Organic trace gas composition of the marine boundary layer over the northwest Indian Ocean in April 2000

In April 2000 atmospheric trace gas measurements were performed on the western Indian Ocean on a cruise of the Dutch research vessel Pelagia from the Seychelles (5°S, 55°E) to Djibouti (12°N, 43°E). The measurements included analysis of dimethyl sulfide (DMS), acetone and acetonitrile every 40 s using PTR-MS (proton-transfer-reaction mass spectrometry) and gas chromatographic analyses of C₂–C₇ hydrocarbons in air samples taken during the cruise. The measurements took place at the end of the winter monsoon season and the sampled air masses came predominantly from the Southern Hemisphere, resulting in low concentrations of some long-lived hydrocarbons, halocarbons, acetone (350 pptv) and acetonitrile (120 pptv). On three consecutive days a diurnal cycle in DMS concentration was observed, which was used to estimate the emission of DMS (1.5±0.7×10¹³ molecules m⁻² s⁻¹) and the 24 h averaged concentration of hydroxyl (OH) radicals (1.4±0.7×10⁶ molecules cm⁻³). A strongly increased DMS concentration was found at a location where upwelling of deeper ocean waters took place, coinciding with a marked decrease in acetone and acetonitrile. In the northwestern Indian Ocean a slight increase of some trace gases was noticed showing a small influence of pollution from Asia and from northeast Africa as indicated with back trajectory calculations. The air masses from Asia had elevated acetonitrile concentrations showing some influence of biomass burning as was also found during the 1999 Indian Ocean Experiment, whereas the air masses from northeast Africa seemed to have other sources of pollution.     

C1–C8 volatile organic compounds in the atmosphere of Hong Kong: Overview of atmospheric processing and source apportionment

We present measurements of C₁–C₈ volatile organic compounds (VOCs) at four sites ranging from urban to rural areas in Hong Kong from September 2002 to August 2003. A total of 248 ambient VOC samples were collected. As expected, the urban and sub-urban sites generally gave relatively high VOC levels. In contrast, the average VOC levels were the lowest in the rural area. In general, higher mixing ratios were observed during winter/spring and lower levels during summer/fall because of seasonal variations of meteorological conditions. A variation of the air mass composition from urban to rural sites was observed. High ratios of ethyne/CO (5.6 pptv/ppbv) and propane/ethane (0.50 pptv/pptv) at the rural site suggested that the air masses over the territory were relatively fresh as compared to other remote regions. The principal component analysis (PCA) with absolute principal component scores (APCS) technique was applied to the VOC data in order to identify and quantify pollution sources at different sites. These results indicated that vehicular emissions made a significant contribution to ambient non-methane VOCs (NMVOCs) levels in urban areas (65±36%) and in sub-urban areas (50±28% and 53±41%). Other sources such as petrol evaporation, industrial emissions and solvent usage also played important roles in the VOC emissions. At the rural site, almost half of the measured total NMVOCs were due to combustion sources (vehicular and/or biomass/biofuel burning). Petrol evaporation, solvent usage, industrial and biogenic emissions also contributed to the atmospheric NMVOCs. The source apportionment results revealed a strong impact of anthropogenic VOCs to the atmosphere of Hong Kong in both urban/sub-urban and rural areas.     

Photochemical oxidation and dispersion of gaseous sulfur compounds from natural and anthropogenic sources around a coastal location

The photochemical oxidation and dispersion of reduced sulfur compounds (RSCs: H₂S, CH₃SH, DMS, CS₂, and DMDS) emitted from anthropogenic (A) and natural (N) sources were evaluated based on a numerical modeling approach. The anthropogenic emission concentrations of RSCs were measured from several sampling sites at the Donghae landfill (D-LF) (i.e., source type A) in South Korea during a series of field campaigns (May through December 2004). The emissions of natural RSCs in a coastal study area near the D-LF (i.e., source type N) were estimated from sea surface DMS concentrations and transfer velocity during the same study period. These emission data were then used as input to the CALPUFF dispersion model, revised with 34 chemical reactions for RSCs. A significant fraction of sulfur dioxide (SO₂) was produced photochemically during the summer (about 34% of total SO₂ concentrations) followed by fall (21%), spring (15%), and winter (5%). Photochemical production of SO₂ was dominated by H₂S (about 55% of total contributions) and DMS (24%). The largest impact of RSCs from source type A on SO₂ concentrations occurred around the D-LF during summer. The total SO₂ concentrations produced from source type N around the D-LF during the summer (a mean SO₂ concentration of 7.4 ppbv) were significantly higher than those (≤0.3 ppbv) during the other seasons. This may be because of the high RSC and SO₂ emissions and their photochemistry along with the wind convergence.     

