Semi-arid Urban Site Research Articles

Water-soluble components in rainwater over Xi'an in northwest China: Source apportionment and pollution controls effectiveness evaluation

Rainwater samples were collected to investigate the water-soluble species levels and their sources at a semi-arid urban site of Xi'an, China, during 2015. The precipitation samples were analyzed for pH, electric conductivity (EC), ionic species (Na+, K+, NH4+, Ca2+, Mg2+, F−, Cl−, NO3−, and SO42−), and water-soluble organic carbon and nitrogen (WSOC and WSTN). The annual volume-weighted mean (VWM) of pH and EC values were 6.50 and 46.9 μS cm−1, respectively. The total ionic concentrations in rainwater varied widely, ranging from 121.9 to 2448.2 μeq L−1 with an annual VWM of 686.4 μeq L−1. SO42− was the largest contributor to anions while Ca2+ was the most important cation. SO42− and NO3− levels decreased in a large scale by comparing with prior study in six years ago. Moreover, the total wet depositions flux of water-soluble inorganic nitrogen (sum of NH4+-N and NO3−-N) and SO42--S were reduced by 54.5 and 81.1% in 2015 compared with 2009, respectively. The results indicated that the pollution controls during the past years were more effective over Xi'an. The average concentrations of water-soluble organic carbon and total nitrogen (WSOC and WSTN) were 2.9 and 3.9 mg L−1. The water-soluble organic nitrogen was the dominant component in WSTN. The neutralization factor showed that NH4+ and Ca2+ were the dominant neutralization substances in the rainwater, whereas the effects of Mg2+, K+ and Na+ were negligible. The enrichment factor analysis indicated that SO42− and NO3− were mainly originated from anthropogenic sources, while Ca2+ was associated with terrestrial sources. Source apportionment by a positive matrix factorization receptor model showed that the anthropogenic emissions were the largest contributor to SO42− (61.9%), NO3− (65.3%), and NH4+ (70.5%), followed by crustal sources (14.9, 21.3, and 18.4%) and biomass burning (13.8, 12.1, and 6.8%). For the major cations of Ca2+, crustal source was the dominant contributor (75.6%), followed by biomass burning (14.6%).

An observation of seasonal and diurnal behavior of O 3 –NO x relationships and local/regional oxidant (OX = O 3 + NO 2 ) levels at a semi-arid urban site of western India

Abstract The present study summarizes the continuous measurements of ozone (O3) and oxides of nitrogen (NOx = NO + NO2) along with meteorological conditions in the ambient atmosphere of semi-arid urban site of western India during the year 2012–13. Seasonal and diurnal variations of gaseous pollutants were investigated and compared using the results of time series analysis. The marked seasonal difference was observed for O3 as highest/(lowest) during pre-monsoon/(monsoon), respectively. In contrast, NO and NO2 exhibit the highest and lowest levels during post-monsoon and monsoon, respectively. The diurnal cycle of O3 and NOx exhibited inverse relationship where surface O3 showed mid-day peak and lower night-time concentrations. The dataset was used to examine the association of O3 with the ambient levels of NO, NO2 and NOx during day-time and night-time, separately. The variation of an oxidant OX (O3 + NO2) with the levels of NOx was examined to infer the atmospheric sources of OX as sum of NOx-independent regional and NOx-dependent local contributions. Significantly strong positive correlations were observed of O3 with temperature and solar radiation with strong negative correlation with relative humidity during the studied seasons. Inconsiderable difference (

Contribution of biogenic and photochemical sources to ambient VOCs during winter to summer transition at a semi-arid urban site in India

This paper presents the sources and characteristics of ambient volatile organic compounds (VOCs) measured using PTR-TOF-MS instrument in a metropolitan city of India during winter to summer transition period. Mixing ratios of VOCs exhibited strong diurnal, day-to-day and episodic variations. Methanol was the most dominant species with monthly mean values of 18–22 pbbv. The emission ratios of VOCs relative to benzene calculated from nighttime data were used to estimate the relative contributions of vehicle exhaust and other sources. The increasing daytime ratios of oxygenated-VOCs (OVOCs)/benzene and isoprene/benzene from February to March indicates increasing contribution of photo-oxidation and biogenic sources. Daytime fractions of acetone (18%), acetaldehyde (15%) and isoprene (4.5%) to the sum of measured VOCs in March were higher than those in February. Variations of VOCs at lower temperatures (<25 °C) were predominantly controlled by anthropogenic sources. At high temperatures, particularly in the range of 32–40 °C during March, levels of OVOCs and isoprene were influenced by biogenic emissions. The emissions of OVOCs from vehicle exhaust were estimated to be smaller (20–40%) than those from other sources. The contributions of biogenic and secondary sources to OVOCs and isoprene increased by 10–15% from winter to summer. This study provides evidence that the winter-to-summer transition has an impact on sources and composition of VOCs in tropical urban areas.

