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Air pollution is associated with increased risk of myocardial infarction (MI), but it is unresolved to what extent the association is modified by factors such as socioeconomic status, comorbidities, financial stress, residential green space, or road traffic noise. We formed a cohort of all (n = 1,964,702) Danes, aged 50–85 years, with 65,311 cases of MI during the followed-up period 2005–2017. For all participants we established residential five-year running average exposure to particulate matter 2.5), ultrafine particles (UFP, 2). We evaluated risk in population strata, using Aalen additive hazards models to estimate absolute risk and Cox proportional hazards models to estimate relative risk of MI with 95% confidence intervals (CI). PM2.5 and the other pollutant were associated with MI. Lower education and lower income were associated with higher absolute risks of MI from air pollution, whereas no clear effect modification was apparent for relative risk estimates. For example, 5 μg/m3 higher PM2.5 was associated with HR for MI of 1.16 (95% CI: 1.10–1.22) among those with only mandatory education and 1.13 (95% CI: 1.03–1.24) among those with long education. The corresponding rate differences per 100,000 person years were 243 (95% CI: 216–271) and 358 (95% CI: 338–379), respectively. Higher level of comorbidity was consistently across all four pollutants associated with both higher absolute and relative risk of MI. In conclusion, people with comorbid conditions or of lower SES appeared more vulnerable to long-term exposure to air pollution and more cases of MI may be prevented by focused interventions in these groups.

Modelling of ambient particle number concentrations (PNC) has been implemented in the Danish air quality modelling system DEHM/UBM/AirGIS and evaluated with long-term measurements. We implemented particle dynamical processes in the regional scale model DEHM using the M7 aerosol dynamics module (presented in the accompanying article by Frohn et al., 2021), and we developed models for PNC at the local scale (UBM) and street scale (OSPM), in a first approximation without including the particle dynamics as presented in this article. Outdoor concentration estimates are provided at the front door of all residential address locations in Denmark for the past 40 years (1979–2018) with a spatial resolution of 1 km × 1 km taking all emission sectors in Denmark into account and additionally at the street location, with significant traffic (>500 vehicles/day). We evaluated our model with up to 18-year long measurement time series of particle number size distributions (PNSD) at Danish street, urban and rural background stations. Two particle size ranges were used for evaluation: PNC>10 (count of particles with diameter larger than 10 nm) and PNC30_250 (diameter range 30–250 nm), in order to exclude nucleation events from the measurements and to obtain a consistent long-term measured time series. When comparing our model estimates with PNC30_250 measurements, we obtain Pearson correlation coefficients (Rp) in the range 0.39–0.95 depending on station location (street, urban background, rural) and averaging time (hour, day, month, year). The highest correlations were found for yearly averages at a monitoring station located at a street with dense traffic (Rp = 0.95) whereas shorter time averages and comparisons with monitoring stations at urban and rural background locations provided lower correlations. The model performance for PNC in terms of correlation coefficients with respect to measurements is comparable to the performance for other pollutants such as NOX, PM2.5 and better than the performance for PM10. The model generally overestimated the observed concentrations, Normalized Mean Bias (NMB) was in the range 6%–190% compared to PNC>10 and 90%–290% compared to PNC30_250. These relatively high NMBs are probably caused by uncertainties in the modelling process, especially the estimation of particle number emissions, which largely determine the ambient concentrations of PNC. Furthermore, uncertainties might as well originate from the complexity of modelling particle dynamical processes accurately and the great challenges in performing long-term PNC measurements. The presented model can estimate PNC at all Danish addresses over the last 40 years with a 1-h time resolution. The data seem to provide a good indication of the relative differences in PNC at Danish addresses.

Physical and chemical properties of submicrometer aerosol particles were measured in summer 2004 (June/July) and winter 2005 (January/February) in Beijing, Peoples Republic of China, using a Twin-Differential Mobility Particle Sizer (T-DMPS), a Hygroscopicity-Tandem Differential Mobility Analyzer (H-TDMA), and a Micro Orifice Uniform Deposit Impactor (MOUDI). Particle number–size distributions were measured in the diameter range Dp = 3–800 nm and hygroscopic properties were determined at initial dry particle diameters of Dp j ( j = 30, 50, 80, 150, 250, and 350 nm) at a relative humidity (RH) of 90%. Hygroscopic properties were compared with chemical analyses of aerosol samples taken with the MOUDI. Based on the hygroscopicity data, the total hygroscopic particle volume was modeled, including dependence on dry particle size, season and level of pollution using a simple approach. Overall, the chemical analysis showed ammonium sulfate to be the major inorganic component of the urban submicrometer aerosol in Beijing along with relatively high fractions of elemental carbon (10–25%) and organic matter (15–60%) depending on particle size and season. The hygroscopic growth distributions (H-TDMA) subdivided the aerosol population into three different groups of particles with varying growth factors depending on dry particle size, namely nearly hydrophobic (growth factor = 0.96–1.07), less hygroscopic (1.06–1.29) and more hygroscopic (1.26–1.62). Hydrophobic particle fractions indicating freshly emitted soot/carbonaceous particles varied between 10 and 32% depending on dry particle size and season. During heavily polluted times, a decreasing number of hydrophobic particle fractions indicated that the urban submicrometer aerosol in Beijing was highly influenced by more aged aerosol transported from the industrial regions around Beijing containing sulfate as a major component. Based on model calculations, the urban submicrometer aerosol in Beijing showed strong compositional variations. The calculated total hygroscopic volume fractions varied between 16 and 65% depending on size, level of pollution and season.