The influence of tropical cyclone Melor on PM10 concentrations during an aerosol episode over the Pearl River Delta region of China: Numerical modeling versus observational analysis

In this article, numerical simulations and observational analyses have been made for the aerosol episode that occurred over the Pearl River Delta (PRD) region in China during 1–3 November 2003. An air quality modeling system that consisted of the mesoscale model MM5, chemical transport model MODELS-3/CMAQ, and air pollutant emission model SMOKE, was employed. Studies have shown that this particulate matter (PM) pollution episode was apparently associated with the activity of tropical cyclone (TC) Melor. Model simulations revealed that Melor spawned this PM episode through dynamic and thermodynamic processes. The strong compensating subsidence induced by Melor's peripheral circulations created favorable meteorological conditions that enhanced local aerosol pollution. This strong downward motion produced significant adiabatic warming (2–4 °C daily) and dramatic drying in the low-level troposphere over the PRD. As a result, the PRD region was blanketed with a dry and warm air layer that strengthened the static stability of the lower troposphere. The descending motion also tended to dramatically lower the heights of the planetary boundary layer (PBL) through its dynamic effect. The fair weather created by this synoptic pattern further intensified the nocturnal temperature inversions through enhanced radiative cooling. All of these factors promoted a stagnant local atmosphere with very light winds near the surface. The horizontal and vertical dispersions of locally emitted aerosol particles were largely suppressed, leading to the accumulation of large amounts of PMs near local emission sources in the PRD region. As Melor drew near, changes in surface winds strengthened the horizontal transport of aerosol particles from inland sources to the area of Hong Kong downstream. This horizontal advection greatly contributed to the high PM₁₀ (particulate matters less than 10 μm in diameters) concentrations in Hong Kong.     

Emission strengths for primary pollutants as estimated from an aircraft study of Hong Kong air quality

An aircraft study of air quality in the Hong Kong region during the fall of 1994 has allowed for an estimation of the daytime source strengths for CO and NO_y from the Hong Kong metropolitan center. Emission rate estimates for the Hong Kong urban plume for NO_y and CO were 5.4×10e(25) molecules s^-1 and 1.8×10e(26) molecules s^-1 as determined for the case study of 18 October. All emission rate estimates have uncertainties of a factor of 2. On one occasion a distinct plume emanating from Shenzhen in the People’s Republic of China was encountered. While plume delimitation was insufficient for source strength calculations, transect integrals did allow for a CO/NO_y ratio of about 16 to be determined. The CO/NO_y ratio for the Hong Kong urban plume was about 3.3. The difference in these ratios indicates differences in the overall combustion processes and efficiencies taking place within Hong Kong and the PRC.     

Monitoring of atmospheric reduced sulfur compounds and their oxidation in two coastal landfill areas

In this study, the distribution characteristics of reduced sulfur compounds (RSCs) in ambient air were investigated in two coastal landfill (LF) facilities and their surrounding areas. The photochemical conversion of RSCs to sulfur dioxide (SO₂) was also evaluated using a photochemical box model (PCBM). Measurements of RSCs were carried out from both in and around areas of two coastal LFs in Gunsan (G) and Donghae (D) city, Korea during several field campaigns (May through December 2004). The dominant RSCs at the Gunsan landfill (G-LF) were found to be DMS and H₂S, whereas those at the Donghae landfill (D-LF) were H₂S and DMDS. The concentrations of DMS at these study sites were likely to be affected not only by LF processes but also by an oceanic source, while such a pattern was more prominent at the D-LF. The chemical species of RSCs that can exert significant influences on the photochemical production of SO₂ in the LF environment were identified to be H₂S, DMS, or DMDS.     

