Peroxyacetyl nitrate measurements during CITE 2: Atmospheric distribution and precursor relationships
H. B. SinghE. P. CondonJ. F. VedderD. O'HaraB. A. RidleyB. W. GandrudJ. D. ShetterLouis J. SalasB. J. HuebertG. HüblerMary Anne CarrollD. L. AlbrittonDouglas D. DavisJ. D. BradshawS. T. SandholmMichael O. RodgersSherwin M. BeckG. L. GregoryP. J. Lebel
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Aircraft measurements of peroxyacetyl nitrate (PAN) and other important reactive nitrogen species (NO, NO 2 , HNO 3 , and NO y ) were performed over the continental United States and the eastern Pacific during August–September 1986 at all altitudes between O and 6 km as part of CITE 2. PAN measurements were conducted by two independent groups, allowing both intercomparisons and greater confidence in its observed atmospheric structure. PAN was found to be a dominant reactive nitrogen species in the troposphere with 98% of the mixing ratios falling in a range of 5–400 ppt. Typically, the highest mixing ratios (100–300 ppt) were observed aloft (4–6 km) with extremely low values (5–20 ppt) in the marine boundary layer. In the lower troposphere, continental air contained significantly more PAN than marine air. The vertical structure of PAN was largely dictated by its thermal destruction rate and equilibrium with available NO 2 . PAN mixing ratios showed a high degree of variability in both continental and marine atmospheres. Westerly marine air trajectories did not guarantee well‐mixed air of uniform composition. Mixing ratios of O 3 , NO y , NO x , HNO 3 , C 2 H 6 , CO, and CFCl 3 were strongly correlated with those of PAN, indicating the important role played by transport processes. High PAN to NO x ratios in the mid‐troposphere further support the importance of long‐range transport from continental sources. Frequently, descending air masses from the upper troposphere suggested that PAN mixing ratios probably continued to increase above the 6‐km ceiling altitude. Air masses with O 3 <20 ppb, CO <60 ppb, and C 2 H 6 <500 ppt contained only miniscule amounts of PAN and are expected to be of tropical origin. Reasons for the observed PAN variability are discussed.Keywords:
Peroxyacetyl nitrate
Mixing ratio
Reactive nitrogen
We report here large‐scale features of the distribution of NO x , HNO 3 , PAN, particle NO 3 − , and NO y in the troposphere from 0.15 to 6 km altitude over central Canada. These measurements were conducted in July–August 1990 from the NASA Wallops Electra aircraft as part of the joint United States‐Canadian Arctic Boundary Layer Expedition (ABLE) 3B‐Northern Wetlands Study. Our findings show that this region is generally NO x limited, with NO x mixing ratios typically 20–30 parts per trillion by volume (pptv). We found little direct evidence for anthropogenic enhancement of mixing ratios of reactive odd nitrogen species and NO y above those in“background”air. Instead, it appears that enhancements in the mixing ratios of these species were primarily due to emissions from several day old or CO‐rich‐NO x ‐poor smoldering local biomass‐burning fires. NO x mixing ratios in biomass‐burning impacted air masses were usually <50 pptv, but those of HNO 3 and PAN were typically 100–300 pptv representing a twofold‐threefold enhancement over “background” air. During our study period, inputs of what appeared to be aged tropical air were a major factor influencing the distribution of reactive odd nitrogen in the midtroposphere over northeastern North America. These air masses were quite depleted in NO y (generally <150 pptv), and a frequent summertime occurrence of such air masses over this region would imply a significant influence on the reactive odd nitrogen budget. Our findings show that the chemical composition of aged air masses over subarctic Canada and those documented in the Arctic during ABLE 3A have strikingly similar chemistries, suggesting large‐scale connection between the air masses influencing these regions.
Reactive nitrogen
Mixing ratio
Nitrogen oxides
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Peroxyacetyl nitrate
Nitrogen oxides
Mixing ratio
Reactive nitrogen
Nitrogen oxides
Atmospheric chemistry
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Measurements of important reactive nitrogen species (NO, NO 2 , HNO 3 , PAN, PPN, NO 3 − , NO y ), C 1 to C 6 hydrocarbons, O 3 , chemical tracers (C 2 Cl 4 , CO), and meteorological parameters were made in the troposphere (0 to 12 km) over the western Pacific (0°–50°N) during the Pacific Exploratory Mission‐West A campaign (September–October 1991). Under clean conditions, mixing ratios of NO, NO 2 , NO y , and O 3 increased with altitude and showed a distinct latitudinal gradient. PAN showed a midtropospheric maximum, while nitric acid mixing ratios were generally highest near the surface. Measured NO y concentrations were significantly greater than the sum of individually measured nitrogen species (mainly NO x , PAN, and HNO 3 ), suggesting that a large fraction of reactive nitrogen present in the atmosphere is made up of hitherto unknown species. This shortfall was larger in the tropics (≈65%) compared to midlatitudes (≈40%) and was minimal in air masses with high HNO 3 mixing ratios (>100 ppt). A global three‐dimensional photochemical model has been used to compare observations with predictions and to assess the significance of major sources. It is possible that the tropical lightning source is much greater than commonly assumed, and both lightning source and its distribution remain a major area of uncertainty in the budgets of NO y and NO x . A large disagreement between measurement and theory exists in the atmospheric distribution of HNO 3 . It appears that surface‐based anthropogenic emissions provide nearly 65% of the global atmospheric NO y reservoir. Relatively constant NO x /NO y ratios imply that NO y and NO x are in chemical equilibrium and the NO y reservoir may be an important in situ source of atmospheric NO x . Data are interpreted to suggest that only about 20% of the upper tropospheric (7–12 km) NO x is directly attributable to its surface NO x source, and free tropospheric sources are dominant. In situ release of NO x from the NO y reservoir, lightning, direct transport of surface NO x , aircraft emissions, and small stratospheric input collectively maintain the NO x balance in the atmosphere. It is shown that atmospheric ratios of reactive nitrogen and sulfur species, along with trajectory analysis, can be used to pinpoint the source of Asian continental outflow. Compared to rural atmospheres over North America, air masses over the Pacific are highly efficient in net O 3 production. Sources of tropospheric NO x cannot yet be accurately defined due to shortcomings in measurements and theory.
