Aerosol Concentration Profiles Research Articles

Aircraft study of aerosol vertical distributions over Beijing and their optical properties

A set of 152 vertical profiles of aerosol number concentration and size distribution with diameter ranging from 0.12 to 3.0 μm observed by the airborne optical spectrometer probe in Beijing, China, between February 2005 and September 2006 is analysed and discussed. The statistic of aerosol number concentration (Na) reveals a high aerosol number density in this region with average surface level number concentration (N0) of about 6600 cm-3 (0.12–3.0 μm). The average vertical profile of Na approximately satisfies an exponential decline function with a scale height of 1419 m. The Na profiles are influenced by the structures of planetary boundary layer (PBL) significantly and two typical types of Na profile under different conditions of PBL are presented and parametrized in this study. The observations of aerosol size distribution show that, in most cases the aerosol size distributions are not very sensitive to altitude, with effective radii ranging from 0.16 to 0.28 μm. Comparison between aircraft-derived aerosol optical depth (AOD) and Moderate Resolution Imaging Spectroradiometer-derived AOD shows good agreement. The Mie model calculations suggest that the surface level number concentration, the PBL height and the structure of PBL can influence the AOD significantly.

Reply to comment by Edgar L Andreas on “Vertical coarse aerosol fluxes in the atmospheric surface layer over the North Polar Waters of the Atlantic”

[1] We are grateful for the interest Edgar L Andreas took in our work. His comments [Andreas, 2007] on our paper [Petelski and Piskozub, 2006] are insightful and confirm the methodology we chose, the aerosol concentration gradient, as a step forward toward achieving a reliable estimation of the aerosol production function. The difference between our approach and the recalculation of our results by Andreas [2007] is the value of the von Karman constant analogue for the aerosol flux. [2] It is not obvious that the dimensionless parameter of the logarithmic profile of aerosol concentration needs to be identical to the von Karman constant. The physics of momentum flux and aerosol flux are identical because we assume Monin-Obukhov similarity, but that implies only the existence of such a constant, not its value. Andreas gives theoretical reasons for its value to be identical to the Karman constant (approximately 0.40). We implicitly assumed it to be 1.0. Hence the approximate 2.5-fold difference of calculated fluxes in our original paper and in the comment by Andreas (approximate because we presented best fit flux functions for each wind speed bin data separately, which makes each of them differ from values of the ‘‘universal’’ function, namely, equation (5) of Petelski and Piskozub [2006], derived from the complete data set). However, we believe that the only way this problem can be solved is by experimental determination of the constant value. Until better data are available (one possibility is eddy correlation measurements in the open sea), our aerosol concentration data processed using the dry deposition method [Smith et al., 1993] may be the best calibration of the coefficient. Results presented in Figures 9 and 10 of Petelski and Piskozub [2006] (the ‘‘Eq(10)’’ lines) seem to suggest that the coefficient may be higher than the von Karman constant, possibly even closer to the value we implicitly chose (1.0). Also, the only eddy correlation measurements of aerosol concentration [Nilsson and Rannik, 2001] in the open sea, measured in the same areas as our data, seem to suggest that the flux functions favored by Andreas [2002] may be underestimated, further strengthening our estimation of the coefficient value. [3] Even with the above disagreements with Andreas [2007], we believe that his analysis of the influence of the flux coefficient choice on the results moves us forward toward a consensus on the marine aerosol source function. Even a 2.5-fold difference in the function value is minor compared to the several orders of magnitude range of functions reviewed by Andreas [2002]. However, further studies using more than one method of aerosol flux estimation are needed to narrow the von Karman constant–related uncertainty.

Particulate matter composition and emission rates from the disk incorporation of class B biosolids into soil

