This study examined spatial variations of precipitation accumulation and chemistry for six sites located on the West and East Coasts of the U.S., and one site each on the islands of Hawaii, Bermuda, ...and Luzon of the Philippines (specifically Manila). The nine coastal sites ranged widely in both mean annual precipitation accumulation, ranging from 40 cm (Mauna Loa, Hawaii) to 275 cm (Washington), and in terms of monthly profiles. The three island sites represented the extremes of differences in terms of chemical profiles, with Bermuda having the highest overall ion concentrations driven mainly by sea salt, Hawaii having the highest SO42− mass fractions due to the nearby influence of volcanic SO2 emissions and mid-tropospheric transport of anthropogenic pollution, and Manila exhibiting the highest concentration of non-marine ions (NH4+, non-sea salt nss SO42−, nss Ca2+, NO3−, nss K+, nss Na+, nss Mg2+) linked to anthropogenic, biomass burning, and crustal emissions. The Manila site exhibited the most variability in composition throughout the year due to shifting wind directions and having diverse regional and local pollutant sources. In contrast to the three island sites, the North American continental sites exhibited less variability in precipitation composition with sea salt being the most abundant constituent followed by some combination of SO42−, NO3−, and NH4+. The mean-annual pH values ranged from 4.88 (South Carolina) to 5.40 (central California) with NH4+ exhibiting the highest neutralization factors for all sites except Bermuda where dust tracer species (nss Ca2+) exhibited enhanced values. The results of this study highlight the sensitivity of wet deposition chemistry to regional considerations, elevation, time of year, and atmospheric circulations.
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•Bermuda exhibited the highest ion concentrations due to sea salt.•Anthropogenic and biomass burning tracer levels were the highest at Manila.•Mauna Loa's high SO4 fraction due to mid-tropospheric transport and volcanic emissions.•North American continental sites had less monthly variability than the islands.•Ammonium had the highest neutralization capacity at all sites except Bermuda (Ca2+).
North American pollution outflow is ubiquitous over the western North
Atlantic Ocean, especially in winter, making this location a suitable
natural laboratory for investigating the impact of ...precipitation on aerosol
particles along air mass trajectories. We take advantage of observational
data collected at Bermuda to seasonally assess the sensitivity of aerosol
mass concentrations and volume size distributions to accumulated
precipitation along trajectories (APT). The mass concentration of
particulate matter with aerodynamic diameter less than 2.5 µm
normalized by the enhancement of carbon monoxide above background
(PM2.5/ΔCO) at Bermuda was used to estimate the degree of
aerosol loss during transport to Bermuda. Results for December–February
(DJF) show that most trajectories come from North America and have the highest
APTs, resulting in a significant reduction (by 53 %) in PM2.5/ΔCO under high-APT conditions (> 13.5 mm) relative to low-APT
conditions (< 0.9 mm). Moreover, PM2.5/ΔCO was most
sensitive to increases in APT up to 5 mm (−0.044 µg m−3 ppbv−1 mm−1) and less sensitive to increases in APT over 5 mm.
While anthropogenic PM2.5 constituents (e.g., black carbon, sulfate,
organic carbon) decrease with high APT, sea salt, in contrast, was comparable
between high- and low-APT conditions owing to enhanced local wind and sea
salt emissions in high-APT conditions. The greater sensitivity of the fine-mode volume concentrations (versus coarse mode) to wet scavenging is evident
from AErosol RObotic NETwork
(AERONET) volume size distribution data. A combination of GEOS-Chem model
simulations of the 210Pb submicron aerosol tracer and its gaseous precursor
222Rn reveals that (i) surface aerosol particles at Bermuda are most
impacted by wet scavenging in winter and spring (due to large-scale
precipitation) with a maximum in March, whereas convective scavenging plays
a substantial role in summer; and (ii) North American 222Rn tracer
emissions contribute most to surface 210Pb concentrations at Bermuda in
winter (∼ 75 %–80 %), indicating that air masses arriving at
Bermuda experience large-scale precipitation scavenging while traveling from
North America. A case study flight from the ACTIVATE field campaign on 22 February 2020 reveals a significant reduction in aerosol number and volume
concentrations during air mass transport off the US East Coast associated
with increased cloud fraction and precipitation. These results highlight the
sensitivity of remote marine boundary layer aerosol characteristics to
precipitation along trajectories, especially when the air mass source is
continental outflow from polluted regions like the US East Coast.
