The climate effects of atmospheric aerosol particles serving as cloud condensation nuclei (CCN) depend on chemical composition and hygroscopicity, which are highly variable on spatial and temporal scales. Here we present global CCN measurements, covering diverse environments from pristine to highly polluted conditions. We show that the effective aerosol hygroscopicity,κ, can be derived accurately from the fine aerosol mass fractions of organic particulate matter (ϵ_org) and inorganic ions (ϵ_inorg) through a linear combination,κ = ϵ_org ⋅ κ_org + ϵ_inorg ⋅ κ_inorg. In spite of the chemical complexity of organic matter, its hygroscopicity is well captured and represented by a global average value ofκ_org = 0.12 ± 0.02 withκ_inorg = 0.63 ± 0.01 as the corresponding value for inorganic ions. By showing that the sensitivity of global climate forcing to changes inκ_organdκ_inorgis small, we constrain a critically important aspect of global climate modelling.

A number of inorganic (nitryl chloride, ClNO₂; chlorine, Cl₂; and hypochlorous acid, HOCl) and chlorinated, oxygenated volatile organic compounds (ClOVOCs) have been measured in Manchester, UK during October and November 2014 using time-of-flight chemical ionisation mass spectrometry (ToF-CIMS) with the I⁻ reagent ion. ClOVOCs appear to be mostly photochemical in origin, although direct emission from vehicles is also suggested. Peak concentrations of ClNO₂, Cl₂ and HOCl reach 506, 16 and 9 ppt respectively. The concentrations of ClNO₂ are comparable to measurements made in London, but measurements of ClOVOCs, Cl₂ and HOCl by this method are the first reported in the UK. Maximum HOCl and Cl₂ concentrations are found during the day and ClNO₂ concentrations remain elevated into the afternoon if photolysis rates are low. Cl₂ exhibits a strong dependency on shortwave radiation, further adding to the growing body of evidence that it is a product of secondary chemistry. However, night-time emission is also observed. The contribution of ClNO₂, Cl₂ and ClOVOCs to the chlorine radical budget suggests that Cl₂ can be a greater source of Cl than ClNO₂, contributing 74 % of the Cl radicals produced on a high radiant-flux day. In contrast, on a low radiant-flux day, this drops to 14 %, as both Cl₂ production and loss pathways are inhibited by reduced photolysis rates. This results in ClNO₂ making up the dominant fraction (83 %) on low radiant-flux days, as its concentrations are still high. As most ClOVOCs appear to be formed photochemically, they exhibit a similar dependence on photolysis, contributing 3 % of the Cl radical budget observed here.

Anthropogenic biomass burning is poorly represented in models due to a lack of observational data but represents a significant source of short‐lived toxic gases. Guy Fawkes Night (bonfire night) is a regular UK‐wide event where open fires are lit and fireworks are set off on 5 November. Previous gas phase studies of bonfire night focus on persistent organic pollutants primarily using off‐line techniques. Here the first simultaneous online gas phase measurements of several classes of compounds including isocyanates, amides, nitrates, and nitro‐organics are made during bonfire night (2014) in Manchester, UK, using a time‐of‐flight chemical ionization mass spectrometer (ToF‐CIMS) using iodide reagent ions. A shallow boundary layer and low wind speeds favor pollutant buildup with typical HCN, HNCO, and CH₃NCO concentrations of tens of parts per thousand increasing by a factor of 13 to potentially harmful levels >1 ppb. Normalized excess mixing ratios relative to CO for a range of isocyanates and amides are reported for the first time. Using a HNCO:CO ratio of 0.1%, we distinguish emissions from flaming and smoldering combustion and report more accurate normalized excess mixing ratios for the distinct burning phases. While bonfire night is a highly polluting event, NO₂ concentrations measured at this location are higher at other times, highlighting the importance of traffic as an NO₂ emission source at this location. A risk communication methodology is used to equate enhancements in hourly averaged black carbon and NO₂ concentrations caused by bonfire night as an equivalent of 26.1 passively smoked cigarettes.

The multilinear engine (ME-2) factorization tool is being widely used following the recent development of the Source Finder (SoFi) interface at the Paul Scherrer Institute. However, the success of this tool, when using the a value approach, largely depends on the inputs (i.e. target profiles) applied as well as the experience of the user. A strategy to explore the solution space is proposed, in which the solution that best describes the organic aerosol (OA) sources is determined according to the systematic application of predefined statistical tests. This includes trilinear regression, which proves to be a useful tool for comparing different ME-2 solutions. Aerosol Chemical Speciation Monitor (ACSM) measurements were carried out at the urban background site of North Kensington, London from March to December 2013, where for the first time the behaviour of OA sources and their possible environmental implications were studied using an ACSM. Five OA sources were identified: biomass burning OA (BBOA), hydrocarbon-like OA (HOA), cooking OA (COA), semivolatile oxygenated OA (SVOOA) and low-volatility oxygenated OA (LVOOA). ME-2 analysis of the seasonal data sets (spring, summer and autumn) showed a higher variability in the OA sources that was not detected in the combined March–December data set; this variability was explored with the triangle plots f44 : f43 f44 : f60, in which a high variation of SVOOA relative to LVOOA was observed in the f44 : f43 analysis. Hence, it was possible to conclude that, when performing source apportionment to long-term measurements, important information may be lost and this analysis should be done to short periods of time, such as seasonally. Further analysis on the atmospheric implications of these OA sources was carried out, identifying evidence of the possible contribution of heavy-duty diesel vehicles to air pollution during weekdays compared to those fuelled by petrol.