Tropospheric Ultraviolet-Visible Research Articles

Aerosol Layering in the Free Troposphere over the Industrial City of Raciborz in Southwest Poland and Its Influence on Surface UV Radiation

Atmospheric aerosol and ultraviolet index (UVI) measurements performed in Racibórz (50.08° N, 18.19° E) were analyzed for the period June–September 2019. Results of the following observations were taken into account: columnar characteristics of the aerosols (aerosol thickness, Angstrom exponent, single scattering albedo, asymmetry factor) obtained from standard CIMEL sun-photometer observations and parameters of aerosol layers (ALs) in the free troposphere (the number of layers and altitudes of the base and top) derived from continuous monitoring by a CHM-15k ceilometer. Three categories of ALs were defined: residues from the daily evolution of the planetary boundary layer (PBL) aerosols, from the PBL-adjacent layer, and from the elevated layer above the PBL. Total column ozone measurements taken by the Ozone-Monitoring Instrument on board NASA’s Aura satellite completed the list of variables used to model UVI variability under clear-sky conditions. The aim was to present a hybrid model (radiative transfer model combined with a regression model) for determining ALs’ impact on the observed UVI series. First, a radiative transfer model, the Tropospheric Ultraviolet–Visible (TUV) model, which uses typical columnar characteristics to describe UV attenuation in the atmosphere, was applied to calculate hypothetical surface UVI values under clear-sky conditions. These modeled values were used to normalize the measured UVI data obtained during cloudless conditions. Next, a regression of the normalized UVI values was made using the AL characteristics. Random forest (RF) regression was chosen to search for an AL signal in the measured data. This explained about 55% of the variance in the normalized UVI series under clear-sky conditions. Finally, the UVI values were calculated as the product of the RF regression and the relevant UVIs by the columnar TUV model. The root mean square error and mean absolute error of the hybrid model were 1.86% and 1.25%, respectively, about 1 percentage point lower than corresponding values derived from the columnar TUV model. The 5th–95th percentile ranges of the observation/model differences were [−2.5%, 2.8%] and [−3.0%, 5.3%] for the hybrid model and columnar TUV model, respectively. Therefore, the impact of ALs on measured surface UV radiation could be demonstrated using the proposed AL characteristics. The statistical analysis of the UVI differences between the models allowed us to identify specific AL configuration responsible for these differences.

Observations of trace gases, photolysis rate coefficients and model simulations over semi-arid region, India

Continuous ground-based measurements of CO, SO2 and NO2 were carried out in a semi-arid rural area, Anantapur [14.62 0N, 77.65 0E], Southern India, for the period January 2012–December 2012. The maximum CO concentration was observed in winter (310 ± 17 ppbv) followed by summer (180 ± 21 ppbv) and post monsoon (174 ± 20 ppbv), while the minimum in monsoon (72 ± 9 ppbv). Seasonal mean NO2/NOx ratios for monsoon, post monsoon, winter and summer were about 0.88, 0.91, 0.76 and 0.80 respectively, indicating a higher conversion of NO to NO2 over the measurement site. Monthly mean low SO2 mixing ratio was found (0.46 ± 0.02 ppbv) in monsoon and high (2.42 ± 0.21 ppbv) in winter. Keeping the emissions aside, the levels of CO, SO2 and NO2 were influenced by meteorology, urban effects and trans-boundary transport in the lower troposphere. Atmospheric boundary layer (ABL) had the good correlation coefficient (R = 0.76) with solar radiation during daytime, while it was mainly correlated with wind speed during night time (R = 0.42). Diurnal trend of atmospheric visibility was found to be maximum during noon times at around (14:00–16:00 h) about 76 k.m. and minimum during morning periods (06:00–08:00 h) about 45 k.m. A strong positive correlation was observed between BC and CO (R = 0.71) with an average slope, suggesting common or proximate sources likely to be traffic emissions contribution for the production of BC and CO. The SO2/NOx and CO/NOx study were strongly suggested that mobile sources were larger contributors over the site and the evidence of transport of emissions from other surrounding regions. Tropospheric Ultraviolet Visible (TUV) radiative transfer model was used to calculate the Photolysis rate coefficients (J(O3), J(NO2)). Chemical box model (NCAR-MM) was used to simulate diurnal variation of CO and the results were reported.

