(160h) The Effect of Urban Biomass Combustion for Space Heating On PM Exposure

Particulate matter (PM) is nowadays one of the most serious urban environmental pressures to public health. PM can be directly emitted from a variety of sources, both natural and anthropogenic, or generated by atmospheric reactions, leading to a complex mixture of solid particles and liquid droplets with different size and chemical composition, met in both outdoor and indoor environments. Macro and micro environmental concentrations of PM are affected by seasonality, reflecting both changes in the prevalent meteorological conditions as well as PM emission patterns, traffic emissions and domestic heating being dominant among them. The concentrations of PM10 at urban sites increase during the winter due to the combination of strictly anthropogenic urban emissions such as traffic and domestic heating, and meteorological conditions such as the prevalence of anticyclonic scenarios which limit air mass circulation from the more polluted urban sites towards the regional environment. Over the last couple of years, the use of biomass as heating source was allowed in Greece as a CO₂-neutral means of space heating in the large metropolitan areas of Athens and Thessaloniki affecting more than half of the country’s population. At the same time the use of light heating diesel was heavily taxed. In the same period Greece faces a financial crisis with significant repercussions on the average household income. This combination resulted in reduced traffic loads but excessive biomass use for domestic heating during wintertime. In this context, the current study deals with the assessment of the seasonal variability of PM exposure in the city of Thessaloniki.

A combination of measured and modeled data of outdoor and indoor PM10 and PM2.5 concentrations was fed into an integrative exposure assessment framework. Indoor concentrations were estimated with the computational tool INTERA. The latter is based on a multi-zonal mass-balance model, that takes into account the major processes governing particles concentration i.e. emissions, deposition, indoor/outdoor exchange rate, and outdoor infiltration. Exposure to PM was estimated based on the indoor/outdoor concentration data for two clusters of exposed individuals, based on whether they live in a house using a fireplace or not.  Different microenvironments were treated in different way in terms of time spent within them, and time-weighted factors were used, based on the time-activity data. By using detailed activity patterns and linking them to specific microenvironments, our methodology takes stock of an additional factor influencing the exposure-effect continuum, the inhalation rate. Different types of activities demand different levels of effort which correspond to different inhalation rates.

The measurements campaign included the assessment of outdoor and indoor air quality and the evaluation of biomass use (in open fireplaces) for domestic heating. With regard to the outdoor measurements, PM10 and PM2.5 samplers were installed at 3.5 m above ground, located at one traffic and one background site. The traffic site is surrounded by several streets with typical city heavy-traffic levels and the background site is located in an urban environment without direct traffic emissions in accordance to the EU Air Quality Directive spectifications. The measurements lasted for six months, covering the transition from the warm to the cold period and back (average daily temperatures 23 and 4 ⁰C respectively).

Parallel measurements of indoor air quality (for PM10 and PM2.5) were carried out in 30 houses allocated close to the traffic and the urban background station respectively, so as to validate INTERA model estimates. PM2.5 and PM10 samples were collected using six low-flow air samplers (ENCO PM, TCR TECORA, Italy). The samplers used sampling heads meeting the EN 12341 (PM10) and EN 14907 (PM2.5) standards, and operated at a flow-rate of 38 L/min, with a collection time of 24 h per sample. Samples were collected on PTFE membranes filters with PMP supporting ring (PALL Life Sciences, Ø 47mm, pore size 2 μm, USA). Teflon filters were analyzed gravimetrically for particle mass concentrations using an electronic microbalance with a sensitivity of ± 1 μg after 24-h equilibration at a temperature between 20 °C and 23 °C and a relative humidity (RH) between 30 and 40%. Each filter was weighed at least three times before and after sampling, and the net mass was obtained by subtracting the average of the pre-sampling weights from the average of the post-sampling weights. Differences among replicate weightings were <5 μg for the blanks and the samples. Prior to the start of the sampling campaign, the flow rate of the PM2.5 and PM10 samplers was calibrated. Field blank filters were also collected and used to correct for background concentrations or influences from handling and transport.

The same PM samplers were used for 24 h sampling of PM10 and PM2.5 in 30 indoor locations, with and without the operation of fireplace.

Ambient air data showed a significant increase of PM10 (from 30.1 to 73.1 μg/m³) and PM2.5 (from 19.4 to 62.7 μg/m³) concentrations during the transition from the warm to the cold period of the year 2012, in contrast to the previous year (2011) when this transition was accompanied by a smaller increase by 12 μg/m³ for both PM10 and PM2.5. Between the two years, significant differences exist in PM emission patterns; in 2012 the contribution of traffic is reduced, while during the colder period the contribution of biomass heating increases up to 34%. The results of the indoor measurements revealed that PM concentrations are close to the ones of the outdoor air if no strong emission source (e.g smoking indoors or combustion) is present. In any other case (i.e use of fireplace), the concentrations tend to rise significantly. Similar results were obtained also for the the days of the cold period, when the fireplaces were burning for 2-5 hours daily. In this case, average daily concentrations tended to be similar or higher than the ones outside, with an average daily increase of about 10 μg/m³ and 14 μg/m³ for PM2.5 and PM10 respectively compared to the houses where the fireplace was not in use. However, this variability is subjected to many parameters, such as the daily duration of fireplace usage, the type of wood used and the burning conditions. Very close agreement was noticed between the measured and the modeled concentrations.

Personal exposure variability follows a pattern that is strongly influenced by indoor air concentrations; this was expected considering that almost 85% of exposed individuals activities are performed indoors and 55% of time is spent in the home environment (where an additional contribution to the overall personal exposure came from the use of open fireplaces or wood stoves). A very interesting result is that for 2012, exposure to PM (mean of 47 and 42.7 μg/m³ for PM10 and PM2.5 respectively) for population living in houses not equipped with fireplaces, is higher than the one for people living in a house equipped with fireplace for year 2011 (mean of 39.6 and 34.6 μg/m³ for PM10 and PM2.5 respectively) during the cold season. For individuals exposed to indoor biomass burning in 2012, average daily exposure is up to 50.1 and 46.8 μg/m³ for PM10 and PM2.5 respectively. During the warm period, exposure is clearly higher in 2011, due to the higher ambient air concentrations, the difference being prominent for PM 2.5 (32.1 μg/m³ for 2011 instead of 20.9 μg/m³ for 2012). Beyond average exposure, intra-day variability of exposure was also investigated. For a typical intra-day winter time profile of an exposed individual living in a house with fireplace, during most of the day, exposure follows the indoors concentration profile; the latter is highly dependent on ambient air concentrations, which in turn is affected by the intensity of outdoor emission sources, mainly traffic during the day time and domestic heating during the evening and night hours. However, Figure 5 shows that actual PM intake (as expressed by the grey line that represents the personal exposure concentration modified by the activity relevant inhalation coefficient), occurs mostly during the daytime; during sleep inhalation rate is lower, thus, despite the elevated outdoor concentrations, actual intake is lower than the one estimated by the monitoring ambient and indoor air concentrations. Tracking intra-day variability, shows that peaks of ambient PM concentrations do not necessarily reflect peaks of exposure, since the timing and the location where people are performing their activities is also important. In this way late night peaks of ambient PM concentrations (related to domestic heating emissions) do not correspond to peaks of actual intake (especially for people not operating a fireplace), since people stay indoors, most of the time sleeping (thus inhalation rate is low) in an environment with minimized indoor/outdoor interaction. On the contrary, peaks of exposure (and potential intake) are related to specific activities performed outdoor (e.g. commuting by any transportation means) or indoors (use of open fireplace), verifying the need for targeted policy interventions.