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Lidar observations of metal layers play a significant role in research on the chemistry and dynamics of the mesosphere and lower thermosphere. This work reports on Fe lidar observations conducted in Beijing and Mohe. Utilizing the same laser emission system, a 1064 nm seed laser was injected into an Nd: YAG laser to generate a single longitudinal-mode pulse 532 nm laser, which pumped a dye laser to produce a 572 nm laser. The 572 nm laser and the remaining 1064 nm fundamental frequency laser passed through a sum–frequency module to generate a 372 nm laser to detect the Fe layer. According to a total of 52.6 h of observations for 10 nights in Beijing, the Fe layer has an average column density of 1.24 × 1010 cm−2, an RMS width of 4.4 km and a centroid altitude of 89.4 km. In Mohe, observed for 16 nights and a total of 91.5 h, the Fe layer has an average column density of 1.08 × 1010 cm−2, an RMS width of 4.6 km and a centroid altitude of 89.5 km. The probability of the occurrence of sporadic Fe layers was 42.4% in Beijing and 29.4% in Mohe. Compared to simultaneously observed Na layers, the occurrence probabilities of sporadic Fe layers were higher than those of sporadic Na layers in both stations. Based on the two cases observed in Beijing, it is conjectured that the formation mechanism of sporadic metal layers above approximately 100 km has a more significant influence on sporadic Fe layers than on sporadic Na layers. The lower thermospheric Fe layers with densities significantly larger than those of the main layer were observed during two nights in Mohe. This work contributes to the refinement of the global distribution of Fe layers and provides abundant observational data for the modeling and study of the metal layers. [ABSTRACT FROM AUTHOR]

Nitrous acid (HONO), as a major precursor of the hydroxyl radical (OH), plays an importantrole in tropospheric photochemistry. Due to the low OH concentration, the heterogeneousconversion of NO2 is known as the primary source of HONO at night. Nevertheless, theconversion process primarily occurs on ground surface or aerosol surface is still controversial.Therefore, we measured simultaneously vertical profiles of HONO and NO2 using theincoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS) which wasmounted on a mobile carriage of the 325-m meteorological observation tower in winterin Beijing [116˚ 23’05"N, 39˚ 58’55"E]. The carriage ascended and descendedat a rate of 0.16 ms−1 with a height limit of 240m at night, and the full verticalprofiles were measured within 1h. The time resolution of IBBCEAS was 15s (verticalresolution of 2.4m for this work) and measured the mean concentration within 2.4m.For comparison with HONO at the ground, another IBBCEAS with measurementtime resolution of 30s was mounted in a temperature-stabilized container on theground. During the entire measurement periods, based on the level of PM1.0, four differentmeteorological conditions were characterized including the clean period (P2), the transitionalperiod (P3) and the haze periods (P1 and P4). The vertical distribution of HONO is consistentwith stratification in several hundred meters of atmosphere at night. The analysis of verticalprofiles of HONO and the contribution of aerosol surface to the production of nocturnalHONO revealed that the heterogeneous conversion of NO2 on ground surface is primarysource of nocturnal HONO, and the significant HONO production on aerosol surfaces couldbe excluded at the measurement site. Acknowledgements. This work was supported by National Nature of Science Foundationof China (Grant Nos. 911544104, 41571130023). We are grateful to the team ofInstitute of Atmospheric Physics for their support, which enabled the vertical profileobservation. We also thank to researcher Pingqing Fu, researcher Yele Sun and theirgroups for providing the meteorological parameters, PM1.0 and other parameters. [ABSTRACT FROM AUTHOR]