Ground-based Multi-Axis Differential Optical Absorption Spectroscopy(MAX-DOAS) measurements of nitrous acid (HONO) and its precursor NO(nitrogen dioxide) as well as aerosols have been performed daily in Beijingcity centre (39.98° N, 116.38° E) from July 2008 toApril 2009 and at the suburban site of Xianghe (39.75° N,116.96° E) located ~60 km east of Beijing from March2010 to December 2012. This extensive dataset allowed for the first timethe investigation of the seasonal cycle of HONO as well as its diurnalvariation in and in the vicinity of a megacity. Our study was focused on theHONO and NO near-surface concentrations (0–200 m layer) and totalvertical column densities (VCDs) and also aerosol optical depths (AODs) andextinction coefficients retrieved by applying the Optimal Estimation Methodto the MAX-DOAS observations. Monthly averaged HONO near-surfaceconcentrations at local noon display a strong seasonal cycle with a maximumin late fall/winter (~0.8 and 0.7 ppb at Beijing and Xianghe,respectively) and a minimum in summer (~0.1 ppb at Beijingand 0.03 ppb at Xianghe). The seasonal cycles of HONO and NO appear tobe highly correlated, with correlation coefficients in the 0.7–0.9 and0.5–0.8 ranges at Beijing and Xianghe, respectively. The strongercorrelation of HONO with NO and also with aerosols observed in Beijingsuggests possibly larger role of NO conversion into HONO in theBeijing city center than at Xianghe. The observed diurnal cycle of HONOnear-surface concentration shows a maximum in the early morning (about 1 ppbat both sites) likely resulting from night-time accumulation, followed by adecrease to values of about 0.1–0.4 ppb around local noon. The HONO / NOratio shows a similar pattern with a maximum in the early morning (values upto 0.08) and a decrease to ~0.01–0.02 around local noon. Theseasonal and diurnal cycles of the HONO near-surface concentration are foundto be similar in shape and in relative amplitude to the corresponding cyclesof the HONO total VCD and are therefore likely driven mainly by the balancebetween HONO sources and the photolytic sink, whereas dilution effectsappear to play only a minor role. The estimation of OH radical productionfrom HONO and O photolysis based on retrieved HONO near-surfaceconcentrations and calculated photolysis rates indicate that in the 0–200 maltitude range, HONO is by far the largest source of OH radicals in winteras well as in the early morning at all seasons, while the contribution ofO dominates in summer from mid-morning until mid-afternoon.
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