Role of long-range transport and local meteorology in seasonal variation of surface ozone and its precursors at an urban site in India

This study is based on the continuous measurements of ozone (O3), carbon monoxide (CO) and nitrogen oxides (NOx) at a semi-arid urban site in Udaipur, India during the years 2011–2012. The mixing ratios of trace gases show strong and weak diurnal variations during the winter and monsoon seasons, respectively. The temporal changes in local emission and PBL depth play an important role in the diurnal variation of trace gases. The daily means of O3, CO and NOx were in the ranges of 5–53 ppbv, 121–842 ppbv and 3–29 ppbv, respectively. The mixing ratios of trace gases were highest and lowest during the winter/pre-monsoon and monsoon seasons, respectively. In the winter season, the lowest of O3 during night–morning hours was caused by the efficient loss due to titration and deposition compared to other seasons. During the winter to the pre-monsoon period, higher levels of trace gases were due to regional biomass burning and long-range transport of continental pollutants. However, strong convection, rainfall and transport of oceanic air resulted in the lowest concentrations of trace gases during the monsoon season. The O3min values tend to increase slightly with increasing values of Tmin while COmax and NOxmax decrease rapidly with increasing values of Tmin. The levels of CO and NOx decreased with increasing wind speed, while O3 tends to increase with wind speed. The rates of change of O3 (dO3/dt) were about 3.7 ppbv h−1 and −4.5 ppbv h−1 during the morning and evening hours, respectively. Exceptionally high levels of trace gases during the Diwali (festival) period were due to extensive use of firecrackers from evening till morning hours. The enhancements of O3, CO and NOx compared to normal days were about 61%, 62% and 23%, respectively.

Variability of atmospheric carbonyl sulfide at a semi-arid urban site in western India

Atmospheric carbonyl sulfide (COS) is a major precursor for sulfate aerosols that play a critical role in climate regulation. Recent studies have highlighted the importance of COS measurements as a reliable means to constrain biospheric carbon assimilation. In a scenario of limited availability of COS data around the globe, we present gas-chromatographic measurements of atmospheric COS mixing ratios over Ahmedabad, a semi-arid, urban region in western India. These measurements, being reported for the first time over an Indian site, enable us to understand the diurnal and seasonal variation in atmospheric COS with respect to its natural, anthropogenic and photochemical sources and sinks. The annual mean COS mixing ratio over Ahmedabad is found to be 0.83±0.43ppbv, which is substantially higher than free tropospheric values for the northern hemisphere. Inverse correlation of COS with soil and skin temperature, suggests that the dry soil of the semi-arid study region is a potential sink for atmospheric COS. Positive correlations of COS with NO2 and CO during post-monsoon and the COS/CO slope of 0.78pptv/ppbv reveals influence of diesel combustion and tire wear. The highest concentrations of COS are observed during pre-monsoon; COS/CO2 slope of 44.75pptv/ppmv combined with information from air mass back-trajectories reveal marshy wetlands spanning over 7500km2 as an important source of COS in Ahmedabad. COS/CO2 slopes decrease drastically (8.28pptv/ppmv) during post-monsoon due to combined impact of biospheric uptake and anthropogenic emissions.

Trace gases at a semi-arid urban site in western India: variability and inter-correlations

