nitrate prediction, which affects the aerosol liquid water abundance and chemistry driving sulfate–nitrate–ammonium partitioning. An additional simulation of the haze pollution with heterogeneous uptake of SO2 to aerosol and simple improvements to the model nitrate simulation results in 30 % less sulfate due to 40 % less nitrate and aerosol water, and this results in an underestimate of sulfate during the haze event. Future studies need to better consider the impact of model physical processes such as dry deposition and nighttime boundary layer mixing on the simulation of nitrate and the effect of improved nitrate simulations on the overall simulation of secondary inorganic aerosol (sulfate + nitrate + ammonium) in East Asia. Foreign emissions are rapidly changing, increasing the need to understand the impact of local emissions on PM2.5 in South Korea to ensure continued air quality improvements.
Ozone pollution in the Southeast US involves complex chemistry driven by emissions of anthropogenic nitrogen oxide radicals (NO(x) triple bond NO + NO2) and biogenic isoprene. Model estimates of ...surface ozone concentrations tend to be biased high in the region and this is of concern for designing effective emission control strategies to meet air quality standards. We use detailed chemical observations from the SEAC(exp 4)RS aircraft campaign in August and September 2013, interpreted with the GEOS-Chem chemical transport model at 0.25 deg x 0.3125 deg horizontal resolution, to better understand the factors controlling surface ozone in the Southeast US. We find that the National Emission Inventory (NEI) for NO(x) from the US Environmental Protection Agency (EPA) is too high. This finding is based on SEAC(exp 4)RS observations of NO(x) and its oxidation products, surface network observations of nitrate wet deposition fluxes, and OMI satellite observations of tropospheric NO2 columns. Our results indicate that NEI NO(x) emissions from mobile and industrial sources must be reduced by 30-60%, dependent on the assumption of the contribution by soil NO(x) emissions. Upper-tropospheric NO2 from lightning makes a large contribution to satellite observations of tropospheric NO2 that must be accounted for when using these data to estimate surface NO(x) emissions. We find that only half of isoprene oxidation proceeds by the high-NO(x) pathway to produce ozone; this fraction is only moderately sensitive to changes in NO(x) emissions because isoprene and NO(x) emissions are spatially segregated. GEOS-Chem with reduced NO(x) emissions provides an unbiased simulation of ozone observations from the aircraft and reproduces the observed ozone production efficiency in the boundary layer as derived from a regression of ozone and NO(x) oxidation products. However, the model is still biased high by 6 plus or minus 14 ppb relative to observed surface ozone in the Southeast US. Ozonesondes launched during midday hours show a 7 ppb ozone decrease from 1.5 km to the surface that GEOS-Chem does not capture. This bias may reflect a combination of excessive vertical mixing and net ozone production in the model boundary layer.
PDF
