Spatial variation of secondary inorganic PM_(2 5) exposure: from exposure magnitude to exposure distance

摘要

Secondary PM_(2.5) exposure, particles formed from precursor emissions, is currently poorly characterized. We estimate secondary PM_(2.5) exposure and exposure travel distances due to precursors such as NH_3, NOx, and SO_2 and evaluate the respective spatial variability within the US. We characterize PM_(2.5) exposure due to NH_3, NOx, and SO_2 using intake fractions (iF), the fraction of PM_(2.5) inhaled by population from a unitary precursor emission. We use a source-receptor (S-R) matrix approach based on the InMAP (Intervention Model for Air Pollution), a reduced-complexity air pollution model, to characterize exposure from ground level emissions in >40,000 source locations in the US. To investigate exposure travel distance we estimate intake travel distance of x (ITDx), distance from the source at which we reach x% of total cumulative iF, for each source. Results indicate substantial spatial variability for PM_(2.5) exposure due to NH_3 emissions in the US. Estimates of iF_(PM2.5,NH3) range between 0.01 and 41.5 ppm. Sources locations in close proximity to big cities (e.g. NY, LA) result in the highest PM_(2.5) exposure from NH_3 emissions. Similar trends are observed for SO_2 and NOx emissions, with iF(PM2.5,SO2) and iF(PM2.5,NOx) estimates spanning between 0.009-3.0 ppm and 0.002-1.3 ppm, respectively. Our analysis produced higher secondary iF(PM2.5,NH3) estimates with higher variability compared to published estimates. In addition, our analysis suggest that exposure radius is precursor dependent with NH3 having on average the shortest (ITD_(50,NH3)=70 km, 95% Cl: 0-450) and SO_2 having on average the largest (ITD_(50,SO2)=200 km, 95% Cl: 0-740) travel distance. Our results improve the exposure characterization of secondary PM_(2.5), especially from NH_3 emissions. This S-R exposure assessment approach enables us to calculate sector specific (e.g. agriculture, electricity production) average iF at local, regional or nation level that can be used in alternative chemical assessment or life cycle assessment.