Effect of heavy haze and aerosol pollution on rice and wheat productions in China.

In China, regional haze pollution is a serious environmental problem. The impact on ecosystem, however, is not clearly understood. This study investigates the effect of regional haze pollution on the yields of rice and wheat in China. The spatial and temporal distributions of aerosol optical depth (AOD) show high particulate pollution in the North China Plain region, Yangtze River Delta region, the central eastern China, and the Si Chuan Basin, coexisted largely with crop growth in time and space. The solar irradiance reaching these regions is estimated to reduce by up to 28–49%, calculated using the AOD distributions and tropospheric ultraviolet-visible (TUV) model. Reduction of solar irradiance in these regions can depress optimal yields of about 45% of rice and 75% of wheat growth in China, leading to 2% reduction in total rice production and 8% reduction in total wheat production in China. However, there are large uncertainties of the estimate related to the diffuse solar radiation. For high diffuse radiation case, the estimate reductions of rice and wheat decrease to 1% and 4.5%, respectively. A further detailed study is needed to clearly understand this effect to meet the growing food demand in the nation in the coming decades.

Analysis of the photochemical production of ozone using Tropospheric Ultraviolet-Visible (TUV) Radiation Model in an Asian megacity

A study of surface layer ozone (O3) and its primary precursors (NOx = NO + NO2) was carried out at Kolkata (22° 33′ N, 88° 30′ E), an urban site in eastern India from October 2010 to April 2011. A simple NOx cycle-based photochemical model for the net rate of production of tropospheric ozone was studied. Photolytic rate constants for NO2 (\( {j}_{{\mathrm{NO}}_2} \)) were estimated using the Tropospheric Ultraviolet and Visible (TUV) Radiation Model-Version 4.1. Relationships between predicted and measured ozone data were found to be sensitive with respect to time of the day. A correlation between the observed ozone and \( {j}_{{\mathrm{NO}}_2} \) during some case study days in the morning (R2 range, 0.34 to 0.96) and the late afternoon (R2 range, 0.79 to 0.99) implies that the NOx chemistry is predominant due to enhanced automobile emissions during the peak traffic hours. VOCs and some peroxy radicals play a vital role in the chemistry of ozone production (net). Computed air mass backward trajectories using HYSPLIT model established the source and transport pathways of the trace gases. The results obtained on different days indicate the importance of advection of gases from both continental as well as marine air mass on case-specific circumstances. Tropospheric columnar NO2 outputs were also compared to ground-based measurements, and these point towards a good regional contribution of NO2.

Effect of aerosols and NO<sub>2</sub> concentration on ultraviolet actinic flux near Mexico City during MILAGRO: measurements and model calculations

Abstract. Urban air pollution absorbs and scatters solar ultraviolet (UV) radiation, and thus has a potentially large effect on tropospheric photochemical rates. We present the first detailed comparison between actinic fluxes (AF) in the wavelength range 330–420 nm measured in highly polluted conditions and simulated with the Tropospheric Ultraviolet-Visible (TUV) model. Measurements were made during the MILAGRO campaign near Mexico City in March 2006, at a ground-based station near Mexico City (the T1 supersite) and from the NSF/NCAR C-130 aircraft. At the surface, measured AF values are typically smaller than the model by up to 25% in the morning, 10% at noon, and 40% in the afternoon, for pollution-free and cloud-free conditions. When measurements of PBL height, NO2 concentration and aerosols optical properties are included in the model, the agreement improves to within ±10% in the morning and afternoon, and ±3% at noon. Based on daily averages, aerosols account for 68% and NO2 for 25% of AF reductions observed at the surface. Several overpasses from the C-130 aircraft provided the opportunity to examine the AF perturbations aloft, and also show better agreement with the model when aerosol and NO2 effects are included above and below the flight altitude. TUV model simulations show that the vertical structure of the actinic flux is sensitive to the choice of the aerosol single scattering albedo (SSA) at UV wavelengths. Typically, aerosols enhance AF above the PBL and reduce AF near the surface. However, for highly scattering aerosols (SSA > 0.95), enhancements can penetrate well into the PBL, while for strongly absorbing aerosols (SSA < 0.6) reductions in AF are computed in the free troposphere as well as in the PBL. Additional measurements of the SSA at these wavelengths are needed to better constrain the effect of aerosols on the vertical structure of the AF.