Biosolids contain metal, synthetic organic compound, endotoxin, and pathogen concentrations that are greater than concentrations in the agricultural soils to which they are applied. Once applied, biosolids are incorporated into soils by disking and the aerosols produced during this process may pose an airborne toxicological and infectious health hazard to biosolids workers and nearby residents. Field studies at a Central Arizona biosolids land application site were conducted to characterize the physical, chemical, and biological content of the aerosols produced during biosolids disking and the content of bulk biosolids and soils from which the aerosols emanate. Arrayed samplers were used to estimate the vertical source aerosol concentration profile to enable plume height and associated source emission rate calculations. Source aerosol concentrations and calculated emission rates reveal that disking is a substantial source of biosolids-derived aerosols. The biosolids emission rate during disking ranged from 9.91 to 27.25 mg s −1 and was greater than previously measured emission rates produced during the spreading of dewatered biosolids or the spraying of liquid biosolids. Adding biosolids to dry soils increased the moisture content and reduced the total PM 10 emissions produced during disking by at least three times. The combination of bulk biosolids and aerosol measurements along with PM 10 concentrations provides a framework for estimating aerosol concentrations and emission rates by reconstruction. This framework serves to eliminate the difficulty and inherent limitations associated with monitoring low aerosol concentrations of toxic compounds and pathogens, and can promote an increased understanding of the associated biosolids aerosol health risks to workers and nearby residents.

Remote Sensing Methods to Investigate Boundary-layer Structures relevant to Air Pollution in Cities

Simultaneous surface-based remote sensing with optical (Ceilometer) and acoustic (Sodar) methods allows the diurnal variation of the structure of the atmospheric boundary layer to be deduced with high temporal resolution. Primarily the convective boundary layer, the nocturnal stable surface layer, and the residual layer can be identified from the measured vertical profiles of aerosol concentration and thermal fluctuations. The ability of the two remote sensing techniques is shown in different examples from two different locations and two seasons. The impact on urban air quality assessment is addressed.

Comparison of aerosol chemistry transport model simulations with lidar and Sun photometer observations at a site near Paris

The ability of the aerosol chemistry transport model CHIMERE to simulate the vertical aerosol concentration profiles at a site near the city of Paris is evaluated using routine elastic backscatter lidar and Sun photometer measurements. The comparisons of model aerosols with measurements are carried out over a full year time period between October 2002 and September 2003. The methodology we propose here is new: From the model concentration outputs (optical properties varying with chemical composition and mass vertical distribution) we simulate the lidar backscattering profiles and compare them with the observed ones. The comparisons demonstrate the ability of the model to reproduce correctly the aerosol vertical distributions and their temporal variability. However, the aerosol load within the boundary layer is generally underestimated by the model, in particular during the afternoon hours and the summertime period. Several sensitivity tests indicate that this underestimation may have two origins: the lack of secondary organic and, to a lesser extent, mineral aerosols inside the model. The second deficiency is due to the absence of erosion/resuspension of soil material in the primary aerosol sources considered here; the first deficiency is probably due to incomplete knowledge about the formation of organic species in a photochemically active atmosphere. The results also show that the particles ranging from 0.08 to 1.25 μm in radius represent more than 89% of the volume backscattering at 532 nm, while the coarse particles are not efficient in terms of optical properties. The missing aerosol mass must therefore be found within the accumulation mode.

Remote sensing of spectral signatures of tropospheric aerosols

With the launch of the German Aerospace Agency's (DLR) Modular Opto-electronic Scanner (MOS) sensor on board the Indian Remote Sensing satellite (IRS-P3) launched by the Indian Space Research Organization (ISRO) in March 1996, 13 channel multi-spectral data in the range of 408 to 1010 nm at high radiometric resolution, precision, and with narrow spectral bands have been available for a variety of land, atmospheric and oceanic studies. We found that these data are best for validation of radiative transfer model and the corresponding code developed by one of the authors at Space Applications Centre, and called ATMRAD (abbreviated for ATMospheric RADiation). Once this model/code is validated, it can be used for retrieving information on tropospheric aerosols over ocean or land. This paper deals with two clear objectives, viz., 1 Validation of ATMRAD model/code using MOS data and synchronously measured atmospheric data, and if found performing well, then to 2 derive relationship between MOS radiances and Aerosol Optical Thickness (AOT). The data validation procedure essentially involves •near-synchronous measurements of columnar aerosol optical thickness and altitude profiles of aerosol concentration using ground-based multi-filter solar radiometers and Argon-ion Lidar, respectively and •computation of the top-of-the-atmosphere (TOA) radiances from a low reflecting target (near clear water reservoir in the present study) using the ATMRAD model. The results show that the model performance is satisfactory and a relationship between the spectral parameters of MOS radiances and aerosol optical thickness can be established. In this communication, we present the details of the experiments conducted, database, validation of the ATMRAD model and development of the relationship between AOT and MOS radiance.

