Steven Poon

Multielemental analysis and characterization of fine aerosols at several key ACE‐Asia sites

loadings of 29, 16, and 9.1 mg/m 3 and coarse mass loadings of 33, 14, and 11 mg/m 3 were measured at Hong Kong, Cheju, and Sado Island sites, respectively, during the study period. The corresponding maximum PM2.5 and coarse mass values for the three sites were 109, 81, and 78 mg/m 3 and 101, 162, and 253 mg/m 3 , respectively. Accelerator-based ion beam analysis (IBA) techniques were used to quantify major components as well as significant trace elements. These included total hydrogen, black carbon, F, Na, Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Br, and Pb, with detection limits close to or below 1 ng/m 3 . The average PM2.5 percentage composition by weight across the three sites was estimated to be around (8.4 ± 4)% black carbon, (7.7 ± 7)% soil, (43 ± 14)% ammonium sulfate, (11 ± 16)% organic matter, (10 ± 12)% salinity, and (0.6 ± 0.3)% trace elements. Soil fingerprints for the east Asian region were generated using oxides of measured Al, Si, K, Ca, Ti, Mn, and Fe concentrations. The coarse fraction was dominated by wind blown soil (23%) and sea salts (48%). [PM10/PM2.5] mass ratios were typically (2.1 ± 0.4) averaged across all sites for the whole year. [PM10/PM2.5] mass ratios for the 21 IBA elements analyzed were also provided. This quantitative data providing both masses and dates over an 18-month period provide useful input for aerosol transport modeling for the east Asia region. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 0399 Atmospheric Composition and Structure: General or miscellaneous; 1610 Global Change: Atmosphere (0315, 0325); 1620 Global Change: Climate dynamics (3309); KEYWORDS: PM2.5, PM10, aerosols, fine particle characterization, Asian region, ACE-Asia, IBA techniques

Observations of Nitryl Chloride and Modeling its Source and Effect on Ozone in the Planetary Boundary Layer of Southern China

Nitryl chloride (ClNO2) plays potentially important roles in atmospheric chemistry, but its abundance and effect are not fully understood due to the small number of ambient observations of ClNO2 to date. In late autumn 2013, ClNO2 was measured with a chemical ionization mass spectrometer (CIMS) at a mountain top (957 m above sea level) in Hong Kong. During 12 nights with continuous CIMS data, elevated mixing ratios of ClNO2 (>400 parts per trillion by volume) or its precursor N2O5 (>1000 pptv) were observed on six nights, with the highest ever reported ClNO2 (4.7 ppbv, 1 min average) and N2O5 (7.7 ppbv, 1 min average) in one case. Backward particle dispersion calculations driven by winds simulated with a mesoscale meteorological model show that the ClNO2/N2O5-laden air at the high-elevation site was due to transport of urban/industrial pollution north of the site. The highest ClNO2/N2O5 case was observed in a later period of the night and was characterized with extensively processed air and with the presence of nonoceanic chloride. A chemical box model with detailed chlorine chemistry was used to assess the possible impact of the ClNO2 in the well-processed regional plume on next day ozone, as the air mass continued to downwind locations. The results show that the ClNO2 could enhance ozone by 5–16% at the ozone peak or 11–41% daytime ozone production in the following day. This study highlights varying importance of the ClNO2 chemistry in polluted environments and the need to consider this process in photochemical models for prediction of ground-level ozone and haze.

Nighttime chemistry at a high altitude site above Hong Kong

Nighttime reactions of nitrogen oxides influence ozone, volatile organic compounds, and aerosol and are thus important to the understanding of regional air quality. Despite large emissions and rapid recent growth of nitrogen oxide concentrations, there are few studies of nighttime chemistry in China. Here we present measurements of nighttime nitrogen oxides, NO3 and N2O5, from a coastal mountaintop site in Hong Kong adjacent to the megacities of the Pearl River Delta region. This is the first study of nighttime chemistry from a site within the residual layer in China. Key findings include the following. First, highly concentrated urban NOx outflow from the Pearl River Delta region was sampled infrequently at night, with N2O5 mixing ratios up to 8 ppbv (1 min average) or 12 ppbv (1 s average) in nighttime aged air masses. Second, the average N2O5 uptake coefficient was determined from a best fit to the available steady state lifetime data as γ(N2O5) = 0.014 ± 0.007. Although this determination is uncertain due to the difficulty of separating N2O5 losses from those of NO3, this value is in the range of previous residual layer determinations of N2O5 uptake coefficients in polluted air in North America. Third, there was a significant contribution of biogenic hydrocarbons to NO3 loss inferred from canister samples taken during daytime. Finally, daytime N2O5 mixing ratios were in accord with their predicted photochemical steady state. Heterogeneous uptake of N2O5 in fog is determined to be an important production mechanism for soluble nitrate, even during daytime.

Atmospheric peroxides in a polluted subtropical environment: seasonal variation, sources and sinks, and importance of heterogeneous processes.

