Characteristics of PM1 over Shanghai, relationships with precursors and meteorological variables and impacts on visibility

The long-term characteristics of submicron particles (PM₁) over Shanghai and their contributing factors (including precursor gases and meteorological variables), as well as their impact on visibility, were investigated using in situ measurements from Jan 1st, 2015, to Dec 31st, 2016. A discretization method was introduced to identify the impact of each contributing factor on PM₁. The results show that the annual mean PM₁ concentration over Shanghai is ∼28 μgm^-3, which accounts for 69% of fine particles (PM_2.5). The PM₁ concentration shows obvious temporal variations on the scales of days, weeks, months, and years. Its diurnal pattern shows higher values in the daytime (with two peaks) than in the nighttime, which is different from the pattern for PM_2.5 with high/low values in the nighttime/daytime. During a week, the PM₁ concentration is the lowest on Tuesday and the highest on Friday. The discretized approach reveals that PM₁ shows good linear relationships with its gaseous precursors and with meteorological variables under most conditions. The concentration of PM₁ increases with increases in SO₂, NO₂, and NO (<34 ppb) with slopes of 3.37, 1.17, and 1.08 μgm^-3 per ppb precursor, respectively. This approach and the slopes were confirmed by the comparison of the observed and calculated PM₁ changes with the day of the week. PM₁ is negatively (positively) correlated with ozone (O₃) when O₃ is <30 (>30) ppb. PM₁ are negatively correlated with precipitation intensity, relative humidity (RH, >35%), and wind speed (>1.5 ms^-1), and their rates of decrease are 3.3, 0.26, and 5.9 μgm^-3 per 1 mmh^-1, 1%, and 1 ms^-1, respectively. Other factors (e.g., temperature and pressure) show nonlinear relationships with PM₁ concentration, presumably due to their indirect influence on the transport, formation, or accumulation of PM₁. The PM₁ concentration has a distinct impact on visibility, and the PM₁/PM_2.5 ratio is a key indicator to represent the impact of particulate matter hygroscopicity on visibility. The PM₁/PM_2.5 ratio shows an exponential relationship (i.e., PM₁/PM_2.5 = 0.76 [(1-RH)/(1-40%)]^0.11) with RH with a determination coefficient of 0.98. This parameter combined with the PM_2.5 concentration well describes the impact of particulate matter and its hygroscopicity on visibility.