Nitric Acid Mixing Ratios Research Articles

Simultaneous airborne nitric acid and formic acid measurements using a chemical ionization mass spectrometer around the UK: Analysis of primary and secondary production pathways

The first simultaneous measurements of formic and nitric acid mixing ratios around the United Kingdom were measured on the FAAM BAe-146 research aircraft with a chemical ionization mass spectrometer using I− reagent ions at 0.8 Hz. Analysis of the whole dataset shows that formic acid and nitric acid are positively correlated as illustrated by other studies (e.g. Veres et al., 2011). However, initial evidence indicates a prominent direct source of formic acid and also a significant source when O3 levels are high, suggesting the importance of the ozonolysis of 1-alkenes. A trajectory model was able to reproduce the formic acid concentrations by both the inclusion of a primary vehicle source and production via ozonolysis of propene equivalent 1-alkene levels. Inspection of data archives implies these levels of 1-alkene are possible after 11 am, but formic acid and nitric acid plumes early in the flight are too high for the model to replicate. These data show the relationship between nitric acid and formic acid cannot solely be attributed to related photochemical production. The simultaneous measurement of HCOOH and HNO3 has been implemented to estimate OH levels along the flight track assuming a relationship between formic and nitric acid in photochemical plumes and a constant source of 1-alkene.

Growth of upper tropospheric aerosols due to uptake of HNO<sub>3</sub>

Abstract. The effect of nitric acid on the equilibrium size distributions of upper tropospheric aerosols is calculated as a function of relative humidity. It is shown that HNO3 concentrations above a few tenths of a ppb can cause substantial increases in haze mode particle concentrations at relative humidities at about 60% and above. The effect can be strongly magnified when letovicite particles are present in addition to sulfuric acid aerosols. Letovicite particles are less acidic than the sulfuric acid particles and so more nitric acid can be absorbed. This effect can be seen even at RH below 50% due to the lowering of the deliquescence RH of letovicite in the presence of gaseous nitric acid at low temperatures. We have also compared equilibrium calculations of the HNO3 effect with observations of increased haze mode concentrations at relative humidities above 50% (Petzold et al., 2000). Nitric acid mixing ratios on the order of 0.5-2ppb may explain the observed increase of haze mode particles at least partially.

Composition and distribution of aerosols over the North Atlantic during the Subsonic Assessment Ozone and Nitrogen Oxide Experiment (SONEX)

We report the mixing ratios of aerosol‐associated soluble ions (focusing on SO4= and NO3−) and HNO3 over the North Atlantic during NASA's Subsonic Assessment Ozone and Nitrogen Oxide Experiment (SONEX). The SONEX campaign was designed to quantify the impacts of jet emissions in the North Atlantic Flight Corridor (NAFC) by sampling both directly within and far removed from the organized track system. Beryllium‐7 activities were also measured to assess the magnitude of stratospheric influence in the SONEX study region. Mixing ratios of aerosol‐associated SO4= and NO3− above 8 km during SONEX were lower than recent measurements over the central United States during the Subsonic Aircraft Contrail and Cloud Effects Special Study (SUCCESS) and the same as those over the remote South Pacific during the Pacific Exploratory Mission‐Tropics (PEM‐Tropics), suggesting that aircraft emissions cannot yet be a major source of these ions. Furthermore, mean SO4= mixing ratios at high altitudes were 65% higher in regions away from the NAFC than they were directly in the track system just a few hours after peak traffic. Nitric acid mixing ratios at the highest DC‐8 sampling altitudes were elevated during SONEX compared to PEM‐Tropics, but there was no clear signal of enhancement by jet exhaust. Strong correlations with 7Be indicate that a large fraction of HNO3 and aerosol‐associated SO4= measured at high altitudes during SONEX were derived from a stratospheric source.