Reactive nitrogen
Mixing ratio
Chemical Transport Model
Peroxyacetyl nitrate
Middle latitudes
Tropospheric ozone
Atmospheric chemistry
Lightning
Nitrogen dioxide
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The aim of this project was to calibrate a thermal-decomposition chemiluminescence (TD-Chem) instrument, capable of measuring the composition of the reactive nitrogen pool at the Cape Verde Atmospheric Observatory (CVAO) in the remote tropical troposphere. This data will be used in global atmospheric models in an attempt to better understand the sources of NOx in the remote troposphere and how these could affect background ozone (O3) levels.
Existing nitrogen oxides (NOx = NO + NO2) data from the CVAO was analysed for the measurement period of October 2006 to December 2011. The aim of this analysis was to identify the cause of the NO2 diurnal, which exhibits a maximum mixing ratio at solar noon, in contrast to the minimum expected due to NO2 photolysis in a clean environment. This anomaly was referred to as ΔNO2. It was found that between 4.12.2007 and 28.2.2009, ΔNO2 was significantly higher and caused an average increase in ΔNO2 of 5.09 ± 0.94 pptv to the entire dataset. This period corresponded with the orientation of the inlet, resulting in the heating of the sample and potentially significant levels of thermal dissociation of peroxyacetyl nitrate (PAN) to produce the NO2 observed. Future speciated measurements of the reactive nitrogen pool will help address the ΔNO2 anomaly fully.
Calibrations of the inlet in the TD-Chem instrument were carried out using PAN, n-propyl nitrate (NPN) and nitric acid (HNO3) standards to represent peroxyacyl nitrate (PNs), alkyl nitrate (ANs) and HNO3 reservoirs. Quantification of the standards was achieved using a molybdenum oven and a gold oven in conjunction with a small flow of carbon monoxide, both heated to 300 C. Both methods are known to cause ~ 100 % conversion of NOy compounds to NO, to allow detection via chemiluminescence (NOy = NOx + PNs, ANs, HNO3, aerosol nitrate, halogen nitrates etc.). Both ovens agreed on the concentration of the standards to > 99 %. Temperature ramp experiments quantified the temperature range at which each standard thermally dissociated to form NO2 and a companion radical in each of the quartz ovens used in the TD-Chem instrument. All experiments show thermal dissociation kinetics consistent with current understanding and kinetic theory. Deviations that do occur are either known and can be quantified, have been experimentally deduced, or a work schedule is in place in order to quantify them in the near future. Completion of the instrument calibration and subsequent installation of the TD-Chem instrument at the CVAO is projected to be in the summer of 2013.
Peroxyacetyl nitrate
Reactive nitrogen
Tropospheric ozone
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Peroxyacetyl nitrate
Mixing ratio
Reactive nitrogen
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Measurements of the title species were made during the Mauna Loa Observatory Photochemistry Experiment (MLOPEX) conducted between May 1 and June 4, 1988, at the Geophysical Monitoring for Climatic Change (GMCC) station at 3.4‐km elevation on the Island of Hawaii. Diurnal changes in the organic nitrates primarily resulted from the transition between downslope flow (usually free tropospheric air) and upslope flow (marine boundary layer or a mix of marine boundary layer and free tropospheric air, both influenced by island sources of precursors) characteristic of the site. Longer term trends in the mixing ratios reflected changes in air mass origins from mid‐latitudes to more tropical latitudes. The average mixing ratios in free tropospheric samples were peroxyacetyl nitrate (PAN, 17 pptv), peroxypropionyl nitrate (PPN, 0.3 pptv), methyl nitrate (MN, 4 pptv), and O 3 (43 ppbv). The organic nitrates (PAN, PPN, MN) represent minor components of the total odd nitrogen budget at the site. In free tropospheric samples, PAN, PPN, and MN constituted average percentages of 7%, <1%, and 2% of total odd nitrogen. In more tropical air masses, MN could constitute as much as 10% of total odd nitrogen. A photochemical model is used to investigate the sensitivity of free tropospheric PAN to local precursor concentrations. The observed mixing ratios of PAN are also contrasted with measurements made at continental surface sites and during aircraft programs.
