Pan Measurements Research Articles

Evaluating potential evapotranspiration methods in a rapidly warming Arctic region, SW Spitsbergen (1983–2023)

Study regionsHornsund, SW Spitsbergen, Svalbard – area representing atlantic sector of High Arctic Study focusSvalbard's warming climate significantly alters its hydrological conditions. Evapotranspiration, a crucial hydrological component, is understudied in this High Arctic environment. In this study, daily potential evapotranspiration (PET) estimates for the period 1983–2023 were calculated using meteorological data from the Polish Polar Station Hornsund (SW Spitsbergen). 11 different PET methods were applied, those include radiation-based, temperature-based, radiation-temperature-based, and combined methods. New hydrological insights for the regionThe results show a large spread in the annual sum of PET ranging from ∼20 mm/y (Kharrufa) through ∼300 mm/y (Penman-Monteith) up to ∼450 mm/y (Abtew). Trends analysis shows different outcomes depending on the length of the averaging period. Using a daily timescale, PET methods tend to show more similar patterns of changes than using monthly timescales. The changes in PET estimates differ between the models, hence classifying PET methods should consider their sensitivity to meteorological changes. PET estimates were compared with pan measurements at a daily time scale in 2022–2023. The Penman method produced the best results in relation to pan measurements. In other cases, despite a relatively high linear correlation, calibration to local conditions is needed to scale the outcomes and limit biases. This study improves understanding of how PET models perform in the rapidly changing High Arctic climate.

Priestly-Taylor Coefficient Evaluation for Konya Closed Basin

Measurement of evaporation in the field is difficult and expensive; thus, the empirical evaporation estimation methods have been developed. However, these estimation methods have both advantages and disadvantages. The main disadvantage is that their coefficients were determined by the climatic conditions of the study areas. One of these methods is Penman. The Penman method, accepted as a reference, has reached the closest estimations to the measurement of evaporation in the field of the different parts of the world. However, it needs lots of measured climatic data. The Priestley-Taylor method was derived to reduce the measured data needs of the Penman method. Priestly and Taylor represented the variables such as saturated and actual vapor pressures and wind speed with coefficient of 1.26. The researchers have continued to study on the calibration of the  coefficient for their studies’ area since this method has been known to underestimate evaporation value in areas where advection is effective. The present study consists of two stages. First, evaporation was tried to be estimated with these two methods by using the measured climatic data of five meteorological stations in the Konya Closed Basin. Estimated values were evaluated making comparison with the pan measurements. Although slightly higher values were estimated from the pan measurements with each method, the Penman method was found to be relatively more consistent on the basis of statistical indicators. Second,  coefficient was obtained as 1.28 for the study area by using three artificial intelligence-based optimization algorithms. The Penman method was used for comparison in this stage. It was concluded that there was no need for any calibration of the  coefficient and the original one was found to be valid for the study area as well.

Reservoir evaporation in a Mediterranean climate: comparing direct methods in Alqueva Reservoir, Portugal

Abstract. Alqueva Reservoir is one of the largest artificial lakes in Europe and is a strategic water storage for public supply, irrigation, and energy generation. The reservoir is integrated within the Multipurpose Alqueva Project (MAP), which includes almost 70 reservoirs in a water-scarce region of Portugal. The MAP contributes to sustainability in southern Portugal and has an important impact on the entire country. Evaporation is the key component of water loss from the reservoirs included in the MAP. Evaporation from Alqueva Reservoir has been estimated by indirect methods or pan evaporation measurements; however, specific experimental parameters such as the pan coefficient were never evaluated. Eddy covariance measurements were performed at Alqueva Reservoir from June to September in 2014 as this time of the year provides the most representative evaporation volume losses in a Mediterranean climate. This period is also the most important period for irrigated agriculture and is, therefore, the most problematic period of the year in terms of managing the reservoir. The direct pan evaporation approach was first tested, and the results were compared to the eddy covariance evaporation measurements. The total eddy covariance (EC) evaporation measured from June to September 2014 was 450.1 mm. The mean daily EC evaporation in June, July, August, and September was 3.7, 4.0, 4.5, and 2.5 mm d−1, respectively. A pan coefficient, Kpan, multivariable function was established on a daily scale using the identified governing factors: air temperature, relative humidity, wind speed, and incoming solar radiation. The correlation between the modelled evaporation and the measured EC evaporation had an R2 value of 0.7. The estimated Kpan values were 0.59, 0.57, 0.57, and 0.64 in June, July, August, and September, respectively. Consequently, the daily mean reservoir evaporation (ERes) was 3.9, 4.2, 4.5, and 2.7 mm d−1 for this 4-month period and the total modelled ERes was 455.8 mm. The developed Kpan function was validated for the same period in 2017 and yielded an R2 value of 0.68. This study proposes an applicable method for calculating evaporation based on pan measurements in Alqueva Reservoir, and it can be used to support regional water management. Moreover, the methodology presented here could be applied to other reservoirs, and the developed equation could act as a first evaluation for the management of other Mediterranean reservoirs.