Determination of gas-phase nitric acid using a tracer technique

A tracer technique has been developed in this laboratory which allows the measurement of aqueous-phase oxidation of SO 2 in clouds. We are now extending this technique to the chemistry of nitrogen species in clouds. Using a conservative tracer such as selenium we can determine the fraction of cloudwater nitrate that is the result of aerosol scavenging. At remote sites this allows the calculation of the nitric acid mixing ratio in the precloud air from measurements made during a cloud event. We have used this technique to determine the pre-cloud nitric acid partial pressure at Whiteface Mountain (WFM) for three cloud events. The calculated concentrations for an event on 2 August 1997 agree with measurements made at a second site below cloud level using filterpacks. Nitric acid concentrations for cloud events during July 1995, yielded values of <0.1–10.0 ppbv. At highly polluted sites closer to NO x sources the technique may provide a method of determining if there is any aqueous-phase oxidation of N(III) occurring.

Stratospheric chemical ionization mass spectrometry: nitric acid detection by different ion molecule reaction schemes

Detailed height profiles of stratospheric nitric acid mixing ratios have been derived with a baloon borne chemical ionization mass spectrometer by applying several ion molecule reaction schemes, each associated to a specific and selective ion source. These ions (CO 3 −, Cl n −, CF 3O −, and CF 3O −H 2O) give rise to specific product ions (mainly CO 3 −HNO 3, NO 3 −HCl, NO 3 −HF, and CF 3O −HNO 3) upon reaction with ambient nitric acid molecules. This paper reports on the instrumental details as well as on the results obtained during two balloon flights with the instrument. Within the accuracy of the measurements, the nitric acid height profiles obtained with the three different ion sources are in good agreement with one another as well as with literature data.

Nitric acid (HNO3) in the upper troposphere and lower stratosphere at midlatitudes: New results from aircraft‐based mass spectrometric measurements

Extensive measurements of gaseous nitric acid (HNO3) have been performed in the upper troposphere and lower stratosphere using aircraft‐based ion‐molecule reaction mass spectrometry (IMRMS). The measurements, which took place in summer and winter between November 1994 and July 1996, cover latitudes between 29°N and 57°N and altitudes between 5.5 and 13 km. According to meteorological analyses, potential vorticity values up to 9.5 PVU (1 PVU = 10−6 m2 s−1 K kg−1) were reached. Stratospheric HNO3 mixing ratios ranged between 300 and 2200 parts per trillion by volume (pptv). In the upper troposphere, nitric acid mixing ratios ranged between 100 and 2000 pptv, with the largest values influenced by fast vertical transport from the planetary boundary layer. These values exceed previous measurements and model calculations. The relatively high upper tropospheric nitric acid mixing ratio indicates a large rate for NOx recycling from gaseous nitric acid, and possibly also an increased efficiency of aerosol activation, which may lead to an increased albedo of cirrus clouds.

Reactive nitrogen and ozone over the western Pacific: Distribution, partitioning, and sources

Measurements of important reactive nitrogen species (NO, NO2, HNO3, PAN, PPN, NO3−, NOy), C1 to C6 hydrocarbons, O3, chemical tracers (C2Cl4, 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, NO2, NOy, and O3 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 NOy concentrations were significantly greater than the sum of individually measured nitrogen species (mainly NOx, PAN, and HNO3), 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 HNO3 mixing ratios (&gt;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 NOy and NOx. A large disagreement between measurement and theory exists in the atmospheric distribution of HNO3. It appears that surface‐based anthropogenic emissions provide nearly 65% of the global atmospheric NOy reservoir. Relatively constant NOx/NOy ratios imply that NOy and NOx are in chemical equilibrium and the NOy reservoir may be an important in situ source of atmospheric NOx. Data are interpreted to suggest that only about 20% of the upper tropospheric (7–12 km) NOx is directly attributable to its surface NOx source, and free tropospheric sources are dominant. In situ release of NOx from the NOy reservoir, lightning, direct transport of surface NOx, aircraft emissions, and small stratospheric input collectively maintain the NOx 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 O3 production. Sources of tropospheric NOx cannot yet be accurately defined due to shortcomings in measurements and theory.

