Airborne measurements of volatile organic compounds (VOCs) were performed during CalNex 2010 (California Research at the Nexus of Air Quality and Climate Change) in the Los Angeles (LA) basin in ...May–June 2010 and during ITCT2k2 (Intercontinental Transport and Chemical Transformation) in May 2002. While CO2 enhancements in the basin were similar between the two years, the ΔCO/ΔCO2 ratio had decreased by about a factor of two. The ΔVOC/ΔCO emission ratios stayed relatively constant between the two years. This indicates that, relative to CO2, VOCs in the LA basin also decreased by about a factor of two since 2002. These data are compared with the results from various previous field campaigns dating back as early as 1960 and from the extensive air quality monitoring system in the LA basin going back to 1980. The results show that the mixing ratios of VOCs and CO have decreased by almost two orders of magnitude during the past five decades at an average annual rate of about 7.5%. Exceptions to this trend are the small alkanes ethane and propane, which have decreased slower due to the use and production of natural gas. A comparison with trends in London, UK shows that, due to stricter regulations at the time, VOC mixing ratios in LA decreased earlier than in London, albeit at a slower rate, such that typical mixing ratios in both cities in 2008 were at about the same level.
Key Points
VOCs and CO have decreased by a large factor in LA since 1960s
VOC emission ratios have not changed
Rate of decrease in London is more rapid, but started later
Halogen atoms and oxides are highly reactive and can profoundly affect atmospheric composition. Chlorine atoms can decrease the lifetimes of gaseous elemental mercury and hydrocarbons such as the ...greenhouse gas methane. Chlorine atoms also influence cycles that catalytically destroy or produce tropospheric ozone, a greenhouse gas potentially toxic to plant and animal life. Conversion of inorganic chloride into gaseous chlorine atom precursors within the troposphere is generally considered a coastal or marine air phenomenon. Here we report mid-continental observations of the chlorine atom precursor nitryl chloride at a distance of 1,400 km from the nearest coastline. We observe persistent and significant nitryl chloride production relative to the consumption of its nitrogen oxide precursors. Comparison of these findings to model predictions based on aerosol and precipitation composition data from long-term monitoring networks suggests nitryl chloride production in the contiguous USA alone is at a level similar to previous global estimates for coastal and marine regions. We also suggest that a significant fraction of tropospheric chlorine atoms may arise directly from anthropogenic pollutants.
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DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Atmospheric oxidation of natural and anthropogenic volatile organic compounds (VOCs) leads to secondary organic aerosol (SOA), which constitutes a major and often dominant component of atmospheric ...fine particulate matter (PM2.5). Recent work demonstrates that rapid autoxidation of organic peroxy radicals (RO₂) formed during VOC oxidation results in highly oxygenated organic molecules (HOM) that efficiently form SOA. As NOₓ emissions decrease, the chemical regime of the atmosphere changes to one in which RO₂ autoxidation becomes increasingly important, potentially increasing PM2.5, while oxidant availability driving RO₂ formation rates simultaneously declines, possibly slowing regional PM2.5 formation. Using a suite of in situ aircraft observations and laboratory studies of HOM, together with a detailed molecular mechanism, we show that although autoxidation in an archetypal biogenic VOC system becomes more competitive as NOₓ decreases, absolute HOM production rates decrease due to oxidant reductions, leading to an overall positive coupling between anthropogenic NOₓ and localized biogenic SOA from autoxidation. This effect is observed in the Atlanta, Georgia, urban plume where HOM is enhanced in the presence of elevated NO, and predictions for Guangzhou, China, where increasing HOM-RO₂ production coincides with increases in NO from 1990 to 2010. These results suggest added benefits to PM2.5 abatement strategies come with NOₓ emission reductions and have implications for aerosol–climate interactions due to changes in global SOA resulting from NOₓ interactions since the preindustrial era.
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The formation of secondary organic aerosol (SOA) in an anthropogenic‐influenced region in the southeastern United States is investigated by a comparison with urban plumes in the northeast. The ...analysis is based on measurements of fine‐particle organic compounds soluble in water (WSOC) as a measure of secondary organic aerosol. Aircraft measurements over a large area of northern Georgia, including the Atlanta metropolitan region, and in plumes from New York City and surrounding urban regions in the northeast show that fine‐particle WSOC are spatially correlated with vehicle emission tracers (e.g., CO), yet the measurements indicate that vehicles do not directly emit significant particulate WSOC. In addition to being correlated, WSOC concentrations were in similar proportions to anthropogenic tracers in both regions, despite biogenic volatile organic compounds (VOCs) that were on average 10–100 times higher over northern Georgia. In contrast, radiocarbon analysis on WSOC extracted from integrated filters deployed in Atlanta suggests that roughly 70–80% of the carbon in summertime WSOC is modern. If both findings are valid, the combined results indicate that in northern Georgia, fine‐particle WSOC was secondary and formed through a process that involves mainly modern biogenic VOCs but which is strongly linked to an anthropogenic component that may largely control the mass of SOA formed. Independent of the radiocarbon results, a strong association between SOA and anthropogenic sources has implications for control strategies in urban regions with large biogenic VOC emissions.
