Seasonal variations of C-1-C-4 alkyl nitrates at a coastal site in Hong Kong: Influence of photochemical formation and oceanic emissions

Five C-1-C-4 alkyl nitrates (RONO2) were measured at a coastal site in Hong Kong in four selected months of 2011 and 2012. The total mixing ratios of C-1-C-4 RONO2 (Sigma 5RONO2) ranged from 15.4 to 143.7 pptv with an average of 65.9 +/- 33.0 pptv. C-3-C-4 RONO2 (2-butyl nitrate and 2-propyl nitrate) were the most abundant RONO2 during the entire sampling period. The mixing ratios of C-3-C-4 RONO2 were higher in winter than those in summer, while the ones of methyl nitrate (MeONO2) were higher in summer than those in winter. Source analysis suggests that C-2-C-4 RONO2 were mainly derived from photochemical formation along with biomass burning (58.3-71.6%), while ocean was a major contributor to MeONO2 (53.8%) during the whole sampling period. The photochemical evolution of C-2-C-4 RONO2 was investigated, and found to be dominantly produced by the parent hydrocarbon oxidation. The notable enrichment of MeONO2 over C-3-C-4 RONO2 was observed in a summer episode when the air masses originating from the South China Sea (SCS) and MeONO2 was dominantly derived from oceanic emissions. In order to improve the accuracy of ozone (O-3) prediction in coastal environment, the relative contribution of RONO2 from oceanic emissions versus photochemical formation and their coupling effects on O-3 production should be taken into account in future studies. (C) 2017 Elsevier Ltd. All rights reserved

Reconciling discrepancies in the source characterization of VOCs between emission inventories and receptor modeling

Emission inventory (EI) and receptor model (RM) are two of the three source apportionment (SA) methods recommended by Ministry of Environment of China and used widely to provide independent views on emission source identifications. How to interpret the mixed results they provide, however, were less studied. In this study, a cross-validation study was conducted in one of China's fast-developing and highly populated city cluster- the Pearl River Delta (PRD) region. By utilizing a highly resolved speciated regional EI and a region-wide gridded volatile organic compounds (VOCs) speciation measurement campaign, we elucidated underlying factors for discrepancies between EI and RM and proposed ways for their interpretations with the aim to achieve a scientifically plausible source identification. Results showed that numbers of species, temporal and spatial resolutions used for comparison, photochemical loss of reactive species, potential missing sources in EI and tracers used in RM were important factors contributed to the discrepancies. Ensuring the consensus of species used in EIs and RMs, utilizing a larger spatial coverage and longer time span, addressing the impacts of photochemical losses, and supplementing emissions from missing sources could help reconcile the discrepancies in VOC source characterizations acquired using both approaches. By leveraging the advantages and circumventing the disadvantages in both methods, the EI and RM could play synergistic roles to obtain robust SAs to improve air quality management practices

Efficient control of atmospheric sulfate production based on three formation regimes

The formation of sulfate (SO₄²⁻) in the atmosphere is linked chemically to its direct precursor, sulfur dioxide (SO₂), through several key oxidation paths for which nitrogen oxides or NO_x (NO and NO₂) play essential roles. Here we present a coherent description of the dependence of SO₄²⁻ formation on SO₂ and NO_x under haze-fog conditions, in which fog events are accompanied by high aerosol loadings and fog-water pH in the range of 4.7–6.9. Three SO₄²⁻ formation regimes emerge as defined by the role played by NO_x. In the low-NO_x regime, NO_x act as catalyst for HO_x, which is a major oxidant for SO₂, whereas in the high-NO_x regime, NO₂ is a sink for HO_x. Moreover, at highly elevated NO_x levels, a so-called NO₂-oxidant regime exists in which aqueous NO₂ serves as the dominant oxidant of SO₂. This regime also exists under clean fog conditions but is less prominent. Sensitivity calculations using an emission-driven box model show that the reduction of SO₄²⁻ is comparably sensitive to the reduction of SO₂ and NO_x emissions in the NO₂-oxidant regime, suggesting a co-reduction strategy. Formation of SO₄²⁻ is relatively insensitive to NO_x reduction in the low-NO_x regime, whereas reduction of NO_x actually leads to increased SO₄²⁻ production in the intermediate high-NO_x regime

High resolution of black carbon and organic carbon emissions in the Pearl River Delta region, China

