Fast particulate nitrate formation via N₂O₅ uptake aloft in winter in Beijing

Particulate nitrate (pNO3-) is an important component of secondary aerosols in urban areas. Therefore, it is critical to explore its formation mechanism to assist with the planning of haze abatement strategies. Here we report vertical measurements of NOx and O3 by in situ instruments on a movable carriage on a tower during a winter heavy-haze episode (18 to 20 December 2016) in urban Beijing, China. Based on the box model simulation at different heights, we found that pNO3- formation via N2O5 heterogeneous uptake was negligible at ground level due to N2O5 concentrations of near zero controlled by high NO emissions and NO concentration. In contrast, the contribution from N2O5 uptake was large at high altitudes (e.g., > 150 m), which was supported by the lower total oxidant (NO2 + O3) level at high altitudes than at ground level. Modeling results show the specific case that the nighttime integrated production of pNO3- for the high-altitude air mass above urban Beijing was estimated to be 50 µg m−3 and enhanced the surface-layer pNO3- the next morning by 28 µg m−3 through vertical mixing. Sensitivity tests suggested that the nocturnal NOx loss by NO3–N2O5 chemistry was maximized once the N2O5 uptake coefficient was over 2 × 10−3 on polluted days with Sa at 3000 µm2 cm−3 in wintertime. The case study provided a chance to highlight the fact that pNO3- formation via N2O5 heterogeneous hydrolysis may be an important source of particulate nitrate in the urban airshed during wintertime.

Large particulate nitrate formation from N₂O₅ uptake in a chemically reactive layer aloft during winter time in Beijing

Particulate nitrate (pNO3−) is a dominant component of secondary aerosols in urban areas. Therefore, it is critical to explore its formation mechanism to assist with the planning of haze abatement strategies. Simultaneous ground-based and tower-based measurements were conducted during a winter heavy haze episode in urban Beijing, China. We found pNO3− formation via N2O5 heterogeneous uptake was negligible at ground level, due to the presence of high NO concentrations limiting the production of N2O5. In contrast, the contribution from N2O5 uptake was larger at higher altitudes (e.g., > 150 m), which was supported by the observed large total oxidant (NO2 + O3) missing aloft compared with ground level. The nighttime integrated production potential of pNO3− for the higher altitude air mass overhead urban Beijing was estimated to be 50 μg m−3, and enhanced the surface pNO3− significantly with 28 μg m−3 after nocturnal boundary layer broken in the next morning. In this case, the oxidation of NOX to nitrate was maximized once N2O5 uptake coefficient over 0.0017, since N2O5 uptake dominated the fate of NO3 and N2O5 with the presence of large aerosol surface concentrations. These results highlight that pNO3− formation via N2O5 heterogeneous hydrolysis at higher altitude air masses aloft could be an important source for haze formation in the urban airshed during winter time. Accurately describing the formation and development of reactive air masses aloft is a critical task for improving current chemical transport models.

Estimating Trends of Mean Monthly Ozone Emission in Urbanised Areas of Malaysia

A 21 year (1992 – 2012) daily ozone emission data of a highly urbanised district in Malaysia was analysed with the aim of estimating the trend of ozone emission and relating this trend to the socio – economic and climatic characteristics of the area. Daily ozone emission dataset used in this study were obtained from the World Ozone and Ultraviolet Data Centre (WOUDC). The data were aggregated to obtain the mean monthly emission data. Descriptive and inferential statistical analyses were conducted to describe the datasets. Trend of the ozone emission was estimated with the use of MANN - KENDALL test. The magnitude of the trend was derived by the use of ordinary least-square fitting and the significance of trend was also tested with the use of MANN-KENDALL tool. The results of the statistical analysis indicated that the highest ozone emission occurred during the south western monsoon (May to August) period and these mean monthly ozone emission differed significantly over the study period. The trend analysis indicated a yearly decrease of between 0.069 ppt to 9.45 ppt for all the months except for the month of June when the predicted ozone concentration increased between 0.403 ppt and 0.414 ppt over 2020 to 2100. Furthermore, the results indicated that the ozone emission datasets yielded good estimates (predictive power of over 90%) with polynomial regression model. It could be concluded that the results of this study provided useful evidence for the importance of the climatic factors such as ambient air temperature, relative humidity on ozone formation. More so, this study could be useful in developing baseline information for assessing the health impact of ozone emission and for urban airshed modelling.

The MUMBA campaign: measurements of urban, marine and biogenic air

The Measurements of Urban, Marine and Biogenic Air (MUMBA) campaign took place in Wollongong, New South Wales (a small coastal city approximately 80 km south of Sydney, Australia) from 21 December 2012 to 15 February 2013. Like many Australian cities, Wollongong is surrounded by dense eucalyptus forest, so the urban airshed is heavily influenced by biogenic emissions. Instruments were deployed during MUMBA to measure the gaseous and aerosol composition of the atmosphere with the aim of providing a detailed characterisation of the complex environment of the ocean–forest–urban interface that could be used to test the skill of atmospheric models. The gases measured included ozone, oxides of nitrogen, carbon monoxide, carbon dioxide, methane and many of the most abundant volatile organic compounds. The aerosol characterisation included total particle counts above 3 nm, total cloud condensation nuclei counts, mass concentration, number concentration size distribution, aerosol chemical analyses and elemental analysis.The campaign captured varied meteorological conditions, including two extreme heat events, providing a potentially valuable test for models of future air quality in a warmer climate. There was also an episode when the site sampled clean marine air for many hours, providing a useful additional measure of the background concentrations of these trace gases within this poorly sampled region of the globe. In this paper we describe the campaign, the meteorology and the resulting observations of atmospheric composition in general terms in order to equip the reader with a sufficient understanding of the Wollongong regional influences to use the MUMBA datasets as a case study for testing a chemical transport model. The data are available from PANGAEA (http://doi.pangaea.de/10.1594/PANGAEA.871982).