Continuous measurements of O3, CO, NOx and SO2 were made in Ahmedabad, a semi-arid, urban region in western India, during May-October, 2011 to study their levels, variability and inter-relationships during this period of changing meteorological conditions. The levels of CO and SO2 observed in Ahmedabad were in general not very high compared to other major urban regions in India. On the other hand, NOx levels, impacted by vehicular emissions, were found to be substantially elevated. However, these values are still lower than levels in megacities like Beijing, Hong-Kong, Mexico etc. A sudden increase in trace gas levels was observed during post-monsoon months due to a concurrent change in prevailing winds. The levels, variability as well as diurnal amplitudes for all the gases were particularly high during October. While the primary pollutants were negatively correlated with wind speed, O3 interestingly showed a positive association. Low slope and relatively lower correlation between O3 and CO in Ahmedabad indicate incomplete photochemical processes in the ambient air. Despite an overall inverse relationship between O3 and NOx, daytime O3 was found to be positively correlated with night-time NOx as well as night-time CO. Levels of CO, NOx and SO2 showed unique relationships with wind direction depending on source locations. Low CO-NOx slope for Ahmedabad indicate relatively fresh emissions from local sources. SO2 values as well as SO2/NOx ratios were higher when wind was from the eastern sector of the city, which contains a thermal power station and a few industrial clusters. The SO2/NOx slope for point sources in Ahmedabad is estimated to be 0.4 while for mobile sources, it is 0.026.

Impact of Nitrogen Oxides, Volatile Organic Compounds and Black Carbon on Atmospheric Ozone Levels at a Semi Arid Urban Site in Hyderabad

The surface level ozone (O3), nitrogen oxides (NOx = NO2 + NO), volatile organic compounds (VOCs), black carbon (BC) and meteorological parameters were continuously monitored for the period of one year (2010) at an urban site in Hyderabad using different trace gas monitors. The local emissions and meteorology have been found to play a significant role in diurnal variations of O3. The average peak time mixing ratios of O3 were observed in the range of 23 ± 8 to 67 ± 13 ppbv. Diurnalseasonal variation of O3 and its precursors were also reported. The observed O3 concentrations were higher during the day than in the night. Maximum O3 concentrations in the three seasons i.e., summer, winter and monsoon were 56 ± 14 ppbv, 50 ± 9 ppbv, and 28 ± 10 ppbv, respectively. Weekday/weekend variation of O3 and its precursors were studied during the three seasons. High O3 concentrations were observed on weekends compared to weekdays, however, NOx and BC levels were found low during weekends. The weekend effect of O3 was high during winter. The annual average afternoon peak time O3 levels on weekends were 11% higher than the weekday concentrations. Weekday/weekend variations of NOx at morning traffic rush hour were 14%, 9% and 8% in winter, summer and monsoon, respectively. The annual mean of NOx and BC concentrations at weekend were observed to be lower than weekday about 10% and 9% respectively. VOC/NOx ratio is more (5) during weekend than weekday (4) which resulted in enhanced O3 formation.

Long-Term Measurements of NO3Radical at a Semiarid Urban Site: 2. Seasonal Trends and Loss Mechanisms

This study is the first to present long-term measurements of the nitrate radical in an urban location. Extensive nitrate radical measurements were conducted together with ancillary parameters during a continuous two year campaign (2005-2007) in the semiarid location of Jerusalem. The average nighttime NO3 concentration was 27.3+/-43.5 ppt, the highest ever reported, with a seasonal average peak during summer (33.3+/-55.8 pptv) with maximum levels exceeding 800 pptv. Significant diurnal changes in NO3 concentrations were observed, caused by an unusual nighttime increase in ozone concentrations. The NO3 loss processes exhibited strong seasonal variability. Homogeneous gas-phase losses were the main removal processes during summer and spring. The heterogeneous losses of N2O5, averaged over the entire campaign, contributed to less than half of the direct losses even though they dominated the winter seasons and part of the autumn months. Statistical regression analysis showed that NO3 was inversely correlated with relative humidity and positively correlated with temperature and to a lesser extent with NO2 and O3, indicating that the heterogeneous removal processes were also important. The diurnal behavior of NO3 was examined using a one-dimensional chemical transport model. The simulations showed that NO3 trends and concentrations were influenced mainly by changes in ozone and nitrogen oxide levels and that the very high levels of NO3 can be explained by the entrainment of fresh ozone from the upper atmospheric levels. After sunset and in the early morning, the homogeneous processes are the major loss pathways, while the heterogeneous N2O5 removal pathway dominates the intermediate times.