A network of 411 ground stations across Luzon Island, Philippines (12.5–20° N, 119–126.5° E) was used to characterize the diurnal cycles of summer precipitation, in terms of amount (PA), frequency ...(PF), and intensity (PI), during the southwest monsoon season (SWM; May–September) between 2011 and 2018. In addition to monsoon exposure, the effect of topography on the diurnal cycle of precipitation also was investigated by comparing a valley, plain, west- and east-facing coasts near mountains. Results show that monsoon exposure significantly influenced diurnal precipitation such that PA and PF decreased (PI increased) toward the leeward side of Luzon Island. Most topographies showed late afternoon-early evening peaks; however, the east-facing coast exhibited a late night-early morning peak. Orographic effects led to a high PA over mountains and enhanced the spatiotemporal propagation of PA in monsoon-exposed areas. The first (second) half of the diurnal peak exhibited high PI/low PF (low PI/high PF), suggesting both PI and PF are important indicators of PA. Finally, graded analysis revealed that light precipitation (0.01–2.5 mm h
−1
) captured overall precipitation trends across Luzon Island, highlighting the importance of this intensity of precipitation. Heavy precipitation (2.5–7.5 mm h
−1
) peaked in the morning; however, underlying mechanisms remain unknown. The study presents the first examination of the diurnal precipitation cycle in Luzon Island using a dense network of synoptic stations. The study demonstrates the complex effect of topography on precipitation and the importance of the SWM in the diurnal cycle of precipitation.
The tropical Northwest Pacific (TNWP) is a receptor for pollution sources throughout Asia and is highly susceptible to climate change, making it imperative to understand long-range transport in this ...complex aerosol-meteorological environment. Measurements from the NASA Cloud, Aerosol, and Monsoon Processes Philippines Experiment (CAMP2Ex; 24 August to 5 October 2019) and back trajectories from the National Oceanic and Atmospheric Administration Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT) were used to examine transport into the TNWP from the Maritime Continent (MC), peninsular Southeast Asia (PSEA), East Asia (EA), and the West Pacific (WP). A mid-campaign monsoon shift on 20 September 2019 led to distinct transport patterns between the southwest monsoon (SWM; before 20 September) and monsoon transition (MT; after 20 September). During the SWM, long-range transport was a function of southwesterly winds and cyclones over the South China Sea. Low- (high-) altitude air generally came from MC (PSEA), implying distinct aerosol processing related to convection and perhaps wind shear. The MT saw transport from EA and WP, driven by Pacific northeasterly winds, continental anticyclones, and cyclones over the East China Sea. Composition of transported air differed by emission source and accumulated precipitation along trajectories (APT). MC air was characterized by biomass burning tracers while major components of EA air pointed to Asian outflow and secondary formation. Convective scavenging of PSEA air was evidenced by considerable vertical differences between aerosol species but not trace gases, as well as notably higher APT and smaller particles than other regions. Finally, we observed a possible wet scavenging mechanism acting on MC air aloft that was not strictly linked to precipitation. These results are important for understanding the transport and processing of air masses with further implications for modeling aerosol lifecycles and guiding international policymaking to public health and climate, particularly during the SWM and MT.
