## 2 General Context
### Overview of BC/PAH/PM\({}_{2.5}\) Emissions  
In the EU27+UK, the main emitting sector is the residential sector (Table 3), followed by road transport and open burning of waste (Figure 2). The lowest ratio BC/PM\({}_{2.5}\) (10%) reported in the EEA guidebook [30; 47] is also for the residential combustion sectors (Figure 3) for solid fuels. This low ratio refers to previous works of [48; 49; 50] and is based on an emission factors measurement method which accounts for condensables (see Section 3.5). There are rather major issues surrounding condensable emissions for the residential sector in Europe [51]. [PERSON] et al. [52] summarises the issues in the context of national reporting under the CLRTAP and the EMEP. The concept of condensable is explained later in this review (see Section 3.5).  
\begin{table}
\begin{tabular}{l c c c c c c} \hline \hline  & \multicolumn{2}{c}{**LDVs, LCVs Class 1 (a)**} & \multicolumn{2}{c}{**LCVs Class 2**} & \multicolumn{2}{c}{**LCVs Class 3**} \\  & **Gasoline (b)** & **Diesel (c)** & **Gasoline** & **Diesel** & **Gasoline** & **Diesel** \\ \hline NMHC * & 68 & - & 90 & - & 108 & - \\ THC * & 100 & - & 130 & - & 160 & - \\ NOC * & 60 & 80 & 75 & 105 & 82 & 125 \\ THC + NOx * & - & 170 & - & 195 & - & 215 \\ _CO *_ & 1000 & 500 & 1810 & 630 & 2270 & 740 \\ _PM *_ & 4.5 (d) & 4.5 (d) & 4.5 (d) & 4.5 (d) & 4.5 (d) & 4.5 (d) & 4.5 \\ _PN **_ & \(6\times 10^{11}\)(d) & \(6\times 10^{11}\)(d) & \(6\times 10^{11}\)(d) & \(6\times 10^{11}\)(d) & \(6\times 10^{11}\)(d) & \(6\times 10^{11}\) \\ \hline \hline \end{tabular} Notes: (a) Classes 1 through 3 are weight classes. (b) Gasoline is used as a proxy term for positive-ignition (PI) engines. (c) Diesel is used as a proxy term for compression-ignition (CI) engines. (d) Applicable to direct-injection engines.* unit in mg km\({}^{-1}\), ** unit in \(\#\) km\({}^{-1}\).  
\end{table}
Table 2: Euro 6 emission limits for passenger cars and light commercial vehicles; table from [PERSON] et al. [44].  
Figure 1: Simplified diagram of emission limits and phase-in timing in four of the main world regions (LEV: low—emission vehicle, ULEV: ultra-low-emission vehicle, WLTP: World LDV Test Procedure, FTP: Federal Test Procedure, SFTP: supplemental FTP, and RDE: real driving emissions).  
Figure 3: Ratio BC/PM\({}_{2.5}\) for EU28 official emissions in 2018 computed from the EMEP database [53] for the main emitting NFR sectors (refer to Table 3).  
\begin{table}
\begin{tabular}{l l} \hline \hline
**Code** & **Name** \\ \hline
1A4 bi & Residential: stationary \\
1A3 bi & Road transport: passenger car \\
5C2 & Open burning of waste \\
11B & Forest fires (natural wildfires) \\
1A3 di(i) & International maritime shipping \\
1A4 cii & Agriculture/Forestry/Fishing: off-road vehicles and other machinery \\
1A3 bii & Road transport: light-duty vehicles \\
1A3 biii & Road transport: heavy-duty vehicles and buses \\
1A2 gviii & Other stationary combustion in manufacturing industries and construction \\
1A2 gvii & Off-road mobile sources in manufacturing industries and construction \\ \hline \hline \end{tabular}
\end{table}
Table 3: Top ten BC-emitting activity sectors in EU28 in 2018 (sorted from the highest to the lowest emitter) according to the EMEP database [53].  
Figure 2: EU28 official emissions in kt year\({}^{-1}\) or t year\({}^{-1}\) (Gg or Mg year\({}^{-1}\)) for BC, PM\({}_{2.5}\) and PAHs (as the sum of benzo(a)pyrene, benzo(b)fluoranthene, benzo(k)fluoranthene and indeno(1,2,3-cd)pyrene) in 2018 for the main emitting NFR sectors (refer to Table 3). Emissions are computed from the EMEP database [53].