Development of a semi-continuous method for the measurement of nitric acid vapor and particulate nitrate and sulfate

An automated method for the measurement of nitric acid vapor, particulate nitrate, and particulate sulfate in the atmosphere has been developed. The method is based on a semi-continuous wet effluent denuder (WED) and wet effluent frit (WEF) sampling system, followed by ion chromatography with conductivity detection. The method provides separate measures of nitric acid and particulate nitrate, and is characterized by a sample frequency of 15 min and a detection limit of 10 pptv (parts per trillion by volume) for each species. A variety of laboratory tests used to characterize the system performance are described. Among them, the particle passing efficiency of the denuder was evaluated using NH 4NO 3 particles (diameters < 1 μm) generated by a new method. Ambient measurements using this method were compared with the Nylon and Teflon filter pack method at three field sites: a loblolly pine forest in rural Alabama; a suburban site in Boulder, CO; and a rural mountain site: near Nederland, CO. The methods intercomparison was carried out at sites with varying nitric acid mixing ratios, with the whole data set ranging from < 0.1 to 4 ppbv nitric acid. The nitric acid measurements collected using the WED method compared very well with the filter pack method (linear regression results: slope = 1.00 ± 0.05, r = 0.97). Particulate nitrate and sulfate measurements were compared pnly in Alabama. The particulate sulfate measured with the wet effluent frit and with the filter pack agreed within 12% on average. The particulate nitrate measured using the frit was much higher than that measured by the filter pack, and in addition, the WEF nitrate exhibited a strong diurnal variation while the filter pack nitrate did not. Principal component analysis has been performed on the data set as a means to elucidate the cause of this difference, and to examine the atmospheric variability observed during the Alabama study.

Polar stratospheric clouds at the South Pole in 1990: Lidar observations and analysis

In December 1989 a Rayleigh/sodium lidar (589 nm) was installed at the Amundsen‐Scott South Pole station, and was used to measure stratospheric aerosol, temperature, and mesospheric sodium profiles through October 1990. Observations of stratospheric aerosol and temperature are presented in this paper. Polar stratospheric clouds (PSCs) were first observed in late May at about 20 km. As the lower stratosphere cooled further, PSCs were observed throughout the 12–27 km altitude region, and remained there from mid‐June until late August. Observations in early September detected no PSCs above 21 km. An isolated cloud was observed in mid‐October. Throughout the winter the clouds had small backscatter ratios (&lt; 10). Observations made at two wavelengths in July show that the clouds are predominately composed of nitric acid trihydrate with associated Angstrom coefficients between 0.2 and 3.7. Comparison of the lidar data and balloon borne frost point measurements in late August indicate that the nitric acid mixing ratio was less than 1.5 ppbv. Observations over periods of several hours show downward motions in the cloud layers similar to the phase progressions of upwardly‐propagating gravity waves. The vertical phase velocities of these features (≈ 4 cm/s) are significantly faster than the expected settling velocities of the cloud particles. Both the backscatter ratio profiles and the radiosonde horizontal wind profiles show 1–4 km vertical structures. This suggests that the kilometer‐scale vertical structure of the PSCs is maintained by low frequency gravity waves propagating through the cloud layers.

Radiative effects of polar stratospheric clouds during the Airborne Antarctic Ozone Experiment and the Airborne Arctic Stratospheric Expedition