Sources and characteristics of carbonaceous aerosol in two largest cities in Pearl River Delta Region,China

PM_2.5 samples were collected at five sites in Guangzhou and Hong Kong, Pearl River Delta Region (PRDR), China in both summer and winter during 2004–2005. Elemental carbon (EC) and organic carbon (OC) in these samples were measured. The OC and EC concentrations ranked in the order of urban Guangzhou > urban Hong Kong > background Hong Kong. Total carbonaceous aerosol (TCA) contributed less to PM_2.5 in urban Guangzhou (32–35%) than that in urban Hong Kong (43–57%). The reason may be that, as an major industrial city in South China, Guangzhou would receive large amount of inorganic aerosol from all kinds of industries, however, as a trade center and seaport, urban Hong Kong would mainly receive organic aerosol and EC from container vessels and heavy-duty diesel trucks. At Hong Kong background site Hok Tsui, relatively lower contribution of TCA to PM_2.5 may result from contributions of marine inorganic aerosol and inland China pollutant. Strong correlation (R²=0.76–0.83) between OC and EC indicates minor fluctuation of emission and the secondary organic aerosol (SOA) formation in urban Guangzhou. Weak correlation between OC and EC in Hong Kong can be related to the impact of the long-range transported aerosol from inland China. Averagely, secondary OC (SOC) concentrations were 3.8–5.9 and 10.2–12.8 μg m⁻³, respectively, accounting for 21–32% and 36–42% of OC in summer and winter in Guangzhou. The average values of 4.2–6.8% for SOA/ PM_2.5 indicate that SOA was minor component in PM_2.5 in Guangzhou.     

Heavy metals and Pb isotopic composition of aerosols in urban and suburban areas of Hong Kong and Guangzhou,South China—Evidence of the long-range transport of air contaminants

Rapid urbanization and industrialization in South China has placed great strain on the environment and on human health. In the present study, the total suspended particulate matter (TSP) in the urban and suburban areas of Hong Kong and Guangzhou, the two largest urban centres in South China, was sampled from December 2003 to January 2005. The samples were analysed for the concentrations of major elements (Al, Fe, Mg and Mn) and trace metals (Cd, Cr, Cu, Pb, V and Zn), and for Pb isotopic composition. Elevated concentrations of metals, especially Cd, Pb, V and Zn, were observed in the urban and suburban areas of Guangzhou, showing significant atmospheric trace element pollution. Distinct seasonal patterns were observed in the heavy metal concentrations of aerosols in Hong Kong, with higher metal concentrations during the winter monsoon period, and lower concentrations during summertime. The seasonal variations in the metal concentrations of the aerosols in Guangzhou were less distinct, suggesting the dominance of local sources of pollution around the city. The Pb isotopic composition in the aerosols of Hong Kong had higher ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁷Pb ratios in winter, showing the influence of Pb from the northern inland areas of China and the Pearl River Delta (PRD) region, and lower ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁷Pb ratios in summer, indicating the influence of Pb from the South Asian region and from marine sources. The back trajectory analysis showed that the enrichment of heavy metals in Hong Kong and Guangzhou was closely associated with the air mass from the north and northeast that originated from northern China, reflecting the long-range transport of heavy metal contaminants from the northern inland areas of China to the South China coast.     

Use of high-resolution MM5/CALMET/CALPUFF system: SO2 apportionment to air quality in Hong Kong

