Fresh Emissions Research Articles

Perturbation of the European free troposphere aerosol by North American forest fire plumes during the ICARTT-ITOP experiment in summer 2004

Abstract. During the ICARTT-ITOP Experiment in summer 2004 plumes from large wildfires in North America were transported to Central Europe at 3–8 km altitude above sea level (a.s.l.). These plumes were studied with the DLR (Deutsches Zentrum fuer Luft- und Raumfahrt) research aircraft Falcon which was equipped with an extensive set of in situ aerosol and trace gas instruments. Analyses by the Lagrangian dispersion model FLEXPART provided source regions, transport times and horizontal extent of the fire plumes. Results from the general circulation model ECHAM/MADE and data from previous aerosol studies over Central Europe provided reference vertical profiles of black carbon (BC) mass concentrations for year 2000 conditions with forest fire activities below the long-term average. Smoke plume observations yielded a BC mass fraction of total aerosol mass with respect to PM 2.5 of 2–8%. The ratio of BC mass to excess CO was 3–7.5 mg BC (g CO)−1. Even after up to 10 days of atmospheric transport, both characteristic properties were of the same order as for fresh emissions. This suggests an efficient lifting of BC from forest fires to higher altitudes with only minor scavenging removal of particulate matter. Maximum aerosol absorption coefficient values were 7–8 Mm−1 which is about two orders of magnitude above the average European free tropospheric background value. Forest fire aerosol size distributions were characterised by a strong internally mixed accumulation mode centred at modal diameters of 0.25–0.30 µm with an average distribution width of 1.30. Nucleation and small Aitken mode particles were almost completely depleted.

Photochemical production of ozone and control strategy for Southern Taiwan

An observation-based method (OBM) is developed to evaluate the ozone (O 3) production efficiency (O 3 molecules produced per NO x molecule consumed) and O 3 production rate ( P(O 3)) during a field campaign in southern Taiwan. The method can also provide an estimate of the concentration of OH. A key step in the method is to use observed concentrations of two aromatic hydrocarbons, namely ethylbenzene and m, p-xylene, to estimate the degree of photochemical processing and amounts of photochemically consumed NO x and NMHCs by OH. In addition, total oxidant (O 3+NO 2) instead of O 3 itself turns out to be very useful for representing ozone production in the OBM approach. The average O 3 production efficiency during the field campaign in Fall (2003) is found to be about 10.2±3.9. The relationship of P(O 3) with NO x is examined and compared with a one-dimensional (1D) photochemical model. Values of P(O 3) derived from the OBM are slightly lower than those calculated in the 1D model. However, OH concentrations estimated by the OBM are about a factor of 2 lower than the 1D model. Fresh emissions, which affect the degree of photochemical processing appear to be a major cause of the underestimate. We have developed a three-dimensional (3D) OBM O 3 production diagram that resembles the EKMA ozone isopleth diagram to study the relationship of the total oxidant versus O 3 precursors. The 3D OBM O 3 production diagram suggests that reducing emissions of NMHCs are more effective in controlling O 3 than reducing NO x . However, significant uncertainties remain in the OBM, and considerable more work is required to minimize these uncertainties before a definitive control strategy can be reached. The observation-based approach provides a good alternative to measuring peroxy radicals for evaluating the production of O 3 and formulating O 3 control strategy in urban and suburban environments.

The contributions of “minimum primary emissions” and “new particle formation enhancements” to the particle number concentration in urban air

In an urban site affected by fresh vehicle exhaust emissions, the ambient air number concentrations of particles coarser than 3 nm (N) was split into two components, N = N 1 + N 2 . This was done using a method based on the high correlation between black-carbon (BC) and number (N) concentrations which is typically observed in ambient air and is the result of vehicle exhaust emissions. The component N1 accounts for “those aerosol components directly emitted in the particle phase” and “those components nucleating immediately after emission”. The component N2 accounts for the new particle formation enhancements during the “dilution and cooling of the vehicle exhaust” and is also influenced by “in situ new particle formation in ambient air”. The contribution of N1 to N exhibits a maximum of 55% during the morning rush hours (07:00–08:00). The contribution of N2 to N exhibits a daily evolution with a broad maximum during daylight (as solar radiation intensity), while for about 7 h (11:00–17:00) the N2 contribution to N is about 70%. During some “afternoon N2 events”, N2 contributions exceeded 90%. Enhancements in the new particle formation processes may increase the N/BC concentrations ratio in one order of magnitude, from 4.82 × 10 6 particles/ng BC to 47 × 10 6 particles/ng BC and during some events up to 97 × 10 6 particles/ng BC. The results show evidence of the high potential of the vehicle exhausts and of the urban atmosphere to trigger new particle formation if the ambient air conditions are favourable. The method used in this study is useful in assessing future changes in the number to BC relationship due to forthcoming regulations in the vehicle exhaust emissions.