Comparison of formulating apparent potential evaporation with pan measurements and Penman methods

The apparent potential evaporation Epa represents the atmospheric evaporation demand which is essential in estimating the actual evaporation. In practice, Epa is usually formulated by different functions based on Penman method or by pan measurements. However, it remains unclear which formulation is preferred for different ecosystems. In this study, we adopted two formulations of Epa by the Penman methods (EpMO based on the Monin–Obukhov similarity theory and Epu2 based on the Rome wind function) and another two formulations by the pan evaporation measurements (ED20 observed by the D20 pan and EE601 observed by the E601B pan) at multi eddy covariance (EC) stations including different ecosystem types. The four formulations were compared in estimating actual evaporation (E) with the aid of a generalized complementary function. Our results show that: 1) For short vegetations (i.e., grass and crop), ED20 is a competitive candidate because this in-situ measurement can accurately reflect the dynamics of the underlying surface through observation, and it is widely accessible and easy to deploy; 2) For high vegetations like forests, EpMO is the best candidate because this theoretical formulation incorporates both evaporation demands from the high canopy and ground surface, which cannot be reasonably captured by the other three candidates; 3) When the canopy information (e.g., displacement height and roughness lengths) is not available, Epu2 can be regarded as a useful alternative formulation of Epa with moderate accuracy in evaporation estimation.

Response of black cumin (Nigella sativa L.) to deficit irrigation in a semi-arid region: Growth, yield, quality, and water productivity

Given the growing and urgent need for more effective irrigation strategies in arid and semi-arid regions, a field study was conducted in 2014 and 2015 in eastern Anatolia, Turkey, to evaluate the effects of water deficit treatments on the growth, yield, yield components, quality, and water productivity of black cumin (Nigella sativa L.). Four irrigation treatments (I1, I2, I3, and I4), based on Class A pan measurements, corresponded to 100, 80, 60 and 40 % cumulative evaporation were studied. The treatments were tested under a randomized, complete block design with three replications. The results indicated that considerable variation in yield and yield components of black cumin occurred, and that yield significantly increased with decreasing water deficit. Maximum seed and oil yields were recorded for the plots grown under the I1 (full irrigation) treatment. The results showed that under full irrigation (I1), seed yield was 1413.5 kg ha−1 and decreased by 13.5 %, 18.9 %, and 30.1 % in I2, I3, and I4 treatments, respectively. Irrigation water use efficiency under the full irrigation treatment was determined to be 6.2 %, 23.0 %, 37.6 % lower than I2, I3, and I4 treatments, although full irrigation provided the highest yield. Water use efficiency increased significantly when the water deficit was more than or equal to 40 %. The study, therefore, suggests that for black cumin grown in areas with sufficient water resources, full irrigation may be the most productive practice while deficit irrigation practices can improve water productivity.

Improvements of the spatially distributed hydrological modelling using the HBV model at 1 km resolution for Norway

