We characterize the temporal variation and vertical distribution of nitrogen dioxide (NO₂), sulfur dioxide (SO₂), formaldehyde (HCHO) and aerosol extinction based on long-term multi-axis differential optical absorption spectroscopy (MAX-DOAS) observations from May 2011 to November 2014 in Wuxi, China. A new inversion algorithm (PriAM) is implemented to retrieve profiles of the trace gases (TGs) and aerosol extinction (AE) from the UV spectra of scattered sunlight recorded by the MAX-DOAS instrument. We investigated two important aspects of the retrieval process. We found that the systematic seasonal variation of temperature and pressure (which is regularly observed in Wuxi) can lead to a systematic bias of the retrieved aerosol profiles (e.g. up to 20 % for the AOD) if it is not explicitly considered. In this study we take this effect into account for the first time. We also investigated in detail the reason for the differences of tropospheric vertical column densities derived from either the geometric approximation or by the integration of the retrieved profiles, which were reported by earlier studies. We found that these differences are almost entirely caused by the limitations of the geometric approximation (especially for high aerosol loads). The results retrieved from the MAX-DOAS observations are compared with independent techniques not only under cloud-free sky conditions, but also under various cloud scenarios. Under most cloudy conditions (except fog and optically thick clouds), the trace gas results still show good agreements. In contrast, for the aerosol results, only near-surface AE could be still well retrieved under cloudy situations. After applying a quality control procedure, the MAX-DOAS data are used to characterize the seasonal, diurnal and weekly variations of NO₂, SO₂, HCHO and aerosols. A regular seasonality of the three trace gases is found, but not for aerosols. Similar annual variations of the profiles of the trace gases appear in different years. Only NO₂ shows a significant seasonality of the diurnal variations. Considerable amplitudes of weekly cycles occur for NO₂ and SO₂, but not for HCHO and aerosols. The TGs and aerosols show good correlations, especially for HCHO in winter. More pronounced wind direction dependencies, especially for the near-surface concentrations, are found for the trace gases than for the aerosols, which implies that the local emissions from nearby industrial areas (including traffic emissions) dominate the local pollution, while long-distance transport might also considerably contribute to the local aerosol levels.

Mobile passive differential optical absorption spectroscopy measurements of SO₂ and NO₂ were performed in the Guangzhou eastern area (GEA) during the Guangzhou Asian Games 2010 from November 2010 to December 2010. The observations were carried out between 10:00 to 13:00 (local time, i.e., during daylight). Spatial and temporal distributions of SO₂ and NO₂ in this area were obtained and emission sources were determined using wind field data. The NO₂ vertical column densities were found to agree with Ozone Monitoring Instrument values. The correlation coefficient (referred to as R²) was 0.88 after cloud filtering within a specific ground pixel. During the Guangzhou Asian Games and Asian Paralympics (Para) Games, the SO₂ and NO_x emissions in the area were quantified using averaged wind speed and wind direction. For times outside the games the average SO₂ emission was estimated to be 9.50 ± 0.90 tons per hour and the average NO_x emission was estimated to be 5.87 ± 3.46 tons per hour. During the phases of the Asian and Asian Para Games, the SO₂ and NO_x emissions were reduced by 53.50% and 43.50%, respectively, compared to the usual condition. We also investigated the influence of GEA on Guangzhou University Town, the main venue located northwest of the GEA, and found that SO₂ concentrations here were about tripled by emissions from the GEA.

A ground-based system for measuring tropospheric OH radical based on laserinduced fluorescence (AIOFM-LIF) was developed in this work. In this system, ambient air is expanded through a 0.4 mm nozzle to low pressure in a detection chamber, where OH radical is irradiated by the 308 nm laser pulse at a repetition rate of 8.5 kHz. Then, the resultant fluorescence corresponding to the A2S (0) X2. (0) i +. =.. = transition at 308 nm is detected using gated photon counting. The AIOFM-LIF system was integrated into a mobile observing platform for the field observation following the series of laboratory characterization. A portable standard OH radical source by water photolysis-ozone actinometry was established and optimized for accurate system calibration. The factors affecting the system sensitivity were quantified. It was shown that the ultimate system sensitivity is 9.9 x 10(-8) cps (molecules cm(-3))-1 mw-1; the minimum detection limits are (1.84 +/- 0.26) x 10(5) cm(-3) and (3.69 +/- 0.52) x 10(5) cm(-3) at night and noon, respectively; and the whole error of AIOFM-LIF system is about 16%. Then, the system was deployed in Shenzhen, China, during the " A comprehensive STudy of the Ozone foRmation Mechanism in Shenzhen" (STORM) campaign. Valid OH radical concentrations for 31 days were obtained, and the peak of the daily average concentration was 6.6 x 10(6) cm(-3) around 12: 00. And a high correlation (R-2 = 0.77) between OH and j(O1D) was also observed in this field campaign. The relationship between OH concentration and NOx was attentively discussed. The deployment of AIOFM-LIF system in STORM campaign has demonstrated its capability of measuring tropospheric OH radical with high sensitivity and accuracy in a polluted environment. (c) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Recently, Chinese cities have suffered severe events of haze air pollution, particularly in the North China Plain (NCP). Investigating the temporal and spatial distribution of pollutants, emissions, and pollution transport is necessary to better understand the effect of various sources on air quality. We report on mobile differential optical absorption spectroscopy (mobile DOAS) observations of precursors SO₂ and NO₂ vertical columns in the NCP in the summer of 2013 (from 11 June to 7 July) in this study. The different temporal and spatial distributions of SO₂ and NO₂ vertical column density (VCD) over this area are characterized under various wind fields. The results show that transport from the southern NCP strongly affects air quality in Beijing, and the transport route, particularly SO₂ transport on the route of Shijiazhuang–Baoding–Beijing, is identified. In addition, the major contributors to SO₂ along the route of Shijiazhuang–Baoding–Beijing are elevated sources compared to low area sources for the route of Dezhou–Cangzhou–Tianjin–Beijing; this is found using the interrelated analysis between in situ and mobile DOAS observations during the measurement periods. Furthermore, the discussions on hot spots near the city of JiNan show that average observed width of polluted air mass is 11.83 and 17.23 km associated with air mass diffusion, which is approximately 60 km away from emission sources based on geometrical estimation. Finally, a reasonable agreement exists between the Ozone Monitoring Instrument (OMI) and mobile DOAS observations, with a correlation coefficient (R²) of 0.65 for NO₂ VCDs. Both datasets also have a similar spatial pattern. The fitted slope of 0.55 is significantly less than unity, which can reflect the contamination of local sources, and OMI observations are needed to improve the sensitivities to the near-surface emission sources through improvements of the retrieval algorithm or the resolution of satellites.