Technical Note: Description and Use of the New Jump Mass Spectrum Mode of Operation for the Aerodyne Quadrupole Aerosol Mass Spectrometers (Q-AMS)

A new mode of operation for the Aerodyne Quadrupole Aerosol Mass Spectrometer (Q-AMS) has been developed and used to improve the detection limits and time resolution of the instrument. The Jump Mass Spectrum (JMS) mode works by stepping through a small number of specific user defined positions within the mass spectrum, increasing the time spent scanning specific m/z's. The JMS mode is conceptually similar to the Selected Ion Monitoring mode of some commercial quadrupole-based instrumentation and can be used for direct quantification when the fragmentation pattern is known. The JMS mode can also be used to augment the standard Q-AMS operation in Mass Spectrum mode when the fragmentation pattern is not known, improving the effective signal-to-noise ratio (SNR) and in turn the detection limits and time resolution. A decrease in detection limits for the Q-AMS by factors of 4.6, 3.9, 1.3, and 3.5 for nitrate, sulphate, total organics, and m/z 43 mass loadings respectively was achieved for 1 minute sampling (20 s in each of the three Q-AMS modes, monitoring 10 m/z in JMS mode). Although the benefit to the SNR of the total organic mass concentration measured by the Q-AMS is smaller, sensitivity to organic fragments which can act as markers for various sources and processes (such as fresh primary anthropogenic emissions, aged secondary organics, and biomass burning aerosol), is greatly increased by the JMS mode. Example data from applications that benefit from this technique are presented, including an aircraft platform and in smog chamber experiments, alongside high time-resolution, ground-based data.

Characteristics and diurnal variations of NMHCs at urban, suburban, and rural sites in the Pearl River Delta and a remote site in South China

The Pearl River Delta (PRD) is one of the most industrialized and urbanized regions in China. With rapid growth of the economy, it is suffering from deteriorating air quality. Non-methane hydrocarbons (NMHCs) were investigated at urban and suburban sites in Guangzhou (GZ), a rural site in PRD and a clean remote site in South China, in April 2005. Additional roadside samples in GZ and Qingxi (QX, a small industrial town in PRD), ambient air samples at the rooftop of a printing factory in QX and exhaust samples from liquefied petroleum gas (LPG)—fueled taxis in GZ were collected to help identify the source signatures of NMHCs. A large fraction of propane (47%) was found in exhaust samples from LPG-fueled taxis in GZ and extremely high levels of toluene (2.0–3.1 ppmv) were found at the rooftop of the printing factory in QX. Vehicular and industrial emissions were the main sources of NMHCs. The effect of vehicular emission on the ambient air varied among the three PRD sites. The impact of industrial emissions was widespread and they contributed greatly to the high levels of aromatic hydrocarbons, especially toluene, at the three PRD sites investigated. Leakage from vehicles fueled by LPG contributed mainly to the high levels of propane and n-butane at the urban GZ site. Ethane and ethyne from long-range transport and isoprene from local biogenic emission were the main contributors to the total hydrocarbons at the remote site. Diurnal variations of NMHCs showed that the contribution from vehicular emissions varied with traffic conditions and were more influenced by fresh emissions at the urban site and by aged air at the suburban and rural sites. Isoprene from biogenic emission contributed largely to the ozone formation potential (OFP) at the remote site. Ethene, toluene and m/ p-xylene were the main contributors to the OFP at the three PRD sites.