Recent research has highlighted the substantial health-related costs of air pollution in the Hong Kong Special Administrative Region (HKSAR) and the potential threat from air pollution to HKSARs economic competitiveness. In order to address the air pollution problems, this paper seeks to analyse the individual contributions of major sulphur dioxide (SO₂) sources in the Pearl River Delta Region (PRDR) (Pearl River Delta + HKSAR) on the air quality in the HKSAR. This study employed the coupling of the MM5/CALMET system with the CALPUFF, the multi-layer, non-steady-state puff dispersion model, where major power plants, marine vessels and vehicles, all in the PRDR, are taken into account. The observation data and simulation results at 11 Hong Kong Environmental Protection Department (HKEPD) general (non-road-side) stations are analyzed. Urban (in-zone) and Rural (out-zone) stations are defined in order to examine the SO₂ contribution of different emission sources in different regions of the HKSAR. The model results show that the contribution of the HKSAR marine sources is significant both in summer and in winter, especially for the locations around the in-zone stations (in general 60%, and 55% in summer and winter respectively). In addition, the contribution of the HKSAR power plants is slightly higher than that of the PRD power plants in early summer, with a contribution difference of up to 20% when the prevailing wind is from the south. However, in late summer, this situation reverses. In winter, the contribution of the PRD power plants is two to three times greater than that of the HKSAR power plants. Moreover, Yantian port affects the northern part of the HKSAR when a northeasterly wind dominates the HKSAR. In order to solve these air pollution problems, the main implication of these results suggests that the HKSAR government, in close co-operation with the Guangdong government, needs to take immediate action.     

Trends of ambient carbonyl compounds in the urban environment of Hong Kong

The concentrations of C₁–C₈ carbonyl compounds were measured at two urban sites in Hong Kong from October 1997 to September 2000. The daily total carbonyl concentrations were found to range from 2.4 to 37 μg m⁻³. Formaldehyde was the most abundant species, which comprised from 36 to 43% of the total detected carbonyls, followed by acetaldehyde (18–21%) and acetone (8–20%). The highest 24-hour average concentrations measured were 10 and 7.7 μg m⁻³ for formaldehyde and acetaldehyde, respectively. Seasonal and temporal variations in the concentrations of formaldehyde and acetaldehyde were not obvious, but lowest concentrations often occurred from June to August. The mean formaldehyde/acetaldehyde molar ratios at the two sites in summer (2.8±1.1 and 2.5±1.2) were significantly higher (p⩽0.01) than those in winter periods (1.9±0.6 and 2.0±0.6). The phenomena were explained by influences of both photochemical reactions and local meteorological conditions. Better correlations between formaldehyde and acetaldehyde, and between NO_x and each of the two major carbonyls were obtained in winter periods indicating direct vehicular emissions were the principal sources. The ambient formaldehyde and acetaldehyde concentrations in the urban atmosphere of Hong Kong were within the normal ranges reported in the literature for other urban sites world-wide.     

Influence of ship emissions on air quality and input of contaminants in southern Alaska National Parks and Wilderness Areas during the 2006 tourist season

The impact of ship emissions on air quality in Alaska National Parks and Wilderness Areas was investigated using the Weather Research and Forecasting model inline coupled with chemistry (WRF/Chem). The visibility and deposition of atmospheric contaminants was analyzed for the length of the 2006 tourist season. WRF/Chem reproduced the meteorological situation well. It seems to have captured the temporal behavior of aerosol concentrations when compared with the few data available. Air quality follows certain predetermined patterns associated with local meteorological conditions and ship emissions. Ship emissions have maximum impacts in Prince William Sound where topography and decaying lows trap pollutants. Along sea-lanes and adjacent coastal areas, NO_x, SO₂, O₃, PAN, HNO₃, and PM_2.5 increase up to 650 pptv, 325 pptv, 900 pptv, 18 pptv, 10 pptv, and 100 ng m^?3. Some of these increases are significant (95% confidence). Enhanced particulate matter concentrations from ship emissions reduce visibility up to 30% in Prince William Sound and 5–25% along sea-lanes.     