Diurnal variations of reactive chlorine and nitrogen oxides observed by MIPAS-B inside the January 2010 Arctic vortex

Abstract. The winter 2009/2010 was characterized by a strong Arctic vortex with extremely cold mid-winter temperatures in the lower stratosphere associated with an intense activation of reactive chlorine compounds (ClOx) from reservoir species. Stratospheric limb emission spectra were recorded during a flight of the balloon version of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS-B) from Kiruna (Sweden) on 24 January 2010 inside the Arctic vortex. Several fast limb sequences of spectra (in time steps of about 10 min) were measured from nighttime photochemical equilibrium to local noon allowing the retrieval of chlorine- and nitrogen-containing species which change rapidly their concentration around the terminator between night and day. Mixing ratios of species like ClO, NO2, and N2O5 show significant changes around sunrise, which are temporally delayed due to polar stratospheric clouds reducing the direct radiative flux from the sun. ClO variations were derived for the first time from MIPAS-B spectra. Daytime ClO values of up to 1.6 ppbv are visible in a broad chlorine activated layer below 26 km correlated with low values (below 0.1 ppbv) of the chlorine reservoir species ClONO2. Observations are compared and discussed with calculations performed with the 3-dimensional Chemistry Climate Model EMAC (ECHAM5/MESSy Atmospheric Chemistry). Mixing ratios of the species ClO, NO2, and N2O5 are well reproduced by the model during night and noon. However, the onset of ClO production and NO2 loss around the terminator in the model is not consistent with the measurements. The MIPAS-B observations along with Tropospheric Ultraviolet-Visible (TUV) radiation model calculations suggest that polar stratospheric clouds lead to a delayed start followed by a faster increase of the photodissoziation of ClOOCl and NO2 near the morning terminator since stratospheric clouds alter the direct and the diffuse flux of solar radiation. These effects are not considered in the EMAC model simulations which assume a cloudless atmosphere.

Effects of stratocumulus, cumulus, and cirrus clouds on the UV-B diffuse to global ratio: Experimental and modeling results

Broadband measurements of global and diffuse UV-B irradiance (280–315nm) together with modeled and measured diffuse to global ratios (DGR) have been used to characterize the influence of different types of clouds on irradiance at the surface. Measurements were carried out during 2000–2001 in Córdoba City, Argentina. The Tropospheric Ultraviolet Visible (TUV) model was used to analyze the behavior of the modeled DGRs for different cloud optical depths and at different altitudes and solar zenith angles (SZA). Different cloud altitudes were also tested, although only the results for a cloud placed at 1.5–2.5km of altitude are shown. A total of 16 day with stratocumulus, 12 with cumulus, and 16 with cirrus have been studied and compared among them and also against 21 clear sky days. Different behaviors were clearly detected and also differentiated through the analysis of the averages and the standard deviations of the DGRs: 1.02±0.06 for stratocumulus, 0.74±0.18 for cumulus, 0.63±0.12 for cirrus, and 0.60±0.13 for the clear sky days, respectively. Stratocumulus clouds showed a low variability in the DGR values, which were concentrated close to one at all SZAs. DGR values for cumulus clouds presented a large variability at all SZAs, mostly associated with the different optical depths. Finally, the closeness between the DGR values for cirrus clouds and the DGR values for clear days showed that these clouds generally do not strongly affect the UV-B irradiance at the surface at any SZA. In the opposite side, stratocumulus clouds were identified as those with the largest effects, at all SZAs, on the UV-B irradiance at the surface.