Detection of particle-phase polycyclic aromatic hydrocarbons in Mexico City using an aerosol mass spectrometer

We report the quantification of ambient particle-bound polycyclic aromatic hydrocarbons (PAHs) for the first time using a real-time aerosol mass spectrometer. These measurements were carried out during the Mexico City Metropolitan Area field study (MCMA-2003) that took place from March 29 to May 4, 2003. This was the first time that two different fast, real-time methods have been used to quantify PAHs alongside traditional filter-based measurements in an extended field campaign. This paper focuses on the technical aspects of PAH detection in ambient air with the Aerodyne AMS equipped with a quadrupole mass analyzer (Q-AMS), on the comparison of PAHs measured by the Q-AMS to those measured with the other two techniques, and on some features of the ambient results. PAHs are very resistant to fragmentation after ionization. Based on laboratory experiments with eight PAH standards, we show that their molecular ions, which for most particulate PAHs in ambient particles are larger than 200 amu, are often the largest peak in their Q-AMS spectra. Q-AMS spectra of PAH are similar to those in the NIST database, albeit with more fragmentation. We have developed a subtraction method that allows the removal of the contribution from non-PAH organics to the ion signals of the PAHs in ambient data. We report the mass concentrations of all individual groups of PAHs with molecular weights of 202, 216, 226 + 228, 240 + 242, 250 + 252, 264 + 266, 276 + 278, 288 + 290, 300 + 302, 316 and 326 + 328, as well as their sum as the total PAH mass concentration. The time series of the Photoelectric Aerosol Sensor (PAS) and Q-AMS PAH measurements during MCMA-2003 are well correlated, with the smallest difference between measured PAH concentrations observed in the mornings when ambient aerosols loadings are dominated by fresh traffic emissions. The Q-AMS PAH measurements are also compared to those from GC–MS analysis of filter samples. Several groups of PAHs show agreement within the uncertainties, while the Q-AMS measurements are larger than the GC–MS ones for several others. In the ambient Q-AMS measurements the presence of ions tentatively attributed to cyclopenta[ cd]pyrene and dicyclopentapyrenes causes signals at m/ z 226 and 250, which are significantly stronger than the signals in GC–MS analysis of filter samples. This suggests that very labile, but likely toxic, PAHs were present in the MCMA atmosphere that decayed rapidly due to reaction during filter sampling, and this may explain at least some of the differences between the Q-AMS and GC–MS measurements.

Large upper tropospheric ozone enhancements above midlatitude North America during summer: In situ evidence from the IONS and MOZAIC ozone measurement network

The most extensive set of free tropospheric ozone measurements ever compiled across midlatitude North America was measured with daily ozonesondes, commercial aircraft and a lidar at 14 sites during July‐August 2004. The model estimated stratospheric ozone was subtracted from all profiles, leaving a tropospheric residual ozone. On average the upper troposphere above midlatitude eastern North America contained 15 ppbv more tropospheric residual ozone than the more polluted layer between the surface and 2 km above sea level. Lowest ozone values in the upper troposphere were found above the two upwind sites in California. The upper troposphere above midlatitude eastern North America contained 16 ppbv more tropospheric residual ozone than the upper troposphere above three upwind sites, with the greatest enhancement above Houston, Texas, at 24 ppbv. Upper tropospheric CO measurements above east Texas show no statistically significant enhancement compared to west coast measurements, arguing against a strong influence from fresh surface anthropogenic emissions to the upper troposphere above Texas where the ozone enhancement is greatest. Vertical mixing of ozone from the boundary layer to the upper troposphere can only account for 2 ppbv of the 16 ppbv ozone enhancement above eastern North America; therefore the remaining 14 ppbv must be the result of in situ ozone production. The transport of NOx tracers from North American anthropogenic, biogenic, biomass burning, and lightning emissions was simulated for the upper troposphere of North America with a particle dispersion model. Additional box model calculations suggest the 24 ppbv ozone enhancement above Houston can be produced over a 10 day period from oxidation reactions of lightning NOx and background mixing ratios of CO and CH4. Overall, we estimate that 69–84% (11–13 ppbv) of the 16 ppbv ozone enhancement above eastern North America is due to in situ ozone production from lightning NOx with the remainder due to transport of ozone from the surface or in situ ozone production from other sources of NOx.