Daily, three dimensional fields of both Type I nitric acid trihydrate (NAT) and Type II water ice polar stratospheric clouds (PSCs) have been generated in the polar regions during the period of the Airborne Antarctic Ozone Experiment (AAOE) and the Airborne Arctic Stratospheric Expedition (AASE) aircraft missions. Mission data on particulate composition and size, together with National Meteorological Center (NMC) analyzed temperatures, have been used. A comparison of computed l‐μm extinction coefficients with those observed by the Stratospheric Aerosol Measurement (SAM II) satellite served to constrain the water ice particle size and the nitric acid mixing ratio profiles. For AAOE, both Type I and Type II clouds were formed for the time period August 23 to September 17, after which only Type I clouds formed. During AASE, while Type I clouds were formed for each day between January 3 and February 10, Type II clouds formed on only two days, January 24 and 31. Mie theory and a radiative transfer model were used to compute the radiative heating rates during the mission periods, for clear and cloudy lower sky cases. Only the Type II water ice clouds have a significant radiative effect, with the Type I NAT PSCs generating a net heating or cooling of 0.1 K/d or less. The major effect of PSCs during AAOE appears to be one of net heating, with maximum heating rates as large as 0.5 K/d for particular days. Heating rates of 0.3 K/d or greater were computed during the periods September 1–6 and September 11–14. This additional heating was generally found over the western part of Antarctica and near the coastal regions, while a small cooling was computed over the Antarctic Plateau. However, when the heating rates are averaged over the AAOE mission, a net heating on the order of only 0.1 K/d is generated by the PSCs over western Antarctica, with other regions exhibiting a very small additional cooling, less than 0.05 K/d. Tropospheric clouds turn this net heating to a small amount of net cooling, roughly 0.1 K/d. The average effect of PSCs to the radiative budget during AASE is extremely small. Therefore, on the whole, the radiative effects of PSCs appear to be too small to have an appreciable impact on vertical motions except over time scales of a few days during AAOE. This result makes it unlikely that the radiative‐dynamical mechanisms proposed for the Antarctic ozone depletion play any significant role.

A search for trends in measurements of stratospheric nitric acid, hydrochloric acid and methane

The nitric acid mixing ratio at 25 km, close to the concentration peak of the nitric acid layer, has been measured with a balloon-borne spectro-radiometer from 1974 to 1987 over a complete solar cycle. The mixing ratio shows an 8% variation over this cycle, with a minimum value close to the maximum solar activity at the end of 1979. There is no obvious long-term trend evident in the data. Interferograms recorded on balloon flights from 1976 to 1985 have been processed to yield altitude profiles for hydrochloric acid and methane. An analysis of these profiles has not revealed any significant trends in either of these gases. However, the trend exhibited by the hydrochloric acid is consistent with our scenario model which gives a 3% increase per year in the column amount above 23 km.

Response of middle atmosphere to short‐term solar ultraviolet variations: 2. Theory

Ozone and temperature responses to solar variability, based on satellite data, have been reported in a companion paper (Keating et al., this issue). The present paper is intended to present a theoretical interpretation of this analysis with the purpose of better understanding the chemical behavior of the stratosphere and the coupling between temperature and ozone concentration, when a periodic forcing is applied to the solar ultraviolet (UV) flux. The response of the temperature and of the trace species concentrations, including ozone, to short‐term variations in the solar UV irradiance is calculated by a one‐dimensional chemical‐radiative time‐dependent model. The applied solar variability is assumed to be sinusoidal with a period of 27 days (in accordance with the rotation period of the sun) or 13.5 days (when two active regions are on opposite sides of the sun). The amplitude varies with wavelength, which is consistent with observations made by the Nimbus 7 solar backscattered ultraviolet (SBUV) experiment. The maximum ozone sensitivity in the stratosphere appears to be located near 3 mbar. The calculated amplitude and phase of the ozone response are significantly modified when the feedback between ozone and temperature is taken into account. The ozone/temperature coupling tends to modify the ozone phase lag such that, in the upper stratosphere and in the mesosphere, the ozone peak occurs a few days before the UV peak. Comparison of the model results with the observed ozone and temperature responses, based on satellite data, shows that the theory is consistent in many respects with observations. The calculated time lag of the temperature response, however, is approximately a factor of 2–4 smaller than the time lags derived from the measurements, suggesting evidence for some additional process not included in the model calculation. Large negative ozone sensitivities and positive temperature responses are predicted in the mesosphere as a result of the absorption by O2 of the solar irradiance at the Lyman α wavelength. The model also shows that the expected variation in the stratospheric nitric acid mixing ratio is a factor 2 larger than the corresponding opposite variation in the ozone concentration.