Ultraviolet actinic flux in clear and cloudy atmospheres: model calculations and aircraft-based measurements

Abstract. Ultraviolet (UV) actinic fluxes measured with two Scanning Actinic Flux Spectroradiometers (SAFS) aboard the NASA DC-8 aircraft are compared with the Tropospheric Ultraviolet-Visible (TUV) model. The observations from 17 days in July-August 2004 (INTEX-NA field campaign) span a wide range of latitudes (28° N–53° N), longitudes (45° W–140° W), altitudes (0.1–11.9 km), ozone columns (285–353 DU), and solar zenith angles (2°–85°). Both cloudy and cloud-free conditions were encountered. For cloud-free conditions, the ratio of observed to clear-sky-model actinic flux (integrated from 298 to 422 nm) was 1.01±0.04, i.e. in good agreement with observations. The agreement improved to 1.00±0.03 for the down-welling component under clear sky conditions. In the presence of clouds and depending on their position relative to the aircraft, the up-welling component was frequently enhanced (by as much as a factor of 8 relative to cloud-free values) while the down-welling component showed both reductions and enhancements of up to a few tens of percent. Including all conditions, the ratio of the observed actinic flux to the cloud-free model value was 1.1±0.3 for the total, or separately 1.0±0.2 for the down-welling and 1.5±0.8 for the up-welling components. The correlations between up-welling and down-welling deviations are well reproduced with sensitivity studies using the TUV model, and are understood qualitatively with a simple conceptual model. This analysis of actinic flux observations illustrates opportunities for future evaluations of photolysis rates in three-dimensional chemistry-transport models.

Atmospheric Degradation of Perfluoro-2-methyl-3-pentanone: Photolysis, Hydrolysis and Hydration

Perfluorinated carboxylic acids are widely distributed in the environment, including remote regions, but their sources are not well understood. Perfluoropropionic acid (PFPrA, CF(3)CF(2)C(O)OH) has been observed in rainwater but the observed amounts can not be explained by currently known degradation pathways. Smog chamber studies were performed to assess the potential of photolysis of perfluoro-2-methyl-3-pentanone (PFMP, CF(3)CF(2)C(O)CF(CF(3))(2)), a commonly used fire-fighting fluid, to contribute to the observed PFPrA loadings. The photolysis of PFMP gives CF(3)CF(2)C·(O) and ·CF(CF(3))(2) radicals. A small (0.6%) but discernible yield of PFPrA was observed in smog chamber experiments by liquid chromatography-mass spectrometry offline chamber samples. The Tropospheric Ultraviolet-Visible (TUV) model was used to estimate an atmospheric lifetime of PFMP with respect to photolysis of 4-14 days depending on latitude and time of year. PFMP can undergo hydrolysis to produce PFPrA and CF(3)CFHCF(3) (HFC-227ea) in a manner analogous to the Haloform reaction. The rate of hydrolysis was measured using (19)F NMR at two different pHs and was too slow to be of importance in the atmosphere. Hydration of PFMP to give a geminal diol was investigated computationally using density functional theory. It was determined that hydration is not an important environmental fate of PFMP. The atmospheric fate of PFMP seems to be direct photolysis which, under low NO(x) conditions, gives PFPrA in a small yield. PFMP degradation contributes to, but does not appear to be the major source of, PFPrA observed in rainwater.

Observations and model calculations of direct solar UV irradiances in the Schirmacher region of east Antarctica

Measurements of direct UV irradiances (using a MICROTOPS II Sunphotometer) carried out from a high‐latitude site, Antarctica are presented. The instantaneous irradiances at 305±0.9, 312±0.9 and 320±1.0 nm during a no‐ozone‐hole (13 December 2004) and an ozone‐hole (4 October 2004) period have been observed to be about 0.031, 0.150 and 0.299 W m−2 and 0.010, 0.049 and 0.102 W m−2, respectively at local noontime. The observations of the direct UV irradiances at 305±0.9, 312±0.9 and 320±1.0 nm are compared with tropospheric ultraviolet visible (TUV) radiation transfer model calculations. The model estimate shows that, during the ozone‐hole period, a loss of ozone of the order of 44% leads to an increase in irradiance of the order of 410%, 90% and 25% at 305±0.9, 312±0.9 and 320±1.0 nm, respectively. The relationship between change in UV irradiance due to a change in column ozone is obtained using a TUV model and irradiances thus estimated from this relationship are found to compare well with the observed irradiances.