Tropospheric O3 has been decreasing across much of the eastern U.S. but has remained steady or even increased in some western regions. Recent increases in VOC and NOx emissions associated with the ...production of oil and natural gas (O&NG) may contribute to this trend in some areas. The Northern Front Range of Colorado has regularly exceeded O3 air quality standards during summertime in recent years. This region has VOC emissions from a rapidly developing O&NG basin and low concentrations of biogenic VOC in close proximity to urban‐Denver NOx emissions. Here VOC OH reactivity (OHR), O3 production efficiency (OPE), and an observationally constrained box model are used to quantify the influence of O&NG emissions on regional summertime O3 production. Analyses are based on measurements acquired over two summers at a central location within the Northern Front Range that lies between major regional O&NG and urban emission sectors. Observational analyses suggest that mixing obscures any OPE differences in air primarily influenced by O&NG or urban emission sector. The box model confirms relatively modest OPE differences that are within the uncertainties of the field observations. Box model results also indicate that maximum O3 at the measurement location is sensitive to changes in NOx mixing ratio but also responsive to O&NG VOC reductions. Combined, these analyses show that O&NG alkanes contribute over 80% to the observed carbon mixing ratio, roughly 50% to the regional VOC OHR, and approximately 20% to regional photochemical O3 production.
Key Points
Modeled photochemical O3 production is Nox‐sensitive at a central location in the Colorado Northern Front Range
Oil and natural gas VOC emissions contribute over 80% to the observed carbon mixing ratio and 17.4% to maximum modeled photochemical O3
Observed O3 production efficiencies are variable but show an influence of less than 1.8 ppbv/ppbv from oil and natural gas VOC emissions
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Airborne observations from the California Research at the Nexus of Air Quality and Climate Change (CalNex) campaign in May and June 2010 are used to investigate the role of ammonia (NH3) in fine ...particulate matter (PM2.5) formation and surface air quality in California and test the key processes relevant to inorganic aerosol formation in the GEOS‐Chem model. Concentrations of ammonia throughout California, sulfur dioxide (SO2) in the Central Valley, and ammonium nitrate in the Los Angeles (LA) area are underestimated several‐fold in the model. We find that model concentrations are relatively insensitive to uncertainties in gas‐particle partitioning and deposition processes in the region. Conversely, increases to anthropogenic livestock ammonia emissions (by a factor of 5) and anthropogenic sulfur dioxide emissions in the Central Valley (by a factor of 3–10) and a reduction of anthropogenic NOx emissions (by 30%) substantially reduce the bias in the simulation of gases (SO2, NH3, HNO3) throughout California and PM2.5 near LA, although the exact magnitudes of emissions in the region remain uncertain. Using these modified emissions, we investigate year‐round PM2.5 air quality in California. The model reproduces the wintertime maximum in surface ammonium nitrate concentrations in the Central Valley (regional mean concentrations are three times higher in December than in June), associated with lower planetary boundary layer heights and colder temperatures, and the wintertime minimum in the LA region (regional mean concentrations are two times higher in June than December) associated with ammonia limitation. Year round, we attribute at least 50% of the inorganic PM2.5 mass simulated throughout California to anthropogenic ammonia emissions.
Key Points
Simulate effects of ammonia and fine PM formation on surface air quality
Increasing livestock ammonia emissions reduces bias throughout California
Surface PM peaks in winter in Central Valley with summer peak near LA
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Recent studies suggest overestimates in current U.S. emission inventories of nitrogen oxides (NO x = NO + NO2). Here, we expand a previously developed fuel-based inventory of motor-vehicle emissions ...(FIVE) to the continental U.S. for the year 2013, and evaluate our estimates of mobile source emissions with the U.S. Environmental Protection Agency’s National Emissions Inventory (NEI) interpolated to 2013. We find that mobile source emissions of NO x and carbon monoxide (CO) in the NEI are higher than FIVE by 28% and 90%, respectively. Using a chemical transport model, we model mobile source emissions from FIVE, and find consistent levels of urban NO x and CO as measured during the Southeast Nexus (SENEX) Study in 2013. Lastly, we assess the sensitivity of ozone (O3) over the Eastern U.S. to uncertainties in mobile source NO x emissions and biogenic volatile organic compound (VOC) emissions. The ground-level O3 is sensitive to reductions in mobile source NO x emissions, most notably in the Southeastern U.S. and during O3 exceedance events, under the revised standard proposed in 2015 (>70 ppb, 8 h maximum). This suggests that decreasing mobile source NO x emissions could help in meeting more stringent O3 standards in the future.
