Brown carbon (BrC) associated with aerosol particles in western United
States wildfires was measured between July and August 2019 aboard the NASA
DC-8 research aircraft during the Fire Influence on ...Regional to Global
Environments and Air Quality (FIREX-AQ) study. Two BrC measurement methods
are investigated, highly spectrally resolved light absorption in solvent
(water and methanol) extracts of particles collected on filters and in situ
bulk aerosol particle light absorption measured at three wavelengths (405,
532 and 664 nm) with a photoacoustic spectrometer (PAS). A light-absorption
closure analysis for wavelengths between 300 and 700 nm was performed. The
combined light absorption of particle pure black carbon material, including
enhancements due to internally mixed materials, plus soluble BrC and a
Mie-predicted factor for conversion of soluble BrC to aerosol particle BrC,
was compared to absorption spectra from a power law fit to the three PAS
wavelengths. For the various parameters used, at a wavelength of roughly 400
nm they agreed, at lower wavelengths the individual component-predicted
particle light absorption significantly exceeded the PAS and at higher
wavelengths the PAS absorption was consistently higher but more variable.
Limitations with extrapolation of PAS data to wavelengths below 405 nm and
missing BrC species of low solubility that more strongly absorb at higher
wavelengths may account for the differences. Based on measurements closest
to fires, the emission ratio of PAS-measured BrC at 405 nm relative to
carbon monoxide (CO) was on average 0.13 Mm−1 ppbv−1; emission
ratios for soluble BrC are also provided. As the smoke moved away from the
burning regions, the evolution over time of BrC was observed to be highly
complex; BrC enhancement, depletion or constant levels with age were all
observed in the first 8 h after emission in different plumes. Within 8 h following emissions, 4-nitrocatechol, a well-characterized BrC
chromophore commonly found in smoke particles, was largely depleted relative
to the bulk BrC. In a descending plume where temperature increased by 15 K,
4-nitrocatechol dropped, possibly due to temperature-driven evaporation, but
bulk BrC remained largely unchanged. Evidence was found for reactions with
ozone, or related species, as a pathway for secondary formation of BrC under
both low and high oxides of nitrogen (NOx) conditions, while BrC was
also observed to be bleached in regions of higher ozone and low NOx,
consistent with complex behaviors of BrC observed in laboratory studies.
Although the evolution of smoke in the first hours following emission is
highly variable, a limited number of measurements of more aged smoke (15 to
30 h) indicate a net loss of BrC. It is yet to be determined how the
near-field BrC evolution in smoke affects the characteristics of smoke over
longer timescales and spatial scales, where its environmental impacts are likely
to be greater.
Aircraft observations of meteorological, trace gas, and aerosol properties were made during May–September 2013 in the southeastern United States (US) under fair-weather, afternoon conditions with ...well-defined planetary boundary layer structure. Optical extinction at 532 nm was directly measured at relative humidities (RHs) of ∼ 15, ∼ 70, and ∼ 90 % and compared with extinction calculated from measurements of aerosol composition and size distribution using the κ-Köhler approximation for hygroscopic growth. The calculated enhancement in hydrated aerosol extinction with relative humidity, f(RH), calculated by this method agreed well with the observed f(RH) at ∼ 90 % RH. The dominance of organic aerosol, which comprised 65 ± 10 % of particulate matter with aerodynamic diameter < 1 µm in the planetary boundary layer, resulted in relatively low f(RH) values of 1.43 ± 0.67 at 70 % RH and 2.28 ± 1.05 at 90 % RH. The subsaturated κ-Köhler hygroscopicity parameter κ for the organic fraction of the aerosol must have been < 0.10 to be consistent with 75 % of the observations within uncertainties, with a best estimate of κ = 0.05. This subsaturated κ value for the organic aerosol in the southeastern US is broadly consistent with field studies in rural environments. A new, physically based, single-parameter representation was developed that better described f(RH) than did the widely used gamma power-law approximation.