Trends in size classified particle number concentration in subtropical Brisbane, Australia, based on a 5 year study

Particle number size distribution data in the range from 0.015 to 0.630 μm were collected over a 5-year period in the central business district (CBD) of Brisbane, Australia. Particle size distribution was summarised by total number concentration and number median diameter (NMD) as well as the number concentration of the 0.015–0.030 ( N 15–30), 0.030–0.050 ( N 30–50), 0.050–0.100 ( N 50–100), 0.100–0.300 ( N 100–300) and 0.300–0.630 ( N 300–630) μm size classes. Morning (6:00–10:00) and afternoon (16:00–19:00) measurements, the former representing fresh traffic emissions (based on the local meteorological conditions) and the latter well-mixed emissions from the CBD, during weekdays were extracted and the respective monthly mean values were estimated for time series analysis. For all size fractions, average morning concentrations were about 1.5 higher than in the afternoon whereas NMD did not vary between the morning and afternoon. The trend and seasonal components were extracted through weighted linear regression models, using the monthly variance as weights. Only the morning measurements exhibited significant trends. During this time of the day, total particle number increased by 105.7% and the increase was greater for larger particles, resulting in a shift in NMD by 7.9%. Although no seasonal component was detected the evidence against it remained weak due to the limitations of the database.

Size-resolved, real-time measurement of water-insoluble aerosols in metropolitan Atlanta during the summer of 2004

During the month of August 2004, the size-resolved number concentration of water-insoluble aerosols (WIA) from 0.25 to 2.0 μm was measured in real-time in the urban center of Atlanta, GA. Simultaneous measurements were performed for the total aerosol size distribution from 0.1 to 2.0 μm, the elemental and organic carbon mass concentration, the aerosol absorption coefficient, and the aerosol scattering coefficient at a dry (RH=30%) humidity. The mean aerosol number concentration in the size range 0.1–2.0 μm was found to be 360±175 cm −3, but this quantity fluctuated significantly on time scales of less than one hour and ranged from 25 to 1400 cm −3 during the sample period. The mean WIA concentration (0.25–2.0 μm) was 13±7 cm −3 and ranged from 1 to 60 cm −3. The average insoluble fraction in the size range 0.25–2.0 μm was found to be 4±2.5% with a range of 0.3–38%. The WIA population was found to follow a consistent diurnal pattern throughout the month with concentration maxima concurring with peaks in vehicular traffic flow. WIA concentration also responded to changes in meteorological conditions such as boundary layer depth and precipitation events. The temporal variability of the absorption coefficient followed an identical pattern to that of WIA and ranged from below the detection limit to 55 Mm −1 with a mean of 8±6 Mm −1. The WIA concentration was highly correlated with both the absorption coefficient and the elemental carbon mass concentration, suggesting that WIA measurements are dominated by fresh emissions of elemental carbon. For both the total aerosol and the WIA size distributions, the maximum number concentration was observed at the smallest sizes; however the WIA size distribution also exhibited a peak at 0.45 μm which was not observed in the total population. Over 60% of the particles greater than 1.0 μm were observed to be insoluble in the water sampling stream used by this instrumentation. Due to the refractive properties of black carbon, it is highly unlikely that these particles could be composed of elemental carbon, suggesting a crustal source for super-micron WIA.

Evolution of submicron organic aerosol in polluted air exported from Tokyo

Ground‐based measurements of aerosols and trace gases were conducted at an urban site in Tokyo (Komaba) and a site 50 km to the north (Kisai) during July–August 2004. An Aerodyne aerosol mass spectrometer (AMS) was deployed at each measurement site to investigate the chemical evolution of submicron organic aerosol (OA) in polluted air. The mass concentrations of OA at the Kisai site were systematically higher than those at the Komaba site and were correlated with ozone under southerly conditions. The rate of increase of OA at the Kisai site is investigated using the photochemical age derived from the ratio of alkyl nitrates to their parent hydrocarbons. The OA concentrations in processed air (age of 8–16 h) were 4–5 times larger than those in fresh emissions (age ∼ 0), suggesting that the OA concentrations can be significantly enhanced within ∼0.5 days under conditions of high photochemical activity.