A robust hydrological modeling at a fine spatial resolution is a vital tool for Norway to simulate river discharges and hydrological components for climate adaptation strategies. However, it requires improvements of modelling methods, detailed observational data as input and expensive computational resources. This work aims to set up a distributed version of the HBV model with a physically based evapotranspiration scheme at 1 km resolution for mainland Norway and to calibrate/validate the model for 124 catchments using regionalized parameterizations. The Penman-Monteith equation was implemented in the HBV model and vegetation characteristics were derived from the Norwegian forest inventory combined with multi-source remote sensing data at 16 m spatial resolution. The estimated potential evapotranspiration (Ep) was compared with pan measurements and estimates from the MODerate Resolution Imaging Spectrometer (MOD16) products, the Global Land Evaporation Amsterdam Model (GLEAM) and Variable Infiltration Capacity (VIC) hydrological model. There are 5 climatic zones in Norway classified based on 4 temperature and precipitation indices. For each zone, the model was calibrated separately by optimizing a multi-objective function including the Nash-Sutcliff efficiency (NSE) and biases of selected catchments. In total, there are 85 catchments for calibration and 39 for validation. The Ep estimates showed good agreement with the measurements, GLEAM and VIC outputs. However, the MOD16 product significantly overestimates Ep compared to the other products. The discharge was well reproduced with the median daily NSE of 0.68/0.67, bias of −3%/−1%, Kling-Gupta efficiency (KGE) of 0.70/0.69 and monthly NSE of 0.80/0.78 in the calibration/validation periods. Our results showed a significant improvement compared to the previous HBV application for all catchments, with an increase of 0.08–0.16 in the median values of the daily NSE, KGE and monthly NSE. Both the temporal and spatial transferability of model parameterizations were also enhanced compared to the previous application.

Estimation of natural water body’s evaporation based on Class A pan measurements in comparison to reference evapotranspiration

Estimation of natural water body’s evaporation based on Class A pan measurements in comparison to reference evapotranspiration

EFFECTS OF SOIL CONDITIONER ON WATER CONTENT OF SANDY SOIL AND PEANUT PRODUCTION UNDER DIFFERENT IRRIGATION RATES

Effects of soil conditioner on water content of sandy soil and peanut production under different irrigation rates were investigated during the two growing summer seasons of 2015 and 2016 at Abdel Moniem Reyad Village (Latitude 30°41'8 N, Longitude 30°18'0 E, and altitude of 7m above sea level), Al-Bustan area, El-Beheira Governorate, Egypt. Also, the validity of a simulation model to predict changes in soil moisture contents with time as well as moisture distribution within the soil profile was tested. Four soil conditioner levels (SC0: control, SC1: 1 ton/fed, SC2: 2 ton/fed, and SC3: 3 ton/fed), and three irrigation rates (full irrigation: 100% ETo, moderate deficit: 75% ETo and severe deficit: 50% ETo) were tested. Reference evapotranspiration (ETo) values were based on class A pan measurements. Peanut (Ismailia 2 cultivar) was used in the experiments. Results indicated that, the performance of the sprinkler irrigation system at the experimental site is considered acceptable. The additions of soil conditioner to the sandy soil led to decreasing the bulk density and saturated hydraulic conductivity values and increasing the saturated soil moisture contents, field capacity, welting point, available water, and air entry values. The tested simulation model proved to be accurate to predict changes in soil moisture contents with time as well as moisture distribution within the soil profile. Average seasonal values of applied irrigation water were 2986, 2240, and 1506 m3/fed for the 100, 75, and 50% ETo treatments, respectively. The tested treatments significantly affected peanut yield and yield components parameters tested under the experimental conditions. Applying amount of irrigation water equals to 100% ETo with 2 ton/fed of hydro-gel soil conditioner produced the highest pod yield of 1.83 and 1.89 ton/fed and straw yields of 1.33 and 1.32 ton/fed from the peanut crop grown in sandy soil. Results indicated also that, an average water productivity value of 0.65 kg pod/m3 and 0.43 kg straw/m3 can be achieved from the interaction between 100% ETo * SC2 treatment. The results indicated that, in sandy soils of poor water retentive capacity, high yields of peanut crop can be obtained with the application of 2-3 ton/fed soil conditioner and irrigating with amount of water equals to 100% ETo.

The hidden hydrology of Groenvlei, a lacustrine wetland on the southern Cape coast of South Africa

Following Parsons’ (2009) challenge of the hypothesis that Groenvlei is sustained by discharge from the underlying Table Mountain Group Aquifer, research was undertaken to better understand the hydrology of Groenvlei. Using a daily water balance model coupled to Darcian flow calculations it was possible to show that direct rainfall and evapotranspiration losses were the key hydrological drivers of the system, with groundwater playing an important but secondary role. It was found that Groenvlei is not endorheic and that S pan measurements are best upscaled to lake evaporation using coefficients offered by Midgley et al. (1994). The reed collar plays an important role in the functioning of the lake, but is dormant during winter. The outcome of the research demonstrated that sound conceptualisation using all available data, information and knowledge remains a critical aspect of any hydrological modelling. The improved understanding of the hydrology of Groenvlei is expected to facilitate a better understanding of the nine Ramsar wetlands located on similar, but more complicated, hydrogeological settings elsewhere on the South African coastline.