Long-Term Measurements of NO3 Radical at a Semiarid Urban Site: 1. Extreme Concentration Events and Their Oxidation Capacity

Nitrate radical (NO(3)), an important nighttime tropospheric oxidant, was measured continuously for two years (July 2005 to September 2007) in Jerusalem, a semiarid urban site, by long-path differential optical absorption spectroscopy (LP-DOAS). From this period, 21 days with the highest concentrations of nitrate radical (above 220 pptv) were selected for analysis. Joint measurements with the University of Heidelberg's LP-DOAS showed good agreement (r = 0.94). For all daytime measurements, NO(3) remained below the detection limit (8.5 pptv). The highest value recorded was more than 800 pptv (July 27, 2007), twice the maximum level reported previously. For this subset of measurements, mean maximum values for the extreme events were 345 pptv (SD = 135 pptv). Concentrations rose above detection limits at sunset, peaked between midnight and early morning, and returned to zero at sunrise. These elevated concentrations of NO(3) were a consequence of several factors, including an increase in ozone concentrations parallel to a substantial decrease in relative humidity during the night; Mean nighttime NO(2) levels above 10 ppbv, which prevented a deficiency in NO(3) precursors; Negligible NO levels during the night; and a substantial decrease in the loss processes, which led to a lower degradation frequency and allowed NO(3) lifetimes to build up to a maximum mean of 25 min. The results indicate that the major sink pathway for NO(3) was direct homogeneous gas phase reactions with VOC, and a smaller indirect pathway via hydrolysis of N(2)O(5). The Jerusalem measurements were used to estimate the oxidation potential of extreme NO(3) levels at an urban location. The 24 h average potential of NO(3), OH, and O(3) to oxidize hydrocarbons was evaluated for 30 separate VOCs. NO(3) was found to be responsible for approximately 70% of the oxidation of total VOCs and nearly 75% of the olefinic VOCs; which was more than twice the VOC oxidation potential of the OH radical. These results establish the NO(3) radical as an important atmospheric oxidant in Jerusalem.

Chemical Composition of Atmospheric Aerosol (PM10) at a Semi-arid Urban Site: Influence of Terrestrial Sources

Atmospheric aerosol (PM(10)) measurements were made at a regional representative semi-arid urban site, Tirupati, India over one-year period i.e. from October, 2001 to September, 2002. The samples were collected on polyflex filters, and analyzed for the major water-soluble ions - F, Cl, NO(3), SO(4), Na, NH(4), K, Ca and Mg, employing ion chromatograph. The average mass of PM(10) is found to be 32.75 mug/m(3) with a total water-soluble aerosol load (total anion + total cation) of 13.56 mug/m(3). Composition of aerosol showed higher concentration of SO(4) followed by Na, Ca and NO(3). Very good correlation is observed between crustal ions Ca and Mg (r=0.82) as well as between crustal and acidic ions; Ca and SO(4) (r=0.75) and NO(3) (r=0.67) and Mg and NO(3) (r=0.78) and SO(4) (r=0.73), suggest that the ionic composition was influenced by local terrestrial sources. The presence of SO(4) and NO(3) may be due to re-suspension of soil particles (formation by heterogeneous oxidation). Ca, Mg and Na are mainly soil derived ones. Correlation matrix with meteorological factors, as well as seasonal distribution of PM(10) and its ionic components present a clear trend of higher concentrations during summer due to greater particle release and lowering atmospheric levels during the rainy season due to washout effect. ANOVA results showed the significant variation of composition from season to season. Paired comparisons (DMRT) revealed the occurrence of significant difference in pairs of mean concentration from season to season except within monsoon i.e. between S-W and N-E monsoon.

A study on trace elemental composition of atmospheric aerosols at a semi-arid urban site using ICP-MS technique

A study has been carried out on trace elemental composition of atmospheric aerosols (PM 10) over a 1-year period i.e. from October 2001 to September 2002 at a semi-arid urban site, Tirupati, southern peninsular India. The samples were collected on paliflex filters with a frequency once a week using Mini-Partisol air sampler equipped with PM 10 inlet. Aerosol loaded filters were extracted by the hot extraction method and the elemental concentrations were determined using inductively coupled plasma mass spectrometry (ICP-MS). Detection limits were found to be in a sub-ppt range and the precision about <5% RSD with comparable levels of accuracy. ICP-MS results are in good agreement with the certified data (<20% RSD). Elemental composition of PM 10 showed a higher contribution of Fe followed by Mn and the lowest contribution of Be. Multivariate receptor model results (PC matrix) demonstrated that the trace elemental components of atmospheric aerosol were largely contributed from crustal material and road dust (69.41%), followed by metallurgical and other industrial processes (11.76%) and fuel oil combustion (6.52%). Regression models suggested the significant impact of meteorological factors on elemental concentration, which ranged between 14% and 53.6%. ANOVA results showed a clear trend of atmospheric levels of elements with variation in climatological conditions.