Clean Tropospheric Air Research Articles

Overshooting of clean tropospheric air in the tropical lower stratosphere as seen by the CALIPSO lidar

Abstract. The evolution of aerosols in the tropical upper troposphere/lower stratosphere between June 2006 and October 2009 is examined using the observations of the space borne CALIOP lidar aboard the CALIPSO satellite. Superimposed on several volcanic plumes and soot from an extreme biomass-burning event in 2009, the measurements reveal the existence of fast-cleansing episodes in the lower stratosphere to altitudes as high as 20 km. The cleansing of the layer, which extends from 14 to 20 km, takes place within 1 to 4 months during the southern tropics convective season that transports aerosol-poor tropospheric air into the lower stratosphere. In contrast, the convective season of the Northern Hemisphere summer shows an increase in the particle load at the tropopause consistent with a lofting of air rich with aerosols. These aerosols can consist of surface-derived material such as mineral dust and soot as well as liquid sulfate and organic particles. The flux of tropospheric air during the Southern Hemisphere convective season derived from CALIOP observations is shown to be 5 times at 16 km and 20 times at 19 km larger, respectively, than that associated with flux caused by slow ascent through radiative heating. These results suggest that convective overshooting is a major contributor to troposphere-to-stratosphere transport with concomitant implications for the Tropical Tropopause Layer top height, the humidity, the photochemistry and the thermal structure of the layer.

High frequency new particle formation in the Himalayas

Rising air pollution levels in South Asia will have worldwide environmental consequences. Transport of pollutants from the densely populated regions of India, Pakistan, China, and Nepal to the Himalayas may lead to substantial radiative forcing in South Asia with potential effects on the monsoon circulation and, hence, on regional climate and hydrological cycles, as well as to dramatic impacts on glacier retreat. An improved description of particulate sources is needed to constrain the simulation of future regional climate changes. Here, the first evidence of very frequent new particle formation events occurring up to high altitudes is presented. A 16-month record of aerosol size distribution from the Nepal Climate Observatory at Pyramid (Nepal, 5,079 m above sea level), the highest atmospheric research station, is shown. Aerosol concentrations are driven by intense ultrafine particle events occurring on >35% of the days at the interface between clean tropospheric air and the more polluted air rising from the valleys. During a pilot study, we observed a significant increase of ion cluster concentrations with the onset of new particle formation events. The ion clusters rapidly grew to a 10-nm size within a few hours, confirming, thus, that in situ nucleation takes place up to high altitudes. The initiation of the new particle events coincides with the shift from free tropospheric downslope winds to thermal upslope winds from the valley in the morning hours. The new particle formation events represent a very significant additional source of particles possibly injected into the free troposphere by thermal winds.

Determination of the Tropospheric Hydroxyl Radical by Liquidphase Scrubbing and HPLC: Preliminary Results

A preliminary study was carried out toexamine the feasibility of measuring tropospherichydroxyl radicals (OH) by liquidphase scrubbing andhigh performance liquid chromatography (HPLC). Thepotential advantages of this approach are itssimplicity, portability, and low expense. Thesampling system employs glass bubblers to trapatmospheric OH into a buffered solution of salicylicacid (o-hydroxybenzoic acid, OHBA). Rapidreaction of OH with OHBA produces a stable fluorescentproduct, 2,5-dihydroxybenzoic acid (2,5-DHBA), whichis determined by reverse-phase HPLC and fluorescencedetection. Our preliminary field results indicatethat this method is most suitable for OH measurementsin clean tropospheric air, where interferences fromother atmospheric species appear to be negligible orminor relative to polluted air. In clean air, thesampling period is about 45–90 minutes, which yieldsa detection limit of approximately 3–6 ×105 radicalscm-3. During an OHintercomparison experiment at the Caribou samplingsite in Colorado, our liquidphase scrubber method wascompared with the ion-assisted mass spectrometry (MS)method. Our results were within the same range asthose of the ion-assisted MS method (1–5 ×106 radicals cm-3) within our precision atthat time (about ±30–50%). Preliminary testsin Pullman, WA indicated that the method might alsofunction in moderately polluted air by acidifying thescrubbing solution or by adding a scavenger tosuppress interferences. In Pullman, mid-day OHconcentrations were usually in the range of 2–20 ×106 radicals cm-3. Nighttime OHconcentrations were always low, either at or slightlyabove the detection limit.

The error in conventionally reported 13C/12C ratios of atmospheric CO due to the presence of mass independent oxygen isotope enrichment

An inherent disadvantage of stable isotope mass spectrometry on CO2 is that 13C‐ and 17O‐substituted CO2 are not resolved. Therefore, CO2‐based mass spectrometry relies on the widely applicable assumption of mass dependent fractionation in the oxygen isotopes, rendering the measurement of 17O superfluous. For atmospheric CO, however, recently published determinations of its oxygen isotopic composition reveal the presence of significant deviation from mass dependent fractionation. For instance, the observed 17O excess of tropospheric CO at high northern latitudes shows a prominent seasonal cycle peaking at 7.5‰ in late summer. This implies that the CO2 based determination of the 13C content of CO introduces a systematical error that needs to be corrected for, and a correction formula is derived. Based on the available Δ17O data and the now identified sources of mass independent fractionation in CO, the extent of this correction for CO isotope analysis in clean tropospheric air is estimated.