Estimation of uv Exposure in Croatia Over The Summer Using A Simple Approximate Formula

The Tropospheric Ultraviolet-Visible (TUV) model, version 4.2 developed by Madronich (2003) was used to estimate the extent of ultraviolet (UV) exposure of general population in Croatia over the summer. Solar noon values (13 h local time, CEST) of the ultraviolet index (UVI) for the period April to October 2004 were calculated for 61 cities in Croatia. The results showed that the risk of sunburn at 13 h local time in clear weather was high between April and September (UVI > 7) and very high in July (UVI > 10). In July, the UVI exceeded 8 between 11 h and 15 h local time. In this study, we developed a simple approximate formula to estimate UVI. The formula includes data on the time, date, altitude and clouds. The difference between our estimate and the TUV model for the summer months of June, July and August at 10 h to 16 h local time was less than 10%.

Photolysis rate coefficients in the upper atmosphere: Effect of line by line calculations of the O 2 absorption cross section in the Schumann–Runge bands

A line by line (LBL) method to calculate highly resolved O 2 absorption cross sections in the Schumann–Runge (SR) bands region was developed and integrated in the widely used Tropospheric Ultraviolet Visible (TUV) model to calculate accurate photolysis rate coefficients ( J values) in the upper atmosphere at both small and large solar zenith angles (SZA). In order to obtain the O 2 cross section between 49,000 and 57,000 cm −1, an algorithm which considers the position, strength, and half width of each spectral line was used. Every transition was calculated by using the HIgh-resolution TRANsmission molecular absorption database (HITRAN) and a Voigt profile. The temperature dependence of both the strength and the half widths was considered within the range of temperatures characteristic of the US standard atmosphere, although the results show a very good agreement also at 79 K. The cross section calculation was carried out on a 0.5 cm −1 grid and the contributions from all the lines lying at ±500 cm −1 were considered for every wavelength. Both the SR and the Herzberg continuums were included. By coupling the LBL method to the TUV model, full radiative transfer calculations that compute J values including Rayleigh scattering at high altitudes and large SZA can now be done. Thus, the J values calculations were performed for altitudes from 0 to 120 km and for SZA up to 89°. The results show, in the J O 2 case, differences of more than ±10% (e.g. at 96 km and 30°) when compared against the last version of the TUV model (4.4), which uses the Koppers and Murtagh parameterization for the O 2 cross section. Consequently, the J values of species with cross sections overlapping the SR band region show variable differences at lower altitudes. Although many species have been analyzed, the results for only four of them (O 2, N 2O, HNO 3, CFC12) are presented. Due to the fact that the HNO 3 absorption cross section extends up to 350 nm this molecule was used to verify the consistency of the new TUV–LBL at lower altitudes. Thus, it shows differences up to 5.7% at 21 km but 0% in the troposphere. Because of the more accurate consideration of the Rayleigh scattering the distribution of the actinic flux in its direct and diffuse components (in the SR bands wavelength interval) is also modified.

Direct solar ultraviolet irradiance over Nainital, India, in the central Himalayas for clear‐sky day conditions during December 2004

From a high‐altitude station, Nainital, India (29.4°N, 79.5°E, 1958 masl), located in the central part of lower Himalayas, the observations made during December 2004 using a pair of Sun photometers (Microtops II) at wavelengths ranging from 305 to 1020 nm are reported. The observed parameters are the direct solar UV irradiance, column ozone, water vapor, and aerosol optical depths (AOD). The results are presented for the full day clear‐sky conditions that prevailed for about 16 days during the whole month. It is found that Nainital is a comparatively pristine site with average AOD at 500 nm ranging between 0.03 and 0.09 and Angstrom exponent generally close to 1. The high AOD values occurred on 2 and 25 December due to winds from populated north Indian plains as revealed by the Hybrid Single‐Particle Lagrangian Integrated Trajectory (HYSPLIT) backward trajectory model. The total column ozone varies between 251 and 308 DU during the entire period of observations. The maximum diurnal UV irradiance values in the 2.4 nm bandwidth centered at 305.5, 312.5, and 320.0 nm varied between 0.027 and 0.049, 0.15 and 0.20, and 0.29 and 0.37 W m−2, respectively. The measured UV irradiances are compared with the Tropospheric Ultraviolet Visible (TUV) radiation model and show a good agreement.

