Variations Of Meteorological Parameters Research Articles

Effect of inter- and intra-seasonal variations in meteorological parameters on wheat yields in Punjab

A study was conducted to evaluate the effect of variations in meteorological parameters on wheat (Triticum aestivum L. emend. Flori & Paol) yields during 1970-2005 and also as simulated with CERES-wheat model. The technology trend model of historical wheat yields in Punjab indicated that over the past 2 to 3 decades, at Ballowal Saunkhri, Amritsar, Ludhiana, Patiala and Bathinda the wheat yields have increased @ 64.2, 76.3, 62.8, 87.3 and 71.1 kg/ ha/year respectively. An analysis of historical meteorological data and past wheat yields revealed that for January, February and March the most favourable maximum temperatures for wheat yields were in the range of 16.1-18.0 oC, <21.0 oC and 28.1-30.0 oC respectively while minimum temperatures were in the range of 3.1-5.0 oC, 5.1-7.0 oC and 11.1- 13.0 oC respectively. The simulation study using CERES-wheat model revealed that the temperature increase mostly affected the early (October) sown crop during fourth week of January up to first fortnight of March; the timely (November) sown crop during February and March; the late (fourth week of November) sown crop during March; and very late(December) sown crop during March and first week of April. An analysis of historical wheat yields and weekly meteorological parameters at Ludhiana revealed that during the high yield (>5 000 kg/ha) crop years, the maximum and minimum temperatures remained near normal (+ 2oC); the rainfall was also normal or slightly above/below normal under assured irrigated conditions. From mid-February to March, dry and clear weather proves beneficial for grain filling in wheat. On the other hand, during low yield (<4 600 kg/ha) crop years, although the temperatures were favourable but very heavy rainfall showers were received during the anthesis and grain-filling period of wheat crop. The weather also remained cloudy and sunshine hours were invariably below or near normal while the relative humidity remained above (3-10 %) or near normal during most part of the crop season. Hence, these conditions were ascertained to be the most pertinent reason for relatively low-wheat yields.

Secondary organic aerosol formation in urban air: Temporal variations and possible contributions from unidentified hydrocarbons

Quantitative evaluation of the performance of one of the most advanced mechanistic secondary organic aerosol (SOA) modules/models, the Model of Aerosol Dynamics, Reaction, Ionization, and Dissolution 2 (MADRID2) in the three‐dimensional Models‐3/Community Multiscale Air Quality (CMAQ), in urban air is made. Model calculations are compared for the Tokyo, Japan, metropolitan area with measurements made using an Aerodyne quadrupole aerosol mass spectrometer (Q‐AMS) at an urban site for 9 days in July and August 2003. In general, model calculations reproduced absolute values and temporal variations of meteorological parameters, C2–C8 volatile organic compounds (VOCs), NOx (NO + NO2), inorganic aerosols, and O3 concentrations reasonably well at this site. However, model‐calculated SOA concentrations are a factor of 5 smaller than observed oxygenated organic aerosol (OOA) concentrations, and calculated total organic aerosol (OA = SOA + primary organic aerosol) concentrations are smaller by a factor of 2, indicating missing processes or sources in the current organic aerosol model calculations. On the other hand, observed features of diurnal and day‐to‐day variations of OOA were captured by our model calculations. Because of the large quantity of unidentified total nonmethane VOCs (NMVOCs) in urban air, a possible contribution of SOA formation from high‐molecular‐weight VOCs is examined through simple sensitivity studies, in which emissions are increased to account for unidentified NMVOCs. It is found that they are potentially one of the missing SOA sources, demonstrating the importance of reliable measurements of high‐molecular‐weight VOCs and total NMVOCs. Relationships between SOA and O3, including regional (∼150 × 150 km2) enhancements around the Tokyo metropolitan area, are also discussed.