Characterization of ambient aerosols in Mexico City during the MCMA-2003 campaign with Aerosol Mass Spectrometry: results from the CENICA Supersite

Abstract. An Aerodyne Aerosol Mass Spectrometer (AMS) was deployed at the CENICA Supersite, during the Mexico City Metropolitan Area field study (MCMA-2003) from 31 March-4 May 2003 to investigate particle concentrations, sources, and processes. The AMS provides real time information on mass concentration and composition of the non-refractory species in particulate matter less than 1 µm (NR-PM1) with high time and size-resolution. In order to account for the refractory material in the aerosol, we also present estimates of Black Carbon (BC) using an aethalometer and an estimate of the aerosol soil component obtained from Proton-Induced X-ray Emission Spectrometry (PIXE) analysis of impactor substrates. Comparisons of AMS + BC + soil mass concentration with other collocated particle instruments (a LASAIR Optical Particle Counter, a PM2.5 Tapered Element Oscillating Microbalance (TEOM), and a PM2.5 DustTrak Aerosol Monitor) show that the AMS + BC + soil mass concentration is consistent with the total PM2.5 mass concentration during MCMA-2003 within the combined uncertainties. In Mexico City, the organic fraction of the estimated PM2.5 at CENICA represents, on average, 54.6% (standard deviation σ=10%) of the mass, with the rest consisting of inorganic compounds (mainly ammonium nitrate and sulfate/ammonium salts), BC, and soil. Inorganic compounds represent 27.5% of PM2.5 (σ=10%); BC mass concentration is about 11% (σ=4%); while soil represents about 6.9% (σ=4%). Size distributions are presented for the AMS species; they show an accumulation mode that contains mainly oxygenated organic and secondary inorganic compounds. The organic size distributions also contain a small organic particle mode that is likely indicative of fresh traffic emissions; small particle modes exist for the inorganic species as well. Evidence suggests that the organic and inorganic species are not always internally mixed, especially in the small modes. The aerosol seems to be neutralized most of the time; however, there were some periods when there was not enough ammonium to completely neutralize the nitrate, chloride and sulfate present. The diurnal cycle and size distributions of nitrate suggest local photochemical production. On the other hand, sulfate appears to be produced on a regional scale. There are indications of new particle formation and growth events when concentrations of SO2 were high. Although the sources of chloride are not clear, this species seems to condense as ammonium chloride early in the morning and to evaporate as the temperature increases and RH decreases. The total and speciated mass concentrations and diurnal cycles measured during MCMA-2003 are similar to measurements during a previous field campaign at a nearby location.

An experimental study of aerosol distribution over a Mediterranean urban area

In this study an attempt is made to investigate the aerosol spatial and size distributions at different heights over the Greater Athens Area (GAA), Greece, under sea breeze conditions and clear sky and to further discuss possible implications for aerosol characteristics. The data used are airborne measurements of aerosol collected during two flights that were performed within the context of the 1997 STAAARTE experimental campaign. The aerosol measurements cover particle diameters from 0.1 to 45.5 μm. The horizontal and vertical distribution revealed that higher concentrations exist within or just above the atmospheric boundary layer, while greater concentrations are observed over the sea compared to land at high altitudes. At all altitudes the number size distributions show dominant diameter ranges between 0.1 and 0.3 μm at all altitudes. The volume distributions are characterised by two modes, one in the accumulation and the other in the coarse particle regime. At lower altitudes, fresh combustion emissions more likely cause the predominance of the size range 0.1–0.3 μm while enhanced physical and chemical processes that favour the growth of smaller particles to larger sizes could also act. The relative humidity does not seem to affect the observed number size distributions at low altitudes, where relative humidity is below 70% while at 4000 m the distributions seem to change over the sea where the humidity increases.

Fresh Gasoline Emissions, Not Paved Road Dust, Alter Cardiac Repolarization in ApoE<sup>−/−</sup> Mice

Fresh vehicular emissions potentially represent a ubiquitous environmental concern for cardiovascular health. We compared electrocardiographic effects of fresh gasoline engine emissions with resuspended paved road dust in a mouse model of coronary insufficiency. Apolipoprotein E (ApoE)-/- mice on a high fat diet were exposed by whole-body inhalation to either gasoline emissions at 60 microg/m3 particulate matter (PM), an equivalent atmosphere with particles filtered out of the whole exhaust, or paved road dust at 0.5 and 3.5 mg /m3 for 6 h/d for 3 d. Radiotelemetry recordings of electrocardiogram (ECG) were analyzed for changes in T-wave morphology (QT interval, T-wave amplitude, and T-wave Area). Following exposures, lung lavage and blood samples were obtained to assay for markers of pulmonary and systemic inflammation. No exposure induced significant changes in heart rate and only the high concentration of road dust induced signs of pulmonary inflammation. T-wave area exhibited significant deviation from baseline values during exposure to gasoline exhaust particulates, but not to either concentration of road dust or gasoline emissions sans particulates. Gasoline-exposed mice demonstrated elevated plasma endothelin-1, but did not cause systemic inflammation. These data support the hypothesis that freshly-generated engine emissions, as opposed to resuspended paved road dust, may drive cardiac effects that have been observed at road-sides in the environment. The absence of ECG effects for both very high concentrations of road dust PM and equivalent concentrations of the vapor/gas phase of gasoline engine exhaust further indicate the specific risk conferred by fresh vehicular PM.