Aerosol particles in Southeast Asia are challenging to characterize due to their complex life cycle within the diverse topography and weather of the region. An emerging aerosol climatology was ...established based on AErosol RObotic NETwork (AERONET) data (December 2009 to October 2018) for clear-sky days in Metro Manila, the Philippines. Aerosol optical depth (AOD) values were highest from August to October, partly from fine urban aerosol particles, including soot, coinciding with the burning season in insular Southeast Asia when smoke is often transported to Metro Manila during the southwest monsoon. Clustering of AERONET volume size distributions (VSDs) resulted in five aerosol particle sources based on the position and magnitude of their peaks in the VSD and the contributions of specific particle species to AOD per cluster based on MERRA-2. The clustering showed that the majority of aerosol particles above Metro Manila were from a clean marine source (58 %), which could be related to AOD values there being relatively low compared to other cities in the region. The following are the other particle sources over Metro Manila: fine polluted sources (20 %), mixed-dust sources (12 %), urban and industrial sources (5 %), and cloud processing sources (5 %). Furthermore, MERRA-2 AOD data over Southeast Asia were analyzed using empirical orthogonal functions. Along with AOD fractional compositional contributions and wind regimes, four dominant aerosol particle air masses emerged: two sulfate air masses from East Asia, an organic carbon source from Indonesia, and a sulfate source from the Philippines. Knowing the local and regional aerosol particle air masses that impact Metro Manila is useful in identifying the sources while gaining insight into how aerosol particles are affected by long-range transport and their impact on regional weather.
Nucleation in the free troposphere (FT) and subsequent growth of new particles represent a globally important source of cloud condensation nuclei (CCN). Whereas new particle formation (NPF) has been ...shown to occur frequently in the upper troposphere over tropical oceans, there have been few studies of NPF at lower altitudes. In addition, the impact of urban emissions and biomass burning on the NPF in tropical marine FT remains poorly understood. In this study, we examine NPF in the lower and mid-troposphere (3–8.5 km) over the tropical ocean and coastal region using airborne measurements during the recent Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP2Ex). NPF was mostly observed above 5.5 km and coincided with elevated relative humidity (RH) and reduced condensation sink (CS), suggesting that NPF occurs in convective cloud outflow. The frequency of NPF increases with altitude, reaching ∼ 50 % above 8 km. An abrupt decrease in NPF frequency coincides with early monsoon transition and is attributed to increased CS resulting from reduced convective activity and more frequent transport of aged urban plumes. Surprisingly, a large fraction of NPF events in background air were observed in the early morning, and the NPF is likely made possible by very low CS despite low actinic flux. Convectively detrained biomass-burning plumes and fresh urban emissions enhance NPF as a result of elevated precursor concentrations and scavenging of pre-existing particles. In contrast, NPF is suppressed in aged urban plumes where the reactive precursors are mostly consumed, while CS remains relatively high. This study shows a strong impact of urban and biomass-burning emissions on the NPF in tropical marine FT. The results also illustrate the competing influences of different variables and interactions among anthropogenic emissions, convective clouds, and meteorology, which lead to NPF under a variety of conditions in tropical marine environments.
Leveraging aerosol data from multiple airborne and surface‐based field campaigns encompassing diverse environmental conditions, we calculate statistics of the oxalate‐sulfate mass ratio (median: ...0.0217; 95% confidence interval: 0.0154–0.0296; R = 0.76; N = 2,948). Ground‐based measurements of the oxalate‐sulfate ratio fall within our 95% confidence interval, suggesting the range is robust within the mixed layer for the submicrometer particle size range. We demonstrate that dust and biomass burning emissions can separately bias this ratio toward higher values by at least one order of magnitude. In the absence of these confounding factors, the 95% confidence interval of the ratio may be used to estimate the relative extent of aqueous processing by comparing inferred oxalate concentrations between air masses, with the assumption that sulfate primarily originates from aqueous processing.
Plain Language Summary
The extent of atmospheric chemical processing remains an uncertain aspect of air mass characterization. Addressing this uncertainty is important because chemical reactions in the atmosphere in the presence of water (aqueous processing) produce a large fraction of global aerosol mass. The oxalate‐to‐sulfate ratio has been proposed as an indicator of aqueous processing, where higher values point to increased processing of an air mass. In this study, we quantify a range in the oxalate‐to‐sulfate mass ratio (0.0154–0.0296) using data from multiple field campaigns encompassing a diverse set of environments. This range is robust near the surface for particles below 1 micrometer in diameter. Larger particles, especially dust, and biomass burning particles significantly affect the oxalate‐to‐sulfate ratio and thus may confound the interpretation of a high oxalate‐to‐sulfate ratio as a signal of aqueous processing. In the absence of dust and biomass burning particles, the oxalate‐to‐sulfate ratio range may be used to compare the relative extent of aqueous processing between different air masses.