The influence of aerosols on crop production: A study using the CERES crop model

The attenuation of solar radiation by atmospheric aerosols simultaneously decreases the amount of radiation reaching the surface and increases the fraction of radiation which is diffuse. Decreasing the total amount of photosynthetically active radiation (PAR, 400–700 nm) tends to decrease the amount of photosynthesis occurring in plant leaves. Increasing the fraction of PAR which is diffuse allows more PAR to reach shaded leaves and can thus increase the total amount of photosynthesis occurring in a plant canopy. In an attempt to quantify these two radiative influences of aerosols, the CERES crop model [Cereal growth, development and yield. In: Tsuji, G.Y., Hoogenboom, G., Thornton, P.K. (Eds.), Understanding Options for Agricultural Production. Kluwer Academic Publishers, Dordrecht, Netherlands.] was modified to estimate the impact on yield for maize, wheat, and rice under varying atmospheric conditions. The influence of aerosols on total and diffuse PAR was modeled using the National Center for Atmospheric Research (NCAR) Tropospheric Ultraviolet Visible (TUV) radiation model [UV radiation in the natural and perturbed atmosphere. In: Tevini, M. (Ed.), Environmental Effects of UV (Ultraviolet) Radiation. Lewis Publisher, Boca Raton, Florida.] under both clear skies and overcast skies and again when cloudiness conditions were allowed to vary to reflect actual meteorological conditions. The PAR intensity and diffuse fraction results from the radiation model were applied to existing meteorological data sets. These data sets were then used as input for the CERES model. The CERES model was modified so that plant radiation use efficiency (RUE) fluctuated as a function of the diffuse fraction calculated by the TUV model. Model simulations were performed using many years of meteorological data and different amounts of atmospheric aerosol loading for several different locations around the world. The change in crop yield due to the influence of aerosols was found to be extremely dependent on the magnitude of the increase in radiation use efficiency (RUE) incurred by increasing the diffuse fraction. The influence of aerosols on simulated crop yield was found to be more negative on overcast days since the diffuse fraction is already quite high even in a clean atmosphere. Consequently, the more overcast days there are in a growing season, the more negative is the influence of aerosols. For the most realistic set of model results based on location-specific aerosol loadings and crop-specific assumptions of RUE change, the influence of aerosols is estimated to be −10% on maize yield, ±5% on wheat yield, and ±10% on rice yield except for when grown under exceptionally sunny conditions as found in California’s central valley, in which case, yields are predicted to increase by up to 30%. Aerosols also tend to decrease plant water stress by reducing soil evaporation and transpiration. When crops are grown under rainfed conditions, this reduction in water loss from evapotranspiration may offset the decrease in the photosynthetic rate and cause aerosols to have a positive influence on final grain yields.

Extinction of UV‐visible radiation in wet midlatitude (maritime) snow: Implications for increased NOx emission

A field and modeling study of the optical properties of wet midlatitude (maritime) mountainous snowpack is presented. The snowpacks were found to have greater UV penetration depths than polar (tundra) snowpacks and consequently may release more NO2 gas to the atmosphere during the photolytic destruction of nitrate anions in the snowpack for a given spherical irradiance. Fluxes of NO2 to the troposphere as a result of NO3− photolysis were calculated for different measured e‐folding depths using tropospheric ultraviolet‐visible (TUV)–modeled actinic flux data assuming all the NO2 can leave the snowpack and the photolysis of nitrate is the rate‐limiting step. These calculated fluxes ranged from 3.3 to 7.6 kg km−2 yr−1 (assuming 150 days of snow cover a year), significantly more than polar snowpacks by a factor of 4–10. The fieldwork measured liquid equivalent e‐folding depths of 3.74–14.66 cm (e‐folding depths of 7.25–32.4 cm−1) at four sites in the Cairngorm mountain range, Scotland (57°07′N, 3°40′W), during the winter of 2003. The wavelength range studied was 300–450 nm. The snowpacks consisted predominantly of windblown rounded grains ranging from 0.1 to 1.5 mm in diameter. The liquid water content of the snowpacks varied between the sites, which were visited up to three times to observe temporal changes in the physical composition and optical properties of the snowpack. The snowpack was modeled using the TUV radiative‐transfer model, calculating scattering cross sections (σscatt) between 1 and 5 m2 kg−1. The absorption coefficient due to impurities (σabs+) was also modeled and was found to be approximately 1.0 cm2 kg−1. Three optically different snowpack categories are suggested: cold, dry polar (tundra) snowpacks, σscatt = 20–30 m2 kg−1; warmer polar coastal (maritime) snowpacks, σscatt = 6–13 m2 kg−1; and melting midlatitude mountainous (maritime) snow, σscatt = 1–5 m2 kg−1. Thus for midlatitude wet snow, 85% of photochemistry is likely to occur in the top 15–60 cm.