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Measurements of peroxyacetyl nitrate (PAN), NO, NO 2 , HNO 3 , NO y , (total odd nitrogen), and O 3 were made in the high‐latitude troposphere over North America and Greenland (35° to 82°N) during the Arctic Boundary Layer Expedition (ABLE 3A) (July–August 1988) throughout 0‐to 6‐km altitudes. These data are analyzed to quantitatively describe the relationships between various odd nitrogen species and assess their significance to global tropospheric chemistry. In the free troposphere, PAN was as much as 25 times more abundant than NO x . PAN to NO x ratio increased with increasing altitude and latitude. PAN was found to be the single most abundant reactive nitrogen species in the free troposphere and constituted a major fraction of NO y , PAN to NO y ratios were about 0.1 in the boundary layer and increased to 0.4 in the free troposphere. A 2‐D global photochemical model with C 1 ‐C 3 hydrocarbon chemistry is used to compare model predictions with measured results. A sizable portion (≈50%) of the gaseous reactive nitrogen budget is unaccounted for, and unknown organic nitrates and pernitrates are expected to be present. Model calculations (August 1, 70°N) show that a major fraction of the observed NO x (50 to 70% of median) may find its source in the available PAN reservoir. PAN and the unknown reservoir species may have the potential to control virtually the entire NO x availability of the high latitude troposphere. It is predicted that the summer NO x and O 3 mixing ratios in the Arctic/sub‐Arctic troposphere would be considerably lower in the absence of the ubiquitous PAN reservoir. Conversely, this PAN reservoir may be responsible for the observed temporal increase in tropospheric O 3 at high latitudes.
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In-situ measurements of a large number of trace chemicals from upper troposphere/lower stratosphere (UT/LS) were performed with the NASA DC-8 aircraft during February/March 1994 over the Pacific Ocean (10 S to 60 N). Mixing ratios in the UT were relatively low in the warm tropical and subtropical air south of the polar jetstream (approx. =28 N) but increased sharply with latitude in the cold polar air north of the jetstream. At about 45 N, high concentrations of PAN (300 ppt) coexisted with extremely low (approx. = 20 ppt) concentrations of NOx. Elevated NOx levels in the UT did not always correspond to continental outflow conditions. Deepest penetrations into the stratosphere (550 ppb O3, 279 ppb NOx, and 350 K potential temperature) corresponded to a region that has been defined as the 'lowermost stratosphere' (LS) by Holton et al. Analysis of data shows that the mixing ratios of long-lived tracer species (e.g., CH4, HNO3, NOy, CFCs, HCFCs) are linearly correlated with those of O3 and N2O. A delta-NOY/delta-O3 of 0.0054 ppb/ppb and delta-NOy/delta-N2O of -0.081 ppb/ppb is in good agreement with other reported measurements from the DC-8. These slopes are however, somewhat steeper than those reported from the ER-2 studies. We find that the reactive nitrogen budget in the UT/LS is largely balanced with shortfalls that are no greater than 15%. A number of oxygenated species (e.g., acetone, H2O2) are present and may provide an important in-situ source of HOx in the UT/LS region.
Ozone Depletion
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High ozone (O3) concentrations at low altitudes (1.5–4 km) were detected from airborne Alpha Jet Atmospheric eXperiment (AJAX) measurements on 30 May 2012 off the coast of California (CA). We investigate the causes of those elevated O3 concentrations using airborne measurements and various models. GEOS-Chem simulation shows that the contribution from local sources is likely small. A back-trajectory model was used to determine the air mass origins and how much they contributed to the O3 over CA. Low-level potential vorticity (PV) from Modern Era Retrospective analysis for Research and Applications 2 (MERRA-2) reanalysis data appears to be a result of the diabatic heating and mixing of airs in the lower altitudes, rather than be a result of direct transport from stratospheric intrusion. The Q diagnostic, which is a measure of the mixing of the air masses, indicates that there is sufficient mixing along the trajectory to indicate that O3 from the different origins is mixed and transported to the western U.S. The back-trajectory model simulation demonstrates the air masses of interest came mostly from the mid troposphere (MT, 76%), but the contribution of the lower troposphere (LT, 19%) is also significant compared to those from the upper troposphere/lower stratosphere (UT/LS, 5%). Air coming from the LT appears to be mostly originating over Asia. The possible surface impact of the high O3 transported aloft on the surface O3 concentration through vertical and horizontal transport within a few days is substantiated by the influence maps determined from the Weather Research and Forecasting–Stochastic Time Inverted Lagrangian Transport (WRF-STILT) model and the observed increases in surface ozone mixing ratios. Contrasting this complex case with a stratospheric-dominant event emphasizes the contribution of each source to the high O3 concentration in the lower altitudes over CA. Integrated analyses using models, reanalysis, and diagnostic tools, allows high ozone values detected by in-situ measurements to be attributed to multiple source processes.
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