This development describes a design of a high humidity tandem differential mobility analyzer (HHTDMA) to determine aerosol hygroscopic growth factors at relative humidities (RH) between 90% and 98% under highly temperature-stabilized conditions. Two temperature baths are used to thermally stabilize the humidity conditioning section and the second DMA and to maintain a slightly lowered temperature in the second DMA relative to the particle humidity conditioning section. The reduced temperature of the second DMA results in constant and higher RHs between 90% and 98%. We describe the basic system design and results from temperature and RH stability tests. In the HHTDMA-system, the absolute temperatures can be maintained within ± 0.1 K and with a long-term stability of 0.02 K. The RH in the second DMA reaches an absolute accuracy of ± 1.2 % for 98% and a long-term stability of ± 0.1 –0.4% of set point values. To demonstrate the capability of the HHTDMA, hygroscopic growth factors of ammonium sulfate aerosol between 84% and 98% RH were determined. The experimental results agree with theoretical values within the uncertainties of the system, with values based on measurements using a single-particle levitation technique, and growth factors of other TDMA experiments in the range up to 94% RH.

Within the ground-based cloud passage experiment FEBUKO, which was carried out at the mountain ridge Thuringer Wald (Germany) during October 2001 and 2002, the dry number size distribution and hygroscopic growth of aerosol particles upwind cloud and the dry number size distributions of interstitial particles and cloud droplet residuals inside cloud were measured at connected flow conditions. The connected flow between the upwind and in-cloud summit site was meteorologically predicted and experimentally confirmed for three selected cloud events. For these events, it could be verified that entrainment and droplet deposition had only a minor influence on the evolution of the particle size distribution between the two sites. Hence, the size resolved soluble volume fraction of the cloud input aerosol particles determined from the hygroscopic growth measurements could be related to the particle activation inferred from the particle size distributions observed inside cloud. The shape and steepness of the scavenging fraction as a function of particle diameter was found to correlate with the increase of soluble volume fraction with size, which had implications for the droplet activation diameter of the cloud condensation nuclei (CCN) that ranged between 110 and 180 nm. The minimum soluble volume fraction e min that was required to serve as CCN was determined for three different dry diameters from the relation of the particle volume fraction and scavenging fraction. From the comparison with e min predictions from classical Kohler theory it is inferred that aerosol particles remained in the interstitial phase although they should have been activated. A discussion of different processes which have the general ability to explain this finding favoured the hypothesis of organic surface films retarding the uptake of water molecules.

As a contribution to the joint research project FEBUKO, hygroscopic properties of atmospheric Aitken and accumulation mode particles were measured in the Thueringer Wald, Germany, using a hygroscopicity-tandem differential mobility analyser (H-TDMA). The hygroscopic growth of particles with initial dry diameters of D p = 50 , 150, and 250 nm at 90% relative humidity was used to calculate average distributions of the soluble volume fraction with respect to the hygroscopic growth of ammonium sulphate. The application of this parameterisation procedure was tested by analysing the dataset with respect to the dependence of the soluble volume fraction on particle size and air masses character. With increasing dry particle size, the fraction of particles containing high soluble volume fractions was found to increase. The number of accumulation mode particles in marine air masses passing the sampling site having a large soluble volume fraction was significantly higher than in air masses of more continental character. For particles with D p = 50 nm , no air mass dependence of the soluble volume fraction was found. In marine air masses, particles with D p = 150 and 250 nm are assumed to undergo similar evolution processes, whereas in continental air masses this seemed to be the case for particles with D p = 50 nm and 150 nm.

The concentration and size distribution of atmospheric particles were traced upwind, inside and downwind a hill cap cloud within the ground-based cloud passage experiment FEBUKO, which was carried out at the mountain ridge Thuringer Wald (Germany) during October 2001 and 2002. Three cloud events were examined in detail for which the connected flow between all three sites was meteorologically confirmed and the influence of entrainment and local sources on particle concentration and size distribution was only of minor importance. Modifications of the number size distributions between 60 and 300 nm particle diameter that are attributed to cloud processing were observed comparing upwind and downwind dry particle size distributions. Despite a small interference of droplet deposition, a maximum in-cloud mass production of 0.38 μg m−3 was found accounting for 5% of the upwind aerosol mass. The corresponding mass production rate was estimated to 1.16 μg m−3 h−1. A formation of new ultra-fine particles was detected in the outflow of the orographic clouds during nighttime yielding number concentrations up to 300 cm−3 at a pre-existing dry particle surface area of about 300 μm2 cm−3. At a more than twice as high particle surface area, i.e. when adsorption of condensable gases on existing particles became more important, no new particle formation was observed. Whereas the in-cloud mass production proceeded rather continuously throughout the cloud events, the particle production occurred during periods of 15 min up to 2 h within the cloud events. In the absence of actinic radiation, ternary nucleation of gaseous substances outgassing from the evaporating droplets at high relative humidity is hypothesized as the most likely particle formation mechanism but the reason for the temporary appearance of the particle production is not known.