Preliminary studies of the effect of aerosols on nitrogen dioxide photolysis rates in the city of São Paulo, Brazil

The main purpose of this study was to perform a first evaluation of the particles and optical parameters influence on photolysis rates in the city of São Paulo, Brazil. In order to achieve this objective an AERONET (AErosol RObotic NETwork—an inclusive federation of ground-based remote sensing aerosol networks) database and TUV (Tropospheric Ultraviolet Visible) model were used to calculate photolysis rates on cloudless days during specific periods (July and August) of the 2002 winter season. The main sources of ozone precursors in MASP (Metropolitam Area of São Paulo) are the burning of fuels—gasoline, ethanol and diesel. This blend of fuels generates a unique photochemical problem. Also in MASP the concentrations of inhalable particles (PM 10) usually exceed the national standard of 150 μg m −3 for 24 h. It is described that the concentration of particulate matter affects the ozone formation, by its interaction with incident radiation and consequently affecting the photolysis rates. It is very important to have a good description of the behavior of these compounds, not only in the surface, but also in the whole column. This study presents the first results related to this subject. To characterize the influence of different particles in the photolysis rates, results were correlated to aerosol composition. Particles that are mostly composed of black carbon had a high impact, decreasing photolysis rates when their concentrations were high. Aerosol optical properties analyzed as single-scattering albedos were found to participate significantly in the reduction of photolysis rates. It was observed that urban aerosols in São Paulo reduce both photolysis rates (round 15%) and ozone concentrations. Comparing July and August, photolysis rates were lower in July due to solar position.

A comparison between radiative transfer calculation and pyranometer data gathered at Observatoire de Haute Provence

The energy received on the ground from the sun is calculated using the Tropospheric Ultraviolet Visible (TUV) model of Madronich and Flocke (Handbook of Environmental Chemistry, Boule, P., (Ed.), Springer-Verlag, Heildelberg; pp. 1–26). Additions are made for running the model up to the infrared domain. Moreover, modifications in the algorithms describing several physical processes experienced by the solar radiations and models of the atmosphere properties are replaced by available local data in the area where the calculation is needed. For that, specific aerosols distribution, spectral absorption cross section of water vapour, local albedo season dependent and a recent extraterrestrial solar spectrum from UV to IR obtained from space measurements, are used. At Observatoire de Haute Provence (44°N, 6°E), a set of instruments provide us with information necessary to run the TUV model. Furthermore, the energy including the scattered and direct solar irradiance is measured by pyranometers in two wavelength ranges. To validate the additions made to the model, comparisons in clear sky conditions between the observations and the model output are made. The additions led to increase the accuracy of the model predictions to about 4%. Furthermore, this study shows that pyranometers characterization have to be improved by taking into account the contribution of the scattered light at large solar zenith angle.

Effect of clouds on photolysis and oxidants in the troposphere

Cloud layers in the troposphere influence photolysis rates (J values) and hence concentrations of chemical species. In order to study the impact of clouds on photolysis rates and oxidants, we have developed a simplified version of the National Center for Atmospheric Research (NCAR) Tropospheric Ultraviolet‐Visible (TUV) model and have coupled the simplified TUV (otherwise known as the fast TUV (FTUV)) into the NCAR/Atmospheric Chemistry Division global transport chemical model (Model for Ozone and Related Chemical Tracers (MOZART‐2)). The FTUV model has the same physical processes as the TUV model, except that the wavelength bins between 121 and 750 nm are reduced from 140 to 17. As a result, FTUV is about 8 times faster than the original TUV. Differences in the calculated photolysis rates between TUV and FTUV are generally less than 5% in the troposphere. Subgrid vertical distributions of clouds are also considered in the calculation of photolysis rates in MOZART‐2. The method used in this study is a mixed maximum and random overlap scheme. The subgrid method increases the computation time for photolysis rates by a factor of 3 compared to a simple method in which clouds are uniformly distributed over the MOZART‐2 grids. Our calculation shows that the uniform cloud distribution method tends to significantly overestimate back scattering on the top of clouds and overestimates the impact on photochemistry in the troposphere. The results suggest that clouds have important impacts on tropospheric chemistry. Global mean OH concentration increases by about 20% due to the impact of clouds. As a result, the calculated CH4 lifetime changes to 11 years for clear sky and 9 years for cloudy sky. The latter value is closer to the methane lifetime estimated from previous studies. Calculated CO surface concentrations are compared with observed values, showing an improvement when the impact of clouds on the photolysis rates is taken into account. Clouds also have important impacts on tropospheric ozone budget. Our calculation suggest that because of clouds, the globally averaged photolysis rates of J[O3], J[CH2O], and J[NO2] are enhanced in the troposphere by about 12, 13, and 13%, respectively, leading to an 8% increase in the tropospheric O3 concentrations. Our study suggests that clouds strongly influence photolysis rates and hence play an important role in controlling the concentrations of the tropospheric oxidants. Such effects should be carefully considered and included in regional and global chemical transport models.