Characteristic features of air ions at Mace Head on the west coast of Ireland

Coastal nucleation events and behavior of cluster ions were characterized through the measurements of air ion mobility distributions at the Mace Head research station on the west coast of Ireland in 2006. We measured concentrations of cluster ions and charged aerosol particles in the size range of 0.34–40 nm. These measurements allow us to characterize freshly nucleated charged particles with diameters smaller than 3 nm. The analysis shows that bursts of intermediate ions (1.6–7 nm) are a frequent phenomenon in the marine coastal environment. Intermediate ion concentrations were generally close to zero, but during some nucleation episodes the concentrations increased to several hundreds per cm 3. Nucleation events occurred during most of the measurement days. We classified all days into one of seven classes according to the occurrence and type of new particle formation. Nucleation events were observed during 207 days in 2006, most prominently in the spring and summer months. Rain-induced events, in turn, were observed during 132 days. Particle formation and growth events mostly coincided with the presence of low tide. Also small cluster ions (0.34–1.6 nm) were characterized. Average concentrations of small ions were 440 cm − 3 for the negative ions and 423 cm − 3 for the positive ions. Average mean mobilities of small ions were 1.86 cm 2V − 1 s − 1 and 1.49 cm 2V − 1 s − 1 for the negative and positive polarities, respectively. Concentrations of small ions were observed to be strongly dependent on the variations of meteorological parameters including wind speed and direction.

Adaptive Tropospheric Delay Modelling in GPS/INS/Pseudolite Integration for Airborne Surveying

Integrated GPS/INS systems have been used for geo-referencing airborne surveying and mapping platforms. However, due to the limited constellation of GPS satellites and their geometric distribution, the accuracy of such integrated systems cannot meet the requirements of precise airborne surveying. This problem can be addressed by including additional GPS-like ranging signals transmitted from the ground-based pseudolites (PLs). As GPS measurement geometry could be strengthened dramatically by the PL augmentation, the accuracy and reliability of an integrated system can be improved, especially in the vertical component. Nevertheless, some modelling challenges exist in PLs augmentation. As PLs are relatively close to receivers, the unit vectors from a PL to he reference and rover receivers can be significantly different. PL tropospheric delay modelling errors cannot be effectively mitigated in a differencing procedure. Furthermore, PL signals propagate through the lower troposphere, where it is very difficult to accurately model the signal delay due to temporal and spatial variations of meteorological parameters. In this paper, an adaptive PL tropospheric delay modelling method is developed to reduce such modelling errors by estimating meteorological parameters in a model. The performance of this adaptive method isevaluated with field test data. The testing results haveshown that the PL tropospheric delay modelling error can be effectively mitigated by the proposed method.

Assessment of land use changes using remote sensing and GIS and their implications on climatic variability for Balachaur watershed in Punjab, India

Abstract Decadal changes in land use/land cover for Balachaur watershed in Nawanshahar district, Punjab, India were studied using black and white aerial photographs for March 1984 on approximately 1:20,000 scale and multidate geocoded false colour composites (FCC) of IRS-1D LISS-III on 1:50,000 scale for March 2002, September 2002, and May 2003 and interpreted visually to prepare land use/land cover maps for the year 1984 and 2003. The results revealed that the area under crop land, moderately dense forest, degraded forest, degraded land in hills and piedmont plains, barren land along choes (seasonal streams) has decreased; whereas, the area under settlements, dense forest, plantations, industrial use, ponds, choes and brick kilns has increased over a span of 20 years. By virtue of afforestation and forest conservation, the density of vegetation in Siwalik hills and piedmont areas has increased. The area under moderately dense forest (1956.7 ha) and degraded forest (755.5 ha) got upgraded to dense forest while 755.5 ha of degraded forest changed to moderately dense forest. The impact of land use/land cover alterations on climatic variability at micro-level revealed that the linear trends computed for the 5-yearly moving average of maximum temperature showed an increasing trend from 1984 to 2003, whereas the minimum temperature, rainfall, and potential evapotranspiration showed a decreasing trend. Inspite of increasing forest cover in the study area, these trends in different meteorological parameters did not corroborate the findings of some earlier reports. Thus land use/ land cover changes over time may not be the only factor which causes variation in meteorological parameters. It implies that there might also be other factors such as CO2 concentration and its effect on green house gases, atmospheric pollution and spectral distribution of the incoming solar radiation, which affect climatic variability in a region. Keywords: Land use change; Climatic variability; Remote sensing; Potential evapotranspiration