Chloride (Cl−) displacement from sea salt particles is an extensively studied phenomenon with implications for human health, visibility, and the global radiation budget. Past works have investigated ...Cl− depletion over the northwest Atlantic (NWA); however, an updated, multi-seasonal, and geographically expanded account of sea salt reactivity over the region is needed. This study uses chemically resolved mass concentrations and meteorological data from the airborne Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) to quantify seasonal, spatial, and meteorological trends in Cl− depletion and to explore the importance of quantifying (1) non-sea salt sources of Na+ and (2) mass concentrations of lost Cl− (instead of relative amounts displaced). Lost Cl− mass concentrations are lowest in December–February and March, moderate around Bermuda in June, and highest in May (median losses of 0.04, 0.04, 0.66, and 1.76 µg m−3, respectively), with losses in May that are high enough to potentially accelerate tropospheric oxidation rates. Inorganic acidic species can account for all Cl− depletion in December–February, March, and June near Bermuda but none of the lost Cl− in May, suggesting that organic acids may be of importance for Cl− displacement in certain months. Contributions of dust to Na+ are not important seasonally but may cause relevant overestimates of lost Cl− in smoke and dust plumes. Higher percentages of Cl− depletion often do not correspond to larger mass concentrations of lost Cl−, so it is highly recommended to quantify the latter to place depletion reactions in context with their role in atmospheric oxidation and radiative forcing.
We use aircraft observations obtained during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) mission to examine the distributions and source ...attributions of O3 and NOy in the Arctic and sub-Arctic region. Using a number of marker tracers, we distinguish various air masses from the background troposphere and examine their contributions to NOx, O3, and O3 production in the Arctic troposphere. The background Arctic troposphere has a mean O3 of ~60 ppbv and NOx of ~25 pptv throughout spring and summer with CO decreasing from ~145 ppbv in spring to ~100 ppbv in summer. These observed mixing ratios are not notably different from the values measured during the 1988 ABLE-3A and the 2002 TOPSE field campaigns despite the significant changes in emissions and stratospheric ozone layer in the past two decades that influence Arctic tropospheric composition. Air masses associated with stratosphere-troposphere exchange are present throughout the mid and upper troposphere during spring and summer. These air masses, with mean O3 concentrations of 140–160 ppbv, are significant direct sources of O3 in the Arctic troposphere. In addition, air of stratospheric origin displays net O3 formation in the Arctic due to its sustainable, high NOx (75 pptv in spring and 110 pptv in summer) and NOy (~800 pptv in spring and ~1100 pptv in summer). The air masses influenced by the stratosphere sampled during ARCTAS-B also show conversion of HNO3 to PAN. This active production of PAN is the result of increased degradation of ethane in the stratosphere-troposphere mixed air mass to form CH3CHO, followed by subsequent formation of PAN under high NOx conditions. These findings imply that an adequate representation of stratospheric NOy input, in addition to stratospheric O3 influx, is essential to accurately simulate tropospheric Arctic O3, NOx and PAN in chemistry transport models. Plumes influenced by recent anthropogenic and biomass burning emissions observed during ARCTAS show highly elevated levels of hydrocarbons and NOy (mostly in the form of NOx and PAN), but do not contain O3 higher than that in the Arctic tropospheric background except some aged biomass burning plumes sampled during spring. Convection and/or lightning influences are negligible sources of O3 in the Arctic troposphere but can have significant impacts in the upper troposphere in the continental sub-Arctic during summer.
High‐resolution in situ CO2 measurements were conducted aboard the NASA DC‐8 aircraft during the ARCTAS/POLARCAT field campaign, a component of the wider 2007–2008 International Polar Year ...activities. Data were recorded during large‐scale surveys spanning the North American sub‐Arctic to the North Pole from 0.04 to 12 km altitude in spring and summer of 2008. Influences on the observed CO2 concentrations were investigated using coincident CO, black carbon, CH3CN, HCN, O3, C2Cl4, and Δ14CO2 data, and the FLEXPART model. In spring, the CO2 spatial distribution from 55°N to 90°N was largely determined by the long‐range transport of air masses laden with Asian anthropogenic pollution intermingled with Eurasian fire emissions evidenced by the greater variability in the mid‐to‐upper troposphere. At the receptor site, the enhancement ratios of CO2 to CO in pollution plumes ranged from 27 to 80 ppmv ppmv−1 with the highest anthropogenic content registered in plumes sampled poleward of 80°N. In summer, the CO2 signal largely reflected emissions from lightning‐ignited wildfires within the boreal forests of northern Saskatchewan juxtaposed with uptake by the terrestrial biosphere. Measurements within fresh fire plumes yielded CO2 to CO emission ratios of 4 to 16 ppmv ppmv−1 and a mean CO2 emission factor of 1698 ± 280 g kg−1 dry matter. From the 14C in CO2 content of 48 whole air samples, mean spring (46.6 ± 4.4‰) and summer (51.5 ± 5‰) Δ14CO2 values indicate a 5‰ seasonal difference. Although the northern midlatitudes were identified as the emissions source regions for the majority of the spring samples, depleted Δ14CO2 values were observed in <1% of the data set. Rather, ARCTAS Δ14CO2 observations (54%) revealed predominately a pattern of positive disequilibrium (1–7‰) with respect to background regardless of season owing to both heterotrophic respiration and fire‐induced combustion of biomass. Anomalously enriched Δ14CO2 values (101–262‰) measured in emissions from Lake Athabasca and Eurasian fires speak to biomass burning as an increasingly important contributor to the mass excess in Δ14CO2 observations in a warming Arctic, representing an additional source of uncertainty in the quantification of fossil fuel CO2.