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IJS, KILJ, NUK, PNG, UL, UM
We develop an improved retrieval of sulfur dioxide (SO2) vertical columns from two satellite instruments (SCIAMACHY and OMI) that measure ultraviolet solar backscatter. For each SCIAMACHY and OMI ...observation, a local air mass factor (AMF) algorithm converts line‐of‐sight “slant” columns to vertical columns using altitude‐dependent scattering weights computed with a radiative transfer model (LIDORT), weighted by relative vertical SO2 profile (shape factor) determined locally with a global atmospheric chemistry model (GEOS‐Chem). The scattering weights account for viewing geometry, surface albedo, cloud scattering, absorption by ozone, and scattering and absorption by aerosols. Absorption of radiation by mineral dust can reduce seasonal mean instrument sensitivity by 50%. Mean SO2 shape factors simulated with GEOS‐Chem and used in the AMF calculation are highly consistent with airborne in situ measurements (INTEX‐A and INTEX‐B); differences would affect the retrieved SO2 columns by 10%. The retrieved vertical columns are validated with coincident airborne in situ measurements (INTEX‐A, INTEX‐B, and a campaign over east China). The annual mean AMF errors are estimated to be 35–70% in polluted regions (e.g., East Asia and the eastern United States) and less than 10% over clear ocean regions. The overall SO2 error assessment is 45–80% for yearly averages over polluted regions. Seasonal mean SO2 columns retrieved from SCIAMACHY and OMI for 2006 are significantly spatially correlated with those from GEOS‐Chem, in particular over the United States (r = 0.85 for SCIAMACHY and 0.82 for OMI). A sensitivity study confirms the sensitivity of SCIAMACHY and OMI to anthropogenic SO2 emissions.
Objective
As inspired oxygen availability falls with ascent to altitude, some individuals develop high‐altitude headache (HAH). We postulated that HAH results when hypoxia‐associated increases in ...cerebral blood flow occur in the context of restricted venous drainage, and is worsened when cerebral compliance is reduced. We explored this hypothesis in 3 studies.
Methods
In high‐altitude studies, retinal venous distension (RVD) was ophthalmoscopically assessed in 24 subjects (6 female) and sea‐level cranial magnetic resonance imaging was performed in 12 subjects ascending to 5,300m. Correlation of headache burden (summed severity scores 0–4 ≤24 hours from arrival at each altitude) with RVD, and with cerebral/cerebrospinal fluid (CSF)/venous compartment volumes, was sought. In a sea‐level hypoxic study, 11 subjects underwent gadolinium‐enhanced magnetic resonance venography before and during hypoxic challenge (fraction of inspired oxygen = 0.11, 1 hour).
Results
In the high‐altitude studies, headache burden correlated with both RVD (Spearman rho = 0.55, p = 0.005) and with the degree of narrowing of 1 or both transverse venous sinuses (r = −0.56, p = 0.03). It also related inversely to both the lateral + third ventricle summed volumes (Spearman rho = −0.5, p = 0.05) and pericerebellar CSF volume (r = −0.56, p = 0.03). In the hypoxic study, cerebral and retinal vein engorgement were correlated, and rose as the combined conduit score fell (a measure of venous outflow restriction; r = ‐0.66, p < 0.05 and r = −0.75, p < 0.05, respectively).
Interpretation
Arterial hypoxemia is associated with cerebral and retinal venous distension, whose magnitude correlates with HAH burden. Restriction in cerebral venous outflow is associated with retinal distension and HAH. Limitations in cerebral venous efferent flow may predispose to headache when hypoxia‐related increases in cerebral arterial flow occur. ANN NEUROL 2013;73:381–389
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
During the Deepwater Horizon (DWH) oil spill, a wide range of gas and aerosol species were measured from an aircraft around, downwind, and away from the DWH site. Additional hydrocarbon measurements ...were made from ships in the vicinity. Aerosol particles of respirable sizes were on occasions a significant air quality issue for populated areas along the Gulf Coast. Yields of organic aerosol particles and emission factors for other atmospheric pollutants were derived for the sources from the spill, recovery, and cleanup efforts. Evaporation and subsequent secondary chemistry produced organic particulate matter with a mass yield of 8 ± 4% of the oil mixture reaching the water surface. Approximately 4% by mass of oil burned on the surface was emitted as soot particles. These yields can be used to estimate the effects on air quality for similar events as well as for this spill at other times without these data. Whereas emission of soot from burning surface oil was large during the episodic burns, the mass flux of secondary organic aerosol to the atmosphere was substantially larger overall. We use a regional air quality model to show that some observed enhancements in organic aerosol concentration along the Gulf Coast were likely due to the DWH spill. In the presence of evaporating hydrocarbons from the oil, NO ₓ emissions from the recovery and cleanup operations produced ozone.
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