The mean physical and optical properties of regional haze dominated by biomass burning aerosol measured from the C‐130 aircraft during SAFARI 2000

Instrumentation on the Met Office C‐130 aircraft measured aerosol physical and optical properties during the Southern African Regional Science Initiative (SAFARI 2000) in September 2002 while flying from Windhoek, Namibia. Filter measurements of aged regional haze suggest a ratio of apparent elemental carbon (ECa) to organic carbon (OC) of 0.12 ± 0.02 and mass fractions of 5% ECa, 25% inorganic compounds, and 70% organic matter (OC plus associated elements). The submicron size distribution of aged regional haze may be fitted with three lognormal distributions with geometric mean radii (rn) of 0.12 ± 0.01, 0.26 ± 0.01, and 0.80 ± 0.01 μm and geometric standard deviations (σ) of 1.3 ± 0.1, 1.5 ± 0.1, and 1.9 ± 0.4. Measurements over 2500 km from the emission region show similar rn and σ for the smallest two modes, while the third mode is absent presumably as a result of sedimentation. At a wavelength (λ) of 0.55 μm, effective medium approximations suggest a refractive index of 1.54 − 0.018i for aged regional haze aerosol. The single scattering albedo (ωoλ) derived using this refractive index and measured size distributions are consistent with those from the nephelometer and Particle Soot Absorption Photometer (PSAP). The optical parameters for aged regional haze a few days old are specific extinction coefficient (keλ=0.55) of 5.0 ± 0.8 m2 g−1, asymmetry factor (gλ=0.55) of 0.59 ± 0.02, and ωoλ=0.55 of 0.91 ± 0.04. Measurements of fresh biomass burning aerosol a few minutes old show smaller more absorbing particles. Vertical profiles of carbon monoxide, aerosol concentration, and aerosol scattering show a good correlation. Over land, aerosols become well mixed in the vertical from the surface to approximately 500 hPa. Over ocean, the aerosols can be separated from underlying stratocumulus cloud by a clear gap and a strong inversion, which may limit the indirect effect.

Recent trends in aerosol climatology and air pollution as inferred from multi‐year lidar observations over a tropical urban station

AbstractRegular nighttime monitoring of aerosol and other atmospheric parameters was initiated in 1985 at the Indian Institute of Tropical Meteorology, Pune. This is a tropical urban station (18°32′N, 73°51′E, 559 m AMSL), situated approximately 100 km inland from the west coast of India. The multi‐year aerosol vertical profile database, utilized in the present study, consisted of more than 1200 vertical aerosol concentration profiles. These data were collected with a computer‐controlled, bistatic, argon‐ion lidar system over a 12 year period from October 1986 to September 1998 and have been utilized to study the morphology of the nighttime atmospheric boundary layer and associated air quality. The recent climatological trend in the aerosol loading at the experimental station has also been studied. The study reveals higher pollution potential during late evenings in the winter and a total increase of about 3% in the aerosol loading over the 12 year observational period. This increase can be attributed partly due to the urban heat island effect and due to growing urbanization and industrialization, as well as to the land‐usage patterns in proximity to the experimental station. Further, it has been found that the long‐term trend in aerosol loading was not uniform, but it changed from year to year depending on meteorological parameters (precipitation, in particular) and local anthropogenic activities. The short‐term variations in aerosol loading and their relationship with concurrent meteorological parameters over the observational site are discussed also. Copyright © 2002 Royal Meteorological Society

A Pacific Aerosol Survey. Part I: A Decade of Data on Particle Production, Transport, Evolution, and Mixing in the Troposphere*

Abstract Integration of extensive aerosol data collected during the past decade around the Pacific basin provides a preliminary assessment of aerosol microphysics for this region and cycling of aerosol in the troposphere. These include aircraft-based data collected as part of numerous field experiments supported by the National Aeronautics and Space Administration (NASA), the National Science Foundation (NSF), and the National Oceanic and Atmospheric Administration (NOAA) [Global Backscatter Experiment (GLOBE), First Aerosol Characterization Experiment (ACE-1), Pacific Exploratory Mission (PEM)-Tropics A and B]. Although these experiments had diverse goals, most included extensive data on aerosol size distributions, optical properties (light scattering and light absorption), and chemistry. Vertical profiles of aerosol concentration, size distribution, and light scattering were used to characterize vertical structure from 70°S to 70°N. The in situ data are placed in the context of meteorological regimes ov...