Comparing Raindrop Size and Velocity Measurement Accuracy Using Shadowgraphy, Disdrometery, and Pie Pan Measurement Techniques

Abstract Local raindrop size and velocity data measurements are often used to quantify hyetographs for rainfall events; the accuracy of the rainfall hyetograph is dependent on the accuracy of the raindrop size and velocity measurement technique. Different raindrop size and velocity measurement techniques were evaluated using simulated raindrops of relatively uniform size in the laboratory. The measurement techniques included a high-speed photography-based imaging system (shadowgraphy), an optical disdrometer, and a pie pan method (raindrop size only). The shadowgraphy system, which had a measurement uncertainty of 2.86 %, served as the comparative standard for the other measurement techniques. The disdrometer raindrop size measurements were relatively accurate except for raindrops passing near the lateral limits of the measurement sheet; the peripheral raindrops sizes were estimated to be 97 % smaller than their actual size. The pie pan method overestimated raindrop sizes by approximately 41 %. The disdrometer raindrop velocity data underestimated actual velocities. Of the techniques evaluated, the disdrometer represents the only practical field deployable method for quantifying rainfall data.

Spatial Patterns and Influence Factors of Conversion Coefficients between Two Typical Pan Evaporimeters in China

Pan measurement is a reliable and efficient method for indicating the evaporative demand of the atmosphere. There are several types of pan evaporimeters worldwide, and the estimation of the conversion coefficients (Kp) between them is necessary in hydrologic research. In China, E601B pans were installed at all meteorological stations beginning in 1998. They replaced the 20 cm pans (φ20). To fully use the records from the two pans and obtain long-term pan evaporation, the spatial patterns of Kp between φ20 and E601B and the factors that influence Kp are investigated based on records from 573 national meteorological stations from 1998 to 2001. In this study, The results show that higher Kp values are found in southwestern regions and lower values are found in northeastern regions during the warm seasons (from May to September), while Kp values are lower during warm seasons than during cold seasons (from October to April the following year). In addition, net radiation was found to be the dominant climate factor that affects variations in Kp, followed by relative humidity and the vapor pressure deficit. This study can improve the benefit of not only the selection of appropriate evaporimeters by meteorological departments, but also of the study of temporal variability and trends in the evaporative demand.

Ozone export from East Asia: The role of PAN

AbstractPeroxyacetyl nitrate (PAN) is an important ozone (O3) precursor. The lifetime of PAN is approximately 1 month in the free troposphere, and this allows O3 production to occur in pollution plumes at intercontinental distances from its source. In this study we use the Goddard Earth Observing System (GEOS)‐Chem global chemical transport model, new satellite measurements of PAN from the Aura Tropospheric Emission Spectrometer (TES), and data from the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) field campaign over North America, to study the role of natural and anthropogenic Asian emissions on free tropospheric (900–400 hPa) PAN distributions and subsequent O3 production. Using the ARCTAS data with GEOS‐Chem, we show that while GEOS‐Chem is unbiased with respect to the aircraft data, TES version 7 PAN data are biased high for regions with surface temperatures colder than 285 K. However, GEOS‐Chem and TES measurements provide a consistent representation (within 15% difference) of PAN abundance over East Asia. Because of the good agreement between model and observations, we use the GEOS‐Chem model to evaluate the sources of PAN precursors and the effect of free tropospheric PAN on the export of O3 from Asia to North America. The GEOS‐Chem model results show that the largest contributors to free tropospheric PAN over Asia and the northern Pacific are anthropogenic and soil NOx emissions. Biomass burning emissions have important contributions to free tropospheric PAN over northern Pacific (25% in April), while the contribution from lightning over northern Pacific is significant in July (40%). Strong springtime transport in April results in more export of free tropospheric PAN and O3 from East Asian emissions. This free tropospheric PAN contributes about 35% to the abundance of free tropospheric O3 over western North America in spring and 25% in summer.