A high-resolution regional black carbon (BC) and organic carbon (OC) emission inventory for the year 2009 was developed for the Pearl River Delta (PRD) region, China, based on the collected activity data and the latest emission factors. PM2.5, BC and OC emissions were estimated to be 303 kt, 39 kt and 31 kt, respectively. Industrial processes were major contributing sources to PM2.5 emissions. BC emissions were mainly from mobile sources, accounting for 65.0%, while 34.1% of OC emissions were from residential combustion. The primary OC/BC ratios for individual cities in the PRD region were dependent on the levels of economic development due to differences in source characteristics, with high ratios in the less developed cities and low ratios in the central and southern developed areas. The preliminary temporal profiles were established, showing the highest OC emissions in winter and relatively constant BC emissions throughout the year. The emissions were spatially allocated into grid cells with a resolution of 3 km x 3 km. Large amounts of BC emissions were distributed over the central-southern PRD city clusters, while OC emissions exhibited a relatively even spatial distribution due to the significant biomass burning emissions from the outlying area of the PRD region. Uncertainties in carbonaceous aerosol emissions were usually higher than in other primary pollutants like SO2, NOx, and PM10. One of the key uncertainty sources was the emission factor, due to the absence of direct measurements of BC and OC emission rates. (C) 2012 Elsevier B.V. All rights reserved

Source Apportionment of Visibility in Hong Kong

A discussion covers visibility across the Victoria Harbor in Hong Kong; post study in Hong Kong, wherein the top three contributors are ammonium sulfate, elemental carbon, and organic carbon; seven pollution sources based on the characteristic explained variance values, i.e., vehicle exhaust, residual oil combustion, fresh sea salt, sea salt, secondary sulfate, secondary nitrate, and biomass burning; locations of meteorological observation stations and sampling site; data description and preprocessing; comparison of PM2.5 source contributions in 00/01 and 04/05 campaigns; mass extraction efficiencies derived from MLR and PMF; and comparison of the source contribution to Bext in 00/01 and 04/05 campaign. This is an abstract of a paper presented at the 103rd Air and Waste Management Association Annual Conference and Exhibition (Calgary, Alberta, Canada 6/22-25/2010)

Modeling inorganic nitrogen deposition in Guangdong province, China

Atmospheric nitrogen deposition is an essential component of acid deposition and serves as one of main sources of nitrogen of the ecosystem. Along with rapidly developed economy, it is expected that the nitrogen deposition in Guangdong province is considerably large, due to substantial anthropogenic reactive nitrogen lost to the Pearl River Delta (PRD) region, one of the most developed region in China. However, characterization of chemical compositions of inorganic nitrogen (IN) deposition and quantification of nitrogen deposition fluxes in time and space in Guangdong province were seldom conducted, especially using a numerical modeling approach. In this study, we established a WRF/SMOKE-PRD/CMAQ model system and expanded 2006-based PRD regional emission inventories to Guangdong provincial ones, including SO2, NOx, VOC, PM10, PM2.5, and NH3 emissions for modeling nitrogen deposition in Guangdong province. Observations, including meteorological observed data, rainfall data, ground-level criteria pollutant measurements, satellite-derived data, and nitrogen deposition fluxes from field measurements were employed in the evaluation of model performance. Results showed that annual nitrogen deposition fluxes in the PRD region and Guangdong province were 31.01 kg N hm(-1) a(-1) and 26.03 kg N hm(-1) a(-1), dominated by NHx (including NH3 and NH4+), with a percentage of 63% and 71% of the total deposition flux of IN, respectively. The ratio of dry deposition to wet deposition was approximately 2:1 in the PRD region and about 3:2 in the whole Guangdong province. IN deposition was mainly distributed in the PRD region, Chaozhou, and Maoming, which was similar to the spatial distributions of NOx and NH3 emissions. The spatial distributions of chemical compositions of IN deposition implied that NH3-N and NOx-N tended to deposit in places close to emission sources, while spatial distributions of aerosol NH4+ - N and NO3- - N usually exhibited broader deposition areas, along with long-range transport of fine particles. Distinct temporal trends were found in IN components, especially for wet depositions, with peak values in August. (C) 2015 Elsevier Ltd. All rights reserved

Insights into factors affecting nitrate in PM2.5 in a polluted high NOx environment through hourly observations and size distribution measurements

