Measurement report: Vertical profiling of particle size distributions over Lhasa, Tibet: Tethered balloon-based in-situ measurements and source apportionment

In-situ measurements of vertically resolved particle size distributions based on a tethered balloon system were carried out for the first time in the highland city of Lhasa over the Tibetan Plateau in summer 2020, using portable optical counters for the size range of 0.124~32 μm. The vertical structure of 112 aerosol profiles was found to be largely shaped by the evolution of the boundary layer (BL), with a nearly uniform distribution of aerosols within the daytime mixing layer and a sharp decline with the height in the shallow nocturnal boundary layer. During the campaign, the average mass concentration of particulate matters smaller than 2.5 μm in aerodynamic diameter (PM2.5) within the BL was around 3 μg m−3, almost four times of the amount in the free troposphere (FT), which was rarely affected by surface anthropogenic emissions. Though there was a lower level of particle mass in the residual layer (RL) than in the BL, a similarity in particle mass size distributions (PMSDs) suggested that particles in the RL might be of the same origin as particles in the BL. This was also in consistence with the source apportionment analysis based on the PMSDs. Three distinct modes were observed in the PMSDs for the BL and the RL. One mode was exclusively coarse particles up to roughly 15 μm and peaked around 5 μm. More than 50 % of total particle mass was often contributed by coarse mode particles in this area, which was thought to be associated with local dust resuspension. The mode peaking over 0.5~0.7 μm was representative of biomass burning on religious holidays and was found to be most pronounced on holiday mornings. The contribution from the religious burning factor rose from about 25 % on non-holidays to nearly 50 % on holiday mornings. The mode dominated by particles smaller than 0.3 μm was thought to be associated with combustion related emissions and/or secondary aerosol formation. In the FT coarse mode particles only accounted for less than 10 % of the total mass and particles larger than 5 μm were negligible. The predominant submicron particles in the FT might be related to secondary aerosol formation and the aging of existed particles. To give a full picture of aerosol physical and chemical properties and better understand the origin and impacts of aerosols in this area, intensive field campaigns involving measurements of vertically resolved aerosol chemical compositions in different seasons would be much encouraged in the future.

Dramatic changes in Harbin aerosol during 2018–2020: the roles of open burning policy and secondary aerosol formation

Despite the growing interest in understanding haze formation in Chinese megacities, air pollution has been largely overlooked for the Harbin–Changchun (HC) metropolitan area, located in the severe cold climate region in northeast China. In this study, we unfolded significant variations of fine particulate matter (PM2.5) in HC's central city (Harbin) during two sequential heating seasons of 2018–2019 and 2019–2020, and we explored major drivers for the observed variations. The two campaigns showed comparable organic carbon (OC) levels but quite different OC sources. The biomass burning (BB) to OC contribution decreased substantially for 2019–2020, which was attributed primarily to the transition of local policies on agricultural fires, i.e., from the “legitimate burning” policy released in 2018 to the “strict prohibition” policy in 2019. Meanwhile, the contribution of secondary OC (OCsec) increased significantly, associated with the much more frequent occurrences of high relative humidity (RH) conditions during the 2019–2020 measurement period. Similar to OCsec, the major secondary inorganic ions, i.e., sulfate, nitrate and ammonium (SNA), also exhibited RH-dependent increases. Given the considerable aerosol water contents predicted for the high-RH conditions, heterogeneous reactions were likely at play in secondary aerosol formation even in the frigid atmosphere in Harbin (e.g., with daily average temperatures down to below −20 ∘C). In brief, compared to 2018–2019, the 2019–2020 measurement period was characterized by a policy-driven decrease of biomass burning OC, a RH-related increase of OCsec and a RH-related increase of SNA, with the first two factors generally offsetting each other. In addition, we found that open burning activities were actually not eliminated by the strict prohibition policy released in 2019, based on a synthesis of air quality data and fire count results. Although not occurring during the 2019–2020 measurement period, agricultural fires broke out within a short period before crop planting in spring of 2020, and this resulted in off-the-charts air pollution for Harbin, with 1 h and 24 h PM2.5 concentrations peaking at ∼ 2350 and 900 µg/m3, respectively. This study indicates that sustainable use of crop residues remains a difficult challenge for the massive agricultural sector in northeast China.

Understanding aerosol composition in an inter-Andean valley impacted by sugarcane intensive agriculture and urban emissions

Agro-industrial areas are frequently affected by various sources of atmospheric pollutants that negatively impact public health and ecosystems. However, air quality in these areas is infrequently monitored because of their lower population density compared to large cities, especially in developing countries. The Cauca River Valley (CRV) is an agro-industrial region in Southwest Colombia, where a large fraction of the area is devoted to sugarcane and derivatives production. CRV is also affected by road traffic and industrial emissions. This study aims to elucidate the chemical composition of particulate matter fine mode (PM2.5) and to identify the main pollutant sources before source attribution. For this, a sampling campaign was carried out at a representative site of the CRV region, where daily-averaged mass concentrations of PM2.5 and the concentrations of water-soluble ions, trace metals, organic and elemental carbon, and various fractions of organic compounds (carbohydrates, n-alkanes, and polycyclic aromatic hydrocarbons – PAHs) were measured. Mean PM2.5 was 14.38 ± 4.35 ug m−3, and the most abundant constituent was organic material (52.99 % ± 17.79 %), followed by ammonium sulfate (16.12 % ± 3.98 %), and elemental carbon (6.95 % ± 2.52 %), which indicates secondary aerosol formation and incomplete combustion. Levoglucosan was present in all samples with a mean concentration of (113.8 ± 147.2 ng m−3) revealing biomass burning as a persistent source. The diagnostic ratios applied to organic compounds revealed the influence of petrogenic and pyrogenic sources. Principal component analysis identified the influence of traffic-generated road dust, secondary aerosol formation, gasoline and diesel combustion vehicle exhaust, vegetative detritus, and resuspended agriculture soil. However, no single component was dominant nor explained the CRV PM2.5 chemical species variance. Many components had equally important roles instead. Likewise, sugarcane pre-harvest burning, a frequent activity in CRV, was not identified as an independent component. This aerosol and trace gas source contributed to various components and was correlated to the formation of secondary aerosols.

