Four years of ground-based MAX-DOAS observations of HONO and NO<sub>2</sub> in the Beijing area

Abstract

Abstract. Ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements of nitrous acid (HONO) and its precursor NO 2 (nitrogen dioxide) as well as aerosols have been performed daily in Beijing city centre (39.98 • N, 116.38 • E) from July 2008 to April 2009 and at the suburban site of Xianghe (39.75 • N, 116.96 • E) located ∼ 60 km east of Beijing from March 2010 to December 2012. This extensive dataset allowed for the first time the investigation of the seasonal cycle of HONO as well as its diurnal variation in and in the vicinity of a megacity. Our study was focused on the HONO and NO 2 near-surface concentrations (0-200 m layer) and total vertical column densities (VCDs) and also aerosol optical depths (AODs) and extinction coefficients retrieved by applying the Optimal Estimation Method to the MAX-DOAS observations. Monthly averaged HONO near-surface concentrations at local noon display a strong seasonal cycle with a maximum in late fall/winter (∼ 0.8 and 0.7 ppb at Beijing and Xianghe, respectively) and a minimum in summer (∼ 0.1 ppb at Beijing and 0.03 ppb at Xianghe). The seasonal cycles of HONO and NO 2 appear to be highly correlated, with correlation coefficients in the 0.7-0.9 and 0.5-0.8 ranges at Beijing and Xianghe, respectively. The stronger correlation of HONO with NO 2 and also with aerosols observed in Beijing suggests possibly larger role of NO 2 conversion into HONO in the Beijing city center than at Xianghe. The observed diurnal cycle of HONO near-surface concentration shows a maximum in the early morning (about 1 ppb at both sites) likely resulting from night-time accumulation, followed by a decrease to values of about 0.1-0.4 ppb around local noon. The HONO / NO 2 ratio shows a similar pattern with a maximum in the early morning (values up to 0.08) and a decrease to ∼ 0.01-0.02 around local noon. The seasonal and diurnal cycles of the HONO near-surface concentration are found to be similar in shape and in relative amplitude to the corresponding cycles of the HONO total VCD and are therefore likely driven mainly by the balance between HONO sources and the photolytic sink, whereas dilution effects appear to play only a minor role. The estimation of OH radical production from HONO and O 3 photolysis based on retrieved HONO near-surface concentrations and calculated photolysis rates indicate that in the 0-200 m altitude range, HONO is by far the largest source of OH radicals in winter as well as in the early morning at all seasons, while the contribution of O 3 dominates in summer from mid-morning until mid-afternoon.

Evaluation of tropospheric SO<sub>2</sub> retrieved from MAX-DOAS measurements in Xianghe, China

“…The temperature-pressure profiles are obtained from the US standard atmosphere. S ε and S a matrices are similar as in Clémer et al (2010) and Hendrick et al (2014). S ε is a diagonal matrix, with variances equal to the square of the DOAS fitting error.…”

“…It is based on the measurement of sunlight scattered at multiple elevation angles towards the horizon, thus increasing the sensitivity to absorbers present close to the ground compared to the zenith viewing geometry (Hönninger et al, 2004). MAX-DOAS studies published so far have been mainly focused on the retrieval of NO 2 (e.g., Wittrock et al, 2004;Vlemmix et al, 2010;Frins et al, 2012;Hendrick et al, 2014;Ma et al, 2013;Wang et al, 2014), halogen oxides like BrO and IO (e.g., Frieß et al, 2011;Großmann et al, 2013), formaldehyde (e.g., Heckel et al, 2005;Wagner et al, 2011), and aerosols (e.g., Wagner et al, 2004;Frieß et al, 2006;Clémer et al, 2010). A lot of work has been done on MAX-DOAS measurements of volcanic SO 2 (e.g., Bobrowski et al, 2007;Galle et al, 2010), but so far only a few studies deal with MAX-DOAS observations of this species in polluted areas (e.g., Irie et al, 2011;Lee et al, 2008;Wu et al, 2013), despite the fact that, as for other trace gases like NO 2 , HCHO, and BrO, the combination of both surface concentration and VCD retrievals makes MAX-DOAS a useful technique for validating SO 2 satellite data.…”

“…For each scan, VCD a is derived using the geometrical approximation method; that is, the SO 2 layer is assumed to be located below the scattering altitude at 30 • EVA, so that tropospheric SO 2 VCDs can be derived by applying a geometrical air mass factor (AMF) to measured 30 • EVA DSCDs (Hönninger et al, 2004;Brinksma et al, 2008; see also Hendrick et al, 2014). In case of aerosols, a fixed AOD of 0.2 is used.…”

“…SO 2 vertical profiles are retrieved for each MAX-DOAS scan by using the bePRO profiling tool developed at BIRA-IASB (Clémer et al, 2010; see also Hendrick et al, 2014). It is based on the optimal estimation method (Rodgers, 2000) and includes the LIDORT radiative transfer model (RTM) as a forward model.…”

Reactive bromine in the low troposphere of Antarctica. Estimations at two research sites

“…The procedure consisted on a two-step approach. First, the AEC were retrieved from the observed O4 dSCDs through an iterative nonlinear process (e.g., Hendrick et al, 2014;Córdoba-Jabonero et al, 2016). The inferred AEC was then used to invert the targeted BrO profiles.…”