Long-term variation of Asian dust and related climate factors

The relationship between long-term variation of Asian dust and climate indices was studied using a regional scale dust-transport model. Simulation results were examined using visibility-based observations by Chinese and Japanese Meteorological Agencies. Those results showed that the model reasonably captured interannual variations of Asian dust during 1972–2004. The long-term trend of dust days in the Gobi desert region showed a remarkable declining trend from the early 1980s–1997; the increasing trend of recent years (2000–02) was more conspicuous in Japan. Analyses of time variation of meteorological parameters in the Gobi region showed that the decreasing trend of dust days in this region is explained by the decreased frequency of strong winds. Additionally, anomaly analyses for dust and meteorological parameters in the Gobi region indicated that invasion of polar cold air played an important role in increasing dust phenomena. To clarify climate factors that affect dust emission and transport, regional climate indices that are appropriate for the scale of Asian dust storms were newly introduced. Correlation analyses between climate indices and simulated dust emissions showed that the dominant climate indices, which are closely correlated with dust emissions, were different in March and April. In March, the climate indices related to divergence of cold air from the polar region to mid-latitudes displayed a strong correlation with dust emissions, but during April, the climate index related to the south-north pressure gradient over the Gobi region exhibited a strong correlation with dust emission. Analyses of correlation between simulated surface dust concentrations and the Southern Oscillation Index (SOI) suggest that the El Niño/Southern Oscillation (ENSO) affects the dust transport path. Analyses of average dust transport flux at 130°E clarified variation of the transport path between La Niña years and El Niño years.

Annual variations in pollen deposition and meteorological conditions on the fell Aakenustunturi in northern Finland: Potential for using fossil pollen as a climate proxy

Annual variation in meteorological parameters (7 years) and pollen deposition (6 years) for 4 sites on a transect across an altitudinal timberline on Aakenus, a fell in northern Finland were monitored, in order to see how pollen production, as reflected by pollen deposition (grains cm year), is related to climate conditions. Wind direction and wind speed before and after estimated flowering time were determined. These indicated that, within the forest and at the physiognomic forest line, pollen deposition is primarily from plants growing within the forested area on the fell and that the contribution of windblown pollen from further south is minimal. Pollen deposition can, therefore, be taken as equivalent to pollen production. Simple linear correlation coefficients between pollen deposition and one-month and two-week mean temperature, effective temperature sums and cumulative effective temperature sums for the current and previous summers were calculated. For Pinus sylvestris the quantity of pollen deposited is affected by July mean temperature, July effective temperature sum and total effective temperature sum, for the year previous to pollen emission. Pollen deposition of Betula pubescens and Picea abies is also affected by thermal factors but of different time periods (Betula of early June and Picea of early July), but always of the year previous to flowering. However, the correlation is not as strong as for Pinus. The results suggest that annual fossil Pinus pollen quantities, if calculated from peat and/or lake sediments, are a potential climate proxy. They also demonstrate that it is possible to use temperature parameters of the current year to make forecasts of the intensity of flowering and pollen production in the following year.

Influence of internal gravity waves on sound propagation in the lower atmosphere

The effects of internal waves on the propagation of acoustic pulses in the lower atmosphere were studied theoretically and by acoustic pulse sounding of the stable atmospheric boundary layer. Due to a control in the experiments of the stratification and time variations of meteorological parameters, such as wind speed, temperature and atmospheric pressure, we were able to observe the influence of the variations of these parameters on a pulse wave form, travel time and time duration. For the travel time and wind speed variations we obtained statistical characteristics (variances, frequency spectra and coherences) in the range of periods from 1 min to 1 h and found several dominant periods, which are inherent to the trapped internal waves in the lower atmosphere. Using a nonlinear model of internal wave spectrum in the atmosphere described here we have made the calculations of variances, frequency spectra and structure functions of travel time fluctuations, which allowed us to interpret some of the observed data.

Climatic patterns in a tropical rainforest in Brunei

Meteorological parameters were monitored on an hourly basis over ten months during 1991–2, from altitudes of 45 and 913 metres in an undisturbed tropical rainforest in Brunei, near the north‐west coast of Borneo. In general, the observed characteristics of the climate accorded with the regional meteorological systems and with published results from other studies in northern Borneo, although a greater degree of temporal variability was apparent. The seasonal variability was probably enhanced by the El Niño Southern Oscillation event of early 1992. Rainfall events were typically highly variable in nature, although storm intensity profiles were broadly similar. High magnitude and high intensity events are common among the large total number of events. More rain, of generally higher intensity, fell on the mountain. The total daily solar energy receipt was similar at both altitudes owing to greater cloud cover on the mountain. Temperatures varied significantly with season at both altitudes and the mean lapse rate was 4.5°C/km. Relative humidity at 913 metres tended to be significantly higher during the day and significantly lower at night than at 45 metres. Wind directions were dominantly controlled by local mountain and valley wind systems. Wind speeds were significantly higher during the dry seasons and at the higher altitude. Previous studies have shown that small but significant variations in meteorological parameters can have distinct ecological responses, and rainfall characteristics are important controls on geomorphological processes.