Temporal,spatial characteristics and uncertainty of biogenic VOC emissions in the Pearl River Delta region,China

Using the Global Biosphere Emissions and Interactions System model (GloBEIS), 3 × 3 km gridded and hourly biogenic volatile organic compound (BVOC) emissions in the Pearl River Delta (PRD) were estimated for the year 2006. The study used newly available land cover database, observed meteorological data, and recent measurements of emission rates for tree species in China. The results show that the total BVOC emission in the PRD region in 2006 was 296 kt (2.2 × 10¹¹ gC), of which isoprene contributes about 25% (73 kt, 6.4 × 10¹⁰ gC), monoterpenes about 34% (102 kt, 8.9 × 10¹⁰ gC), and other VOCs (OVOC) about 41% (121 kt, 6.8 × 10¹⁰ gC). BVOC emissions in the PRD region exhibit a marked seasonal pattern with the peak emission in July and the lowest emission in January, and are mainly distributed over the outlying areas of the PRD region, where the economy and land use are less developed. The uncertainties in BVOC emission estimates were quantified using Monte Carlo simulation; the results indicate high uncertainties in isoprene emission estimates, with a relative error of ?82 to +177%, ranging from 12.4 to 186.4 kt; ?41 to +58% uncertainty for monoterpenes emissions, ranging from 67.7 to 181.9 kt; and ?26 to +30% uncertainty in OVOC emissions, ranging from 88.8 to 156.2 kt on the 95% confidence intervals. The key uncertainty sources include emission factors and the model empirical coefficients α, C_T1, C_L, and E_opt for estimating isoprene emission, and emission factors and foliar density for estimating monoterpenes and OVOC emissions. This implies that determining these empirical coefficient values properly and conducting more field measurements of emission rates of tree species are key approaches for reducing uncertainties in BVOC emission estimates. Improving future BVOC emission inventory work in the PRD region requires giving priority to research on shrub land, coniferous forests, and irrigated cropland and pasture.     

Photochemical oxidation of reduced sulfur compounds in an urban location based on short time monitoring data

This study examines the oxidation of reduced sulfur compounds (RSCs) in urban ambient air. The photochemical conversions of RSC (such as DMS, CS₂, H₂S, DMDS, and CH₃SH) to a further oxidized form (e.g., SO₂, MSA, and H₂SO₄) were assessed using a photochemical box model. For our model simulation of RSC oxidation, measurements were taken at an urban monitoring station in Seoul, Korea (37.6° N, 127° E) during three separate time periods (e.g., Sept. 17–18, Oct. 23, and Oct. 27–28, 2003). The results indicate that DMS and H₂S were the dominant RSCs with concentrations of 370 ± 140 and 110 ± 60 pptv, respectively. The photochemical conversion of DMDS to SO₂ was found to occur more efficiently than other RSCs. The overall results of our study suggest that photochemical conversion of RSCs accounted for less than 15% of the observed SO₂ during the measurement period. The SO₂ production from DMS oxidation (mainly by the reaction with OH) was found to be affected primarily by the abstraction channel due to high NO_x levels during the experimental conditions.     

Abundances and variability of tropospheric volatile organic compounds at the South Pole and other Antarctic locations

Multiyear (2000–2006) seasonal measurements of carbon monoxide, hydrocarbons, halogenated species, dimethyl sulfide, carbonyl sulfide and C₁–C₄ alkyl nitrates at the South Pole are presented for the first time. At the South Pole, short-lived species (such as the alkenes) typically were not observed above their limits of detection because of long transit times from source regions. Peak mixing ratios of the longer lived species with anthropogenic sources were measured in late winter (August and September) with decreasing mixing ratios throughout the spring. In comparison, compounds with a strong oceanic source, such as bromoform and methyl iodide, had peak mixing ratios earlier in the winter (June and July) because of decreased oceanic production during the winter months. Dimethyl sulfide (DMS), which is also oceanically emitted but has a short lifetime, was rarely measured above 5 pptv. This is in contrast to high DMS mixing ratios at coastal locations and shows the importance of photochemical removal during transport to the pole. Alkyl nitrate mixing ratios peaked during April and then decreased throughout the winter. The dominant source of the alkyl nitrates in the region is believed to be oceanic emissions rather than photochemical production due to low alkane levels.Sampling of other tropospheric environments via a Twin Otter aircraft included the west coast of the Ross Sea and large stretches of the Antarctic Plateau. In the coastal atmosphere, a vertical gradient was found with the highest mixing ratios of marine emitted compounds at low altitudes. Conversely, for anthropogenically produced species the highest mixing ratios were measured at the highest altitudes, suggesting long-range transport to the continent. Flights flown through the plume of Mount Erebus, an active volcano, revealed that both carbon monoxide and carbonyl sulfide are emitted with an OCS/CO molar ratio of 3.3 × 10^?3 consistent with direct observations by other investigators within the crater rim.     