Nitrate, a major PM2.5 component in polluted environments, could be greatly elevated during pollution episodes. In this study, nitrate and other inorganic ions on PM2.5 were measured half hourly at a residential location in Hong Kong in December 2009. Hourly nitrate concentrations in PM2.5 varied from 0.8 to 40.5 mu gm(-3). In an episode during which hourly visibility was down to 3.71.0km and NO2 was 80.714.4ppb, PM2.5 NO3- reached 27.88.0 mu gm(-3), similar to 6 times the level during the normal hours. Nitrate was fully balanced by NH4+, indicating abundant presence of NH3. Size-segregated measurements showed 84% of nitrate was in the fine mode during the episode and also suggested that less acidic fine particles and less abundant sea-salt particles were the contributing factors to the dominant presence in the fine mode. An observation-based model for secondary inorganic aerosols was applied to investigate the relative importance of homogeneous and heterogeneous reactions to production of NO3- potential (sum of HNO3 (g) and aerosol nitrate). The modeling analysis shows that both formation pathways were significantly more active during the episode. Gas phase production of HNO3 through reaction of NO2+OH dominated during the initial rapid buildup of nitrate around noon time, but the heterogeneous N2O5 hydrolysis pathway made a sizable contribution in the subsequent few hours due to sustained high-NO2 concentrations combined with reduced photolysis loss of N2O5. This case study illustrates the important role of NH3 and NO2 in elevating PM2.5 in a high-NOx environment through the formation of nitrate

Water-soluble Organic Carbon and Oxalate in Aerosols at a Coastal Urban Site in China: Size Distribution Characteristics, Sources, and Formation Mechanisms

Water-soluble organic compounds in aerosol contribute a significant fraction to organic aerosol mass and influence interactions of aerosols with water in the atmosphere. Despite their importance, the sources and formation mechanisms of these compounds are not well understood. In this work, we measured the size distributions of water-soluble organic carbon (WSOC) and its most abundant single component, oxalate, in the urban area of Shenzhen, a coastal metropolitan city in southern China. In the cloud condensation nuclei size range, organic compounds were found to constitute a significant fraction (roughly one half) of the total water-soluble substance mass. The positive matrix factorization (PMF) model was used to resolve the bulk mass size distributions into condensation, droplet, and coarse modes, with their respective modal peak at 0.4, 1.0, and 5.5 mu m. Both WSOC and oxalate had a dominant droplet mode, a minor condensation mode, and a minor coarse mode. Approximately one half of WSOC and two thirds of oxalate mass were in the droplet mode. The sources and formation mechanisms of oxalate and WSOC were inferred in reference to the well-understood size distribution characteristics of inorganic species (Ca2+, Na+, K+, and SO42-), in conjunction with source identification and contribution estimation by PMF. We found that the droplet mode oxalate was mostly produced from in-cloud aqueous phase reactions. Among significant sources contributing to the total WSOC were biomass burning, in-cloud processing, soil dust particles, and aged sea salt particles. The first two sources were the major contributors to the droplet WSOC, while the latter two were responsible for the coarse mode WSOC. The droplet mode WSOC correlated well with K+ and sulfate, consistent with the source estimates by PMF. Future work on WSOC is suggested to be directed at characterizing the biomass burning aerosols and elucidating the molecular formation pathways in the aqueous phase

An Observation-Based Model for Secondary Inorganic Aerosols

An observation-based model for secondary inorganic aerosol (OBM-SIA) is developed to determine the sensitivity of formation of sulfate (SO42-) and nitrate (NO3-) to changes in their precursors. The model incorporates CB05 chemical mechanism with inclusion of two recently discovered OH enhancement pathways and essential aqueous phase chemical reactions, thermodynamic equilibriums for gas-aerosol phase apportionment and size distribution of SO42- and NO3-. A sequence of present time-frame observations of precursors and particle compositions are used to drive the simulation and to determine responses to perturbed emission rates of precursors. OBM-SIA obviates the need for uncertain emission inventories and boundary layer dynamic conditions, and makes use of data that are increasingly available due to recent advances in online instruments for various gaseous and aerosol components, thereby offering a cost-effective tool for the analysis of SIA-precursor relationships in the atmosphere. OBM-SIA is applied to hourly gaseous and particulate composition data during a wintertime pollution episode in Hong Kong. The major pathway responsible for the production of NO3- is the reaction of OH and NO2 in the gas phase, while the gas phase oxidation of SO2 by OH and aqueous phase oxidation of S((IV)) by O-3 contribute most significantly on SO42- production. NO3- production is more sensitive to the reduction of NOx and anthropogenic VOCs. Reduction of SO42- formation is however most sensitive to SO2 reduction and it becomes more effective as more SO2 is reduced. The work illustrates the utility of OBM-SIA in suggesting evidence-based control strategies for effective reduction of targeted SIAs