Key Points
LRT of CO2 emissions to North Pole
Enriched 14CO2 from fires offsetting ff CO2
CO2 Emission Factor 1634 g per kg DM
This work presents results from the NASA Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) study. Whole air samples were obtained on board research flights ...that flew over California during June 2008 and analyzed for selected volatile organic compounds, including several halogenated species. Samples collected over the South Coast Air Basin of California (SoCAB), which includes much of Los Angeles (LA) County, were compared with samples from inflow air masses over the Pacific Ocean. The levels of many halocarbon species were enhanced significantly over the SoCAB, including compounds regulated by the Montreal Protocol and subsequent amendments. Emissions estimates of HFC-152a (1,1-difluoroethane, CH3CHF2; 0.82 ± 0.11 Gg) and HFC-134a (1,1,1,2-tetrafluoroethane, CH2FCF3; 1.16 ± 0.22 Gg) in LA County for 2008 were obtained using the observed HFC:carbon monoxide (CO) enhancement ratio. Emission rates also were calculated for the SoCAB (1.60 ± 0.22 Gg yr−1 for HFC-152a and 2.12 ± 0.28 Gg yr−1 for HFC-134a) and then extrapolated to the United States (32 ± 4 Gg yr−1 for HFC-152a and 43 ± 6 Gg yr−1 for HFC-134a) using population data. In addition, emission rates of the two HFCs in LA County and SoCAB were calculated by a second method that utilizes air quality modeling. Emissions estimates obtained using both methods differ by less than 25% for the LA County and less than 45% for the SoCAB.
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.
This work evaluates the IASI CO product against independent in-situ aircraft data from the MOZAIC program and the POLARCAT aircraft campaign. The validation is carried out by analysing the impact of ...assimilation of eight months of IASI CO columns retrieved for the period of May to December 2008 into the global chemistry transport model LMDz-INCA. A modelling system based on a sub-optimal Kalman filter was developed and a specific treatment that takes into account the representativeness of observations at the scale of the model grid is applied to the IASI CO columns and associated errors before their assimilation in the model. Comparisons of the assimilated CO profiles with in situ CO measurements indicate that the assimilation leads to a considerable improvement of the model simulations in the middle troposphere as compared with a control run with no assimilation. Model biases in the simulation of background values are reduced and improvement in the simulation of very high concentrations is observed. The improvement is due to the transport by the model of the information present in the IASI CO retrievals. Our analysis also shows the impact of assimilation of CO on the representation of transport into the Arctic region during the POLARCAT summer campaign. A considerable increase in CO mixing ratios over the Asian source region was observed when assimilation was used leading to much higher values of CO during the cross-pole transport episode. These higher values are in good agreement with data from the POLARCAT flights that sampled this plume.
Meteorological and air-quality model simulations are analyzed alongside observations to investigate the role of the Chesapeake Bay breeze on surface air quality, pollutant transport, and boundary ...layer venting. A case study was conducted to understand why a particular day was the only one during an 11-day ship-based field campaign on which surface ozone was not elevated in concentration over the Chesapeake Bay relative to the closest upwind site and why high ozone concentrations were observed aloft by in situ aircraft observations. Results show that southerly winds during the overnight and early-morning hours prevented the advection of air pollutants from the Washington, D.C., and Baltimore, Maryland, metropolitan areas over the surface waters of the bay. A strong and prolonged bay breeze developed during the late morning and early afternoon along the western coastline of the bay. The strength and duration of the bay breeze allowed pollutants to converge, resulting in high concentrations locally near the bay-breeze front within the Baltimore metropolitan area, where they were then lofted to the top of the planetary boundary layer (PBL). Near the top of the PBL, these pollutants were horizontally advected to a region with lower PBL heights, resulting in pollution transport out of the boundary layer and into the free troposphere. This elevated layer of air pollution aloft was transported downwind into New England by early the following morning where it likely mixed down to the surface, affecting air quality as the boundary layer grew.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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