Estimates of synthetic fertilizer N-induced direct nitrous oxide emission from Chinese croplands during 1980-2000

There is increasing concern that agricultural intensification in China has greatly increased N₂O emissions due to rapidly increased fertilizer use. By linking a spatial database of precipitation, synthetic fertilizer N input, cropping rotation and area via GIS, a precipitation-rectified emission factor of N₂O for upland croplands and water regime-specific emission factors for irrigated rice paddies were adopted to estimate annual synthetic fertilizer N-induced direct N₂O emissions (FIE-N₂O) from Chinese croplands during 1980-2000. Annual FIE-N₂O was estimated to be 115.7 Gg N₂O-N year⁻¹ in the 1980s and 210.5 Gg N₂O-N year⁻¹ in the 1990s, with an annual increasing rate of 9.14 Gg N₂O-N year⁻¹ over the period 1980-2000. Upland croplands contributed most to the national total of FIE-N₂O, accounting for 79% in 1980 and 92% in 2000. Approximately 65% of the FIE-N₂O emitted in eastern and southern central China.     

Ground-level ozone in the Pearl River Delta region: Analysis of data from a recently established regional air quality monitoring network

The ambient air quality monitoring data of 2006 and 2007 from a recently established Pearl River Delta (PRD) regional air quality monitoring network are analyzed to investigate the characteristics of ground-level ozone in the region. Four sites covering urban, suburban, rural and coastal areas are selected as representatives for detailed analysis in this paper. The results show that there are distinct seasonal and diurnal cycles in ground-level ozone across the PRD region. Low ozone concentrations are generally observed in summer, while high O₃ levels are typically found in autumn. The O₃ diurnal variations in the urban areas are larger than those at the rural sites. The O₃ concentrations showed no statistically significant difference between weekend and weekdays in contrast to the findings in many other urban areas in the world. The average ozone concentrations are lower in urban areas compared to the sites outside urban centers. Back trajectories are used to show the major air-mass transport patterns and to examine the changes in ozone from the respective upwind sites to a site in the center of the PRD (Wanqingsha). The results show higher average ozone concentrations at the upwind sites in the continental and coastal air masses, but higher 1 h-max O₃ concentrations (by 8–16 ppbv) at the center PRD site under each of air-mass category, suggesting that the ozone pollution in the PRD region exhibits both regional and super-regional characteristics.     

Ozone episodes in urban Hong Kong 1994–1999

Summer pollution episodes in Hong Kong are related to the passage of tropical storms close to the territory. Between 1994 and 1999, there were six territory-wide ozone episodes in Hong Kong during which the Hong Kong Air Quality Objective for ozone (240 μg m⁻³, 1 h) was violated. The maximum O₃ concentration for the period was 334 μg m⁻³ recorded in August 1999. Synoptically, tropical storms were in the vicinity on all the episode days. Northwesterly/westerly winds induced by the storms are believed to cause ozone precursor emissions from local power plants in the western part of Hong Kong to impact the territory, and at the same time allowing the import of emissions from upwind sources along the mainland coast. Other important meteorological factors that contribute to the occurrence of the episode events include: stable atmospheres, morning break-up of nocturnal inversions, low winds, strong solar radiation and high temperatures. Trajectory analysis of airflows at 850 hPa confirms the long-range pollutant transport. The strong correlation between non-sea-salt sulphate (NS-SO₄) and selenium for the summer of 1999 indicates that the main source of high levels of NS-SO₄ in summer in Hong Kong is coal combustion. The correlation between arsenic (As) and vanadium (V) for the summers of 1996–1999 suggests a concomitant influence of coal and residual oil combustion in the region.     

