Abstract
Abstract. Hydrocarbons are key precursors to two priority air pollutants, ozone and particulate matter. Those with two to seven carbons have historically been straightforward to observe and have been successfully reduced in many developed cities through air quality policy interventions. Longer chain hydrocarbons released from diesel vehicles are not considered explicitly as part of air quality strategies and there are few direct measurements of their gaseous abundance in the atmosphere. This study describes the chemically comprehensive and continuous measurements of organic compounds in a developed megacity (London), which demonstrate that on a seasonal median basis, diesel-related hydrocarbons represent only 20–30 % of the total hydrocarbon mixing ratio but comprise more than 50 % of the atmospheric hydrocarbon mass and are a dominant local source of secondary organic aerosols. This study shows for the first time that 60 % of the winter primary hydrocarbon hydroxyl radical reactivity is from diesel-related hydrocarbons and using the maximum incremental reactivity scale, we predict that they contribute up to 50 % of the ozone production potential in London. Comparing real-world urban composition with regulatory emissions inventories in the UK and US highlights a previously unaccounted for, but very significant, under-reporting of diesel-related hydrocarbons; an underestimation of a factor ~4 for C9 species rising to a factor of over 70 for C12 during winter. These observations show that hydrocarbons from diesel vehicles can dominate gas phase reactive carbon in cities with high diesel fleet fractions. Future control of urban particulate matter and ozone in such locations requires a shift in policy focus onto gas phase hydrocarbons released from diesels as this vehicle type continues to displace gasoline world-wide.
Highlights
With an increasing proportion of the world’s population living in cities, rising from only 3 % in the 1800’s to over 47 % by the end of the 20th Century (Gaffney and Marley, 2009), the impact of urban air pollution has become a significant factor in global health (Harrison et al, 2012)
Primary urban air pollution emissions are dominated by particulate matter (PM), nitrogen oxides (NOx), carbon monoxide (CO) and volatile organic compounds (VOCs)
Many of these species can react in the atmosphere to create secondary pollutants, such as ozone (O3), oxygenated VOCs (OVOCs), peroxy acyl nitrates (PANs) and condensed materials in the form of secondary organic aerosol (SOA), which add to the overall PM load (Robinson et al, 2007; Atkinson and Arey, 2003; Odum et al, 1997)
Summary
With an increasing proportion of the world’s population living in cities, rising from only 3 % in the 1800’s to over 47 % by the end of the 20th Century (Gaffney and Marley, 2009), the impact of urban air pollution has become a significant factor in global health (Harrison et al, 2012). Primary urban air pollution emissions are dominated by PM, nitrogen oxides (NOx), carbon monoxide (CO) and volatile organic compounds (VOCs). The efficiency with which O3 and SOA can be formed from diesel or gasoline emissions is dependent on the mass of available organic carbon, and the reactivity and volatility of that material (Gros et al, 2007; Jimenez et al, 2009; Donahue et al, 2012) To quantify this requires individual speciation of VOCs in order that each property can be properly estimated. Several recent field studies have investigated the relative importance of gasoline, diesel and biogenic emissions in generating SOA (Gentner et al, 2012; Bahreini et al, 2012; Gordon et al, 2013; Platt et al, 2013; Gordon et al, 2014a, b; Jathar et al, 2014; Ensberg et al, 2014) These have been carried out predominately in the US, in California, where current diesel usage is rather low by global standards. By comparison to the emission inventories, we highlight a severe underestimation in the impact of gaseous VOC emissions from diesel on urban air quality that is likely replicated across Europe and other cities globally where diesel vehicle use is high
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