Plume Gamma Dose Evaluation under Non-Homogeneous Non-Stationary Meteorological Conditions using Particle Trajectory Model for Short Term Releases

The particle trajectory model is employed to estimate plume gamma dose due to elevated release (100 m) of 41 Ar under non-homogeneous and non-stationary meteorological conditions using time varying vertical profiles of meteorological parameters given by SODAR. Unit release rate (1 Bq.s -1 ) of 41 Ar was assumed for one hour for the estimation of gamma dose. The gamma dose is also computed using the Gaussian plume model for comparison based on one hour average meteorological data. It is observed that the ground level plume gamma dose derived using the Gaussian plume model is lower by a factor of 4 than that derived using the particle trajectory model for typical unstable conditions. It is higher by two orders of magnitude for the typical stable condition studied here. The present study considers only vertical profiles of meteorological parameters and their variation with time. However, the model is capable of including horizontal variation in meteorological parameters on a three-dimensional computational domain, if such data are available. The internal consistency of the particle trajectory model is verified with the Gaussian plume model under homogeneous and stationary meteorological condition. This study assumes great significance during any unplanned releases of radioactivity into the atmosphere from nuclear power plants.

Evaluation of ammonia emission abatement on the basis of measurements and model calculations

Since 1990 regulations were implemented in order to decrease the ammonia emissions in the Netherlands. These regulations include the coverage of animal manure storage basins, the prohibition of application of manure in winter and the obligatory incorporation of manure in the soil after application. Furthermore, investments to decrease emission from housing systems are stimulated. As the result of these regulations, it was estimated that the ammonia emissions were decreased by 37% in 1996 relative to 1990. These estimates were based on statistical data and assumptions about the effectiveness of the regulations. Measurements of air concentrations and depositions between 1992 and 1996 and model results were used to evaluate the decrease in emission estimates. The measurements do not show a distinct decrease. Model results based on the estimated emissions show lower concentrations and depositions than the measurements. These differences cannot be attributed to variations in meteorological parameters or uncertainties in model parameters. For the period 1984 to 1985 model calculations agree reasonably well with measurements (with higher uncertainty). In the more recent years, however, the difference between model results and measurements becomes much bigger. It is concluded that the abatement strategies with an assumed decrease in emission of 30–40% were in fact less effective.

Trends of ambient concentrations and deposition fluxes of particulate trace metals in Detroit from 1982 to 1992

Ambient air concentrations of the particulate trace metals Fe, Zn, Pb, Ni, Cr, Cd, Be and Hg have been measured since the early 1980s at seven urban sites across the metropolitan area of Detroit. Trace metals such as Fe, Zn, Ni and Cr were found to be more abundant at industrial sites, while Pb and Cd concentrations were highest at commercial sites. Be and Hg ambient concentrations did not show any large variations among land use categories on an annual average basis. The largest increases in ambient concentrations between two consecutive years were a 20 fold increase for Fe in industrial areas (Winter 1988), a 21-fold increase for Zn in a residential area (Summer 1986), a 5-fold increase for Ni in commercial areas (Winter 1992), a 10-fold increase for Cr in industrial and commercial areas (Spring 1982), a 5-fold increase for Cd in Commercial areas (Fall 1987), a 27- and 24-fold increase for Be in residential and industrial areas, respectively; and a 6-fold increase for Hg in residential areas. Pb did not show a large increase between 2 consecutive years (< 1%); however a 6-fold decrease was observed in residential areas during the summer of 1992. Selected US market parameters were used as indicators of emission source activity and their temporal trends were compared with those of the ratio between maximum ( C max) and average ( C average) ambient concentrations of trace metals observed in Detroit. Good agreement was found between the trend in the C max C average ratio and the trend in US production and consumption, import and export, and the price of metal-based products. A dynamic model was used to calculate the dry deposition flux of trace metals associated with total suspended particulate matter and its temporal variation by land-use category accounting for the variation of meteorological parameters, surface roughness, and ambient concentration. This model showed that the seasonal variability in meteorological parameters may yield a 40–50% variation in the dry deposition velocity of particulate trace metals on an annual basis. It also showed that deposition fluxes are generally higher in industrial areas than in residential and commercial areas. Finally, temporal trends of the ambient concentration and dry deposition flux of particulate trace metals are provided for the 1982–1992 period using a simple regression of seasonal data obtained at each sampling site. Ambient concentrations and dry deposition fluxes of Fe, Pb, Cr, Cd and Be are characterized by downward trends with annual decreases in the range of −1 to −11% using 1982 as the reference year. Trace metals such as Zn, Ni and Hg show an upward trend with annual increases in the range of 0.4–9%.

