Qiaozhi Zha

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.

Large conversion rates of NO2 to HNO2 observed in air masses from the South China Sea: Evidence of strong production at sea surface?

Nitrous acid (HONO) plays important roles in tropospheric chemistry, but its source(s) are not completely understood. Here we analyze measurements of HONO, nitrogen dioxide (NO2), and related parameters at a coastal site in Hong Kong during September–December 2012. The nocturnal NO2-to-HONO conversion rates were estimated in air masses passing over land and sea surfaces. The conversion rates in the “sea cases” (3.17–3.36 × 10−2 h−1) were significantly higher than those in the “land cases” in our study (1.20–1.30 × 10−2 h−1) and in previous studies by others. These results suggest that air-sea interactions may be a significant source of atmospheric HONO and need to be considered in chemical transport models.

Oxidizing capacity of the rural atmosphere in Hong Kong, Southern China

Atmospheric oxidizing capacity (AOC), dominated by the hydroxyl radical (OH), is an important index of the self-cleaning capacity of atmosphere and plays a vital role in the tropospheric chemistry. To better understand the key processes governing the chemistry of rural atmosphere of southern China, we analyzed the oxidation capacity and radical chemistry at a regional background site in Hong Kong from 23 August to 22 December 2012, which covered the summer, autumn and winter seasons. A chemical box model built on the latest Master Chemical Mechanism (v3.3) was used to elucidate the OH reactivity and sources of ROX radicals (ROX=OH+HO2+RO2). The AOC showed a clear seasonal pattern with stronger intensity in late summer compared to autumn and winter. Reactions with NO2 (30%) and oxygenated volatile organic compounds (OVOCs) (31%) together dominated the OH loss in summer, while reactions with CO (38% in autumn and 39% in winter) and OVOCs (34% in autumn and 25% in winter) made larger contributions in autumn and winter. Photolysis of O3 (36%-47%) presented the major ROX source during all three seasons. The second largest ROx source was HONO photolysis (25%) in summer compared to HCHO photolysis in autumn (20%) and winter (21%). Besides, photolysis of other OVOCs was another important primary source of ROx radicals with average contributions of 14%, 13% and 20% for the summer, autumn and winter cases, respectively. Overall, the present study evaluates the oxidizing capacity of the rural atmosphere of South China and elucidates the varying characteristics of photochemical processes in different air masses.

Observations of biogenic ion-induced cluster formation in the atmosphere.

On the ability of biogenic vapors to initiate ion-induced cluster formation in the boreal forest. A substantial fraction of aerosols, which affect air quality and climate, is formed from gaseous precursors. Highly oxygenated organic molecules (HOMs) are essential to grow the newly formed particles and have been evidenced to initiate ion-induced nucleation in chamber experiments in the absence of sulfuric acid. We investigate this phenomenon in the real atmosphere using an extensive set of state-of-the-art ion and mass spectrometers deployed in a boreal forest environment. We show that within a few hours around sunset, HOMs resulting from the oxidation of monoterpenes are capable of forming and growing ion clusters even under low sulfuric acid levels. In these conditions, we hypothesize that the lack of photochemistry and essential vapors prevents the organic clusters from growing past 6 nm. However, this phenomenon might have been a major source of particles in the preindustrial atmosphere and might also contribute to particle formation in the future and consequently affect the climate.

Revisiting nitrous acid (HONO) emission from on-road vehicles: A tunnel study with a mixed fleet

ABSTRACT Nitrous acid (HONO) is an important precursor of OH radicals in the atmosphere. In urban areas, emissions from vehicles are the main source of air pollutants, including reactive nitrogen. Previously reported emission ratios of HONO (HONO/NOx) from vehicles were measured in the late 1990s and need to be updated due to the significant changes in emission control technologies. We measured the emission ratio of a fleet of vehicles (38% diesel on average) from March 11 to 21, 2015, in a road tunnel in Hong Kong. The emission ratio of 1.24% (±0.35%) obtained is greater than the commonly adopted 0.8% or 0.3%. The elevated emission ratio is found to be related to the presence of vehicles equipped with diesel particle filters (DPFs). Positive correlation between HONO and black carbon (BC) shows that HONO and BC were emitted together, while the lack of correlation or even anticorrelation between HONO/NOx and BC indicates that the BC-mediated conversion of NO2 to HONO in the dark was insignificant in the immediate vicinity of the emission sources. Implications: Vehicular emission is a key source for HONO in the urban atmosphere. However, the most commonly used emission ratio HONO/NOx in modeling studies was measured more than 15 years ago. Our tunnel study suggests that a mixed fleet nowadays has a higher emission ratio, possibly because of the diesel particle filter (DPF) retrofit program and the growing share of Euro IV or more advanced diesel vehicles. Our study also provides new insight into the role of black carbon in HONO formation from vehicles.

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.

Field chemical sensing with LEDs

Reliable concentration assessment of the major atmospheric oxidants (hydroxyl free radicals, nitrate radicals, and ozone, for example), and their precursors (nitrous acid, nitrogen dioxide, and formaldehyde) is essential for understanding and predicting chemical processes that affect regional air quality and global climate change. Monitoring these short-lived atmospheric constituents in real-time in situ is challenging because of their high reactivity, which results in lifetimes of around 1-100 seconds (this is short by comparison with greenhouse gases, for example). Furthermore, these short-lived oxidant species have ultralow concentrations that measure in parts per billion by volume (ppbv) to parts per quadrillion by volume (ppqv). Since the last decade, atmospheric environmental monitoring has benefited from the development of novel sensors with high sensitivity and specificity. These breakthroughs were made possible by significant breakthroughs in photonic technology, including greater availability of light sources at desired wavelengths, as well as high performance photodetectors that provide fast response and shot-noise-limited sensitivity. In our work, we monitored atmospheric nitrous acid (HONO) and nitrogen dioxide (NO2/ using LED-based incoherent broadband cavity-enhanced absorption spectroscopy (LED-IBBCEAS). The IBBCEAS technique1 relies on the use of a high-finesse optical cavity to provide the necessary ultra-high sensitivity, in conjunction with a broadband light source, such as an LED or xenon arc lamp, to monitor trace gas species in the visible and UV spectral regions. LEDs, widely used today in lighting and video displays, are advantageous in optical sensing because they allow access to spectral regions that involve strong fundamental electronic transition (see Figure 1). The LED-IBBCEAS technique offers several advantages. The 1m long optical cavity has an effective absorption path length of 1–10km, which allows for enhanced detection sensitivity while keeping the setup very compact for high spatial resolution Figure 1. (top) Strong and structured broadband absorptions of atmospheric molecules in the UV and visible spectral regions allow for high sensitivity detection of key atmospheric species. (bottom) Characterization of some LEDs (a-c) used in IBBCEAS testing.2–4