A multi-lake study of seasonal variation in lake surface evaporation using MODIS satellite-derived surface temperature

Knowledge of the evaporative loss from lakes and reservoirs is critical to water resources managers as well as to the overall understanding of the water balance in a given basin, geographical region, or continent. Existing methods for ascertaining evaporation from lakes and reservoirs include point measurements, water balance and mass transfer calculations, and proxy measurements using a pan. Point measurements using the eddy flux covariance method can be accurate, but are resource intensive and unsuited for determining spatial variation over a lake, or for obtaining measurements over many lakes. Mass balance methods cannot provide spatial variability and their accuracy depends on other portions of the water balance that can be challenging to obtain, such as leakage. Similarly, relatively recently deployed scintillation methods provide only an average for a strip across a lake and are also resource intensive and not suited for multi-lake studies. Evaporation pan measurements can also be used, though their accuracy is poor. Herein, we use a combination of Moderate Resolution Imaging Spectroradiometer (MODIS) satellite measurements of water surface temperature, measurements of wind speed, air temperature, and relative humidity from local NWS stations, and a mass transfer method, to demonstrate multi-lake evaporation measurements. Specifically, the seasonal variation in evaporation is obtained for the five major lakes in the Savannah River Basin (in South Carolina, USA): Lakes Jocassee, Keowee, Hartwell, Russell, and Thurmond. Since this approach requires only an existing satellite resource with global coverage and existing NWS stations, this method can potentially be ported to any lake where there is a nearby meteorology station. Hence, this method could be used by both water resource managers and limnologists alike. The possibility is discussed of extending this approach beyond a single basin to encompass an entire geographical region or continent.

Estimating evaporation in semi-arid areas facing data scarcity: Example of the El Haouareb dam (Merguellil catchment, Central Tunisia)

The El Haouareb dam (Merguellil catchment) in central Tunisia, which is typical of semi-arid environments. Most estimates of evaporation from water bodies located in semi-arid environments suffer from the lack of data, or biased field measurements. It is thus important for hydrologists to assess the relative performance of the various available methods used to estimate this water loss, as well as their uncertainties. We confronted physical approaches based on contrasted theoretical formulae (Dalton, simplified BREB, Penman) and geochemical approaches based on mass conservation (stable isotopes and chloride). We compared the results with Colorado pan measurements, and tested the methods’ sensitivity to various physical parameters and data gaps. In this region, where mean annual rainfall is 300 mm, estimates of evaporation of the El Haouareb Dam lake ranged from 1400 to 1900 mm a−1, depending on the method and the year. The Penman approach was found to be the most robust and gave an annual mean of 1600 mm a−1. Evaporation values were refined by combining results from the different methods. Mean interannual evaporation was estimated to be 1700 mm a−1, with an uncertainty of 15%. From this work, we propose an annual Colorado pan conversion coefficient of 0.8 which can be adjusted, 1.0 for spring and 0.76 for the rest of the year.

A gas chromatograph for quantification of peroxycarboxylic nitric anhydrides calibrated by thermal dissociation cavity ring-down spectroscopy

Abstract. The peroxycarboxylic nitric anhydrides (PANs, molecular formula: RC(O)O2NO2) can readily be observed by gas chromatography (PAN-GC) coupled to electron capture detection. Calibration of a PAN-GC remains a challenge, because the response factors differ for each of the PANs, and because their synthesis in sufficiently high purity is non-trivial, in particular for PANs containing unsaturated side chains. In this manuscript, a PAN-GC and its calibration using diffusion standards, whose output was quantified by blue diode laser thermal dissociation cavity ring-down spectroscopy (TD-CRDS), are described. The PAN-GC peak areas correlated linearly with total peroxy nitrate (ΣPN) mixing ratios measured by TD-CRDS (r > 0.96). Accurate determination of response factors required the concentrations of PAN impurities in the synthetic standards to be subtracted from ΣPN. The PAN-GC and its TD-CRDS calibration method were deployed during ambient air measurement campaigns in Abbotsford, BC, from 20 July to 5 August 2012, and during the Fort McMurray Oil Sands Strategic Investigation of Local Sources (FOSSILS) campaign at the AMS13 ground site in Fort McKay, AB, from 10 August to 5 September 2013. The PAN-GC limits of detection for PAN, PPN, and MPAN during FOSSILS were 1, 2, and 3 pptv, respectively. For the Abbotsford data set, the PAN-GC mixing ratios were compared, and agreed with those determined in parallel by thermal dissociation chemical ionization mass spectrometry (TD-CIMS). Advantages and disadvantages of the PAN measurement techniques used in this work and the utility of TD-CRDS as a PAN-GC calibration method are discussed.

