Min Qin

Observation of the nighttime nitrate radical in Hefei, China

Observation of nighttime nitrate radical (NO3) was performed by using long path differential optical absorption spectroscopy (LP-DOAS), on the outskirts of Hefei, China. The time series of NO3 and supporting parameters were simultaneously measured for a week (31 May-7 June 2006). The results indicated that the average concentration of NO3 was 15.6 pptv with an average lifetimes of 96 s, whereas, NO3 production rates varied from 8 x 10(5)/(cm3 x s) to 2.98 x 10(7)/(cm3 x s). Furthermore, the calculated N2O5 concentration averaged at 380 pptv. Analysis of data indicated that direct sinks were probably dominating the NO3 loss process during this campaign. The results were compared with other campaigns in the boundary layer.

Measurement of Ammonia by a Portable UV-DOAS Gas Sensor Based on Multi-Pass Cell

Ammonia, the third most important abundant nitrogen compound, is a primary alkaline gas in the atmosphere. It has strong absorption bands in the deep ultraviolet (DUV) spectral range and so can be reliably detected by the differential optical absorption spectroscopy (DOAS) technique. A portable UV-DOAS gas sensor based on multi-pass cell has been designed to detected trace gases, especially for ammonia, in the DUV spectral range, with good performance using a broad-band Deuterium source and high-sensitivity spectrometer. With the optical path as long as 20m, such a sensor could detected NH3 concentrations as low as 100ppb according to the result of in-situ measurement. Fast response time and low measurement error of this portable gas sensor could be competent for emergency monitoring.

Monitoring of SO2 emissions from industry by passive DOAS

Sulfur dioxide is a highly toxic air contaminant that harms human health and damages the environment. It is easily converted to sulfuric acid which is the major component in acid rain and to sulfate particles. Coal-burning power plants are the main sources of SO2 pollution. It is necessary to evaluate the emissions from industry for emission reduction strategies. Passive DOAS method has been successfully applied in volcanic plume and atmosphere monitoring because of its advantage of relative simple system with no light source. Here we report the measurement of SO2 total flux from a chimney in plant area in Hefei city (China) with a compact passive DOAS system. The system consists of a small telescope pointing zenith direction and a fibre-coupled OceanOptics USB2000 spectrometer. In the measurement the system was mounted on a mobile platform moving under the plume approximately perpendicular to the plume transport direction and the spectra of the zenith direction were recorded. By combining the integrated gas concentration over the plume cross section with wind velocity data SO2 flux was estimated.

Development of stripping coil-ion chromatograph method and intercomparison with CEAS and LOPAP to measure atmospheric HONO.

Nitrous acid (HONO) is the major precursor of OH radicals in polluted areas. Accurate measurement of HONO provides vital evidence for exploring the formation of secondary pollution. Stripping coil (SC) equipped with ion chromatograph (IC) or spectrograph as one of wet chemical methods has been already used to measure HONO. The reliability of the method mainly depends on the collection efficiency and the interference from other species. In this study, a SC-IC method was set up to measure HONO. The performance of the method was assessed in the chamber using two kinds of absorption solutions i.e. ultrapure water and 25 μM Na2CO3 solution under different concentrations of SO2. Results indicated that HONO concentrations absorbed by ultrapure water and Na2CO3 solution were almost identical in the absence of SO2 in the chamber and both the collection efficiencies were >99%. However, the collection efficiency of ultrapure water decreased with the increase of SO2, indicating that the presence of SO2 resulted in the penetration of HONO. The collection efficiency kept >90% when the concentration of SO2 was no >23 ppbv. Comparing with the situation without SO2, HONO performed a remarkable increase with the presence of SO2 when using Na2CO3 absorption solution, indicating that the extra generation of HONO from the reaction between SO2 and NO2 in alkaline solution. Consequently, ultrapure water as the absorption solution could provide a high collection efficiency and avoid the interferences from SO2 when the concentration of SO2 was below 23 ppbv. High correlations (slope = 0.94-1.06, r2 > 0.90) were found during the intercomparisons between SC-IC and other three techniques, suggesting the SC-IC method developed in this study was able to measure atmospheric HONO in the field campaigns.