Indirect acoustic impedance computations involving fixed free-space attenuation or fixed boundary loss factors: Experimental model validation studies

An experimental verification of a surface acoustic impedance estimation theory and method is hereby presented. Two distinct estimation methods are evaluated. Each method utilizes a search algorithm to estimate the impedance that will validate analytical expressions for ground attenuation of sound, using experimental measurements of continuous-wave sound attenuation changes between two above-ground sound source locations with respect to a single microphone close to the ground. With each of the two methods evaluated, reliable ground impedance estimates were computed with considerable reduction in computation and experimental complexity: The first method requires an experimental arrangement that allowed spherical spreading components of the excess attenuation terms to essentially cancel out. With the second method, the arrangement allowed the boundary loss factor to remain constant while the source sound location changes. Variations in meteorological parameters between the two sound source locations were assumed negligible in each case. These successfully verified analytical and experimental arrangements display the potential to provide a means of obtaining speedy and efficient estimates of acoustic impedance for various surfaces. [Work was supported by the US Army Research Office and the Human Engineering Laboratory, Aberdeen Proving Ground, MD.]

On indirect acoustic impedance computations involving fixed free-space attenuation or fixed boundary loss factors.

A class of methods for speedily estimating the acoustic impedance of surfaces is presented. The methods utilize a lattice search algorithm to estimate the impedance that will validate analytical expressions for ground attenuation of sound, using experimental measurements of continuous-wave sound attenuation changes between two above-ground sound source locations with respect to a single microphone close to the ground. In the two cases considered, opportunities appear to exist for a considerable reduction in computation complexity: in the first case, the arrangement allows spherical spreading components of the excess attenuation terms to essentially cancel out. In the second case, the arrangement allows the boundary loss factor to remain constant while the source sound location changes. Variations in meteorological parameters between the two sound source locations are assumed negligible. These proposed analytical and experimental arrangements display the potential to provide a means of obtaining speedy and efficient estimates of ground impedance for various surfaces. [Work supported by the U.S. Army Research Office.]

States and interstate switching in meteorological parameters

We show theoretically and also through actual records that variations in air/surface temperature and associated meteorological parameters take place in three different states termed herein «state 1», «state 2» and «state 3». For a given place, one of the two states (i.e. states 1 and 2) is accompanied by high (or increasing) rainfall while the other state is accompanied by relatively small (or decreasing) rainfall. Systematic switching from one state to another does take place at several places under physical conditions detailed in the text. Available evidence indicates that interstate switching between states 1 and 2 is primarily caused by the 11 year solar cycle and some of those other cycles responsible for variations in the solar energy incident onto the earth-atmosphere system. Finally it is shown that one of the possible causes of double sunspot cycle variations in meteorological parameters is interstate switching between state 1 and state 2. This conclusion is both consistent with and supported by past observations.

Seasonal and geothermal production variations in concentrations of He and CO 2 in soil gases, Roosevelt Hot Springs Known Geothermal Resource Area, Utah, U.S.A.

To increase understanding of natural variations in soil gas concentrations, CO 2, He, O 2 and N 2 were measured in soil gases collected regularly for several months from four sites at the Roosevelt Hot Springs Known Geothermal Resource Area, Utah. Soil temperature, air temperature, per cent relative humidity, barometric pressure and amounts of rain and snowfall were also monitored to determine the effect of meteorological parameters on concentrations of the measured gases. Considerable seasonal variation existed in concentrations of CO 2 and He. The parameters that most affected the soil-gas concentrations were soil and air temperatures. Moisture from rain and snow probably affected the soil-gas concentrations also. However, annual variations in meteorological parameters did not appear to affect measurements of anomalous concentrations in samples collected within a time period of a few days. Production from some of the geothermal wells probably affected the soil-gas concentrations.