An observational case study of ozone and precursors inflow to South East England during an anticyclone

Summertime photochemical air pollution episodes within the United Kingdom have been proposed via modelling studies to be strongly influenced by regional scale inflow of air from the continental European boundary layer. We present a vertically resolved case study using measurements made from the NERC/Met Office BAe 146 research aircraft on 18th August 2005 over the South East of England and the North Sea during a weak anticyclone centred over Northern Europe. The vertical distribution of ozone, CO, NO(x), peroxyacetyl nitrate (PAN) and a wide range of both nonmethane hydrocarbons (NMHC) and oxygenated volatile organic compounds (OVOCs) were determined between 500 ft (approximately 152 m) and 7000 ft (approximately 2134 m) over the East Anglia coastline and 50 km inland. In excess of 80 ppbV ozone was observed within inflowing boundary layer air over the North Sea coast in a broad N-S sloping feature around 60 km wide. The inflowing feature of European origin was also observed further inland within the boundary layer albeit with lower, more variable, ozone mixing ratios. The increased variability in ozone over land was a product of titration by fresh surface emissions of NO via rapid upward transport in thermals, a hypothesis supported by the observed vertical wind speed component. Fast boundary layer mixing over land was further illustrated by a uniform distribution in reactive alkenes. A comparison between aircraft and surface O(3) UK AUN (Automatic Urban Network) measurements showed good agreement with the inland site, Sibton, but marked differences with the coastal monitoring site at Weybourne, potentially due to gradients established by ocean deposition in stably stratified marine air.

Hydrocarbon-like and oxygenated organic aerosols in Pittsburgh: insights into sources and processes of organic aerosols

Abstract. A recently developed algorithm (Zhang et al., 2005) has been applied to deconvolve the mass spectra of organic aerosols acquired with the Aerosol Mass Spectrometer (AMS) in Pittsburgh during September 2002. The results are used here to characterize the mass concentrations, size distributions, and mass spectra of hydrocarbon-like and oxygenated organic aerosol (HOA and OOA, respectively). HOA accounts for 34% of the measured organic aerosol mass and OOA accounts for 66%. The mass concentrations of HOA demonstrate a prominent diurnal profile that peaks in the morning during the rush hour and decreases with the rise of the boundary layer. The diurnal profile of OOA is relatively flat and resembles those of SO42− and NH4+. The size distribution of HOA shows a distinct ultrafine mode that is commonly associated with fresh emissions while OOA is generally concentrated in the accumulation mode and appears to be mostly internally mixed with the inorganic ions, such as SO42− and NH4+. These observations suggest that HOA is likely primary aerosol from local, combustion-related emissions and that OOA is secondary organic aerosol (SOA) influenced by regional contributions. There is strong evidence of the direct correspondence of OOA to SOA during an intense new particle formation and growth event, when condensational growth of OOA was observed. The fact that the OOA mass spectrum from this event is very similar to that from the entire study suggests that the majority of OOA in Pittsburgh is likely SOA. O3 appears to be a poor indicator for OOA concentration while SO42− is a relatively good surrogate for this dataset. Since the diurnal averages of HOA track those of CO during day time, oxidation/aging of HOA appears to be very small on the time scale of several hours. Based on extracted mass spectra and the likely elemental compositions of major m/z's, the organic mass to organic carbon ratios (OM:OC) of HOA and OOA are estimated at 1.2 and 2.2 μg/μgC, respectively, leading to an average OM:OC ratio of 1.8 for submicron OA in Pittsburgh during September. The C:O ratio of OOA is estimated at 1:0.8. The carbon contents in HOA and OOA estimated accordingly correlate well to primary and secondary organic carbon, respectively, estimated by the OC/EC tracer technique (assuming POC-to-EC ratio=1). In addition, the total carbon concentrations estimated from the AMS data agree well with those measured by the Sunset Laboratory Carbon analyzer (r2=0.87; slope=1.01±0.11). Our results represent the first direct estimate of the OM:OC ratio from highly time-resolved chemical composition measurements.