From 2D to 3D: Construction of a 3D Parametric Model for Detection of Dental Roots Shape and Position from a Panoramic Radiograph—A Preliminary Report

Objectives. To build a 3D parametric model to detect shape and volume of dental roots, from a panoramic radiograph (PAN) of the patient. Materials and Methods. A PAN and a cone beam computed tomography (CBCT) of a patient were acquired. For each tooth, various parameters were considered (coronal and root lengths and widths): these were measured from the CBCT and from the PAN. Measures were compared to evaluate the accuracy level of PAN measurements. By using a CAD software, parametric models of an incisor and of a molar were constructed employing B-spline curves and free-form surfaces. PAN measures of teeth 2.1 and 3.6 were assigned to the parametric models; the same two teeth were segmented from CBCT. The two models were superimposed to assess the accuracy of the parametric model. Results. PAN measures resulted to be accurate and comparable with all other measurements. From model superimposition the maximum error resulted was 1.1 mm on the incisor crown and 2 mm on the molar furcation. Conclusion. This study shows that it is possible to build a 3D parametric model starting from 2D information with a clinically valid accuracy level. This can ultimately lead to a crown-root movement simulation.

Estimation of land surface evapotranspiration over the Heihe River basin based on the revised three‐temperature model

AbstractEvaporation and heat transfer mechanisms differ for different types of soils. In this study, Evapotranspiration (ET) models for the bare soil, vegetation land, and the mixed regions were developed based on the Land Surface Energy Balance Theory. Surface energy fluxes and daily ET of the Heihe River Basin were estimated using Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data and meteorological observation data, including the daily duration time of the sun, air temperature, etc. based on the revised three‐temperature model (3T model), and then the distribution maps of ET were retrieved. Thereafter, Evaporator pan (E‐601) measurements obtained from meteorological observation stations in the study area were chosen to validate the accuracy of the calculated ET. Results showed that the maximum absolute error was 1·70 mm/d, whereas the minimum was 0·05 mm/d, which indicated that the revised 3T model was of adequate accuracy in estimating ET in the study area and could be extended to the regional scale. Also, the spatial distribution of surface ET was different in the upper, middle and lower reaches in the study area and fit with vegetation conditions. On the whole, the ET increased from desert of northeast to mountain of southwest. The results showed that the ET in the study area ranged from 0·06 to 7·00 mm/d, with an average value of 2·43 mm/d, on 2 August 2008. The highest ET values was from the forest or alpine meadow areas in the upper reaches, with more than 4 mm/d, the lowest values was from the Gobi desert in the lower reaches, with below 2 mm/d, and ET of oasis in the middle reaches was not only changed with the crop growth stage and vegetation conditions, but also controlled greatly by human activities, with more than 3·5 mm/d. Copyright © 2011 John Wiley & Sons, Ltd.

Evaluating the calculated dry deposition velocities of reactive nitrogen oxides and ozone from two community models over a temperate deciduous forest

Hourly measurements of O3, NO, NO2, PAN, HNO3 and NOy concentrations, and eddy-covariance fluxes of O3 and NOy over a temperate deciduous forest from June to November, 2000 were used to evaluate the dry deposition velocities (Vd) estimated by the WRF-Chem dry deposition module (WDDM), which adopted Wesely (1989) scheme for surface resistance (Rc), and the Noah land surface model coupled with a photosynthesis-based Gas-exchange Evapotranspiration Model (Noah-GEM). Noah-GEM produced better Vd(O3) variations due to its more realistically simulated stomatal resistance (Rs) than WDDM. Vd(O3) is very sensitive to the minimum canopy stomatal resistance (Ri) which is specified for each seasonal category assigned in WDDM. Treating Sep-Oct as autumn in WDDM for this deciduous forest site caused a large underprediction of Vd(O3) due to the leafless assumption in ‘autumn’ seasonal category for which an infinite Ri was assigned. Reducing Ri to a value of 70 s m �1 , the same as the default value for the summer season category, the modeled and measured Vd(O3) agreed reasonably well. HNO3 was found to dominate the NOy flux during the measurement period; thus the modeled Vd(NOy )w as mainly controlled by the aerodynamic and quasi-laminar sublayer resistances (Ra and Rb), both being sensitive to the surface roughness length (z0). Using an appropriate value for z0 (10% of canopy height), WDDM and Noah-GEM agreed well with the observed daytime Vd(NOy). The differences in Vd(HNO3) between WDDM and Noah-GEM were small due to the small differences in the calculated Ra and Rb between the two models; however, the differences in Rc of NO2 and PAN between the two models reached a factor of 1.1e1.5, which in turn caused a factor of 1.1e1.3 differences for Vd. Combining the measured concentrations and modeled Vd ,N Ox, PAN and HNO3 accounted for 19%, 4%, and 70% of the measured NOy fluxes, respectively.