Thermal performance of underground air pipe: Different earth surface treatments

Taking into account the effect of earth storage and annual variation of meteorological parameters, the thermal performance of air pipes used for heating/cooling of buildings has been analysed. It is seen that, for given values of air pipe parameters, there is an optimum length of the pipe for which the annual heating potential per unit pipe length is maximum. The optimum length of the pipe for which the annual cooling potential of the pipe is maximum is, in general (not always), also nearly the same. The effect of different earth surface treatments on the thermal performance of air pipes and their optimum length has been investigated for hot-dry, composite and dry-cold climates typified by Jodhpur, Delhi and Leh, respectively.

Climatology of the middle atmosphere of the Southern hemisphere

Main features of spatial distribution and time variations of meteorological parameters in the Southern hemisphere at altitudes 25–80 km are reviewed on the basis of zonal empirical models revised in 1982. Meridional distribution and seasonal variations are analysed. For comparison purposes with the Northern hemisphere, a model developed by Cole and Kantor in 1978 is used. It is revealed that the compilation of new models of the Southern hemisphere atmosphere has not resulted in substantial revision of hemispheric-structure outlined earlier in studies conducted in the Central Aerological Observatory. Meridional distribution of the parameters in summer is characterized by higher values of temperature, pressure and density gradients in the stratosphere of the Southern hemisphere than in that of the Northern hemisphere. This resulted in greater advancement of the core of the summer-time easterly (low towards the equator in the Southern hemisphere than in its northern counterpart. In winter, meridional temperature gradients in the middle stratosphere are greater in the Southern hemisphere than those in the Northern hemisphere, however, rapid attenuation of the gradients with height is observed in the Southern hemisphere, and above 35–40 km they become negative near 50–60°S, in contrast to positive values typical for the Northern hemisphere stratosphere. In the wind field, specific features of the Southern hemisphere westerly flow are high intensity and relatively low altitude of the maximum speed (as compared to the Northern hemisphere). The phases of the annual temperature wave at low latitudes are similar south and north of the equator; south of 30°S a reversal of the phase is observed. The semi-annual oscillation of temperature and wind is less pronounced in middle and high latitudes of the Southern hemisphere than in the Northern hemisphere. The origin of the primary differences between the hemispheres is related mainly to lower intensity of large-scale stratospheric processes in the Southern hemisphere as compared to those in the Northern hemisphere, which is confirmed by values of the standard deviation of the parameters in the two hemispheres. In summer, temperature and pressure fields based on satellite data are symmetric relative to the poles in both hemispheres. In winter, the distortion of the mean pressure field in the mesosphere may be as great in the Southern as in the Northern hemisphere.

Bioassay of Extracts of Ambient Particulate Matter

Organic extracts from airborne particles collected at various sites in Scandinavia have been tested for mutagenicity in the Ames Salmonella/microsome assay. Extracts from particles in the respirable size fraction (diameter less than 3 microns) were mutagenic with and without metabolic activation. The mutagenic activity varied from day to day, mainly due to variations in meteorological parameters, especially wind speed and atmospheric stability. A seasonal variation could also be observed, with the highest average values in winter time. Samples collected in urban areas were considerably more mutagenic than samples from background areas. The results suggest that exhaust from motor vehicles are the most important source of mutagenic particles in urban areas. Comparison of roof top samples with street level samples indicated that atmospheric reactions cause transformation of nonpolar compounds in the primary emission to more oxygenated mutagenic compounds. It is, however, not known to which degree this causes an overall increase of the mutagenic activity. The mutagenic activity of emissions from stationary combustion sources have also been studied, and residential heating by burning solid fuels in small combustion units have been shown to be a major contributor to mutagens in the environment.

Bioassay of extracts of ambient particulate matter.

Organic extracts from airborne particles collected at various sites in Scandinavia have been tested for mutagenicity in the Ames Salmonella/microsome assay. Extracts from particles in the respirable size fraction (diameter less than 3 microns) were mutagenic with and without metabolic activation. The mutagenic activity varied from day to day, mainly due to variations in meteorological parameters, especially wind speed and atmospheric stability. A seasonal variation could also be observed, with the highest average values in winter time. Samples collected in urban areas were considerably more mutagenic than samples from background areas. The results suggest that exhaust from motor vehicles are the most important source of mutagenic particles in urban areas. Comparison of roof top samples with street level samples indicated that atmospheric reactions cause transformation of nonpolar compounds in the primary emission to more oxygenated mutagenic compounds. It is, however, not known to which degree this causes an overall increase of the mutagenic activity. The mutagenic activity of emissions from stationary combustion sources have also been studied, and residential heating by burning solid fuels in small combustion units have been shown to be a major contributor to mutagens in the environment.