Tunable diode laser absorption measurements of H2O2 and HCHO during the Mauna Loa Observatory Photochemistry Experiment

The tunable diode laser absorption spectrometric technique was employed to measure H2O2 and HCHO during the Mauna Loa Observatory Photochemistry Experiment 2 (MLOPEX 2) fall and winter intensives. Comparisons were made between the measurements made with the Tunable Diode Laser Absorption Spectrometer (TDLAS) technique and several other methods. The half‐hourly average mixing ratios varied between 1300 parts per trillion by volume (pptv) and the instrument detection limit of 50 – 100 pptv for H2O2 and between 400 pptv and the instrument detection limit ∼25 pptv for HCHO. Daily average mixing ratios for H2O2 were 550 ± 79 pptv (fall) and 540 ± 80 pptv (winter), and for HCHO were 130 ± 21 pptv (fall) and 200 ± 35 (winter). HCHO showed strong diurnal behaviour with maxima in the 1200 – 1700 time period whereas H2O2 showed essentially no diurnal variation. Clean tropospheric air sampled during downslope conditions between 2000 and 0600 had HCHO mixing ratios averaging 80 pptv in the fall and 160 pptv in the winter. These values are considerably lower than model predictions.

Analysis of trace oxygenated hydrocarbons in the environment

A technique has been developed which can measure low-molecular-weight (C2−C5) oxygenated hydrocarbons down to concentrations of less than 10 parts per trillion (10-12) in the atmosphere. The method uses cryogenic trapping of trace gases from the air, and two-dimensional gas chromatography (2DGC) with flame ionization and photo-ionization detectors to analyze the samples. The method has been used to make extensive measurements in the field, and it is capable of measuring all of the C2−C5 carbonyl compounds in clean tropospheric air. The 2DGC analytical system also makes it possible to prepare accurate, reproducible standards of the low-molecular-weight oxygenated hydrocarbons at trace levels.

Distribution and geochemistry of aerosols in the tropical north Atlantic troposphere: Relationship to Saharan dust

The spatial distribution and water‐soluble chemical composition of aerosols in the tropical North Atlantic troposphere were examined during the NASA Global Tropospheric Experiment/Atlantic Boundary Layer Experiment (GTE/ABLE 1) conducted in the vicinity of Barbados, West Indies. During the June 1984 study period the atmospheric aerosol at 1.5–3.5 km altitudes, the region composing the so‐called Sahara air layer, consisted primarily of Saharan dust. The aerosol mass concentration reached 400 μg (m3 STP)−1 in the mainstream of a massive Saharan dust outbreak which passed through the longitude of Barbados between June 13 and 18, 1984. Measurements of the particle size distribution of the Saharan dust indicated that about 85% of the mass was contained in the 1‐ to 8‐μm diameter range; a distribution that was consistent among the three dust outbreaks observed during ABLE. Enrichment of water‐soluble aerosol chemical species in Sahara air compared to clean tropospheric air at >4 km altitude, ranged from about tenfold for NH4+ to more than a factor of 400 for NO3−. Calculations suggest that Saharan dust represented a significant source of airborne NO3−, non‐sea‐salt SO42−, and PO43− in boundary layer air over the tropical North Atlantic. Estimates of the annual input of Saharan dust‐related chemical species to the equatorial Atlantic suggested that deposition of mineral aerosol to oligotrophic surface waters may be an important source of the nutrient species NO3− and PO43−. This atmospheric input probably occurs principally in the form of dry deposition.

Calibration and tests of the filter-collection method for measuring clean-air, ambient levels of nitric acid

A portable permeation tube based system has been developed for the field calibration of nitric acid vapor collection on Nylon filters during actual atmospheric sampling. Extensive testing has been undertaken to determine the effects of: transfer line materials, the uptake of nitric acid on the filter holder apparatus, the linearity of dilution of the nitric acid effusing from the permeation tube source achievable and the effects of the permeation of gases, other than nitric acid, on the permeation tube calibration. Data demonstrating significant uptake of HNO 3 on commonly used materials are given, together with specific recommendations on the use of materials to avoid such problems. Nitrogen dioxide is shown to be a significant contaminant of the HNO 3 permeation tubes used requiring corrections of the order of 15%. Nitric acid “spiking” during actual field sampling of clean tropospheric air at a remote site in the Rocky Mountains west of Boulder, Colorado demonstrated agreement between the measured Nylon filter nitrate loading and the calculated loading to + 0.1, −0.4 μg at the 60% confidence level. At the atmospheric sampling rate used of 120 std ℓ min −1, these limits correspond to +5, −20 pptv for a one hour sampling time, with proportionately smaller uncertainties on the mean HNO 3 mixing ratio for longer sampling times.

Volcanic Gases in the April 1979 Soufriere Eruption

Six gas samples from the 17 April 1979 Soufriere eruption plume were analyzed for carbonyl sulfide, carbon disulfide, carbon monoxide, carbon dioxide, methane, nitrous oxide, fluorocarbon-11, fluorocarbon-12, methyl chloroform, and carbon tetrachloride. Only carbon monoxide, carbon dioxide, carbonyl sulfide, and carbon disulfide were found to have increased mixing ratios as compared with those in clean tropospheric air, but the increases were not sufficient to contribute greatly to the global budgets of these four components.

The Effects of Water Vapor, Ozone and Aerosol on Atmospheric Turbidity

Abstract Direct solar radiation intensity was measured with an Eppley sun photometer at 380 and 500 nm near Page, Ariz. The turbidity coefficient B derived from these measurements was anomalous, B380 < B500. This unusual condition seems to be caused by a combination of an appreciable amount of water vapor in very clean tropospheric air plus an almost monodisperse stratospheric aerosol. We recommend the inclusion of a water vapor term in the basic turbidity equation.