Global atmospheric budget of acetaldehyde: 3-D model analysis and constraints from in-situ and satellite observations

Abstract. We construct a global atmospheric budget for acetaldehyde using a 3-D model of atmospheric chemistry (GEOS-Chem), and use an ensemble of observations to evaluate present understanding of its sources and sinks. Hydrocarbon oxidation provides the largest acetaldehyde source in the model (128 Tg a−1, a factor of 4 greater than the previous estimate), with alkanes, alkenes, and ethanol the main precursors. There is also a minor source from isoprene oxidation. We use an updated chemical mechanism for GEOS-Chem, and photochemical acetaldehyde yields are consistent with the Master Chemical Mechanism. We present a new approach to quantifying the acetaldehyde air-sea flux based on the global distribution of light absorption due to colored dissolved organic matter (CDOM) derived from satellite ocean color observations. The resulting net ocean emission is 57 Tg a−1, the second largest global source of acetaldehyde. A key uncertainty is the acetaldehyde turnover time in the ocean mixed layer, with quantitative model evaluation over the ocean complicated by known measurement artifacts in clean air. Simulated concentrations in surface air over the ocean generally agree well with aircraft measurements, though the model tends to overestimate the vertical gradient. PAN:NOx ratios are well-simulated in the marine boundary layer, providing some support for the modeled ocean source. We introduce the Model of Emissions of Gases and Aerosols from Nature (MEGANv2.1) for acetaldehyde and ethanol and use it to quantify their net flux from living terrestrial plants. Including emissions from decaying plants the total direct acetaldehyde source from the land biosphere is 23 Tg a−1. Other terrestrial acetaldehyde sources include biomass burning (3 Tg a−1) and anthropogenic emissions (2 Tg a−1). Simulated concentrations in the continental boundary layer are generally unbiased and capture the spatial gradients seen in observations over North America, Europe, and tropical South America. However, the model underestimates acetaldehyde levels in urban outflow, suggesting a missing source in polluted air. Ubiquitous high measured concentrations in the free troposphere are not captured by the model, and based on present understanding are not consistent with concurrent measurements of PAN and NOx: we find no compelling evidence for a widespread missing acetaldehyde source in the free troposphere. We estimate the current US source of ethanol and acetaldehyde (primary + secondary) at 1.3 Tg a−1 and 7.8 Tg a−1, approximately 60{%} and 480% of the corresponding increases expected for a national transition from gasoline to ethanol fuel.

Closing the peroxy acetyl nitrate budget: observations of acyl peroxy nitrates (PAN, PPN, and MPAN) during BEARPEX 2007

Abstract. Acyl peroxy nitrates (APNs, also known as PANs) are formed from the oxidation of aldehydes and other oxygenated VOC (oVOC) in the presence of NO2. There are both anthropogenic and biogenic oVOC precursors to APNs, but a detailed evaluation of this chemistry against observations has proven elusive. Here we describe measurements of PAN, PPN, and MPAN along with the majority of chemicals that participate in their production and loss, including OH, HO2, numerous oVOC, and NO2. Observations were made during the Biosphere Effects on AeRosols and Photochemistry Experiment (BEARPEX 2007) in the outflow of the Sacramento urban plume. These observations are used to evaluate a detailed chemical model of APN ratios and concentrations. We find that the ratios of APNs are nearly independent of the loss mechanisms and thus an especially good test of our understanding of their sources. We show that oxidation of methylvinyl ketone, methacrolein, methyl glyoxal, biacetyl and acetaldehyde are all significant sources of the PAN+peroxy acetyl (PA) radical reservoir, accounting for 26%, 2%, 7%, 20%, and 45%, of the production rate on average during the campaign, respectively. At high temperatures, when upwind isoprene emissions are highest, oxidation of non-acetaldehyde PA radical sources contributes over 60% to the total PA production rate, with methylvinyl ketone being the most important of the isoprene-derived sources. An analysis of absolute APN concentrations reveals a missing APN sink that can be resolved by increasing the PA+∑RO2 rate constant by a factor of 3.