Long-term observations of tropospheric NO2, SO2 and HCHO by MAX-DOAS in Yangtze River Delta area, China

Yangtze River Delta (YRD) area is one of the important economic zones in China. However, this area faces increasing environmental problems. In this study, we use ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) network in Eastern China to retrieve variations of NO2, SO2, and formaldehyde (HCHO) in the YRD area. Three cities of YRD (Hefei, Nanjing, and Shanghai) were selected for long-term observations. This paper presents technical performance and characteristics of instruments, their distribution in YRD, and results of vertical column densities (VCDs) and profiles of NO2, SO2, and HCHO. Average diurnal variations of tropospheric NO2 and SO2 in different seasons over the three stations yielded minimum values at noon or in the early afternoon, whereas tropospheric HCHO reached the maximum during midday hours. Slight reduction of the pollutants in weekends occurred in all the three sites. In general trace gas concentrations gradually reduced from Shanghai to Hefei. Tropospheric VCDs of NO2, SO2, and HCHO were compared with those from Ozone Monitoring Instrument (OMI) satellite observations, resulting in R2 of 0.606, 0.5432, and 0.5566, respectively. According to analysis of regional transports of pollutants, pollution process happened in YRD under the north wind with the pollution dissipating in the southeast wind. The feature is significant in exploring transport of tropospheric trace gas pollution in YRD, and provides basis for satellite and model validation.

Development of an incoherent broadband cavity enhanced absorption spectrometer for in situ measurements of HONO and NO 2 in China

Gaseous nitrous acid (HONO) is an important source of OH radicals in the troposphere. However, its source, especially that during daytime hours remains unclear. We present an instrument for simultaneous unambiguous measurements of HONO and NO2 with high time resolution based on incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS). To achieve robust performance and system stability under different environment conditions, the current IBBCEAS instrument has been developed with significant improvements in terms of efficient sampling as well as resistance against vibration and temperature change, and the IBBCEAS instrument also has low power consumption and a compact design that can be easily deployed on different platforms powered by a high-capacity lithium ion battery. The effective cavity length of the IBBCEAS was determined using the absorption of O2-O2 to account for the “shortening” effect caused by the mirror purge flows. The wall loss for HONO was estimated to be 2.0 % via a HONO standard generator. Measurement precisions (2σ ) for HONO and NO2 are about 180 and 340 ppt in 30 s, respectively. A field inter-comparison was carried out at a rural suburban site in Wangdu, Hebei Province, China. The concentrations of HONO and NO2 measured by IBBCEAS were compared with a long optical path absorption photometer (LOPAP) and a NOx analyzer (Thermo Fisher Electron Model 42i), and the results showed very good agreement, with correlation coefficients (R2) of HONO and NO2 being ∼ 0.89 and ∼ 0.95, respectively; in addition, vehicle deployments were also tested to enable mobile measurements of HONO and NO2, demonstrating the promising potential of using IBBCEAS for in situ, sensitive, accurate and fast simultaneous measurements of HONO and NO2 in the future.

Measurements of atmospheric NO3 radicals in Hefei using LED-based long path differential optical absorption spectroscopy*

NO3 radicals accumulate during the night, thereby being the most critical night oxidant. Owing to the low concentration and dramatic variation, the detection of atmospheric NO3 radicals is still challenging. In this paper, an LED-based Long Path Differential Optical Absorption Spectroscopy (LPDOAS) instrument is developed for measuring the atmospheric NO3 radicals. This instrument is composed of a Schmidt–Cassegrain telescope, a combined emitting and receiving fiber, and a red LED equipped with a thermostat, and has a center wavelength of 660 nm, covering the NO3 strongest absorption peak (662 nm). The influence of LED temperature fluctuations is discussed. The temperature of the LED lamp with a home-made thermostat is tested, showing a stability of ±0.1 °C. The principle and fitting analyses of LED-LPDOAS are presented. A retrieval example and a time series of NO3 radical concentrations with good continuity for one night are shown. The detection limit of NO3 for 2.6-km optical path is about 10 ppt.

An incoherent broadband cavity enhanced absorption spectrometer on mobile platforms for measurements of HONO and NO 2

HONO is a source of OH via photolysis. An incoherent broadband cavity enhanced absorption spectrometer on mobile platforms has been developed for simultaneous measuring the spatial distribution of HONO and NO2

Atmopsheric NO 3 radical detection by spectroscopic techniques

Three detection methods of Differential optical absorption spectroscopy, cavity ring-down spectroscopy and cavity enhanced absorption spectroscopy for atmospheric NO3 radical measurements are introduced.

Simultaneous measurements of atmospheric NO2 and HONO using IBBCEAS with a near-ultraviolet LED

High sensitivity simultaneous measurements of atmospheric NO2 and HONO using incoherent broadband cavityenhanced absorption spectroscopy (IBBCEAS) were developed. A near-ultraviolet light emitting diode (LED) peaked at 372 nm was used as light source of the IBBCEAS instrument to measure the absorption of NO2 and HONO in the spectral range of 361-378 nm. Concentrations of atmospheric NO2 and HONO were retrieved from the absorption spectra recorded in the absence of atmospheric aerosols and the average detection limits of 2.9 ppbv for NO2 and 1.2 ppbv for HONO with an acquisition time of 5 min were achieved. The results demonstrated high sensitivity of this measurement technique based on IBBCEAS, which is a promising technique for measurements of atmospheric trace gases.