Dramatic changes in Harbin aerosol during 2018–2020: the roles of open burning policy and secondary aerosol formation

Despite the growing interest in understanding haze formation in Chinese megacities, air pollution has been largely overlooked for the Harbin-Changchun (HC) metropolitan area located in the severe cold climate region in Northeast China. In this study, we unfolded significant variations of fine particulate matter (PM2.5) in HC’s central city (Harbin) during two sequential heating seasons of 2018–2019 and 2019–2020, and explored major drivers for the observed variations. The two campaigns showed comparable organic carbon (OC) levels but quite different OC sources. The biomass burning (BB) to OC contribution decreased substantially for 2019–2020, which was attributed primarily to the transition of local policies on agricultural fires, i.e., from the “legitimate burning” policy released in 2018 to the “strict prohibition” policy in 2019. Meanwhile, the contribution of secondary OC (OCsec) increased significantly, associated with the much more frequent occurrences of high relative humidity (RH) conditions during the 2019–2020 measurement period. Similar to OCsec, the major secondary inorganic ions, i.e., sulfate, nitrate and ammonium (SNA), also exhibited RH-dependent increases. Given the considerable aerosol water contents predicted for the high-RH conditions, heterogeneous reactions were likely at play in secondary aerosol formation even in the frigid atmosphere in Harbin (e.g., with daily average temperatures down to below −20 °C). In brief, compared to 2018–2019, the 2019–2020 measurement period was characterized by a policy-driven decrease of biomass burning OC, a RH-related increase of OCsec and a RH-related increase of SNA, with the former two factors generally offsetting each other. In addition, we found that open burning activities were actually not eliminated by the “strict prohibition” policy released in 2019, based on a synthesis of air quality data and fire count results. Although not evident throughout the 2019–2020 measurement period, agricultural fires broke out within a short period before crop planting in spring of 2020, and resulted in off-the-chart air pollution for Harbin, with 1- and 24-hour PM2.5 concentrations peaking at ~2350 and 900 μg/m3, respectively. This study indicates that sustainable use of crop residues remains a difficult challenge for the massive agricultural sector in Northeast China.

Secondary aerosol formation from dimethyl sulfide – improved mechanistic understanding based on smog chamber experiments and modelling

Dimethyl sulfide (DMS) is the dominant biogenic sulfur compound in the ambient marine atmosphere. Low-volatility acids from DMS oxidation promote the formation and growth of sulfur aerosols and ultimately alter cloud properties and Earth's climate. We studied the OH-initiated oxidation of DMS in the Aarhus University Research on Aerosol (AURA) smog chamber and the marine boundary layer (MBL) with the aerosol dynamics and gas- and particle-phase chemistry kinetic multilayer model ADCHAM. Our work involved the development of a revised and comprehensive multiphase DMS oxidation mechanism, capable of both reproducing smog chamber and atmospheric relevant conditions. The secondary aerosol mass yield in the AURA chamber was found to have a strong dependence on the reaction of methyl sulfinic acid (MSIA) and OH, causing a 82.8 % increase in the total PM at low relative humidity (RH), while the autoxidation of the intermediate radical CH3SCH2OO forming hydroperoxymethyl thioformate (HPMTF) proved important at high temperature and RH, decreasing the total PM by 55.8 %. The observations and modelling strongly support the finding that a liquid water film existed on the Teflon surface of the chamber bag, which enhanced the wall loss of water-soluble intermediates and oxidants dimethyl sulfoxide (DMSO), MSIA, HPMTF, SO2, methanesulfonic acid (MSA), sulfuric acid (SA) and H2O2. The effect caused a 64.8 % and 91.7 % decrease in the secondary aerosol mass yield obtained at both dry (0 % RH–12 % RH) and humid (50 % RH–80 % RH) conditions, respectively. Model runs reproducing the ambient marine atmosphere indicate that OH comprises a strong sink of DMS in the MBL (accounting for 31.1 % of the total sink flux of DMS) although less important than the combined effect of halogen species Cl and BrO (accounting for 24.3 % and 38.7 %, respectively). Cloudy conditions promote the production of SO42- particular mass (PM) from SO2 accumulated in the gas phase, while cloud-free periods facilitate MSA formation in the deliquesced particles. The exclusion of aqueous-phase chemistry lowers the DMS sink as no halogens are activated in the sea spray particles and underestimates the secondary aerosol mass yield by neglecting SO42- and MSA PM production in the particle phase. Overall, this study demonstrated that the current DMS oxidation mechanisms reported in literature are inadequate in reproducing the results obtained in the AURA chamber, whereas the revised chemistry captured the formation, growth and chemical composition of the formed aerosol particles well. Furthermore, we emphasize the importance of OH-initiated oxidation of DMS in the ambient marine atmosphere during conditions with low sea spray emissions.