Sudden changes in aerosol and gas concentrations in the central Arctic marine boundary layer: Causes and consequences

Measurements of aerosol number size distributions and concentrations of the precursor gases dimethyl sulfide, sulfur dioxide and ammonia were made within the pack ice region of the central Arctic Ocean during July and August 1996 from the icebreaker Oden. Changes in concentration, sometimes exceeding the entire seasonal variation, often occurred within an hour and attempts to find the reasons for them are described. Vertical profiles of aerosol concentration in Aitken and accumulation mode particles obtained on helicopter flights revealed intense concentration gradients in the lowest 1000 m. Those below 100 m were common. Concentrations of accumulation mode particles were usually greater near the surface than at 100 m. Four representative case studies for which vertical aerosol profiles were obtained are presented. Observations of rapid large changes in near‐surface concentration of aerosols in different size ranges are compared with the vertical profiles, meteorological information, and acoustic or optical remote sensing to infer processes causing the changes. Comparison of simultaneous variations in aerosols and precursor gas concentrations are used to define the vertical profiles of the gases. It was found that dimethyl sulfide and ammonia concentrations usually must have been strongly depleted near the surface relative to concentrations at about 100 m. Sulfur dioxide profiles appeared to be more complex. Turbulence or vertical air motions initiated by atmospheric wave motions trapped within the stable boundary layer appeared to be directly responsible for many of the sudden concentration changes, through interaction with concentration gradients close to the surface. The presence of low‐level jets also had direct or indirect influences on mixing in the lowest few hundred meters. The extent to which aerosols measured near the surface can determine the microphysics of central Arctic marine boundary layer clouds is examined.

Some Measurement Results on Sulfate Aerosol Concentration in and above a Pine Canopy

Profile of sulfate aerosol (SO4 2−) concentration was measured for four days at six heights in and above a 15m-high canopy of pine plantation during 6th and 10th August. 1999. The concentration was the lowest (about 2 nmol/m3) on 6th, and gradually increased to 9th showing the maximum values of about 13 nmol/m3, and then decreased to 2 nmol/m3 on 10th. The vertical profiles of SO4 2− concentration showed mostly higher in the canopy than above the canopy. As for the vertical profiles above the canopy on 8th and 10th, the minimum was observed just above the canopy (16m), showing SO4 2− transport from the upper air layer to the canopy. While on 9th the profiles that are higher concentration just above the canopy and lower at the upper air layer were observed, suggesting SO4 2− emission from the canopy to the upper air layer.

Measurement and modeling of near-ocean surface properties affecting aerosol concentration profiles during EOPACE

Measurement and modeling of near-ocean surface properties affecting aerosol concentration profiles during EOPACE

The Effects of Clouds on Aerosol and Chemical Species Production and Distribution. Part III: Aerosol Model Description and Sensitivity Analysis

A modeling study of the effects of clouds on the evolution and redistribution of aerosol particles in the troposphere is presented. A two-mode, two-moment aerosol evolution model is coupled with a two-dimensional, mixed-phase, two-moment microphysics, Eulerian cloud model and a sulfate cloud chemistry model. The coupled model is used to simulate evolution of a convective cloud with different assumptions about the initial chemical and aerosol fields. In the simulations, SO is convectively transported to the mid- to upper troposphere, where it is oxidized to gas-phase H 2 SO 4 . After cloud processing, cloud condensation nuclei (CCN) particles are removed by precipitation and graupel to form a CCN-depleted region above cloud top and in the cold and humidified cloud outflow region. These conditions are favorable for binary homogeneous nucleation of ultrafine sulfuric acid particles to take place. The new particle formation in the mid- and upper troposphere interacts with cloud processing and transport of aerosol particles and produces a peak of small particle concentration in the outflow region. Sensitivity tests varying initial aerosol composition and mass mixing ratio, initial H 2 SO 4 mass mixing ratio, assumed OH. profile, and nucleation rate factor are discussed. The small particie concentration in the upper troposphere is most sensitive to initial aerosol composition and assumed OH. profile. When the nucleation rate factor is increased, the critical H 2 SO 4 (g) concentration is lowered, and the nucleation rare adjusts to changes in environmental variables more quickly. The model results suggest that both aerosols and aerosol precursors can be transported into the mid- and upper troposphere by convective clouds, affecting vertical profiles of aerosol concentrations.