Distributions of C 2–C 5 NMHCs and related trace gases at a tropical urban site in India

Simultaneous surface measurements of C 2–C 5 non-methane hydrocarbons (NMHCs), O 3, CO and CH 4 were made during the year 2002 at a tropical urban site, Ahmedabad. This is the first time that NMHCs levels have been characterized in detail in India. The diurnal distributions of these species show pronounced variations in the winter months and less during the summer months. The seasonal variations of all these species show substantially higher levels during the winter and lowest during the summer season. The strength (winter to summer ratios) of seasonal variations in NMHCs are observed to be higher than other reported measurements elsewhere. The seasonal changes in transport patterns, boundary layer height and OH concentrations, all contribute in the seasonal variations of these trace gases. The correlation studies of various NMHCs and CO indicate dominant role of local emissions in the observed distributions of trace gases. The natural gas emission and leakage of liquid petroleum gas contribute to elevated levels of ethane and propane. While emissions from vehicular exhaust are found to be dominant sources of ethene, propene and acetylene. The higher C 2H 2/CO ratio of about 6.4 pptv/ppbv indicates influences of fresh emissions at Ahmedabad.

Processing of soot in an urban environment: case study from the Mexico City Metropolitan Area

Abstract. Chemical composition, size, and mixing state of atmospheric particles are critical in determining their effects on the environment. There is growing evidence that soot aerosols play a particularly important role in both climate and human health, but still relatively little is known of their physical and chemical nature. In addition, the atmospheric residence times and removal mechanisms for soot are neither well understood nor adequately represented in regional and global climate models. To investigate the effect of locality and residence time on properties of soot and mixing state in a polluted urban environment, particles of diameter 0.2–2.0 μm were collected in the Mexico City Metropolitan Area (MCMA) during the MCMA-2003 Field Campaign from various sites within the city. Individual particle analysis by different electron microscopy methods coupled with energy dispersed x-ray spectroscopy, and secondary ionization mass spectrometry show that freshly-emitted soot particles become rapidly processed in the MCMA. Whereas fresh particulate emissions from mixed-traffic are almost entirely carbonaceous, consisting of soot aggregates with liquid coatings suggestive of unburned lubricating oil and water, ambient soot particles which have been processed for less than a few hours are heavily internally mixed, primarily with ammonium sulfate. Single particle analysis suggests that this mixing occurs through several mechanisms that require further investigation. In light of previously published results, the internally-mixed nature of processed soot particles is expected to affect heterogeneous chemistry on the soot surface, including interaction with water during wet-removal.

Carbonaceous and ionic components in wintertime atmospheric aerosols from two New Zealand cities: Implications for solid fuel combustion

Wintertime PM10 samples collected in two New Zealand cities (Christchurch and Auckland) have been analysed for total carbon (TC), elemental carbon (EC), organic carbon (OC), water-soluble organic carbon (WSOC), and water-insoluble organic carbon (WIOC) as well as major inorganic ions. Christchurch, which is heavily influenced by residential wood and coal burning in winter, showed significantly higher ambient concentrations of carbonaceous materials than Auckland whose major sources of aerosols are sea-salt and vehicular emissions. In Christchurch, the mean concentrations of TC, EC, WSOC, and WIOC were 21, 6.0, 7.0 and 7.6 μg m−3, respectively. In contrast, those of Auckland were 5.0, 2.0, 1.3 and 1.7 μg m−3, respectively. On average, total carbonaceous materials (EC, water-soluble organic matter and water-insoluble organic matter) accounted for about 70% of PM10 mass in Christchurch but less than 30% in Auckland. Moreover, WSOC/EC and nss-K+/EC mass ratios in Christchurch were found to be substantially higher than those of Auckland. In contrast, nss-SO42−, NO3− and nss-Ca2+ concentrations in Auckland were dramatically enhanced relative to carbonaceous materials. This study suggests that fresh emissions from solid fuel burning are significantly more enriched with WIOC than WSOC, although a significant fraction of WSOC may be a secondary atmospheric product. It is estimated that only one-fifth to one-quarter of the measured OC was attributable to secondary organic aerosols in the two cities in winter.