Hydrogen peroxide (H2O2) and organic peroxides play an important role in atmospheric chemistry, but knowledge of their abundances, sources, and sinks from heterogeneous processes remains incomplete. Here we report the measurement results obtained in four seasons during 2011-2012 at a suburban site and a background site in Hong Kong. Organic peroxides were found to be more abundant than H2O2, which is in contrast to most previous observations. Model calculations with a multiphase chemical mechanism suggest important contributions from heterogeneous processes (primarily transition metal ion [TMI]-HOx reactions) to the H2O2 budget, accounting for about one-third and more than half of total production rate and loss rate, respectively. In comparison, they contribute much less to organic peroxides. The fast removal of H2O2 by these heterogeneous reactions explains the observed high organic peroxide fractions. Sensitivity analysis reveals that the role of heterogeneous processes depends on the abundance of soluble metals in aerosol, serving as a net H2O2 source at low metal concentrations, but as a net sink with high metal loading. The findings of this study suggest the need to consider the chemical processes in the aerosol aqueous phase when examining the chemical budget of gas-phase H2O2.

Peroxyacetyl nitrate measurements by thermal dissociation–chemical ionization mass spectrometry in an urban environment: performance and characterizations

AbstractPeroxyacetyl nitrate (PAN) is an important indicator of photochemical smog and has adverse effects on human health and vegetation growth. A rapid and highly selective technique of thermal dissociation–chemical ionization mass spectrometry (TD-CIMS) was recently developed to measure the abundance of PAN in real time; however, it may be subject to artifact in the presence of nitric oxide (NO). In this study, we tested the interference of the PAN signal induced by NO, evaluated the performance of TD-CIMS in an urban environment, and investigated the concentration and formation of PAN in urban Hong Kong. NO caused a significant underestimation of the PAN signal in TD-CIMS, with the underestimation increasing sharply with NO concentration and decreasing slightly with PAN abundance. A formula was derived to link the loss of PAN signal with the concentrations of NO and PAN, which can be used for data correction in PAN measurements. The corrected PAN data from TDCIMS were consistent with those from the commonly used gas chromatography with electron capture detection, which confirms the utility of TD-CIMS in an urban environment in which NO is abundant. In autumn of 2010, the hourly average PAN mixing ratio varied from 0.06 ppbv to 5.17 ppbv, indicating the occurrence of photochemical pollution in urban Hong Kong. The formation efficiency of PAN during pollution episodes was as high as 3.9 to 5.9 ppbv per 100 ppbv ozone. The efficiency showed a near-linear increase with NOx concentration, suggesting a control policy of NOx reduction for PAN pollution.

Nighttime NO x loss and ClNO 2 formation in the residual layer of a polluted region: Insights from field measurements and an iterative box model

The heterogeneous reaction of dinitrogen pentoxide (N2O5) on aerosols is an important sink of nitrogen oxides (NOx) in the polluted boundary layer, and the production of nitryl chloride (ClNO2) can have significant effects on the atmospheric oxidative capacity. However, the heterogeneous loss of N2O5 and the formation of ClNO2 are still not well quantified, especially in China. In a previous study, we measured ClNO2 and N2O5 concentrations in several air masses at a high-elevation site in Hong Kong, and found the highest levels ever reported at one night. The present study employed an iterative box model to investigate five N2O5/ClNO2-laden nights. We first estimated the N2O5 uptake coefficient and ClNO2 yield and then calculated the relative importance of N2O5 heterogeneous reactions to NOx loss and the accumulated ClNO2 production over the entire night. The average uptake coefficient was 0.004±0.003, and the average yield was 0.42±0.26. As the air masses aged, the accumulated ClNO2 reached up to 6.0ppbv, indicating significant production of ClNO2 in the polluted air from the Pearl River Delta. ClNO2 formation (N2O5+Cl-), N2O5 hydrolysis (N2O5+H2O), and NO3 reactions with volatile organic compounds (NO3+VOCs) consumed 23%, 27%, and 47% of the produced NO3, respectively, as the average for five nights. A significant portion of the NOx in the air masses (70%±10%) was removed during the night via NO3 reactions with VOCs (~40%) and N2O5 heterogeneous loss (~60%).

Photochemical Smog in Southern China: A Synthesis of Observations and Model Investigations of the Sources and Effects of Nitrous Acid

Recent studies have revealed potentially important effects of additional source(s) of hydroxyl radicals on the atmosphere’s oxidative capacity and, in turn, the production of secondary air pollutants. In this paper, we give an overview of our recent efforts in investigating the sources and effects of nitrous acid (HONO) on ozone and some secondary aerosols in southern China by combining field measurements and model simulations. Beginning in 2011, a series of field measurements of HONO were conducted at five sites, with diverse land use and different effects of emission sources. We observed the seasonal characteristics, emission ratios, heterogeneous production, and made simulations with a chemical transport model for the photochemical effects of HONO. The key findings are as follows. The derived emission ratios from vehicles exhibited wide variability and were mostly higher than the more uniform value of 0.8% reported in the literature. Larger nocturnal heterogeneous conversion rates of NO2 to HONO were observed when air masses were passing over sea surfaces, compared with land surfaces. Widely reported daytime sources of HONO also exist in Hong Kong. Moreover, the revised WRF-Chem model with comprehensive HONO sources significantly improved the simulations of the observed HONO, which enhanced regional hydroxyl radicals, O3, and PM2.5 by 10–20, 8–15, and 10–15% over urban areas in the Pearl River Delta region, respectively. Our studies highlight the importance of considering HONO sources when simulating secondary pollutants in polluted atmospheres.