Vertical distribution of aerosol particles near the air-sea interface in coastal zone

Measurements of vertical aerosol concentration profiles in the Mediterranean coastal zone are presented in the size band 0.1 to 20 μm, with focus on aerosol particles smaller than 10 μm. The results show that the profile shape depends mainly on turbulent processes induced by the wind rather thanthe whitecap production of freshly generated aerosols from the wind-wave interaction. Therefore, the data recorded during weak and moderate wind speeds show the prevalence of turbulent mixing processes induced by the wind for profiles in the size range 0.1–1 μm. For the size range 1–10 μm, in addition to the effect of wind velocity and direction (i.e. the coastal influence), we also noted the importance of the relative humidity gradient (very close to the interface). Good agreement was found between vertical concentration profiles recorded for winds of marine origin and the marine mixed layer model (Davidson and Shutz, 1983). However, we show that the model cannot be used for winds of continental origin. For wind speeds above >9 ms −1, a maximum occurs in the concentration profiles (near 2–3 m height) for all particles larger than 1 μm, which confirms the predominant influence of an eddy which appears in the lee of the crest, referred to as the “wave rotor” model (De Leeuw, 1986).

Variations and vertical profiles of trace gas and aerosol concentrations and CO 2 exchange in Eastern Lapland

This article presents results of measurements performed at Värriö environmental research station in Finland during 1992 to 1995. The aim of the station is to obtain more information on air quality as influenced by the Kola industrial areas and the effects on photosynthesis in subarctic climate. The diurnal and annual characteristics and correlations of measured species and their profiles were analyzed. Trace gas (SO 2, O 3, CO 2) and particle concentrations, profiles and weather characteristics were measured continuously, and CO 2 exchange of pine twigs was measured during summer periods using an automatic cuvette system. Practically, no local sources of pollutants existed in Värriö, however regularly occurring episodes of sulphur dioxide were observed during northeasterly winds. These events were presumed to be caused by transport from the Kola industrial areas. Some of the pollutant sources were located less than 200 km away from Värriö. Trajectories related to some typical pollution conditions are presented. Single episodes of SO 2 and aerosols usually occurred simultaneously, but not always. Mean SO 2 concentrations were lower during summer due to more effective deposition of SO 2, sulfur dioxide conversion into sulfate and vertical mixing of air. The mean aerosol concentrations were higher during the summer than during the winter. An annual and a weak diurnal cycle were observed in ozone concentrations. The ozone concentrations in Värriö were not strongly dependent on the wind direction, and signs of long-range transport could not be clearly seen. Profile measurements of gases and particles showed the deposition to and emission from plants and soil. The carbon dioxide gradient was quite strong during the summer, which was related to the photosynthetic activity of the plants. A gradient in the ozone concentrations was also observed. The dependency of Co 2 exchange on irradiation was clear.

Feasibility of Retrieving Aerosol Concentration in the Atmospheric Boundary Layer Using Multitime Lidar Returns and Visual Range

Abstract This paper studies the feasibility of retrieving aerosol concentrations in the planetary boundary layer (PBL) with a variational data assimilation (VDA) algorithm using dual-wavelength lidar returns and visual range simulated at multiple times. Aerosols are assumed to consist of nucleation, accumulation, and coarse modes, with each mode distributed in a gamma distribution. The VDA algorithm retrieves initial vertical profiles of aerosol concentrations of the three modes, which are predicted by a 1D PBL model. The accuracy of retrieved aerosol concentrations of the three modes is examined through a series of identical twin numerical experiments. For the VDA algorithm that uses data from a lidar wavelength pair (0.289, 11.15 μm), results show that 1) if both random and systematic errors in the observed data are less than 1.0 dB and the number densities of accumulation and coarse modes are, respectively, 0.025–0.25 and 0–1.25 × 10−3 times that of the nucleation mode, relative errors in the retrieved...