Comparison of airborne measured and calculated spectral actinic flux and derived photolysis frequencies during the PEM Tropics B mission

Scanning actinic flux spectroradiometer (SAFS) instruments were deployed on the National Aeronautics and Space Administration (NASA) DC‐8 and P‐3B research aircraft for the Pacific Exploratory Mission in the Tropical Pacific (PEM Tropics B mission). The SAFS instruments measured the wavelength‐dependent downwelling and upwelling actinic flux from 282 to 420 nm. From the actinic flux spectra, photolysis frequencies of a number of molecules were calculated. Flight tracks during the PEM Tropics B mission varied in altitude from the surface up to 12 km, with much time spent above, below, and among broken and scattered clouds. Comparisons of the measured photolysis frequencies to the photolysis frequencies calculated with the National Center for Atmospheric Research (NCAR) tropospheric ultraviolet visible (TUV) radiative transfer model, assuming clear skies, demonstrate some expected differences in this complex radiation environment, while in general the agreement between the measurements and the model is within 15%. Differences between the measurements and the model are expected in and around clouds, because the radiation model is not equipped to input broken and scattered cloud effects, except on an individual case‐by‐case basis. The differences in the composite mean ratio of SAFS measurement to TUV model for total photolysis frequencies vary from 6% to 18% for jNO2 and from 6% to 11% for jO(1D) for both aircraft. The differences between the measurements and the models represent the accuracy that can be expected when using photolysis frequencies from clear‐sky radiative transfer modeling in regional and global atmospheric chemistry models for regions with broken and scattered clouds in the troposphere.

Evaluation of nitrogen dioxide photolysis rates in an urban area using data from the 1997 Southern California Ozone Study

The photolysis of nitrogen dioxide and formaldehyde are two of the most influential reactions in the formation of photochemical air pollution, and their rates are computed using actinic flux determined from a radiative transfer model. In this study, we compare predicted and measured nitrogen dioxide photolysis rate coefficients ( j NO 2 ). We used the Tropospheric Ultraviolet-Visible (TUV) radiation transfer model to predict j NO 2 values corresponding to measurements performed in Riverside, California as part of the 1997 Southern California Ozone Study (SCOS’97). Spectrally resolved irradiance measured at the same site allowed us to determine atmospheric optical properties, such as aerosol optical depth and total ozone column, that are needed as inputs for the radiative transfer model. Matching measurements of aerosol optical depth, ozone column, and j NO 2 were obtained for 14 days during SCOS’97. By using collocated measurements of the light extinction caused by aerosols and ozone over the full height of the atmosphere as model input, it was possible to predict sudden changes in j NO 2 resulting from atmospheric variability. While the diurnal profile of the rate coefficient was readily reproduced, j NO 2 model predicted values were found to be consistently higher than measured values. The bias between measured and predicted values was 17–36%, depending on the assumed single scattering albedo. By statistical analysis, we restricted the most likely values of the single scattering albedo to a range that produced bias on the order of 20–25%. It is likely that measurement error is responsible for a significant part of the bias. The aerosol single scattering albedo was found to be a major source of uncertainty in radiative transfer model predictions. Our best estimate indicates its average value at UV-wavelengths for the period of interest is between 0.77 and 0.85.