Abstract. Delhi, India, routinely experiences some of the world's highest urban
particulate matter concentrations. We established the Delhi Aerosol Supersite
study to provide long-term
characterization of the ambient submicron aerosol composition in Delhi. Here
we report on 1.25 years of highly time-resolved speciated submicron
particulate matter (PM1) data, including black carbon (BC) and
nonrefractory PM1 (NR-PM1), which we combine to develop a
composition-based estimate of PM1
(“C-PM1” = BC + NR-PM1) concentrations. We observed marked seasonal and diurnal variability in the concentration and
composition of PM1 owing to the interactions of sources and atmospheric
processes. Winter was the most polluted period of the year, with average
C-PM1 mass concentrations of ∼210 µg m−3. The monsoon was hot and rainy, consequently
making it the least polluted (C-PM1 ∼50 µg m−3) period. Organics constituted more than half
of the C-PM1 for all seasons and times of day. While ammonium, chloride,
and nitrate each were ∼10 % of the C-PM1 for the cooler
months, BC and sulfate contributed ∼5 % each. For the warmer
periods, the fractional contribution of BC and sulfate to C-PM1
increased, and the chloride contribution decreased to less than 2 %. The
seasonal and diurnal variation in absolute mass loadings were generally
consistent with changes in ventilation coefficients, with higher
concentrations for periods with unfavorable meteorology – low
planetary boundary layer height and low wind speeds. However, the variation
in C-PM1 composition was influenced by temporally varying sources,
photochemistry, and gas–particle partitioning. During cool periods when wind
was from the northwest, episodic hourly averaged chloride concentrations
reached 50–100 µg m−3, ranking
among the highest chloride concentrations reported anywhere in the world. We estimated the contribution of primary emissions and secondary processes to
Delhi's submicron aerosol. Secondary species contributed
almost 50 %–70 % of Delhi's C-PM1 mass for the
winter and spring months and up to 60 %–80 % for the warmer summer
and monsoon months. For the cooler months that had the highest C-PM1
concentrations, the nighttime sources were skewed towards primary sources,
while the daytime C-PM1 was dominated by secondary species. Overall,
these findings point to the important effects of both primary emissions and
more regional atmospheric chemistry on influencing the extreme particle
concentrations that impact the Delhi megacity region. Future air quality
strategies considering Delhi's situation in both a regional
and local context will be more effective than policies targeting only local,
primary air pollutants.
Submicron aerosol composition in the world's most polluted
megacity: The Delhi Aerosol Supersite campaign