Airborne aerosol and black carbon measurements over the East Siberian Sea, Spring 1992

In April 1992 we performed measurements of Arctic aerosol properties from an aircraft flying over the East Siberian Sea from Cherskiy (69°N, 161°E) to Bennett Island (76.5°N, 149°E). Eight round-trip flights were made over this 1000 kilometer path, with each flight incorporating several profiling descents from approximately 4.2 km altitude to the surface. On a 10-second time base were recorded measurements of aerosol black carbon (BC), condensation nuclei (CN), air temperature and absolute barometric pressure, from which altitude was deduced. Some profiles showed considerable vertical structure with numerous temperature inversions and stratified aerosol layers. Peak BC and CN concentrations exceeded 1000 ng m −3 and 1000 cm −3 respectively. Occasionally, extremely clean air was found in some layers. Other descents showed stable temperature profiles, with aerosol BC and CN distributed throughout the lower 4 km of the atmosphere. In most cases, the surface temperature-inversion layer contained high concentrations of aerosol species. We conclude that the atmosphere in this remote area is strongly affected by anthropogenic emissions from distant source regions, and that the meteorological and aerosol concentration profile structures were similar to those regularly observed in the western sectors of the Arctic in springtime.

The macrotransport of aerosol particles in the lung: Aerosol deposition phenomena

The macrotransport model of lung dispersion ( Edwards, 1994) is generalized to incorporate phenomena involving the nonconservation of gas and/or aerosol species in the airways and acini of the human lung during a single breath. The three principal parameters measured in single-breath gas or aerosol bolus dispersion experiments, namely, net deposition rate ( K ∗ ), mean bolus velocity ( U ∗ ) and effective dispersion coefficient ( D ∗ ), are theoretically determined in terms of the detailed microtransport, branch-level phenomena underlying them. Explicit formulas are provided for the macroscale coefficients ( K ∗, U ∗, D ∗ ). Additionally, a formula is provided for a coefficient A ∗ that serves to characterize a novel “apparent initial condition” to be imposed upon the aerosol concentration profile as viewed from its coarse-grained level. Numerical calculations are made in the case of aerosols. These are based upon a symmetrical Weibel model A geometry, and employ the deposition model of Landahl (1950) for assigning constitutive laws to the branch-level deposition-rate parameters that underly the theory. Branch-level dispersion coefficients are described by an axial-streaming constitutive law, as in the previous contribution, while taking explicit account of the diminution of axial streaming caused by aerosol deposition. Comparison is made with the aerosol bolus experimental data of Heyder et al. (1988) and Anderson et al. (1989) . Predictions of net aerosol deposition in the human lung as a function of aerosol particle size are compared with predictions made on the basis of former aerosol-deposition theories, as well as with additional aerosol-deposition data appearing in the literature. Very good agreement is found for all of these comparisons.

Incense Smoke: Characterization and Dynamics in Indoor Environments

Burning incense in houses and temples for religious functions is especially common in Asia. Smoke from burning incense is a source of indoor aerosols, and extracts from this smoke are mutagenic in the Ames Salmonella test. This paper describes characterization of the smoke in terms of particle size, shape, concentration, density, and chemical composition. Our data showed that aerosols from incense smoke were spherical droplets with a count median diameter of about 0.13 μm and mass median aerodynamic diameter of 0.28 μm. The particle density was 1.06 ± 0.08 g cm−3. Aerosols produced from incense are therefore similar to other aerosols formed by condensation, such as environmental tobacco smoke found indoors. The aerosol concentration profiles and size distributions within a room during typical burning cycles were monitored. The air ventilation rate in the room was also measured by the SF6 tracer gas method. Respirable aerosol concentrations up to several hundred μg m−3 were produced by burning one to three...

In situ measurements of nitric oxide, ozone, and aerosols in the Scandinavian Arctic stratosphere in January 1992

During the European Arctic Stratospheric Ozone Experiment (EASOE) the Arctic stratosphere was investigated with a set of instruments flown on a CNES high altitude balloon from ESRANGE, northern Sweden (67°52′N, 21°4′E) on January 22, 1992. The vertical profiles to an altitude of 30 km are presented for ozone, nitric oxide, aerosols, and the temperature. The ozone mixing ratio was less than 0.1 ppmv below 13km, and increased significantly only above the 18.5 km level. The aerosol concentration profile shows a large peak between 13 km and 21km, centered at 18.5 km, reaching as high as about 10 particles/cm³. Above 24 km the aerosols were absent. Below 22 km, the NO mixing ratio was lower than 50 pptv; however it increased rapidly from 23 km with a sharp peak at 27 km. The rapid increase in NO occurs just at the altitude where the aerosol concentration falls below the detection limit: on this day, NO was present only in the absence of aerosols.