Key Points
Oxalate‐sulfate mass ratios show similarity across multiple environments (95% confidence interval: 0.0154–0.0296; R = 0.76; N = 2,948)
Oxalate‐sulfate mass ratio is biased toward higher values in presence of coarse aerosol particles and/or biomass burning
Ground‐based, size‐resolved measurements reveal that the ratio can be robust within the mixed layer for the submicrometer mode
The South China Sea (SCS) is a receptor of numerous
natural and anthropogenic aerosol species from throughout greater Asia. A
combination of several developing countries, archipelagic and peninsular
...terrain, a strong Asian monsoon climate, and a host of multi-scale
meteorological phenomena make the SCS one of the most complex
aerosol–meteorological systems in the world. However, aside from the
well-known biomass burning emissions from Indonesia and Borneo, the current
understanding of aerosol sources is limited, especially in remote marine
environments. In September 2011, a 2-week research cruise was conducted near Palawan, Philippines, to sample the remote SCS environment.
Size-segregated aerosol data were collected using a Davis Rotating Uniform
size-cut Monitor (DRUM) sampler and analyzed for concentrations of 28 elements
measured via X-ray fluorescence (XRF). Positive matrix factorization (PMF)
was performed separately on the coarse, fine, and ultrafine size ranges to
determine possible sources and their contributions to the total elemental
particulate matter mass. The PMF analysis resolved six sources across the
three size ranges: biomass burning, oil combustion, soil dust, a
crustal–marine mixed source, sea spray, and fly ash. Additionally, size
distribution plots, time series plots, back trajectories and satellite data
were used in interpreting factors. The multi-technique source apportionment
revealed the presence of biogenic sources such as soil dust, sea spray, and a crustal–marine mixed source. Anthropogenic sources were also identified: biomass burning, oil combustion, and fly ash. Mass size distributions showed elevated aerosol concentrations towards the end of the sampling period, which coincided with a shift of air mass back trajectories to southern Kalimantan. Covariance between coarse-mode soil dust and fine-mode biomass burning aerosols were observed. Agreement between the PMF and the linear regression analyses indicates that the PMF solution is robust. While biomass burning is indeed a key source of aerosol, this study shows the presence of other important sources in the SCS. Identifying these sources is not only key for characterizing the chemical profile of the SCS but, by improving our picture of aerosol sources in the region, also a step forward in developing our understanding of aerosol–meteorology feedbacks in this complex environment.
A 16-month (July 2018–October 2019) dataset of size-resolved aerosol
composition is used to examine the sources and characteristics of five
organic acids (oxalate, succinate, adipate, maleate, ...phthalate) and
methanesulfonate (MSA) in Metro Manila, Philippines. As one of the most
polluted megacities globally, Metro Manila offers a view of how diverse
sources and meteorology impact the relative amounts and size distributions
of these species. A total of 66 sample sets were collected with a
Micro-Orifice Uniform Deposit Impactor (MOUDI), of which 54 sets were
analyzed for composition. Organic acids and MSA surprisingly were less
abundant than in other global regions that are also densely populated. The
combined species accounted for an average of 0.80 ± 0.66 % of total
gravimetric mass between 0.056 and 18 µm, still leaving 33.74 % of
mass unaccounted for after considering black carbon and water-soluble ions
and elements. The unresolved mass is suggested to consist of
non-water-soluble metals as well as both water-soluble and non-water-soluble
organics. Oxalate was approximately an order of magnitude more abundant than
the other five species (149 ± 94 ng m−3 versus others being
< 10 ng m−3) across the 0.056–18 µm size range. Both
positive matrix factorization (PMF) and correlation analysis are conducted
with tracer species to investigate the possible sources of organic acids
and MSA. Enhanced biomass burning influence in the 2018 southwest monsoon
resulted in especially high levels of submicrometer succinate, MSA, oxalate,
and phthalate. Peculiarly, MSA had negligible contributions from marine
sources but instead was linked to biomass burning and combustion. Enhanced
precipitation during the two monsoon seasons (8 June–4 October 2018 and
14 June–7 October 2019) coincided with a stronger influence from local
emissions rather than long-range transport, leading to notable concentration
enhancements in both the sub- and supermicrometer ranges for some species
(e.g., maleate and phthalate). While secondary formation via gas-to-particle
conversion is consistent with submicrometer peaks for the organic acids and
MSA, several species (i.e., phthalate, adipate, succinate, oxalate)
exhibited a prominent peak in the coarse mode, largely owing to their
association with crustal emissions (i.e., more alkaline aerosol type) rather
than sea salt. Oxalate's strong association with sulfate in the
submicrometer mode supports an aqueous-phase formation pathway for the study
region. However, high concentrations during periods of low rain and high
solar radiation suggest photo-oxidation is an important formation pathway.
