The photolysis frequency of NO2, j(NO2), is an important analytical parameter in the study of tropospheric chemistry. A chemical actinometer (CA) was built to measure the ambient j(NO2) based on a high precision NOx instrument with 1 min time resolution. Parallel measurements of the ambient j(NO2) by using the CA and a commercial spectroradiometer (SR) were conducted at a typical urban site (Peking University Urban Environmental Monitoring Station) in Beijing. In general, good agreement was achieved between the CA and SR data with a high linear correlation coefficient (R-2 = 0.977) and a regression slope of 1.12. The regression offset was negligible compared to the measured signal level. The j(NO2) data were calculated using the tropospheric ultraviolet visible radiation (TUV) model, which was constrained to observe aerosol optical properties. The calculated j(NO2) was intermediate between the results obtained with CA and SR, demonstrating the consistency of all the parameters observed at this site. The good agreement between the CA and SR data, and the consistency with the TUV model results, demonstrate the good performance of the installed SR instrument. Since a drift of the SR sensitivity is expected by the manufacturer, we propose a regular check of the data acquired via SR against those obtained by CA for long-term delivery of a high quality series of j(NO2) data. Establishing such a time series will be invaluable for analyzing the long-term atmospheric oxidation capacity trends as well as O-3 pollution for urban Beijing. (C) Higher Education Press and Springer-Verlag Berlin Heidelberg 2016
Particulate pollution is a major air pollution problem in Chinese mega-cities. Under such conditions, the atmospheric gas-phase chemistry is strongly influenced by heterogeneous reactions, of which to quantify the heterogeneous reaction processes of N2O5 is essential for the understanding of the nighttime oxidation capacity, regional NOx budget, photochemical ozone prodution, etc. In this paper, we extensively review the research progress of the N2O5 heterogeneous reaction processes such as its reaction mechanism, measurement techniques of the corresponding uptake coefficient (gamma(N2O5)) and the measurement results on different aerosol substrates. The heterogeneous reaction processes of N2O5 is a typical reactive uptake process which can be ideally studied by the aerosol flow tube system. The corresponding laboratory kinetic studies are started from model aerosols (sulfate), and evolved to be more realistic aerosols according to the accumulated knowledges on the aerosol properties obtained in field studies. It is found that the gamma(N2O5) varied from 0. 001 to 0. 2 on different aerosol substrates, more than two orders of magnitude. The variation is influenced by the ambient temperature, relative humidity, mixing state, phase state, aerosol chemical compositions like NO3-, Cl-, SO42-, liquid water content (LWC), organics, etc., of which the uptake coefficient is higher with higher LWC, Cl-, SO42- while lower with higher NO3- and organics. The avaiable field studies in the United States and Europe showed that, to describe gamma(N2O5), these impact factors can' t be independently expressed; and the dependence seems to be very complicated and cross correlated. Therefore the state of art parameterization methods of gamma(N2O5) developed from lab kinetic studies are still not able to describe the field observations. Since high aerosol loading and high N2O5 are always co-located at urban aeras, more field observations and sucessful parameterization of gamma(N2O5) is proposed to be conducted in typical urban conditions including Chinese megacity regions.
In the last four decades, various techniques including spectroscopic, wet chemical and mass spectrometric methods, have been developed and applied for the detection of ambient nitrous acid (HONO). We developed a HONO detection system based on long path photometry which consists of three independent modules i.e., sampling module, fluid propulsion module and detection module. In the propulsion module, solenoid pumps are applied. With solenoid pumps the pulsed flow can be computer controlled both in terms of pump stroke volume and pulse frequency, which enables the attainment of a very stable flow rate. In the detection module, a customized Liquid Waveguide Capillary Cell (LWCC) is used. The customized LWCC pre-sets the optical fiber in-coupling with the liquid wave guide, providing the option of fast startup and easy maintenance of the absorption photometry. In summer 2014, our system was deployed in a comprehensive campaign at a rural site in the North China Plain. More than one month of high quality HONO data spanning from the limit of detection to 5 ppb were collected. Intercomparison of our system with another established system from Forschungszentrum Juelich is presented and discussed. In conclusion, our instrument achieved a detection limit of 10 pptV within 2 min and a measurement uncertainty of 7%, which is well suited for investigation of the HONO budget from urban to rural conditions in China. (C) 2016 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.