Characteristics and Formation Mechanisms of Winter Particulate Pollution in Lanzhou, Northwest China

Abstract

The formation mechanisms of particulate pollution in Lanzhou, which used to be one of the most polluted cities across the world, remain unclear even though air pollution in Lanzhou has been improved in recent decades. Multiple online data during 2019-2020 winter was used to analyze the characteristics and reveal the formation mechanisms of particulate pollution in Lanzhou. Organic matter, nitrate, sulfate, and ammonium accounted for 35.8%, 16.6%, 12.7%, and 10.8%, respectively, of PM 2.5 mass. The contribution of secondary inorganic aerosols decreased from low to high particulate levels. Nitrate and ammonium concentrations consistently increased with increasing PM 2.5 while sulfate concentrations firstly increased but remained stagnant under high particulate conditions. The sulfur and nitrogen oxidation ratios exhibited relatively low values of 0.18 ± 0.12 and 0.11 ± 0.05, respectively. Nitrogen oxidation ratio showed little dependence on relative humidity because of limited aqueous phase generation of nitrate. The increasing nitrate to sulfate ratio and decreasing organic carbon to elemental carbon ratio under high particulate concentrations indicate relative decreasing contribution of coal burning and increasing contribution of vehicle exhaust emissions on particulate pollution. The rapid decreasing trend of secondary organic carbon to elemental carbon ratio under high particulate concentrations was attributed to a combination of the relatively decreasing photochemical generations and increasing accumulation of primary emissions by the weakened atmospheric diffusion ability. Our results suggest that controlling primary carbonaceous species and vehicle exhaust emissions is the most effective measure in order to sustainably mitigate particulate pollution in Lanzhou in the future.

Changes in physical and chemical properties of urban atmospheric aerosols and ozone during the COVID-19 lockdown in a semi-arid region

“…4 ). Extremely low NOR of 0.11 and very low SOR of 0.19 were found during P1 partially due to semi-arid climate environment ( Du et al, 2020 ). Significant increase in both NOR and SOR was found in the afternoon (i.e., 12:00 to 20:00).…”

“…Multiple online instruments were deployed to continuously measure the PM concentrations with different sizes, aerosol optical properties, chemical compositions, gases, atmospheric visibility, and meteorological parameters. Detailed introduction of the LACMS site, instruments, and data are in Table S1 and available elsewhere ( Du et al, 2020 ; Guan et al, 2021 ; Wang et al, 2021 ). PM 2.5 chemical composition data at the LACMS site were missing from February 10, 2020 to March 3, 2020.…”

“…Lanzhou is a typical valley city in the semi-arid region of northwest China, where coal is the main fuel used for residential heating in winter ( Tan et al, 2017 ). Previous studies suggest that secondary inorganic aerosols (SIAs) in Lanzhou in winter were mainly from photochemical reactions ( Du et al, 2020 ) and motor vehicle emissions were a major contributor to PM pollution in Lanzhou ( Wang et al, 2021 ). A recent study revealed that PM 1 reduced by 50% during the lockdown, mainly contributed by the reduction in secondary species ( Xu et al, 2020 ).…”

Response of Surface Ozone to Atmospheric Aerosol Absorption is More Sensitive than to Scattering

“…The observation results are representative of the cities. Detailed descriptions of the instruments at the LACMS can be found in other studies (Du et al, 2020;Chang et al, 2022).…”

“…Lanzhou is a semi-arid basin with relatively high surface ozone and particulate pollution (Du et al, 2020). The average ozone and PM2.5 concentrations during the field campaign were 81.1 and 44.9 μg•m −3 , respectively.…”