This work focuses on total organic carbon (TOC) and contributing species in cloud water over Southeast Asia using a rare airborne dataset collected during NASA's Cloud, Aerosol and Monsoon Processes ...Philippines Experiment (CAMP2Ex), in which a wide variety of maritime clouds were studied, including cumulus congestus, altocumulus, altostratus, and cumulus. Knowledge of TOC masses and their contributing species is needed for improved modeling of cloud processing of organics and to understand how aerosols and gases impact and are impacted by clouds. This work relies on 159 samples collected with an axial cyclone cloud-water collector at altitudes of 0.2–6.8 km that had sufficient volume for both TOC and speciated organic composition analysis. Species included monocarboxylic acids (glycolate, acetate, formate, and pyruvate), dicarboxylic acids (glutarate, adipate, succinate, maleate, and oxalate), methanesulfonic acid (MSA), and dimethylamine (DMA). TOC values range between 0.018 and 13.66 ppm C with a mean of 0.902 ppm C. The highest TOC values are observed below 2 km with a general reduction aloft. An exception is samples impacted by biomass burning for which TOC remains enhanced at altitudes as high as 6.5 km (7.048 ppm C). Estimated total organic matter derived from TOC contributes a mean of 30.7 % to total measured mass (inorganics + organics). Speciated organics contribute (on a carbon mass basis) an average of 30.0 % to TOC in the study region and account for an average of 10.3 % to total measured mass.The order of the average contribution of species to TOC, in decreasing contribution of carbon mass, is as follows (±1 standard deviation): acetate (14.7 ± 20.5 %), formate (5.4 ± 9.3 %), oxalate (2.8 ± 4.3 %), DMA (1.7 ± 6.3 %), succinate (1.6 ± 2.4 %), pyruvate (1.3 ± 4.5 %), glycolate (1.3 ± 3.7 %), adipate (1.0 ± 3.6 %), MSA (0.1 ± 0.1 %), glutarate (0.1 ± 0.2 %), and maleate (< 0.1 ± 0.1 %). Approximately 70 % of TOC remains unaccounted for, highlighting the complex nature of organics in the study region; in samples collected in biomass burning plumes, up to 95.6 % of TOC mass is unaccounted for based on the species detected. Consistent with other regions, monocarboxylic acids dominate the speciated organic mass (∼ 75 %) and are about 4 times more abundant than dicarboxylic acids.Samples are categorized into four cases based on back-trajectory history, revealing source-independent similarity between the bulk contributions of monocarboxylic and dicarboxylic acids to TOC (16.03 %–23.66 % and 3.70 %–8.75 %, respectively). Furthermore, acetate, formate, succinate, glutarate, pyruvate, oxalate, and MSA are especially enhanced during biomass burning periods, which is attributed to peat emissions transported from Sumatra and Borneo. Lastly, dust (Ca2+) and sea salt (Na+/Cl-) tracers exhibit strong correlations with speciated organics, supporting how coarse aerosol surfaces interact with these water-soluble organics.
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