Control strategies for the reduction of airborne particulate nitrate in California's San Joaquin Valley

Abstract The effect of NO x , volatile organic compound (VOC), and NH 3 emissions control programs on the formation of particulate ammonium nitrate in the San Joaquin Valley (SJV) was examined under the typical winter conditions that existed on 4–6 January, 1996. The UCD/CIT photochemical transport model was used for this study so that the source origin of primary particulate matter and secondary particulate matter could be identified. When averaged across the entire SJV, the model results predict that 13–18% of the reactive nitrogen (NO y =NO x +reaction products of NO x ) emitted from local sources within the SJV was converted to nitrate at the ground level. Each gram of NO x emitted locally within the SJV (expressed as NO 2 ) produced 0.23–0.31 g of particulate ammonium nitrate (NH 4 NO 3 ), which is much smaller than the maximum theoretical yield of 1.7 g of NH 4 NO 3 per gram of NO 2 . The fraction of reactive nitrogen converted to nitrate varied strongly as a function of location. Urban regions with large amounts of fresh NO emissions converted little reactive nitrogen to nitrate, while remote areas had up to 70% conversion (equivalent to approximately 1.2 g of NH 4 NO 3 per gram of NO 2 ). The use of a single spatially averaged ratio of NH 4 NO 3 /NO x as a predictor of how changes to NO x emissions would affect particulate nitrate concentrations would not be accurate at all locations in the SJV under the conditions studied. The largest local sources of particulate nitrate in the SJV were predicted to be diesel engines and catalyst equipped gasoline engines under the conditions experienced on 6 January, 1996. Together, these sources accounted for less than half of the ground-level nitrate aerosol in the SJV. The remaining fraction of the aerosol nitrate originated from reactive nitrogen originally released upwind of the SJV. The majority of this upwind reactive nitrogen was already transformed to nitrate by the time it entered the SJV. The effect of local emissions controls on this upwind material was small. A 50% reduction in NO x emissions applied to sources within the SJV reduced the predicted concentration of total nitrate by approximately 25% during the study episode. VOC emissions controls were less effective, while reasonable NH 3 emissions controls had the smallest effect on the amount of ammonium nitrate produced. A 50% reduction in VOC emissions lowered predicted concentrations of total nitrate by 17.5%, while a 50% reduction in NH 3 emissions lowered predicted concentrations of total nitrate by only 10%. This latter result is expected since the formation of ammonium nitrate aerosol is limited by the availability of gas-phase nitric acid, with large amounts of excess NH 3 available. NO x emissions controls appear to be the most efficient method to reduce the concentration of locally generated particulate nitrate in the SJV under the conditions experienced on 4–6 January, 1996.

Seasonal and spatial variability of the size‐resolved chemical composition of particulate matter (PM10) in the Los Angeles Basin

For a period of almost 3 years, sampling of size‐fractionated ambient particulate matter with diameter below 10 μm (PM10) was performed at urban source sites (Downey and University of Southern California) and inland receptor sites (Claremont and Riverside) in the Los Angeles Basin as part of the Southern California Particle Center and Supersite. Results for size‐resolved PM10 mass, inorganic ions (sulfate and nitrate), metals, elemental carbon, and organic carbon were obtained. Three collocated micro‐orifice uniform deposit impactors (MOUDIs) were deployed to collect 24‐hour samples roughly once a week. Ultrafine particle concentrations (particle diameter dp < 0.1 μm) were found to be the highest at the source sites resulting from fresh vehicular emissions. Mass concentrations in the accumulation mode (0.1 < dp < 2.5 μm) were lower in winter than in summer, especially at the receptor sites. PM concentrations in the coarse mode (2.5 < dp < 10 μm) were lower in winter and were composed mostly of nitrate and crustal elements (iron, calcium, potassium, silicon, and aluminum). Consistent relative levels of these elements indicate a common source of soil and/or road dust. In the accumulation mode, nitrate and organic carbon were predominant with higher nitrate levels found at the receptor sites. The ultrafine mode PM consisted of mostly organic carbon, with higher wintertime levels at the source sites due to increased organic vapor condensation from vehicles at lower temperatures. Conversely, higher ultrafine organic carbon levels at the receptor areas are due to secondary organic aerosol formation by photochemical reactions as well as increased advection of polluted air masses from upwind.