An analysis on abnormally low ozone in the upper troposphere over subtropical East Asia in spring 2004

Abnormally low ozone (O₃) mixing ratios were observed by electrochemical concentration cell (ECC) ozonesondes in the upper troposphere over subtropical East Asia in spring 2004, a season when high tropospheric O₃ is usually observed in the region. Low O₃ with a lowest mixing ratio of 13 ppbv, less than a fourth of the respective seasonal average of 60–100 ppbv, was observed at 11–18 km above ground over Hong Kong (22.31°N, 114.17°E), Sanya (18.23°N, 109.52°E) and Taipei (24.98°N, 121.43°E). The origin of the low O₃ was investigated using meteorological evidence, satellite imagery and three-dimensional backward air trajectory. We found for the first time that the low O₃ resulted from deep convective pumping of low O₃ maritime air masses near the center of typhoon Sudal from the boundary layer of the tropical region to the east of the Philippines to the upper troposphere. The low O₃ air masses were then transported to the higher latitudes far ahead of the typhoon following the long-range transport driven by the circulations associated with the typhoon and the northern Hadley cell. The findings of this study highlight that more research efforts are needed to understand the effect of the circulation associated with tropical cyclones on the distribution and budget of O₃ and other trace gases in the troposphere.     

Formation of submicron sulfate and organic aerosols in the outflow from the urban region of the Pearl River Delta in China

Size-resolved chemical compositions of non-refractory submicron aerosols were measured using a quadrupole Aerodyne aerosol mass spectrometer at a rural site near Guangzhou in the Pearl River Delta (PRD) of China in the summer of 2006. Two cases characterized as the outflows from the PRD urban region with plumes of high SO₂ concentration were investigated. The evolution of sulfate size distributions was observed on a timescale of several hours. Namely mass concentrations of sulfate in the condensation mode (with vacuum aerodynamic diameters (D_va) < 300 nm) increased at a rate of about 0.17–0.37 ppbv h^?1 during the daytime. This finding was consistent with the sulfuric acid production rates of about 0.17–0.3 ppbv h^?1, as calculated from the observed gas-phase concentrations of OH (～3.3 × 10⁶–1.7 × 10⁷ cm^?3) and SO₂ (～3–21.2 ppbv). This implies that the growth of sulfate in the condensation mode was mainly due to gas-phase oxidation of SO₂. The observed rapid increase was caused mainly by the concurrent high concentrations of OH and SO₂ in the air mass. The evolution of the mass size distributions of m/z 44, a tracer for oxygenated organic aerosol (OOA), was very similar to that of sulfate. The mass loadings of m/z 44 were strongly correlated with those of sulfate (r² = 0.99) in the condensation mode, indicating that OOA might also be formed by the gas-phase oxidation of volatile organic compound (VOC) precursors. It is likely that sulfate and OOA were internally mixed throughout the whole size range in the air mass.     

Measurement of organic mass to organic carbon ratio in ambient aerosol samples using a gravimetric technique in combination with chemical analysis

Organic materials make up a significant fraction of ambient particulate mass. It is important to quantify their contributions to the total aerosol mass for the identification of aerosol sources and subsequently formulating effective control measures. The organic carbon (OC) mass can be determined by an aerosol carbon analyzer; however, there is no direct method for the determination of the mass of organic compounds, which also contain N, H, and O atoms in addition to C. The often-adopted approach is to estimate the organic mass (OM) from OC multiplying by a factor. However, this OC-to-OM multiplier was rarely measured for a lack of appropriate methods for OM. We report here a top-down approach to determine OM by coupling thermal gravimetric and chemical analyses. OM is taken to be the mass difference of a filter before and after heating at 550 °C in air for 4 h minus mass losses due to elemental carbon (EC), volatile inorganic compounds (e.g., NH₄NO₃), and loss of aerosol-associated water that arise from the heating treatment. The losses of EC and inorganic compounds are determined through chemical analysis of the filter before and after the heating treatment. We analyzed 37 ambient aerosol samples collected in Hong Kong during the winter of 2003, spring of 2004, and summer of 2005. A value of 2.1±0.3 was found to be the appropriate factor to convert OC to OM in these Hong Kong aerosol samples. If the dominant air mass is classified into two categories, then an OM-to-OC ratio of 2.2 was applicable to aerosols dominated by continent-originated air mass, and 1.9 was applicable to aerosols dominated by marine air mass.